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Liquid-liquid phase separation in atmospherically relevant particles You, Yuan 2014

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LIQUID-LIQUID PHASE SEPARATION IN ATMOSPHERICALLY RELEVANT PARTICLES  by Yuan You  B.S., Shanghai Jiao Tong University, 2009  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DRGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry)  THE UNIVERISTY OF BRITISH COLUMBIA (Vancouver)  September 2014  © Yuan You, 2014   ii  Abstract  Aerosol particles containing both organic material and inorganic salts are abundant in the atmosphere. These particles may undergo phase transitions when the relative humidity fluctuates between high and low values in the atmosphere. This dissertation focuses on liquid-liquid phase separation in atmospherically relevant mixed organic-inorganic salt particles. Liquid-liquid phase separation has potentially important implications in chemical and physical processes in the atmosphere.  A humidity and temperature controlled flow cell coupled to either an optical, fluorescence, or Raman microscope was used to study the occurrence of liquid-liquid phase separation and the phase separation relative humidity (SRH) of particles containing atmospherically relevant organic species mixed with inorganic salts. Organic species in the particles studied include single organic species, such as carboxylic acids, alcohols, and oxidized aromatic compounds, as well as complex laboratory-produced secondary organic material. Material directly collected from the atmospheric environment was also studied. In this dissertation, the effects of oxygen-to-carbon elemental ratio (O:C) of the organic species, salt types, molecular weight of the organic species, and temperature on the occurrence of liquid-liquid phase separation and SRH were studies. The oxygenic-to-carbon elemental ratio was a useful parameter for predicting liquid-liquid phase separation and SRH. Liquid-liquid phase separation did not depend strongly on the molecular weight of the organic species or temperature. The correlation between SRH and O:C in particles containing organic species mixed with different salts were qualitatively similar. Results of this research will help improve iii  the understanding of liquid-liquid phase separation in the atmospheric aerosols, and may, in turn, improve simulations and predictions of atmospheric chemistry and climate.  iv  Preface  Chapter 2, Chapter 3, Chapter 4, Chapter 5, and Chapter 6 are co-authored peer-reviewed journal articles. The details of my contributions to each research chapter mentioned above are described below. Chapter 2 (ninth author of published journal article): Bertram, A. K., Martin, S. T., Hanna, S. J., Smith, M. L., Bodsworth, A., Chen, Q., Kuwata, M., Liu, A., You, Y. and Zorn, S. R. (2011), Predicting the relative humidities of liquid-liquid phase separation, efflorescence, and deliquescence of mixed particles of ammonium sulfate, organic material, and water using the organic-to-sulfate mass ratio of the particle and the oxygen-to-carbon elemental ratio of the organic component. Atmos. Chem. Phys. 11, 10995-11006, doi:10.5194/acp-11-10995-2011. I was involved in the following parts:  Performed experimental measurements and data analysis of relative humidities at which liquid-liquid phase separation (SRH) occur in particles containing single organic species mixed with ammonium sulfate.  Prepared figures, movies, and tables of experimental results of SRH of particles containing single organic species mixed with ammonium sulfate.  Performed Raman microscopy experiments and analysis with Dr. Sarah Hanna and Dr. Ken Wong. Chapter 3 (first author of published journal article): You, Y., Renbaum-Wolff, L., Carreras-Sospedra, M., Hanna, S. J., Hiranuma, N., Kamal, S., Smith, M. L., Zhang, X., Weber, R., Shilling, J. E., Dabdub, D., Martin, S. and Bertram, A. K. (2012), Images reveal that v  atmospheric particles can undergo liquid-liquid phase separations. Proc. Natl. Acad. Sci. USA 109(33): 13188-13193, doi:10.1073/pnas.1206414109  Proposed research questions and designed research project with my supervisor.  Performed optical microscopy and fluorescence microscopy experiments of all the samples in this project.  Completed data and image analysis of optical microscopy and fluorescence microscopy experiments and prepared corresponding figures and movies.  Shared writing of the manuscript mainly with my supervisor, Dr. Lindsay Renbaum-Wolff, Dr. Marc Carreras-Sospedra, Dr. Sarah Hanna, Dr. Donald Dabdub and Dr. Scot Martin.  Additional contributions from co-authors: o Dr. Mackenzie Smith and Dr. Scot Martin provided the filter samples of secondary organic material produced by the ozonolysis of α-pinene, and provided advice on how to extract the filter samples in water. o Dr. Marc Carreras-Sospedra and Dr. Donald Dabbub carried out simulations on the reactive uptake of N2O5 by particles with a phase separated morphology and a homogeneous morphology. (This part is not included in this dissertation since I was not involved with these simulations.) o Dr. Xiaolu Zhang and Dr. Rodney Weber provided filter samples collected in urban Atlanta.  o Dr. Naruki Hiranuma and Dr. John Shilling provided the filter sample of secondary organic material produced by photo-oxidation of 1,2,4-trimethylbenzene. vi  o Dr. Saeid Kamal provided training and help with the fluorescence microscopy measurements. Chapter 4 (second author of published journal article): Smith, M. L., You, Y., Kuwata, M., Bertram, A. K., and Martin, S. T. (2013), Phase transitions and phase miscibility of mixed particles of ammonium sulfate, toluene-derived secondary organic material, and water. J. Phys. Chem. A, 117, 8895-8906, doi:10.1021/jp405095e  Performed optical microscopy experiments.  Completed data and image analysis of optical microscopy experiments and prepared corresponding figures and movies.  Wrote the part of the manuscript on microscopy experiments with my supervisor. Chapter 5 (first author of published journal article): You, Y., Renbaum-Wolff, L., and Bertram, A. K. (2013), Liquid-liquid phase separation in particles containing organics mixed with ammonium sulfate, ammonium bisulfate, ammonium nitrate or sodium chloride. Atmos. Chem. Phys. 13, 11723, doi:10.5194/acp-13-11723-2013  Proposed research questions and designed research project with my supervisor.  Performed all the experiments.  Completed all the data and image analysis of experiments and prepared corresponding figures and movies.  Wrote the manuscript with my supervisor. Dr. Lindsay Renbaum-Wolff provided useful comments on one figure and the section in the manuscript on atmospheric implication. Chapter 6 (first author on a journal article): You, Y., and Bertram, A. K. (2014), Effects of molecular weight and temperature on liquid-liquid phase separation in particles containing vii  organic species and ammonium sulfate. Atmos. Chem. Phys. Discuss. 14, 23341-23373, doi:10.5194/acpd-14-23341-2014   Proposed research questions and designed research project with my supervisor.  Performed all the experiments.  Completed all the data and image analysis of experiments and prepared corresponding figures.  Wrote the manuscript with my supervisor.  viii  Table of Contents Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents ....................................................................................................................... viii List of Tables ............................................................................................................................... xii List of Figures ............................................................................................................................. xiv List of Supplementary Materials ............................................................................................... xx List of Symbols ......................................................................................................................... xxiii List of Abbreviations ............................................................................................................... xxiv Acknowledgements ................................................................................................................... xxv Dedication ................................................................................................................................ xxvii Chapter 1. Introduction ......................................................................................................... 1 1.1 Atmospheric aerosols ................................................................................................... 1 1.2 Sizes and sources of atmospheric aerosols ................................................................... 3 1.3 Compositional complexity of organic species in the mixed organic-inorganic salt particles ................................................................................................................................... 5 1.4 Secondary organic material .......................................................................................... 6 1.5 Phase transitions and liquid-liquid phase separation in mixed organic-inorganic particles ................................................................................................................................... 7 1.6 Atmospheric implications ........................................................................................... 10 ix  1.7 Motivation and overview of dissertation .................................................................... 11 Chapter 2. Liquid-liquid phase separation in mixed organic-ammonium sulfate particles……… ............................................................................................................................ 13 2.1 Introduction ................................................................................................................ 13 2.2 Experimental ............................................................................................................... 14 2.2.1 Preparation of sample particles ........................................................................... 14 2.2.2 Flow cell system .................................................................................................. 16 2.2.3 Calibration of the hygrometer ............................................................................. 19 2.2.4 Raman microscopy .............................................................................................. 19 2.3 Results and discussion ................................................................................................ 19 2.4 Conclusions ................................................................................................................ 28 Chapter 3. Images reveal that atmospheric particles can undergo liquid-liquid phase separations…… ........................................................................................................................... 30 3.1 Introduction ................................................................................................................ 30 3.2 Experimental ............................................................................................................... 31 3.2.1 Production of secondary organic material ........................................................... 31 3.2.2 Collection of samples from Atlanta, Georgia...................................................... 32 3.2.3 Production of particles ........................................................................................ 33 3.2.4 Optical and fluorescence microscopy of particles............................................... 34 3.3 Results and discussion ................................................................................................ 34 x  3.3.1 Liquid-liquid phase separation in particles containing laboratory generated secondary organic material ............................................................................................... 34 3.3.2 Liquid-liquid phase separation in particles containing materials collected from atmosphere ........................................................................................................................ 38 3.4 Conclusions ................................................................................................................ 40 Chapter 4. Phase transitions of particles consisting of toluene-derived secondary organic material and ammonium sulfate .................................................................................. 42 4.1 Introduction ................................................................................................................ 42 4.2 Experimental ............................................................................................................... 44 4.2.1 Secondary organic material generation ............................................................... 44 4.2.2 Particle generation ............................................................................................... 45 4.2.3 Optical microscopy experiments using a flow cell system ................................. 45 4.3 Results and discussion ................................................................................................ 46 4.4 Conclusion .................................................................................................................. 49 Chapter 5. Liquid-liquid phase separation in particles containing organics mixed with ammonium sulfate, ammonium bisulfate, ammonium nitrate or sodium chloride .............. 50 5.1 Introduction ................................................................................................................ 50 5.2 Experimental ............................................................................................................... 51 5.2.1 Preparation of particles........................................................................................ 51 5.2.2 Optical microscopy experiments with a flow cell system ................................... 54 5.3 Results and discussion ................................................................................................ 56 xi  5.3.1 Effect of H:C (hydrogen-to-carbon elemental ratio), O:C, and inorganic salt type on liquid–liquid phase separation ..................................................................................... 56 5.3.2 Atmospheric implications ................................................................................... 66 5.4 Conclusion .................................................................................................................. 67 Chapter 6. Effects of molecular weight and temperature on liquid-liquid phase separation in particles containing organics and ammonium sulfate ...................................... 68 6.1 Introduction ................................................................................................................ 68 6.2 Experimental ............................................................................................................... 69 6.2.1 Sample preparation and apparatus....................................................................... 69 6.2.2 Molecular weight dependent studies ................................................................... 70 6.2.3 Temperature dependent studies ........................................................................... 71 6.3 Results and Discussion ............................................................................................... 73 6.3.1 Effect of molecular weight on liquid-liquid phase separation ............................ 73 6.3.2 Effect of temperature on liquid-liquid phase separation ..................................... 78 6.4 Conclusions ................................................................................................................ 83 Chapter 7. Conclusions ........................................................................................................ 85 7.1 Conclusions ................................................................................................................ 85 7.2 Outlook of future work ............................................................................................... 87 References .................................................................................................................................... 91 xii  List of Tables Table 2.1 List of the organic compounds studied in the mixed organic-ammonium sulfate particles. ............................................................................................................................ 15 Table 2.2 Results of SRH from the optical microscope experiments. The results correspond to particles consisting of one organic compound and ammonium sulfate. ........................... 22 Table 2.3 List of the SRH data used for the parameterizations of SRH in systems consisting of one organic plus ammonium sulfate. ................................................................................ 24 Table 2.4 Parameterizations of the SRH data. SRH results were measured at temperatures ranging from 290 to 298 K................................................................................................ 28 Table 3.1 Relevant information for samples. ................................................................................ 35 Table 5.1 Summary of different organics used in the liquid-liquid phase separation experiments............................................................................................................................................ 52 Table 5.2 Summary of SRH results for an organic-to-inorganic mass ratio (OIR) of 2.0 ± 0.1. Uncertainties represent 2  of multiple SRH measurements and the uncertainty from the calibration. ........................................................................................................................ 56 Table 5.3 Parameterizations of SRH results as a function of the oxygen-to-carbon elemental ratio (O:C) of the organic material. ........................................................................................... 59 Table 5.4 Summary of SRH results as a function of the organic-to-inorganic (OIR) mass ratio for following organics: 2, 2-dimethylsuccinic acid; α, 4-dihydroxy-3-methoxybenzeneacetic acid; and 2, 5-dihydroxybenzonic acid. Included are results from both the current studies and results from Bertram et al. (2011). Uncertainties represent 2σ of multiple SRH measurements and the uncertainty from the calibration. .................................................. 63 xiii  Table 6.1 List of the ten organic species used in molecular weight dependent measurements. Each organic species was separately mixed with ammonium sulfate to make particles, and liquid-liquid phase separation was studied in these particles at 290 ± 1 K. ............... 71 Table 6.2  Summary of the twenty organic species used in temperature dependent experiments. Each organic species was separately mixed with ammonium sulfate to make particles, and liquid-liquid phase separation was studied in these particles at 244 ± 1 K, 263 ± 1 K and 278 ± 1 K. Two of the organic species (poly (ethylene glycol) diacrylate and raffinose) mixed with ammonium sulfate were also studied at 290 ± 1 K. ...................... 72 Table 6.3  Combined data set used to assess the effect of molecular weight on SRH in mixed organic-ammonium sulfate particles.  This data set includes the ten types of particles studied here (see Table 6.1) and the SRH results of twenty-three types of particles containing single organic species and ammonium sulfate studied in Chapter 5. OIR = 2.0 ± 0.1 in all the experiments. Uncertainties include 2σ of multiple SRH measurements and the uncertainty from the calibration. ................................................................................. 77 xiv  List of Figures  Figure 1.1 Average non-refractory submicron particle mass determined from measurements in regions around world. Data were taken from Zhang et al. (2007). ..................................... 5 Figure 1.2 Schematic of liquid-liquid phase separation in a mixed organic-inorganic salt particle. Green represents an organic liquid phase and aqua represents an aqueous inorganic phase............................................................................................................................................ 10 Figure 2.1 A diagram of the system used for generating particles in this study. .......................... 16 Figure 2.2 A diagram of the optical microscope coupled to a temperature and relative humidity controlled flow-cell system used to measure phase transition relative humidities in this study as well as studies in Chapter 3 to Chapter 6. (a) a schematic of the experimental system used to control relative humidity of the particles; (b) A detailed schematic of the flow cell used to measure phase transition relative humidities. ........................................ 18 Figure 2.3 Optical microscope images of a single particle consisting of 1,2,6-hexanetriol and ammonium sulfate during a typical experiment when RH decreased from high to low values (from right to left). Images were recorded with 500 × magnification and at a temperature of 290 ± 1 K. ................................................................................................. 20 Figure 2.4 Raman spectra of (a) a pure ammonium sulfate particle, (b) a pure α,4-dihydroxy-3-methoxybenzeneacetic acid particle, (c) the inner phase of a mixed α,4-dihydroxy-3-methoxybenzeneacetic acid–ammonium sulfate particle after phase separation, and (d) the outer phase of a mixed α,4-dihydroxy-3-methoxybenzeneacetic acid-ammonium sulfate particle after phase separation. The mixed particle used for Raman analysis (org:sulf=1.44) is shown to the right. All Raman spectra were collected at approximately xv  293 K. In addition, during Raman measurements the particles were exposed to room air, and hence the RH was not controlled. .............................................................................. 21 Figure 2.5 Separation relative humidity (SRH) as a function of organic-to-sulfate mass ratio. The colors represent the O:C of the organic species. Axes denote the observed relative humidity (RH) of a transition and the organic-to-sulfate mass ratio of a studied mixed system. (a)  results in the particles containing organic species with O:C < 0.7; and (b) with O:C > 0.7. Open symbols indicate that liquid-liquid phase separation was not observed. In some cases SRH was not probed below 35-40% RH because ammonium sulfate or the organic material crystallized. Measurements were at temperatures ranging from 290 to 298 K except for experiments with particles consisting of 1,2,6-hexanetriol and ammonium sulfate, which were studied at 273 ± 1K . ............................................... 25 Figure 2.6 Average liquid-liquid phase separation relative humidity (SRH) as a function of O:C. An open symbol for SRH indicates that SRH was not observed. In some cases SRH was not probed below 35-40% RH because crystallization of ammonium sulfate or the organic occurred at lower RH values. The line when O:C < 0.7 is a second order polynomial fit to the data (R2 = 0.87). Phase transitions were measured at temperatures ranging from 290 to 298 K. ................................................................................................................................ 26 Figure 3.1 Optical and fluorescence images of ammonium sulfate particles and ammonium sulfate-secondary organic material particles at various relative humidities. Images were collected by optical reflectance microscopy and fluorescence microscopy for decreasing relative humidity. Rows: (a and b) optical and fluorescence images of a reference ammonium sulfate particle; (c and d) optical and fluorescence images of ammonium sulfate-secondary organic material particle produced by α-pinene ozonolysis (org:sulf = xvi  1.4); (e and f) optical and fluorescence images of ammonium sulfate-secondary organic material particle produced by photo-oxidation of 1,2,4-trimethylbenzene photo-oxidation (org:sulf = 3). The light circle in the optical images at the center of all liquid droplets is an optical effect due to scattering by a spherical particle. This circle decreases in intensity or disappears after the inner phase crystallizes. ................................................................ 37 Figure 3.2 Optical and fluorescence images of particles generated from filter samples collected in Atlanta, Georgia, USA. Images were collected for decreasing relative humidity. From the top to bottom: (a and b) optical and fluorescence images of a particle generated from a filter sample collected on July 28 and 29, 2010; (c and d) optical and fluorescence images of a particle generated from a filter sample collected on August 6 and 7, 2010. ............. 40 Figure 4.1 Optical microscopy images and corresponding illustrations when RH was deceased. Columns from left to right represent particles at RH-values ranging from 90% to 0%. The top two rows of images and illustrations are for particles consisting of toluene photo-oxidation secondary organic material and ammonium sulfate with org:sulf = 0.1 and 3.0, respectively. The bottom rows of images and illustrations are for particles containing only toluene photo-oxidation secondary organic material. Optical microscopy images were taken from Smith et al. 2013, and copyright from American Chemical Society 2013............................................................................................................................................ 47 Figure 5.1 Optical microscopy images and illustrations of particles containing 2-methylglutaric acid and one of the inorganic salts with OIR = 2.0 ± 0.1. Shown in the images and illustrations are relative humidities at which the images were recorded. The inorganic salts studied were ammonium sulfate in row (a), ammonium bisulfate in row (b), sodium xvii  chloride in row (c), and ammonium nitrate in row (d). The diameter of the particles shown ranged from 28 to 34 µm. ...................................................................................... 55 Figure 5.2 Van Krevelen Diagram for the different mixed organic-inorganic salt particles (OIR = 2.0 ± 0.1) studied: (a) organic-ammonium sulfate particles, (b) organic-ammonium bisulfate particles, (c) organic-sodium chloride and (d) organic-ammonium nitrate particles. Open circles indicate that liquid-liquid phase separation was observed, while stars indicated that liquid-liquid phase separation was not observed. The vertical hatched regions correspond to the H:C and O:C conditions when liquid-liquid phase separation was always observed and the horizontal hatched regions correspond to the H:C and O:C conditions when liquid-liquid phase separation was never observed. .............................. 58 Figure 5.3 Summary of SRH results (OIR = 2.0 ±0.1) as a function of oxygen-to-carbon elemental ratio (O:C): (a) organic-ammonium sulfate particles, (b) organic-ammonium bisulfate particles, (c) organic-sodium chloride particles and (d) organic-ammonium nitrate particles. Circles represent the relative humidity at which separation occurred. Error bars associated with the circles represent 2σ of multiple SRH measurements and the uncertainty from the calibration. Stars indicate that liquid-liquid phase separation was not observed. The error bars corresponding to the stars indicate that liquid-liquid phase separation could potentially occur within the range indicated by the error bars, but could not be detected due to the occurrence of efflorescence in the particles. The curves in the panels are Sigmoidal-Boltzmann fits to the data. Red triangles represent the results of liquid-liquid phase separation in bulk solution from Marcolli and Krieger 2006). .......... 60 Figure 5.4 Summary of trends of SRH of particles (OIR=2.0 ± 0.1) as a function of inorganic salt type: (a) the SRH of the organics (20 in total) that followed the trend (NH4)2SO4 ≥ xviii  NH4HSO4 ≥ NaCl ≥ NH4NO3 and (b) the SRH for 2,5-dihydroxybenzonic acid, α,4- dihydroxy-3-methoxybenzeneacetic acid, and 2,2-dimethylsuccinic acid. In panel a, colors represent the O:C of individual organics. The organics shown in panel b didn’t follow the trend (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3. Uncertainties in the SRH measurements have been left off for clarity. ..................................................................... 62 Figure 5.5 Summary of SRH results as a function of OIR for the following types of particles: (a) 2,2-dimethylsuccinic acid and inorganic salts, (b) σ,4-dihydroxy-3-methoxybenzeneacetic acid and inorganic salts, and (c) 2,5-dihydroxybenzoic acid and inorganic salts. Closed symbols represent results from current study; while open squares represent results from previous studies by Bertram et al. (2011). Uncertainties in the SRH measurements have been left off for clarity. ..................................................................................................... 64 Figure 6.1 The effect of molecular weight and O:C of the organic species on the occurrence of liquid-liquid phase separation in  mixed organic-ammonium sulfate particles (OIR = 2.0 ± 0.1).  Data plotted are from the current study and Chapter 5 and are summarized in Table 6.3. Open circles indicate liquid-liquid phase separation was observed, while stars indicate liquid-liquid phase separation was not observed. The orange hatched region corresponds to the molecular weight and O:C of the organic species when liquid-liquid phase separation was always observed, and the green hatched region corresponds to the molecular weight and O:C of the organic species when liquid-liquid phase separation was never observed. ................................................................................................................. 74 Figure 6.2 SRH as a function of molecular weight of organic species in the particles at 290 ± 1 K. The SRH results are from the current study and Chapter 5 (see Table 6.3). The colors represent the O:C of different organic species. Squares represent SRH of particles in xix  which liquid-liquid phase separation was observed. Bars for the squares include 2σ of multiple measurements and the uncertainty from the calibration. Stars indicate that liquid-liquid phase separation was not observed. OIR = 2.0 ± 0.1 in all the experiments. ......... 76 Figure 6.3 SRH as a function of O:C of the organic species at 290 ± 1 K. The SRH results are from the current study and Chapter 5 (see Table 6.3). The colors represent the molecular weight of the different organic species. Squares represent the SRH of particles in which liquid-liquid phase separation was observed. Bars for the squares include 2σ of multiple measurements and the uncertainty from the calibration. Stars indicate liquid-liquid phase separation was not observed. OIR = 2.0 ± 0.1 in all the experiments. The black curve is a Sigmoidal-Boltzmann fit to the data. ................................................................................ 76 Figure 6.4 (A) SRH of mixed organic-ammonium sulfate particles as a function of O:C measured at four different temperatures. Different symbols represent the different temperatures. Bars for the data include 2σ of multiple measurements and the uncertainty from the calibration. Data at SRH= 0% indicate liquid-liquid phase separation was not observed. Data plotted are summarized in Table 6.4. The OIR = 2.0 ± 0.1 in all the experiments. (B) Range of the average O:C of organic material in particles from measurements at many locations in the Northern Hemisphere and the Amazon. .................................................. 81 Figure 6.5 Summary of SRH as a function of temperature for mixed organic-ammonium sulfate particles that underwent liquid-liquid phase separation. Data plotted were taken from Table 6.4. Bars for the data include 2σ of multiple measurements and the uncertainty from the calibration. Colors represent the O:C values of different organic species in the particles. OIR = 2.0 ± 0.1 in all the experiments. ............................................................. 82  xx  List of Supplementary Materials Movie 2.1 A movie of a mixed 1,2,6-hexanetriol-ammonium sulfate particle (org:sulf = 2.06 and diameter = 21 µm) recorded using 500  magnification as the RH was decreased from 86% to 35% at a temperature of 290  1 K. The ramp rate was approximately 0.6% RH min–1. Initially the particle was completely liquid (1 liquid phase). Next, liquid-liquid phase separation occurred to form a predominately aqueous ammonium sulfate core and organic shell. After liquid-liquid phase separation the aqueous ammonium sulfate core effloresced. A rough indication of the RH during the movie has been included.  Movie 3.1 A movie of a particle consisting of ammonium sulfate (diameter = 21.6 µm).  Images were recorded as the RH was decreased from 50% to 35% at a temperature of 270  1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, efflorescence occurred at 37.4 ± 4%. A rough indication of the RH during the movie has been included.  Movie 3.2 A movie of a particle consisting of ammonium sulfate (diameter = 21.6 µm).  Images were recorded as the RH was increased from 62 to 80% RH at a temperature of 270  1 K. The ramp rate was approximately 0.6 % RH min–1. For the particle shown in this movie, deliquescence occurred at 78.0 ± 4%.  A rough indication of the RH during the movie has been included.  Movie 3.3 A movie of a particle (org:sulf = 1.4 and diameter = 28.0 µm) consisting of ammonium sulfate mixed with secondary organic material generated by the ozonolysis of α-pinene.  Images were recorded as the RH was decreased from 90% to 35% at a temperature of 270  1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, liquid-liquid phase separation occurred at an RH greater than 90 ± 4% and efflorescence occurred at 40.5 ± 4%. A rough indication of the RH during the movie has been included.  Movie 3.4 A movie of a particle (org:sulf = 1.4 and diameter = 28.0 µm) consisting of ammonium sulfate mixed with secondary organic material generated by the ozonolysis of α-xxi  pinene.  Images were recorded as the RH was increased from 60 to 80% RH at a temperature of 270  1 K. The ramp rate was approximately 0.6 % RH min–1. For the particle shown in this movie, complete deliquescence occurred at 77.0 ± 4%. A rough indication of the RH during the movie has been included.  Movie 3.5 A movie of a particle (org:sulf = 1.0 and diameter = 18.3 µm) generated from a filter sample collected August 6-7, 2010 in Atlanta.  Images are shown as the RH was decreased from 68% to 35% at a temperature of 270  1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, liquid-liquid phase separation occurred at an RH greater than 90% ± 4%, while efflorescence occurred at an RH of 37.0 ± 4%. A rough indication of the RH during the movie has been included.  Movie 3.6 A movie of a particle (org:sulf = 1.0 and diameter = 18.3 µm) generated from filter sample collected August 6-7, 2010 in Atlanta.  Images are shown as the RH was increased from 50% to 80% at a temperature of 270  1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, deliquescence occurred at 71.6 ± 4%. A rough indication of the RH during the movie has been included.  Movie 4.1 A movie of a particle consisting of ammonium sulfate and toluene photo-oxidation secondary organic material of org:sulf = 0.1 recorded for decreasing RH at 289 ± 1 K. The RH was decreased from 90% to 30% at a rate of 0.5% RH min-1. A rough indication of the RH during the movie has been included.  Movie 4.2 A movie of a particle consisting of ammonium sulfate and toluene photo-oxidation secondary organic material of org:sulf = 3.0 recorded for decreasing RH at 289 ± 1 K. The RH was decreased from 90% to 30% at a rate of 0.5% RH min-1 and from 30% to < 0.5% at a rate of 4-5% RH min-1. A rough indication of the RH during the movie has been included.   Movie 5.1 A movie of a particle (OIR = 2.0 and diameter = 28.0 μm) consisting of 2-methylglutaric acid and ammonium sulfate. Images were recorded as the RH was decreased from xxii  79% to 72% at a temperature of 290 ± 1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, liquid-liquid phase separation occurred at an RH 75 ± 2.5%. A rough indication of the RH during the movie has been included.   Movie 5.2 A movie of a particle (OIR = 2.0 and diameter = 32.3 μm) consisting of 2-methylglutaric acid and ammonium bisulfate. Images were recorded as the RH was decreased from 70% to 60% at a temperature of 290 ± 1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, liquid-liquid phase separation occurred at an RH 65.5 ± 2.5%. A rough indication of the RH during the movie has been included.   Movie 5.3 A movie of a particle (OIR = 2.1 and diameter = 33.6 μm) consisting of 2-methylglutaric acid and sodium chloride. Images were recorded as the RH was decreased from 65% to 55% at a temperature of 290 ± 1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, liquid-liquid phase separation occurred at an RH 60.2 ± 2.5%. A rough indication of the RH during the movie has been included.  Movie 5.4 A movie of a particle (OIR = 2.0 and diameter = 32.3 μm) consisting of 2-methylglutaric acid and ammonium nitrate. Images were recorded as the RH was decreased from 43% to 27% at a temperature of 290 ± 1 K. The ramp rate was approximately 0.6% RH min–1. For the particle shown in this movie, liquid-liquid phase separation occurred at an RH 35.3 ± 2.5%. A rough indication of the RH during the movie has been included.   xxiii  List of Symbols Cs the concentration of the inorganic salt ks Setchenov constant NO3 nitrate radical OH hydroxyl radical pH2O     partial vapor pressure of water p˚H2O saturation vapor pressure of water at a certain temperature So     solubility of organic species in water in the absence of inorganic salt S solubility of organic species in inorganic salt aqueous solution                                     xxiv  List of Abbreviations AMS Aerosol Mass Spectrometer CMFR continuously mixed flow reactor CREATE-AAP Collaborative Research and Training Experience - Atmospheric Aerosol Program DRH deliquescence relative humidity ERH efflorescence relative humidity H:C hydrogen-to-carbon elemental ratio HEC Harvard Environmental Chamber HR-ToF-AMS high-resolution time-of-flight Aerosol Mass Spectrometer  NSERC Natural Sciences and Engineering Research Council O:C oxygen-to-carbon elemental ratio org:sulf organic-to-sulfate mass ratio OIR organic-to-inorganic mass ratio  PNNL Pacific Northwest National Laboratory ppbv parts-per-billion by volume RH relative humidity sLpm standard liters per minute SRH separation relative humidity VOC volatile organic compound RH relative humidity   xxv  Acknowledgements  I would like to thank my mentor and research supervisor Dr. Allan Bertram sincerely for providing me with this exciting opportunity to explore atmospheric chemistry with him. He keeps track of my research progress and provides timely help and suggestions. Meanwhile, he gives me opportunities to solve problems and to become independent. He is also patient with my mistakes on writing and speaking. In addition, he cares about student’s development and introduced me into the CREATE-AAP. I enjoyed my Ph.D study mainly because of the active atmosphere in Bertram group. Michael, Donna, Jason, Pedro, Song, Ryan, James, Meng, Yuri, Mijung, Cédric, Stephen, Vickie, Kaitlin, Kristina and Amir, thank all of you for valuable suggestions, encouragements, and the fun we had together. Special thank you to Dr. Sarah Hanna and Aidan who taught me the basics and helped me initiate my first research project, to Dr. Lindsay Renbaum-Wolff who provided helpful comments and suggestions on presenting my results, and to Dr. Richard Iannone who showed me useful skills of computer software and provided me with valuable advice on career development.  My research projects could not proceed well without the instruments and techniques provided by chemistry department. I would like to thank Dr. Keith Mitchell and Dr. Ken Wong for their assistance with the Raman microscopy measurements, and Dr. Saeid Kamal for his assistance and training with fluorescence microscopy measurements. Mechanical shop, glassblowing shop, and electrical shop also provided their technical support. My research projects involved collaborations with different research groups. I would like to thank all the collaborators for their hard work and valuable comments. Special thank you to xxvi  Dr. Mackenzie and Dr. Scot Martin for helpful discussions and samples they provided, and to Dr. Markus Petters for helpful discussions on Hofmeister series.  My research was funded by Natural Sciences and Engineering Research Council of Canada (NSERC) and Collaborative Research and Training Experience - Atmospheric Aerosol Program (CREATE-AAP). Dr. Corinne Schiller taught me a lot on the data analysis and brought me to the sampling site during my internship in Environment Canada. I appreciate all these support. At last, I would like to thank my family. Mom and dad, thank you so much for providing me with the best you have, caring about my interests in science, and encouraging me to pursue what I want and to be myself. I wouldn’t be here without my wonderful family. My sister, I really enjoyed the experience of growing up with you.  xxvii  Dedication         To mom and dad              1  Chapter 1. Introduction 1.1 Atmospheric aerosols An aerosol is defined as a suspension of liquid or solid particles in a gas (Seinfeld and Pandis, 2006). The term atmospheric aerosol is often used as a synonym for atmospheric particles or particulate matter. These three terms are typically used to describe the particles alone without the gas phase (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). This dissertation focuses on atmospheric aerosols in the troposphere, which is the lowest layer of the atmosphere, ranging from the Earth’s surface to about 10-15 km altitude (Seinfeld and Pandis, 2006). In urban and remote regions, the atmosphere often contains particles in concentrations ranging from 102 to 108 cm-3 (Finlayson-Pitts and Pitts, 2000). Atmospheric particles can be divided into primary particles and secondary particles based on the mechanism of their formation (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). Primary particles are emitted directly from sources, while secondary particles are formed in the atmosphere by oxidation and subsequent condensation of gas-phase species. Particles usually stay in the atmosphere for a few days to a few weeks before deposition at the Earth’s surface or removal from the atmosphere by precipitation (Seinfeld and Pandis, 2006). Atmospheric aerosols can have important influences on climate. They can directly influence climate by scattering and absorbing solar radiation, which can lead to a cooling or warming effect (Myhre et al., 2013; Seinfeld and Pandis, 2006; Kanakidou et al., 2005; Finlayson-Pitts and Pitts, 2000). They can also indirectly affect climate by serving as cloud condensation nuclei or ice nuclei, which affect the formation, number, and lifetime of clouds. 2  Changes in the number, properties and lifetime of clouds influences their ability to scatter and absorb radiation from both the Sun and Earth’s surface (Boucher et al., 2013; Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006; Andreae and Rosenfeld, 2008; Hoose and Möhler, 2012; Murray et al., 2012). The sum of the aerosol direct and indirect effect on climate is thought to be a cooling effect and the confidence level of this effect is currently considered medium (Myhre et al., 2013). The extent of the effect of aerosols on climate may be comparable to the extent of warming due to carbon dioxide (Myhre et al., 2013).  Atmospheric aerosols can also have important influences on atmospheric chemistry. Atmospheric aerosols provide a medium for heterogeneous reactions, which can alter the concentrations of gas-phase species in the atmosphere (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006; Dentener et al., 1996; Ravishankara, 1997; Rudich, 2003; Rudich et al., 2007; Kroll and Seinfeld, 2008; George and Abbatt, 2010; Chang et al., 2011; Abbatt et al., 2012; Gross et al., 2009). For example, N2O5 can react quickly with water on or in atmospheric particles and form nitric acid. Dentener and Crutzen calculated the magnitude of the impact of heterogeneous reactions of N2O5 on particles (Dentener and Crutzen, 1993). Their model simulations showed that 90% of the nitric acid burden in the Northern Hemisphere may be produced by this reaction. Their calculations also suggested that O3 concentration could be decreased by up to 25 % due to the heterogeneous reactions on particles. In the troposphere, O3 is a pollutant and greenhouse gas.  Atmospheric aerosols can also influence air quality and human health (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). Research has shown that concentrations of aerosols correlate with respiratory infections and asthma (Samoli et al., 2011; Nastos et al., 2010; Amarillo and Carreras, 2012), as well as mortality rate (Dockery et al., 1993). Aerosols were also 3  found to be related to lung cancer and cardiovascular diseases (Pope et al., 2002; Lepeule et al., 2012). 1.2 Sizes and sources of atmospheric aerosols Atmospheric aerosols typically range from a few nanometers to around 100 micrometers (µm) in diameter (Seinfeld and Pandis, 2006). Aerosols can contain a range of species including sulfate, ammonium, nitrate, sodium, chloride, elemental carbon, organic carbon, trace metals, elements from the Earth’s crust, and water (Seinfeld and Pandis, 2006). The composition depends on the aerosol size and sources. Aerosols are generally divided into two size regimes: coarse aerosol particles (particles with diameters greater than 2.5 µm) and fine aerosol particles (particles with diameters less than 2.5 µm) (Seinfeld and Pandis, 2006). Coarse aerosols are generated mainly by mechanical processes and include wind-blown dust, sea spray, volcanic emissions, and biological particles (Seinfeld and Pandis, 2006; Woodcock et al., 1953; Blanchard and Woodcock, 1957). Coarse aerosols are efficiently removed from the atmosphere due to their large sedimentation velocity.  Fine aerosols include the majority of the number and a large fraction of the mass of particles in the atmosphere (Seinfeld and Pandis, 2006; Finlayson-Pitts and Pitts, 2000). Fine aerosols can come from primary emissions or nucleation and condensation of gas-phase species (Seinfeld and Pandis, 2006). A large fraction of fine aerosols contain both organic species and inorganic salts (Murphy and Thomson, 1997; Murphy et al., 1998; Middlebrook et al., 1998; Buzorius et al., 2002; Murphy, 2005; Murphy et al., 2006; Tolocka et al., 2005; Chen et al., 2009; Pratt and Prather, 2010; Zhang et al., 2007).  4  Figure 1.1 shows the average chemical composition of submicron particles (particles smaller than 1 µm in diameter) measured by Aerosol Mass Spectrometer (AMS) (Zhang et al., 2007). The average is based on measurements at several locations. Figure 1.1 shows that organic materials and inorganic salts are important components in atmospheric particles. Single particle measurements also show that organic species and inorganic salts are often internally mixed in the same particle (Murphy et al., 2006). These particles are referred to as mixed organic-inorganic salts particles. This dissertation focuses mainly on these mixed organic-inorganic salt particles. Ammonium sulfate, ammonium bisulfate, ammonium nitrate and sodium chloride are thought to be important inorganic salts in atmospheric particles (Seinfeld and Pandis, 2006; Finlayson-Pitts and Pitts, 2000). Sulfate and nitrate mainly come from the oxidation of SO2 and NO in the atmosphere (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). The majority of SO2 emissions is from fossil fuel combustion (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). NO is produced by both natural and anthropogenic sources, such as lighting, biomass burning, and fossil fuel combustion (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). Ammonium is incorporated into the particulate phase by neutralization of acidic components in the particles by ammonia gas, which mainly comes from biological and anthropogenic sources (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). Sodium chloride is primarily generated from bubbles bursting at the air-sea interface, and it is most common in particles over, and downwind, of marine environments (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006; Woodcock et al., 1953; Blanchard and Woodcock, 1957; Quinn and Bates, 2005). 5   Figure 1.1 Average non-refractory submicron particle mass determined from measurements in regions around world. Data were taken from Zhang et al. (2007). 1.3 Compositional complexity of organic species in the mixed organic-inorganic salt particles As described in Section 1.2, atmospheric fine particles contain both organic species and inorganic salts. The reported organic-to-inorganic mass ratio (OIR) in atmospheric aerosols at different sampling sites ranges from approximately 0.2 to 3.5 (Chen et al., 2009; Zhang et al., 2007; Jimenez et al., 2009). Compared to the limited number of inorganic salts in atmospheric particles, the organic fraction is much more complex and consists of thousands of different molecules. Only approximately 10 % of which (by mass) have been identified at the molecular level (Hamilton et al., 2004; Goldstein and Galbally, 2007; Decesari et al., 2006; Hallquist et al., 2009).  The oxygen-to-carbon elemental ratio (O:C) of the organic material is a useful parameter in describing the properties of particulate organic species and has been useful for reducing this 6  complexity in previous research (Jimenez et al., 2009; Bertram et al., 2011; Song et al., 2012b). In addition, O:C has been measured in many sampling sites around the world. Typical average O:C values reported in the Northern Hemisphere and in the Amazon range from 0.1 to 1.0 (Chen et al., 2009; Aiken et al., 2008; DeCarlo et al., 2008; Jimenez et al., 2009; Hawkins et al., 2010; Heald et al., 2010; Ng et al., 2010; Takahama et al., 2011).  The most common functional groups of organic species in mixed organic-inorganic salt particles found in previous field measurements include carboxylic acids, alcohols, and oxidized aromatic compounds (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006; Decesari et al., 2006; Hallquist et al., 2009; Takahama et al., 2011; Rogge et al., 1993; Saxena and Hildemann, 1996; Finlayson-Pitts and Pitts, 1997; Day et al., 2009; Gilardoni et al., 2009; Liu et al., 2009; Russell et al., 2009; Russell et al., 2011; Fu et al., 2011; Fuzzi et al., 2001). The molecular weight of organic species in the atmospheric particles ranges from less than 100 Da to around 1000 Da (Gao et al., 2004; Kalberer et al., 2004; Tolocka et al., 2004; Nguyen et al., 2010). 1.4 Secondary organic material  In the atmosphere, low-volatility products from gas-phase oxidation of biogenic and anthropogenic volatile organic compounds (VOCs) condense onto inorganic particle surfaces and form mixed organic-inorganic salt particles (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006; Decesari et al., 2006; Hallquist et al., 2009; Marcolli et al., 2004; Donahue et al., 2006). OH (hydroxyl radical), O3, and nitrate radicals (NO3) are common species for oxidation reactions of VOCs in the atmosphere. Secondary organic material is defined here as the condensed-phase organic material formed by atmospheric oxidation of VOCs. Secondary organic material can make up to about 90% of the mass of organic material in atmospheric submicron particles (Hallquist et al., 2009). Oxidation of VOCs from biogenic emissions contributes a large fraction 7  to the secondary organic material mass concentration (Seinfeld and Pandis, 2006; Kanakidou et al., 2005). Isoprene and terpenes, such as α-pinene, limonene, and β-caryophyllene, are good examples of biogenic VOCs (Kanakidou et al., 2005). Oxidation of aromatic VOCs can also be a dominant source of secondary organic material in urban regions (Pandis et al., 1992; Seinfeld and Pandis, 2006; Henze et al., 2008; Odum et al., 1997). Toluene, benzene, and xylene are good examples of anthropogenic VOCs (Henze et al., 2008; Forstner et al., 1997).  1.5 Phase transitions and liquid-liquid phase separation in mixed organic-inorganic particles Relative humidity (RH) is defined with                                                                                                      (1.1) where pH2O is the partial vapor pressure of water and p˚H2O is the saturation vapor pressure of water at a certain temperature.  With cycles of daily temperature and moving of air parcels, RH fluctuates between high and low values. As an example, a fluctuating RH ranging from approximately 100% to 30% was measured almost daily during the 2010 California Research at Nexus of Air Quality and Climate Change (CalNex)-Los Angeles field campaign from May 15th to June 15th (Liu et al., 2012).  As the relative humidity changes, the water content of particles will change due to their hygroscopic response. More specifically, as the relative humidity in an air parcel decreases, the chemical potential of water in the gas phase decreases and the water molecules migrate from the particle phase to the gas phase. As a result, the water content and chemical potential of water in the particles are changed. The changes can further alter the chemical potential of other species in mixed organic-inorganic salt particles, and this potentially leads to different phase transitions occurring in the particles. 8  In the past, most studies on phase transitions in particles consisting of mixtures of organic species and inorganic salts focused on the deliquescence and efflorescence (Brooks et al., 2002; Choi and Chan, 2002; Chan and Chan, 2003; Brooks et al., 2003; Wise et al., 2003; Braban and Abbatt, 2004; Pant et al., 2004; Parsons et al., 2004a; Badger et al., 2006; Marcolli and Krieger, 2006; Parsons et al., 2006; Salcedo, 2006; Ling and Chan, 2008; Treuel et al., 2009; Bodsworth et al., 2010; Bertram et al., 2011; Smith et al., 2012; Krieger et al., 2012). Deliquescence is a thermodynamic process that occurs when RH increases from a low value to a threshold value.  At this threshold RH value the solid salt spontaneously takes up water and forms a saturated aqueous solution (Martin, 2000). This threshold RH value is defined as the deliquescence relative humidity (DRH). On the other hand, efflorescence is a kinetic process that occurs as the RH is decreased. As the RH is decreased water evaporates from aqueous particles to form a saturated solution and eventually a supersaturated solution (Martin, 2000). If the RH is decreased enough, the supersaturated solution may reach a threshold value at which it may crystallize. This threshold RH value is referred to as the efflorescence relative humidity (ERH) (Martin, 2000).  In many cases, as long as the particles do not first undergo liquid-liquid phase separation, organic species tend to decrease the DRH and ERH of the inorganic salts in mixed organic-inorganic salt particles (Marcolli et al., 2004; Braban and Abbatt, 2004; Parsons et al., 2004a; Bertram et al., 2011; Smith et al., 2012; Song et al., 2013). The reason for the decrease of DRH and ERH of inorganic salts after introducing organic species into the particles containing one organic species mixed with ammonium sulfate can be understood with the Gibbs-Duhem relation. When an organic species is added to an aqueous salt particle, the mole fraction of organic species will increase. According to the Gibbs-Duhem relation, the chemical potential of any inorganic salt in the mixed organic-inorganic salt particles will decrease, assuming the mole 9  fraction of water in the particle remains constant, due to a constant relative humidity. This decrease of chemical potential for the inorganic salt can lead to a decrease in the DRH in order to approach a saturation value of one and to a decrease in the ERH to reach the critical supersaturation necessary for efflorescence. Because of the large number of organic species in a single mixed organic-inorganic salt particle, the mass fraction of a single organic species in a particle is very small. Thus, organic species in the mixed organic-inorganic salt particles are expected to be present in a liquid phase, since the supersaturation with respect to efflorescence of most of the organic species cannot be reached (Marcolli et al., 2004). However, these organic species may form an amorphous solid, glassy, or highly viscous state at low RH values or low temperature (Zobrist et al., 2008; Koop et al., 2011; Renbaum-Wolff et al., 2013; Mikhailov et al., 2009; Power et al., 2013; Virtanen et al., 2010; Vaden et al., 2011; Cappa and Wilson, 2011; Perraud et al., 2012; Saukko et al., 2012; Bones et al., 2012; Adler et al., 2013). Glassy or highly viscous organic species can inhibit phase transitions of inorganic salts in the mixed organic-inorganic salt particles (Bodsworth et al., 2010; Schill and Tolbert, 2013), because the highly viscous state can inhibit or slow down diffusion of water within the particles. In addition to deliquescence and efflorescence,  liquid-liquid phase separation potentially can occur in mixed organic-inorganic salt particles as the relative humidity cycles from high to low values as suggested in earlier theoretical work (e.g. Clegg et al. 2001 and Pankow, 2003). As shown in Figure 1.2, a particle can exist as in two liquid phases after undergoing liquid-liquid phase separation. One phase consists of mainly organic species with trace amounts of inorganic salt and water, and a second phase consists of an aqueous inorganic salt solution with trace amounts of organic species (Bertram et al., 2011; Ciobanu et al., 2009; Song et al., 2012a). 10  Liquid-liquid phase separation relative humidity (SRH) in mixed organic-inorganic salt particles is defined as the threshold RH value at which liquid-liquid phase separation occurs in the mixed organic-inorganic salt particles when RH decreases. If the relative humidity around the phase separated particles increases, liquid-liquid mixing will occur in the particles and there should be another threshold RH value corresponding to the liquid-liquid mixing process. We don’t expect large differences between the RH values for liquid-liquid phase separation and liquid-liquid mixing, because calculations for some simple organic species mixed with inorganic salt in aqueous bulk solutions (e. g. Zuend et al. 2010) suggest the relative humidity range over which two phases are metastable is small. Prior to the research in this dissertation, there were only a few laboratory studies on liquid-liquid phase separation in mixed organic-inorganic salt particles (Marcolli and Krieger, 2006; Buajarern et al., 2007; Anttila et al., 2007; Ciobanu et al., 2009; Kwamena et al., 2010; Prisle et al., 2010).Therefore, this dissertation focuses on liquid-liquid phase separation in particles consisting of organic species and inorganic salts.    Figure 1.2 Schematic of liquid-liquid phase separation in a mixed organic-inorganic salt particle. Green represents an organic liquid phase and aqua represents an aqueous inorganic phase. 1.6 Atmospheric implications  The occurrence of liquid-liquid phase separation in atmospherically relevant particles has important implications for climate, regional visibility, and air quality. Phase separation of 11  organic species and inorganic salts could alter the DRH and ERH of inorganic salts in mixed organic-inorganic salt particles, leading to the change in the extinction of solar radiation by the inorganic salts in the particles (Martin et al., 2004; Wang et al., 2008a). Assuming that the aqueous phase is surrounded by the organic-rich phase, liquid-liquid phase separation could also significantly reduce the reactive uptake of gas-phase species by particles (Anttila et al., 2006; Folkers et al., 2003; Escoreia et al., 2010; Cosman and Bertram, 2008; Park et al., 2007; Thornton and Abbatt, 2005; McNeill et al., 2006; Gaston et al., 2014). Folkers et al. performed experiments to study the heterogeneous hydrolysis of N2O5 on phase separated organic-inorganic salt particles. Their results showed that the reaction probability on phase separated particles was reduced by approximately a factor of five compared to the reaction probability on aqueous inorganic salt particles. Reimer et al. showed in simulations for the European atmosphere that the mixing ratios of NO3, N2O5, VOCs in the gas phase, as well as nitrate in the particulate phase, are changed because of the decrease of N2O5 uptake onto the liquid-liquid phase separated particles (Riemer et al., 2009). Phase separation can also change the partitioning of organic species between particles and the gas phase (Seinfeld et al., 2001; Chang and Pankow, 2006; Zuend et al., 2010; Shiraiwa et al., 2013). The mass of organic species that can partition into particles can increase by as much as 50% when particles are in a two-liquid-phase state compared to particles in a one-liquid-phase state (Chang and Pankow, 2006; Zuend et al., 2010). Phase separation can also change the ice nucleation properties of atmospheric particles (Schill and Tolbert, 2013). 1.7 Motivation and overview of dissertation Prior to the work in this dissertation, liquid-liquid phase separation in particles was studied using only a small number of particle types. This dissertation focuses on liquid-liquid 12  phase separation in mixed organic-inorganic salt particles covering a wider range of particle types and conditions relevant to the atmosphere. In addition, samples from the real atmosphere were studied. The goal of this dissertation is to help improve the understanding of liquid-liquid phase separation in atmospheric aerosols, and in turn improve simulations of atmospheric chemistry, air quality and climate. This dissertation focuses on liquid-liquid phase separation in atmospherically relevant mixed organic-inorganic salt particles using microscopy techniques. Chapters 2-6 are research chapters. Specifically, Chapter 2 is a study of the dependence of liquid-liquid phase separation in mixed organic-ammonium sulfate particles on the O:C ratio of the organic species and OIR of the particle.  The correlation between liquid-liquid phase separation and O:C found in this study provided the base for further studies in this dissertation. The research in Chapter 3 highlights the occurrence of liquid-liquid phase separation in particles consisting of ambient material as well as two types of secondary organic material produced in laboratory chambers. Based on the experiments, we conclude that liquid-liquid phase separation is likely a common occurrence in atmospheric aerosols. The research in Chapter 4 focuses on the liquid-liquid phase separation in particles consisting of ammonium sulfate and secondary organic material produced by photo-oxidation of toluene which is considered to be a contributor to secondary organic material in regions heavily influenced by anthropogenic emissions. The research in Chapter 5 focuses on the effect of inorganic salt type on liquid-liquid phase separation in mixed organic-inorganic salt particles. The research in Chapter 6 focuses on the effects of molecular weight of organic species and temperature on liquid-liquid phase separation in mixed organic-ammonium sulfate particles. Chapter 7 is a summary of this dissertation, and an outlook for future research.   13  Chapter 2. Liquid-liquid phase separation in mixed organic-ammonium sulfate particles 2.1 Introduction Atmospheric aerosols have many important effects on climate, atmospheric chemistry and physics, as well as regional visibility and human health (see examples and references in Chapter 1, Section 1.1). In the atmosphere, single particles having both organic species and sulfate are abundant (Murphy and Thomson, 1997; Murphy et al., 1998; Middlebrook et al., 1998; Buzorius et al., 2002; Murphy, 2005; Murphy et al., 2006; Tolocka et al., 2005; Chen et al., 2009; Pratt and Prather, 2010). Since the sulfate fraction is often partially or fully neutralized by ammonium (Dibb et al., 1996; Huebert et al., 1998; Talbot et al., 1998; Dibb et al., 2000; Lee et al., 2003), mixed organic-ammonium sulfate particles are an important class of atmospheric aerosols. Individual organic-ammonium sulfate particles can undergo liquid-liquid phase separation, efflorescence, and deliquescence as the relative humidity cycles between high and low values during air parcel motion and temperature oscillations in the atmosphere (see Chapter 1, Section 1.5). In the past, researchers trying to understand phase transitions of organic-ammonium sulfate particles found in the atmosphere focused mainly on the individual organic molecules that have been identified in the atmosphere. However, the organic fraction in atmospheric particles consists of thousands of different molecules, with only about 10% by mass molecularly identified (Hamilton et al., 2004; Goldstein and Galbally, 2007; Decesari et al., 2006; Hallquist et al., 2009). Because only a small fraction of the organic material in atmospheric particles have been identified, liquid-liquid phase separation, efflorescence, and 14  deliquescence of mixed organic-ammonium sulfate particles found in the atmosphere have been difficult to anticipate. Herein, given the daunting task of characterizing and studying thousands of individual organic molecules, we explore a different approach by focusing on the organic-to-sulfate mass ratio (org:sulf) of the mixed particles and the oxygen-to-carbon elemental ratio (O:C) of the organic component as possible predictors of liquid-liquid phase separation. The O:C ratio and org:sulf ratio are considered to be useful and simple parameters in describing the properties of complex mixtures of organic species in the particles (Jimenez et al., 2009). A practical advantage of using these ratios is that they are measured by instrumentation that has already been deployed at measurement sites worldwide (Chen et al., 2009; Aiken et al., 2008; DeCarlo et al., 2008; Jimenez et al., 2009; Hawkins et al., 2010; Heald et al., 2010; Ng et al., 2010; Takahama et al., 2011). Liquid-liquid phase separation was studied in particles containing one organic species mixed with ammonium sulfate. Current laboratory results as well as other laboratory results reported in the literature were summarized and discussed. The parameterization of the laboratory results was presented.  2.2 Experimental 2.2.1 Preparation of sample particles Eleven organic species were studied (see Table 2.1). The O:C values of the organics studied  ranged from 0.29 to 1.33, covering the range of O:C values measured in the atmosphere (approximately from 0.1 to 1.0) (Chen et al., 2009; Aiken et al., 2008; DeCarlo et al., 2008; Jimenez et al., 2009; Hawkins et al., 2010; Heald et al., 2010; Ng et al., 2010; Takahama et al., 15  2011). Functional groups in the organic species studied included esters, alcohols, carboxylic acids, ethers, and oxidized aromatic compounds. This selection covered many of the functional groups found in the atmosphere (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006; Decesari et al., 2006; Hallquist et al., 2009; Takahama et al., 2011; Rogge et al., 1993; Saxena and Hildemann, 1996; Finlayson-Pitts and Pitts, 1997; Day et al., 2009; Gilardoni et al., 2009; Liu et al., 2009; Russell et al., 2009; Russell et al., 2011; Fu et al., 2011; Fuzzi et al., 2001). All organic compounds studied herein were purchased from Sigma-Aldrich with purities ≥ 98%, with the exception of suberic acid monomethyl ester and 1,2,6-hexanetriol, which were purchased from Sigma-Aldrich with a purity of  97%, and glycerol, which was obtained from Thermo Fisher Scientific with a purity of 99.9%. All organic compounds were used without further purification.  Table 2.1 List of the organic compounds studied in the mixed organic-ammonium sulfate particles.  Compound Formula Functional groups O:C Diethyl sabacate C14H26O4 esters 0.29 Suberic acid monomethyl ester C9H16O4 carboxylic acid + ester 0.44 1,2,6-hexanetriol C6H14O3 alcohols 0.50 α,4-dihydroxy-3-methoxybenzeneacetic acid C9H10O5 carboxylic acid + alcohols + ether + aromatic 0.56 2,5-dihydroxybenzoic acid C7H6O4 carboxylic acid + alcohols 0.57 2,2-dimethylsuccinic acid C6H10O4 carboxylic acids 0.67 Glutaric acid C5H8O4 carboxylic acids 0.8 Levoglucosan C6H10O5 ethers + alcohols 0.83 Glycerol C3H8O3 alcohols 1.00 Citric acid C6H8O7 carboxylic acids + alcohol 1.17 Malonic acid C3H4O4 carboxylic acids 1.33  16  In each experiment, particles contained an organic species mixed with ammonium sulfate. A solution of ammonium sulfate and an organic species was prepared in high purity water (Millipore, 18.2 MΩ cm) except for experiments with particles containing diethyl sebacate and liquid suberic acid monomethyl ester. These two organic species have a solubility of less than 1 % (weight/weight) in water, so a mixture of water and methanol was used to make homogeneous solutions of ammonium sulfate and each of these two organic species. As shown in Figure 2.1, the solution was then passed through a nebulizer (Meinhard) to produce submicron droplets. These droplets were directed toward the hydrophobic glass slide, upon which they deposited and coalesced into super-micron droplets. The water or the water/methanol mixture was then evaporated to generate organic-ammonium sulfate particles with lateral dimensions ranging from 10 to 30 μm.  Figure 2.1 A diagram of the system used for generating particles in this study. 2.2.2 Flow cell system The relative humidity at which liquid-liquid phase separation occurred (SRH) in particles containing single organic species mixed with ammonium sulfate was studied with an optical 17  microscope coupled to a temperature and relative humidity controlled flow-cell, as shown in Figure 2.2 (Koop et al., 2000; Parsons et al., 2004b; Pant et al., 2006; Bodsworth et al., 2010). The bottom surface of the flow cell was a hydrophobic glass slide upon which the particles were deposited and observed. Underneath the bottom surface was a cooling stage which was used to control the temperature. The phase of particles was monitored by a light reflected microscope (Zeiss Axiotech; 50× objective). Relative humidity in the cell was controlled by a continuous flow of a mixture of humid and dry nitrogen. Ultra-high purity nitrogen (Praxair, 99.999%) was first passed through a hydrocarbon filter (Supelco; Supelpure HC 2-2446) and then was split into two flows. One flow passed through a water bubbler seated inside a refrigerating circulator (Thermo Neslab, RTE-140) to generate humidified flow, and the second flow served as the dry line. Typical total flow rates were approximately 1.5 L min−1. The relative humidity in the cell was adjusted by changing the flow rate of the humidified flow and the dry flow, or by altering the temperature of the refrigerating circulator.  The dew point of the total gas flow was determined using a chilled mirror sensor (General Eastern; Model 1311DR). The temperature of the cell was measured by a thermocouple in the cooling stage underneath the bottom of the cell. In this way, the relative humidity in the cell can be calculated using the dew point of the gas flow and the temperature of the cell. The uncertainty in the reported SRH values were 2.5 %, based on the reproducibility of the data. At the beginning of an experiment the RH in the flow cell was first set to nearly 100 %. Then the RH over the particles was ramped down at a rate of 0.4-0.6 %   min-1, and images of the particles were captured every approximately 20 seconds using a digital camera (Edmund optics, EO-1312C) connected to the microscope. The RH was decreased to approximately 25% if efflorescence was observed and to < 2% if no efflorescence was observed.  18   Figure 2.2 A diagram of the optical microscope coupled to a temperature and relative humidity controlled flow-cell system used to measure phase transition relative humidities in this study as well as studies in Chapter 3 to Chapter 6. (a) a schematic of the experimental system used to control relative humidity of the particles; (b) A detailed schematic of the flow cell used to measure phase transition relative humidities.  All experiments were carried out at a temperature of 290 ± 1 K, except for experiments with 1,2,6-hexanetriol. The results reported for 1,2,6-hexanetriol were carried out at 273 ± 1 K 19  though several experiments were also conducted at 290 ± 1 K. Within experimental uncertainty the SRH values were the same at both temperatures. 2.2.3 Calibration of the hygrometer Calibration of dew point readings from the hygrometer, which was used for calculating RH, was done by measuring the deliquescence relative humidity of pure ammonium sulfate particles and comparing the measured value with values reported in the literature at a given temperature of the cell. 2.2.4 Raman microscopy The spatial distribution of the organic and ammonium sulfate material in the particle after liquid-liquid phase separation was probed with Raman microscopy at approximately 293 K. Raman spectra of a pure ammonium sulfate particle, a pure α,4-dihydroxy-3-methoxybenzeneacetic acid particle, and a mixed α,4-dihydroxy-3-methoxybenzeneacetic acid-ammonium sulfate particle (org:sulf = 3.1) were collected. Spectra were acquired on particles deposited on a hydrophobic glass slide in the same manner as for the optical microscope experiments. The spectra were collected using a Renishaw inVia Raman microscope with excitation using an argon ion laser at 514 nm and a power of 200 mW. All Raman spectra were collected at room temperature. For the Raman experiments the particles were exposed to room air, and hence, the RH was not controlled. 2.3 Results and discussion Shown in Figure 2.3 are examples of images recorded during a typical experiment in which liquid-liquid phase separation and efflorescence were observed.  Movies corresponding to these images are shown in the Supplementary Materials (Movie 2.1). When RH decreased from 20  high to low values (right to left shown in the figure), the particle underwent liquid-liquid phase separation, from a homogeneous liquid phase to two separated liquid phases. The outer liquid phase was an organic-rich, while the inner liquid phase was ammonium sulfate-rich phase. When RH went even lower, the inner aqueous ammonium sulfate phase effloresced and particle transformed into a solid-liquid particle.  Figure 2.3 Optical microscope images of a single particle consisting of 1,2,6-hexanetriol and ammonium sulfate during a typical experiment when RH decreased from high to low values (from right to left). Images were recorded with 500 × magnification and at a temperature of 290 ± 1 K.  Shown in Figure 2.4 are Raman spectra of particles after liquid-liquid phase separation. The morphology was an organic coating surrounding an aqueous ammonium sulfate core, with small amounts of each in the other phase. For some systems we also observed several sulfate rich inclusions with diameters on the order of a few micrometers within an organic rich phase. These results are consistent with images of poly(ethylene glycol)-ammonium sulfate particles after liquid-liquid phase separation (Ciobanu et al., 2009). In our studies, as well as the previous studies with poly(ethylene glycol) ammonium sulfate particles, the hydrophobic glass slide may have influence the morphology. Studies with levitated particles have reported an organic lens on aqueous droplets and several small aqueous ammonium sulfate inclusions suspended in an organic particle after liquid-liquid phase separation (Buajarern et al., 2007; Kwamena et al., 21  2010). Regardless of the morphology, the Raman spectra showed that the organic molecules were largely excluded from the aqueous salt solution after phase separation. This separation allowed efflorescence of inner ammonium sulfate to occur at an RH close to that of pure aqueous ammonium sulfate (Ciobanu et al., 2009; Smith et al., 2011).  Figure 2.4 Raman spectra of (a) a pure ammonium sulfate particle, (b) a pure α,4-dihydroxy-3-methoxybenzeneacetic acid particle, (c) the inner phase of a mixed α,4-dihydroxy-3-methoxybenzeneacetic acid–ammonium sulfate particle after phase separation, and (d) the outer phase of a mixed α,4-dihydroxy-3-methoxybenzeneacetic acid-ammonium sulfate particle after phase separation. The mixed particle used for Raman analysis (org:sulf=1.44) is shown to the right. All Raman spectra were collected at approximately 293 K. In addition, during Raman measurements the particles were exposed to room air, and hence the RH was not controlled.  22  The SRH results from the study are  listed in Table 2.2 and plotted in Figure 2.5 as a function of org:sulf, together with previous studies of SRH of organic-ammonium sulfate systems (Table 2.3 and references therein). Figure 2.5 reveals that a dividing line emerges from the data, specifically for O:C < 0.7 compared to O:C > 0.7. For O:C < 0.7, the particles regularly underwent liquid-liquid phase separation; while for O:C > 0.7, no liquid-liquid phase separation was observed in systems with any org:sulf values.  Table 2.2 Results of SRH from the optical microscope experiments. The results correspond to particles consisting of one organic compound and ammonium sulfate. Organic compound used in the three-component studies Composition ( organic : sulfate) SRHa (% RH) Diethyl sabacate  (O:C = 0.29) 0.248 Not observedb 0.481 97.5 1.443 100c 4.380 100c 7.857 100c Suberic acid monomethyl ester (O:C = 0.44) 0.477 100c 1.401 100c 2.276 100c 4.463 100c 1,2,6-hexanetriol  (O:C = 0.5) 0.355 70.3 0.945 69.0 2.127 72.5 5.673 71.6 α,4-dihydroxy-3-methoxybenzeneacetic acid (O:C =0.56 ) 0.516 80.1 1.375 81.3 3.094 79.0 8.252 Not observed 2,5-dihydroxybenzoic acid (O:C =0.57 ) 0.244 61.5 0.456 64.2 0.734 62.9 1.109 Not observed 1.376 Not observed 2.003 Not observed 2,2-dimethylsuccinic acid (O:C =0.67 ) 0.422 63.8 0.734 61.5 1.361 Not observed 1.673 Not observed 2.128 Not observed 4.197 Not observed Glutaric acid  (O:C = 0.80) 0.464 Not observed 1.416 Not observed 4.207 Not observed 23  Organic compound used in the three-component studies Composition ( organic : sulfate) SRHa (% RH) Levoglucosan (O:C = 0.83) 0.277 Not observed 0.563 Not observed 1.097 Not observed 2.140 Not observed 3.134 Not observed 6.788 Not observed 12.824 Not observed Glycerol  (O:C = 1.0)  0.133 Not observed 0.286 Not observed 0.558 Not observed 0.990 Not observed 1.363 Not observed 2.622 Not observed 7.989 Not observed Citric acid  (O:C = 1.17) 0.222 Not observed 0.353 Not observed 0.500 Not observed 0.667 Not observed 0.857 Not observed 1.997 Not observed 3.314 Not observed 7.635 Not observed Malonic acid  (O:C = 1.33)  0.149 Not observed 0.342 Not observed 0.615 Not observed 1.212 Not observed 2.728 Not observed 8.653 Not observed  aThe uncertainty in the SRH was typically 2.5%. bNot observed indicates that liquid-liquid phase separation was not observed for the range of relative humidities probed. In some cases SRH was not probed below 35-40% RH since at RH values less than 35-40% crystallization of ammonium sulfate or the organic occurred in the three-component particles. Phase transitions were measured at a temperature of 290  1 K.  c100 for SRH indicates that two liquid phases were observed even for the highest relative humidity exposed to the particles, which was 100  2.5%.      24  Table 2.3 List of the SRH data used for the parameterizations of SRH in systems consisting of one organic plus ammonium sulfate.  Compound Formula Functional groups O:C Referencesa Diethyl sabacate C14H26O4 ester 0.29 this study 1,2-hexanediol C6H14O2 alcohol 0.33 (Marcolli and Krieger, 2006) Suberic acid monomethyl ester C9H16O4 carboxylic acid, ester 0.44 this study 1,4-butanediol C4H10O2 alcohol 0.50 (Marcolli and Krieger, 2006) 1,2,6-hexanetriol C6H14O3 alcohol 0.50 this study α,4-dihydroxy-3-methoxybenzeneacetic acid C9H10O5 alcohol, aromatic, carboxylic acid, ether 0.56 this study Polyethylene glycol-400 C2nH4n+2On+1, n = 8.2 to 9.1 alcohol, ether ~0.56 (Marcolli and Krieger, 2006; Ciobanu et al., 2009) 2,5-dihydroxybenzoic acid C7H6O4 alcohol, carboxylic acid 0.57 this study 2,2-dimethylsuccinic acid C6H10O4 carboxylic acid 0.67 this study Glutaric acid C5H8O4 carboxylic acid 0.8 this study Levoglucosan C6H10O5 alcohol, ether 0.83 this study Glycerol C3H8O3 alcohol 1.00 this study Citric acid C6H8O7 alcohol, carboxylic acid 1.17 this study Malonic acid C3H4O4 carboxylic acid 1.33 this study  a SRH were measured for temperatures ranging from 290 to 298 K. SRH results reported by Marcolli and Krieger were measured in bulk solution systems.      25     Figure 2.5 Separation relative humidity (SRH) as a function of organic-to-sulfate mass ratio. The colors represent the O:C of the organic species. Axes denote the observed relative humidity (RH) of a transition and the organic-to-sulfate mass ratio of a studied mixed system. (a)  results in the particles containing organic species with O:C < 0.7; and (b) with O:C > 0.7. Open symbols indicate that liquid-liquid phase separation was not observed. In some cases SRH was not probed below 35-40% RH because ammonium sulfate or the organic material crystallized. Measurements were at temperatures ranging from 290 to 298 K except for experiments with particles consisting of 1,2,6-hexanetriol and ammonium sulfate, which were studied at 273 ± 1K . The SRH value was independent of org:sulf in most systems studied, as illustrated in Figure 2.5 a and b, but correlated with O:C, at least for the parameter space explored here (i.e., 0.1<org:sulf<15 and 0.2<O:C<1.4). The average SRH were calculated by taking the average of all the SRH values of particles containing the same organic species excluding the results when liquid-liquid phase separation was not observed and when the org:sulf < 0.1 and > 15. To explore the correlation between SRH and O:C, the average SRH values were fitted as a function of O:C with a second order polynomial function for O:C values less than ≤ 0.8.  The results of the fitting are shown in Figure 2.6, and Table 2.5. R2 of the fitting was 0.87 for O:C ≤ 0.8. 26   Figure 2.6 Average liquid-liquid phase separation relative humidity (SRH) as a function of O:C. An open symbol for SRH indicates that SRH was not observed. In some cases SRH was not probed below 35-40% RH because crystallization of ammonium sulfate or the organic occurred at lower RH values. The line when O:C < 0.7 is a second order polynomial fit to the data (R2 = 0.87). Phase transitions were measured at temperatures ranging from 290 to 298 K.  The correlation between SRH and O:C shown in Figure 2.5 (a) (b) and Figure 2.6 can be rationalized by the salting out effect of the organic molecule by inorganic salts. The decrease in solubility of an organic species in an aqueous solution due to the addition of a salt is known as the salting out effect. Salting out of organic species by the inorganic salt could be described by the Setchenov equation, which was first used to describe the solubility of small gas molecules in salt solutions (Lee, 1997; Long and Mcdevit, 1952; Grover and Ryall, 2005):                                                                                                               (2.1) 27  where So is solubility of organic species in water in the absence of inorganic salt, S is the solubility of organic species in inorganic salt aqueous solution, Cs is the concentration of the inorganic salt, and ks is Setchenov constant. A negative value of ks indicates salting out of organic species by the inorganic salt. A decreasing negative value of ks indicates a greater tendency of liquid-liquid phase separation. According to the electrostatic theory, the Setchenov constant ks is related to the dipole moment of the organic molecule (Desnoyer and Ichhaporia, 1969; Conway et al., 1964). Since dipole moment should be roughly related to O:C, a correlation between ks and O:C , as well as a correlation between SRH and O:C, are expected. This logic is also consistent with a recent modeling study using alcohols and salts which showed that hydrophilicity, which was reflected in O:C, is a key feature in defining the region of a miscibility gap in alcohol-salt-water systems (Zuend et al., 2010). The difference between 0.87 and unity for the correlation factor of SRH with O:C (shown in Figure 2.6) is plausibly explained by the combination of several different classes of organic molecules in our data set. A better correlation would be expected if a homologous series of organic molecules were studied.   28  Table 2.4 Parameterizations of the SRH data. SRH results were measured at temperatures ranging from 290 to 298 K.   Parameterization Valid Range SRH (%) =0 0.7<(O:C)<1.4 and 0.1<(org:sulf)<15 =35.5+339.9×(O:C)-471.8×(O:C)2 0.2<(O:C)<0.7 and 0.1<(org:sulf)<15  The SRH parameterization correctly predicted SRH within 15% RH for 88% of the measurements. In addition, the parameterization predicted with reasonable accuracy the O:C range at which liquid-liquid phase separation is expected to occur. For more accurate predictions of SRH, additional information (i.e., in addition to O:C) is needed. Useful information would include functional groups and the molecular weight of organic species. However, any additional accuracy would come at the expense of added complexity in the parameterization and may require chemical information that is currently not routinely measured (unlike org:sulf and O:C). 2.4 Conclusions The relative humidity at which liquid-liquid phase separation occurred (SRH) in different organic-ammonium sulfate systems was studied with an optical microscope. Eleven organic species were included. The spatial distribution of the organic and sulfate materials in the phase separated particles was also studied using Raman microscope. The new laboratory SRH results as well as data reported in the literatures showed that SRH is not sensitive to org:sulf in most of the mixed systems studied. New results were combined with data reported in the literatures to develop parameterization of SRH in terms of O:C. The parameterizations correctly predict SRH within 15% RH for 88% of the measurements. Improvements in the predictions of SRH will require additional chemical information which may not be routinely measured. The parameterizations of SRH as a function of O:C  would be useful to incorporation in 29  comprehensive chemical transport models (Wang et al., 2008b). Chemical transport models are used in large-scale predictions of atmospheric chemistry and are coupled in advanced treatments to global climate models.  30  Chapter 3. Images reveal that atmospheric particles can undergo liquid-liquid phase separations 3.1 Introduction Results of laboratory measurements of particles containing ammonium sulfate mixed with one or a few specific organic molecules have shown that oxygen-to-carbon elemental ratio (O:C) of the organic material in the particles is a good predictor of the occurrence of liquid-liquid phase separation and SRH of mixed organic-inorganic salt particles (Bertram et al., 2011; Song et al., 2012b; You et al., 2013). Liquid-liquid phase separation was regularly observed when O:C < 0.7 and was never observed when O:C > 0.7 (Chapter 2). Laboratory experiments and calculations results also suggest, after extrapolation to atmospheric conditions, liquid-liquid phase separations can occur in atmospheric particles when the relative humidity (RH) varies from high to low values in the atmosphere (Zuend et al., 2010; Marcolli and Krieger, 2006; Bertram et al., 2011; Ciobanu et al., 2009; Erdakos et al., 2006; Song et al., 2012a).  Atmospheric particles, however, are far more complex than the simple proxies used in these studies. As discussed in Chapter 1 Section 1.4, organic material in atmospheric particles mainly comes from atmospheric oxidation of volatile organic compound (VOC) precursor gases, which is called secondary organic material. Secondary organic material can be produced in the laboratory by oxidation of VOCs under typical atmospheric conditions. Compared to single organic species or mixtures of a few organic species, secondary organic material produced in the laboratory is more representative of organic material in atmospheric particles. The phase transition behavior of particles containing complex secondary organic material at different relative humidities may be different from particles containing one or a few organic species, even 31  if the average O:C values of the organic material and functional groups are same. For example, Marcolli et al. have shown the deliquescence relative humidities (DRH) of the mixtures of two to five atmospherically relevant carboxylic acids are significantly lower than the DRH of each individual carboxylic acid (Marcolli et al., 2004). Studies of liquid-liquid phase separation in particles containing laboratory produced secondary organic material and inorganic salt are useful to improve the understanding of liquid-liquid phase separation in real atmospheric particles. A few studies have inferred that liquid-liquid phase separation occurs in particles containing ammonium sulfate and secondary organic material generated from the dark ozonolysis of α-pinene (Smith et al., 2011; Prisle et al., 2010). In the present study, liquid-liquid phase separation is explored for real-world samples, as well as particles containing ammonium sulfate and secondary organic material generated from dark ozonolysis of α-pinene and photo-oxidation of 1,2,4-trimethylbenzene in  the laboratory environmental chambers. 3.2 Experimental  3.2.1 Production of secondary organic material Dark ozonolysis of α-pinene was performed in the Harvard Environmental Chamber to produce secondary organic material that then condensed on dry ammonium sulfate seed particles. Ammonium sulfate seed particles were introduced into the chamber by atomization of a solution of (NH4)2SO4, which was then dried using a diffusion dryer. The α-pinene and ozone were introduced to the chamber by a flow of purified air. The setup and experimental conditions of the dark ozonolysis experiments were similar to those employed by Shilling et al. (Shilling et al., 2008). Flow into the continuous-flow chamber was fixed at 20 standard liters per minute (sLpm) during operation. The temperature and relative humidity inside the chamber were maintained at 32  25°C and 40%, respectively. An Aerodyne high-resolution time-of-flight Aerosol Mass Spectrometer (HR-ToF-AMS) was used to determine the composition of the particles. The secondary organic material with ammonium sulfate seeds was collected at the outlet of the continuous-flow chamber onto a quartz fiber filter. The collection time for sampling was 48 hours at a flow rate of 9.0 sLpm. Photo-oxidation of 1,2,4-trimethylbenzene was carried out in the Pacific Northwestern National Laboratory (PNNL) Continuous-Flow Environmental Chamber to produce secondary organic material. 1,2,4-trimethylbenzene was injected into 24 sLpm of pure air flow to produce concentrations of 4.8 ppmv (parts-per-million by volume) in the chamber prior to reaction. An additional 1 sLpm flow of pure air was bubbled through a 50% (vol/vol) solution of hydrogen peroxide in water and added to the chamber. Photo-oxidation was initiated by 105 ultraviolet (Q-labs UVA-340) lights surrounding the chamber, which generated OH radicals from the photolysis of hydrogen peroxide. The chamber temperature and relative humidity were maintained at 18.1 ± 0.2 °C and 6 ± 2 %, respectively over the course of the 4-day experimental run. NOx levels were below the 1 ppbv (parts-per-billion by volume) detection limit of a NOx chemiluminescence detector. Particle chemical composition was analyzed in real time using an Aerodyne HR-ToF-AMS. The secondary organic material was collected at the outlet of the continuous-flow chamber onto a Teflon filter (Pall, R2PL037). The collection time for sampling was 82 hours at a flow rate of 4 sLpm.  3.2.2 Collection of samples from Atlanta, Georgia Particles were collected on quartz fiber filters for approximately 24 hours using a high-volume sampler (Thermo Anderson, flow rate 1.13 × 103 sLpm). Collection occurred on the Georgia Institute of Technology Environmental Science and Technology rooftop laboratory 33  located in Central Atlanta, approximately 15 m above ground level. The concentration of organic carbon and inorganic salts on the filters was determined from 2.54 cm diameter filter punches, using a total organic carbon analyzer and ion chromatography. For the sample collected on July 28 and 29, 2010, the mass composition was the following: organic carbon = 3.99 mg; NH4+=0.859 mg; SO42– = 2.711 mg; NO3– = 0.471 mg; Na+ = 0.097 mg; K+ = 0.043 mg; Mg2+ = 0.019 mg; Ca2+ = 0.068 mg; Cl- = 0.066 mg. For the sample collected on August 6-7, 2010, the composition was the following: organic carbon = 4.58 mg; NH4+=1.974 mg; SO42– = 4.677 mg; NO3– = 0.902 mg; Na+ = 0.074 mg; K+ = 0.012 mg; Mg2+ = 0.012 mg; Ca2+ = 0.043 mg; Cl- = 0.049 mg. The O:C of the organic component of the filters collected in Atlanta were determined by extracting the filters with water and then determining the O:C from the filter extracts using HR-ToF-AMS (Aiken et al., 2008; DeCarlo et al., 2006). 3.2.3 Production of particles Water-soluble species were extracted from the filter samples using high purity water (Millipore, 18.2 MΩ cm). For the samples collected in Atlanta, particles were generated directly from the filter extracts without the addition of ammonium sulfate. Filter extracts were passed through a Particle-on-Demand Generator (Model MJ-ABL-01-120, MicroFab Technologies) to produce particles with sizes ranging from 10-30 µm in diameter. These particles were deposited on Teflon slides and then placed in a flow cell that had temperature and relative humidity control for the microscopy experiments (Chapter 2 Section 2.2.2). To increase the organic-to-sulfate mass ratio (org:sulf) to values prevalent in the atmosphere, ammonium sulfate solution was added to the filter extracts from the environmental chambers. After addition of ammonium sulfate, org:sulf in the samples from -pinene and 1,2,4-trimethylbenzene were 1.4 and 3.0, respectively. Once ammonium sulfate was added to these 34  filter extracts, they were used to produce particles as described above. Ammonium sulfate particles were studied for comparison with the experimental samples. First a solution of ammonium sulfate in high purity water was prepared. This solution was then added to a blank filter and processed the same way as the filter samples discussed above. 3.2.4 Optical and fluorescence microscopy of particles Images of the particles were recorded with fluorescence microscopy (Zeiss LSM510, λexcitation = 543 nm, λemission = 650-710 nm, 293  1 K) and optical microscopy (Zeiss Axiotech; 50× objective, 273  1 K). At the beginning of an experiment, the RH in the flow cell was set to 90-100 %, and the particles were allowed to equilibrate with the flow cell RH for 15-20 min. The RH was then decreased at a constant rate (0.6 % min-1) while images were collected (Chapter 2 Section 2.2.2). Calibration of the absolute RH readings was performed using the deliquescence relative humidity (DRH) values for pure ammonium sulfate particles (Chapter 2 Section 2.2.3).  3.3 Results and discussion 3.3.1 Liquid-liquid phase separation in particles containing laboratory generated secondary organic material Up to 90% of the submicron particle-phase organic material in the atmosphere is secondary organic material (Hallquist et al., 2009). In this section, we investigated the phase behavior of particles consisting of ammonium sulfate mixed with secondary organic material that had been produced for conditions designed to simulate natural and polluted atmospheres. For these experiments, α-pinene was ozonized, a process that is considered to be a major biogenic source of secondary organic material in the atmosphere (Hallquist et al., 2009; Kanakidou et al., 2005). As a surrogate of a polluted urban atmosphere, 1,2,4-trimethylbenzene was photo-35  oxidized (Odum et al., 1997). The O:C of the secondary organic material used in these experiments are listed in Table 3.1. These ratios fall within the range of average O:C values observed in the atmosphere, which is from 0.1 to 1.0 (Chen et al., 2009; Aiken et al., 2008; DeCarlo et al., 2008; Jimenez et al., 2009; Hawkins et al., 2010; Heald et al., 2010; Ng et al., 2010; Takahama et al., 2011). Table 3.1 Relevant information for samples. Particle Type O:C  Ratio Org:Sulf  mass ratio NH4+ mass (mg) SO42- mass (mg) NO3- mass (mg) Other inorganic ions massa (mg) SRH (%) α-pinene dark-ozonolysis SOMc+(NH4)2SO4 0.3 1.4 0.9 2.4 NA NA >90 1,2,4-trimethylbenzene photo-oxidation SOM+(NH4)2SO4 0.4 3.0 2.3 6.2 NA NA >70 Atlanta Organic-Sulfate  July 28-29 2010 0.5b 1.5 0.9 2.7 0.5 0.3 >90 Atlanta Organic-Sulfate  August 6-7 2010 Unknown 1.0 2.0 4.7 0.9 0.2 >90  The table shows oxygen-to-carbon elemental ratio (O:C) of the organic material, organic-to-sulfate mass ratio (org:sulf), mass of inorganic material, and the liquid-liquid separation relative humidities (SRH). Entries “NA” indicate “not applicable” aOther inorganic ions: Na+, K+, Mg2+, Ca2+, and Cl-. bThe O:C is based on a filter collected on July 29-30, 2010. We assume that the O:C was similar on July 28-29, since meteorological conditions and particle chemistry, such as org:sulf, concentrations of water soluble organic carbon, and concentrations of inorganic ions did not change considerably over the two days.  c SOM is the abbreviation of secondary organic material Shown in Figure 3.1 are the optical and fluorescence images of the particles as a function of RH. Also included for comparison are images of pure ammonium sulfate particles. Included in Supplementary Materials are movies of ammonium sulfate particles and ammonium sulfate-36  secondary organic material particles recorded as the RH was decreased and increased (see Movie 3.1, 3.2, 3.3, and 3.4).    37  Figure 3.1 Optical and fluorescence images of ammonium sulfate particles and ammonium sulfate-secondary organic material particles at various relative humidities. Images were collected by optical reflectance microscopy and fluorescence microscopy for decreasing relative humidity. Rows: (a and b) optical and fluorescence images of a reference ammonium sulfate particle; (c and d) optical and fluorescence images of ammonium sulfate-secondary organic material particle produced by α-pinene ozonolysis (org:sulf = 1.4); (e and f) optical and fluorescence images of ammonium sulfate-secondary organic material particle produced by photo-oxidation of 1,2,4-trimethylbenzene photo-oxidation (org:sulf = 3). The light circle in the optical images at the center of all liquid droplets is an optical effect due to scattering by a spherical particle. This circle decreases in intensity or disappears after the inner phase crystallizes. Particles of ammonium sulfate behaved as expected (Figure 3.1 a and b). At 90%, 70%, and 50% RH the particles consisted of a single aqueous phase, and when the relative humidity was decreased from 90% to 50%, the particles decreased in size due to the loss of water. At 30% RH the particles were crystalline. In addition, the fluorescence signal was nearly negligible for the ammonium sulfate particles for all RH values under the experimental conditions. In comparison with these results for ammonium sulfate, images of particles consisting of secondary organic material and ammonium sulfate (Figure 3.1 c-f) indicated the presence of two distinct phases. In the optical images, the outer phase appeared as a dark perimeter ring. The phase separation was especially evident in the fluorescence images because of the contrast between the fluorescent outer phase and the nonfluorescent inner phase. The mixed particles of ammonium sulfate and secondary organic material from the ozonolysis of α-pinene appeared to have two distinct phases, even at the upper end of the RH measurement at 90 % RH (Figure 3.1 c and d). The mixed particles of ammonium sulfate and secondary organic material from the photo-oxidation of 1,2,4-trimethylbenzene appeared to have two phases at RH values up to at least 70 % (Figure 3.1 e and f). Several pieces of evidence indicate that the inner phase was inorganic rich and the outer phase was organic rich. First, the fluorescence signal, practically absent for pure ammonium 38  sulfate particles, was confined mainly to the outer phase. Second, the inner phase crystallized at relative humidities between 35 % and 40 % RH (Supplementary Materials Movie 3.1, 3.2, 3.3, and 3.4). This range overlaps with the crystallization RH range of pure ammonium sulfate particles (∼33-37 %) (Pant et al., 2006; Bodsworth et al., 2010). Third, the outer phase did not crystallize for the full range of RH values studied in our experiments, suggesting that there was negligible ammonium sulfate present in the outer phase. On the basis of these several observations, we conclude that the inner phase was an inorganic-rich phase and that the outer phase was an organic-rich phase. This conclusion is in agreement with previous Raman studies of particles containing ammonium sulfate and oxidized organic species (Ciobanu et al., 2009; Bertram et al., 2011; Song et al., 2012a). We can also conclude from our results that both phases are noncrystalline above 35-40% RH. The presence of two separate noncrystalline phases confirms that liquid-liquid phase separation occurred in the particles, as molecular mobility is required for two phases to separate. A few previous studies have inferred from laboratory studies that liquid-liquid phase separation occurs in particles containing ammonium sulfate and secondary organic material generated from the dark ozonolysis of α-pinene (Anttila et al., 2007; Smith et al., 2011; Prisle et al., 2010). In these previous studies submicron particles were investigated. The consistency between the current results which use super-micron particles and these previous studies suggest that the liquid-liquid phase separations studied here do not depend strongly on particle size. 3.3.2 Liquid-liquid phase separation in particles containing materials collected from atmosphere We investigated the phase behavior of atmospheric samples collected in Atlanta, Georgia during July and August 2010. The atmosphere in and around Atlanta is heavily influenced by 39  both anthropogenic and biogenic emissions (Weber et al., 2007). Single-particle mass spectrometric measurements in Atlanta have shown that ambient particles typically occur as internal mixtures of organic molecules and sulfate (Lee et al., 2002; Liu et al., 2003). For our study, atmospheric samples were collected on filters, and inorganic salts and water-soluble organic material were extracted with high purity water. The filter extracts were almost exclusively ammonium sulfate and organic material, with other species making up less than 10% of the total mass identified (see Section 3.2.2 and Table 3.1 for further details). Additional ammonium sulfate was not added to the Atlanta filter samples. Shown in Figure 3.2 are results from samples collected July 28 and 29 and August 6 and 7, 2010 in Atlanta. Included in the Supplementary Materials are movies for samples collected on August 6 and 7, 2010 in Atlanta, recorded as the RH was decreased and increased (Supplementary Materials Movie 3.5 and 3.6). The sequence of observations is qualitatively similar to that described for the laboratory-generated particles of mixed secondary organic material and ammonium sulfate. Following a similar line of argument, we then conclude that the atmospheric samples from Atlanta also underwent liquid-liquid phase separation. A total of four samples collected on different days in Atlanta were analyzed, and in all cases results similar to those shown in Figure 3.2 were observed. As only the relatively water-soluble species were extracted from the filters, the experiments herein can be anticipated to be enriched in more oxidized organic species in comparison to the atmospheric particles. However, less-oxidized species tend to favor liquid-liquid phase separation more strongly than do more-oxidized species (Zuend et al., 2010; Bertram et al., 2011; Song et al., 2012a). Therefore, our results showing that liquid-liquid phase separation occurs even in particles containing the more-oxidized fraction of secondary organic material suggest that liquid-liquid phase separation will also occur in particles 40  containing the whole composition of secondary organic material, as present in ambient atmospheric particles.  Figure 3.2 Optical and fluorescence images of particles generated from filter samples collected in Atlanta, Georgia, USA. Images were collected for decreasing relative humidity. From the top to bottom: (a and b) optical and fluorescence images of a particle generated from a filter sample collected on July 28 and 29, 2010; (c and d) optical and fluorescence images of a particle generated from a filter sample collected on August 6 and 7, 2010.   3.4 Conclusions The work reported here shows that liquid-liquid phase separation can occur in the atmospheric particles when the O:C ratio of the organic material is roughly ≤ 0.5. Additional 41  studies are needed to determine whether these phase transitions also occur in the atmosphere when the O:C ratio > 0.5. 42   Chapter 4. Phase transitions of particles consisting of toluene-derived secondary organic material and ammonium sulfate  4.1 Introduction As discussed earlier, particle-phase secondary organic material is formed in the atmosphere by oxidation of volatile organic compounds (VOCs) and the subsequent condensation of the oxidation products (see Chapter 1 Section 1.4). Particles consisting of one inorganic salt and laboratory-generated secondary organic material should be better proxies for ambient particles than particles consisting of one inorganic salt mixed with one organic species.   Emissions of biogenic VOC, particularly, α-pinene, isoprene, limonene, and β-caryophyllene contribute to a large fraction of the particle-phase secondary organic material mass concentrations in the atmosphere (Seinfeld and Pandis, 2006; Kanakidou et al., 2005). Phase transitions of particles consisting of ammonium sulfate and laboratory-produced secondary organic material from the oxidation of biogenic VOCs have been investigated in a few studies (Bertram et al., 2011; Smith et al., 2011; Takahama et al., 2007) including the work discussed in Chapter 3 (You et al., 2012). Smith et al. showed that the efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) of ammonium sulfate in the mixed particles changed by less than 4% RH depending on the different mass fractions of α-pinene secondary organic material. Their results also suggested that phase separation occurred in the mixed α-pinene secondary organic material-ammonium sulfate particles (see discussion in Section 2.3). The optical and fluorescence microscopy study in Chapter 3 confirmed that liquid-liquid phase separation occurred in the particles containing ammonium sulfate and secondary organic material 43  produced from ozonolysis of α-pinene (You et al., 2012). In another study Smith et al. showed that the ERH and DRH of ammonium sulfate decreased by more than 20% RH with the increase of isoprene-derived secondary organic material fraction in the mixed particle (Smith et al., 2012). This result suggested that aqueous ammonium sulfate and the secondary organic material from oxidation of isoprene were fully mixed. Emissions of anthropogenic VOCs, such as toluene, benzene, and xylene, contribute to a large fraction of particle-phase secondary organic material in urban environments (Seinfeld and Pandis, 2006; Pandis et al., 1992; Odum et al., 1997; Henze et al., 2008). Even though the secondary organic material derived from anthropogenic VOCs accounts for a large fraction of an organic aerosol in urban environments, there has only been one study of liquid-liquid phase separation in particles containing inorganic salts and secondary organic material derived from anthropogenic VOCs at the time of the research in this chapter. In Chapter 3 the occurrence of liquid-liquid phase separation in the mixed particles of ammonium sulfate and secondary organic material produced by photo-oxidation of 1,2,4-trimethylbenzene was investigated using optical and fluorescence microscopy. Images showed that liquid-liquid phase separation occurred in the mixed particles for high secondary organic material mass fraction (Chapter 3). Since toluene is also an important VOC from anthropogenic sources, in this study liquid-liquid phase separation in particles consisting of ammonium sulfate and secondary organic material produced from photo-oxidation of toluene with two different secondary organic material to sulfate mass ratios (organic-to-sulfate mass ratio (org:sulf) = 0.1 and 3.0, respectively) were investigated.  44  4.2 Experimental 4.2.1 Secondary organic material generation   Secondary organic material were produced in the Harvard Environmental Chamber (HEC). The setup of the HEC has been described in detail previously (King et al., 2009; Shilling et al., 2008), as well as discussed briefly in Chapter 3.  HEC consists of a 4.7 m3 Teflon bag enclosed in a temperature-controlled chamber. The bag was operated as a continuously mixed flow reactor (CMFR), meaning that the inflow and outflow (8 Lpm) were balanced and steady-state conditions were obtained. The mean reactor residence time was 48 h, the temperature within the HEC was 25 °C, and the relative humidity RHCMFR in the bag was maintained at < 5%. Gaseous toluene and gaseous hydrogen peroxide were continuously injected into the CMFR. Liquid toluene (DrySolve, purity > 99.8%) was evaporated from the tip of a syringe in a glass round-bottom flask (100 mL, Ace Glass) and carried into the chamber by a pure air flow. The gas-phase concentration of toluene prior to reaction was 900 ppb. Hydrogen peroxide solution (Sigma-Aldrich, 30 wt %) was introduced into a separate round-bottom flask by a syringe pump. The hydrogen peroxide evaporated and was flushed into the CMFR by an air flow through the flask to give a gas-phase concentration of ∼10 ppm. Approximately 1 mg of secondary organic material was collected on quartz filters (Whatman 1851-047). The O:C of the particle-phase secondary organic material produced by the chemistry was 0.71 ± 0.2.The O:C of the organic material in particle phase were measured using an Aerodyne Aerosol Mass Spectrometer (AMS) (Aiken et al., 2007), which has an uncertainty of 30%  when determining O:C (Aiken et al., 2007). 45  4.2.2 Particle generation The secondary organic material was extracted from the filters in 30 mL high purity water (Millipore, 18.2 MΩ cm), so only the water-soluble fraction of secondary organic material was extracted. Two different filter-samples were used in this study, and different amounts of ammonium sulfate were added to the two filter extracts.  The org:sulf ratios of the two extracts after the addition of ammonium sulfate were 0.11 and 3.0.  After addition of ammonium sulfate, the extracts were atomized onto Teflon slides, resulting in particles having diameters from 28 to 41 μm. Particles consisting of pure ammonium sulfate and particles produced from just the Secondary organic material extract (without the addition of ammonium sulfate) were also prepared for experimental comparison. Because the secondary organic material was extracted from the filters in water, only the water-soluble fraction of the secondary organic material was studied in the microscopy experiments. This material may be enriched in more-oxidized organic species compared to the total secondary organic material collected on the filters. More-oxidized species compared to less oxidized species tend to favor mixing with an aqueous phase. In other words, the observation of liquid-liquid phase separation in particles containing only the water-soluble fraction of the secondary organic material suggests that phase separation will occur in particles containing the whole fraction of the secondary organic material. 4.2.3 Optical microscopy experiments using a flow cell system The slide was placed in a temperature- and RH-controlled flow cell (Chapter 2 Section 2.2.2). All experiments were carried out at 289 ± 1 K. Optical images of the particles were recorded every 10 seconds. At the beginning of the experiment, the particles were equilibrated at around 90% RH for 20 mins. Then the RH was decreased from 90 to 30% at a rate of 0.5-2.0 % 46  min−1. In cases where efflorescence occurred, RH was subsequently increased and scanned from 60 to 85% RH at a rate of 0.5-0.6% min−1 to observe deliquescence. In cases where efflorescence was not observed at RH ≥ 30%, RH was further decreased at a faster rate of 4-5% min−1 to < 0.5%, and kept at 0.5% for about 10 mins to see if the morphology further changed.  4.3 Results and discussion  47  Figure 4.1 Optical microscopy images and corresponding illustrations when RH was deceased. Columns from left to right represent particles at RH-values ranging from 90% to 0%. The top two rows of images and illustrations are for particles consisting of toluene photo-oxidation secondary organic material and ammonium sulfate with org:sulf = 0.1 and 3.0, respectively. The bottom rows of images and illustrations are for particles containing only toluene photo-oxidation secondary organic material. Optical microscopy images were taken from Smith et al. 2013, and copyright from American Chemical Society 2013. Optical microscopy images were collected to probe visually for phase separation at low organic-to-sulfate mass ratio (org:sulf = 0.1) and high organic-to-sulfate mass fraction (org:sulf =3.0) (Figure 4.1). For particles of org:sulf = 0.1, the images (see Supplementary Materials Movie 4.1) show that when decreasing RH one phase separated into two phases at 85 ± 5 % RH.  For this composition the second phase forms as an outer layer and small inclusions in the particle. Similar inclusions have been observed in experiments probing liquid-liquid phase separation of particles containing ammonium sulfate mixed with one or a few model organics (Ciobanu et al., 2009; Song et al., 2012a). A possible mechanism for the formation of the phase-separated inclusions is spinodal decomposition (Ciobanu et al., 2009; Song et al., 2012a). With further decreasing RH after liquid-liquid phase separation occurred, ammonium sulfate in the particle effloresced at 35.4 ± 2.5 % RH (Figure 4.1 and Supplementary Materials Movie 4.1). At high RH even before liquid-liquid phase separation occurred, the amount of secondary organic material was relatively small in these particles, while the ammonium sulfate-rich phase (inner aqueous phase) would have even less secondary organic material after liquid-liquid phase separation (because most of the secondary organic material would separate into second phases). Thereby, ammonium sulfate in the inner aqueous phase is expected to effloresce at close to the relative humidity of efflorescence in pure ammonium sulfate particles (around 35% RH).   For particles of org:sulf = 3.0, the optical images (Supplementary Materials Movie 4.2) show that for decreasing RH one phase separated into a dominant phase and a minor phase 48  between 100 and 70 % RH. In this case, the second phase appeared as a small inclusion within or on the larger droplet.  The small volume of the second phase compared to the large dominant phase indicated that most of the organic material was in the same phase with ammonium sulfate and water.  The majority of the material in the particles was secondary organic material, so even after phase separation the ammonium sulfate-rich phase still had much secondary organic material. No efflorescence was observed when RH decreased until < 0.5% and the minor phase disappeared lower than 50-20 % RH. The absence of efflorescence is consistent with phase mixing at low relative humidities (between 50-20 % RH) since large amounts of organic material in the ammonium sulfate-rich phase would suppress the efflorescence relative humidity (ERH) of ammonium sulfate as suggested by the Gibbs-Duhem equation (as discussed in Chapter 1 Section 1.5).  Also shown in Figure 4.1 for comparison are examples of particles consisting of water-soluble fraction of secondary organic material produced by photo-oxidation of toluene (in absent of ammonium sulfate). No liquid-liquid phase separation or efflorescence was observed when the RH was decreased to < 0.5%. As discussed in Chapter 2, a relationship between the occurrence of liquid-liquid phase separation and O:C of the organic materials in particles consisting of ammonium sulfate and one organic species was presented. Liquid-liquid phase separation in mixed organic-ammonium sulfate particles is expected to occur when O:C < 0.7 (Bertram et al., 2011). Song et al. expanded the result with organic functional groups and compositional complexity, and showed that liquid-liquid phase separation is expected to frequently occur for 0.56 < O:C < 0.8 (Song et al., 2012b). In this study, the O:C of toluene-derived secondary organic material is 0.71 ± 0.2 and liquid-liquid phase separation in the mixed particles was 49  observed. This result is consistent with the range of O:C values where liquid-liquid phase separation is expected to frequently occur.  4.4 Conclusion Optical images showed that toluene-derived secondary organic material-ammonium sulfate particles underwent liquid-liquid phase separation at 85 ± 5 % RH for the low organic-to-sulfate mass ratio (org:sulf = 0.1). When the organic-to-sulfate mass ratio was high (org:sulf = 3.0), a minor phase formed in between 100 and 70 % RH and mixed with the major phase again at RH lower than 50-20%. In Chapter 2, the relationship between occurrence of liquid-liquid phase separation and O:C in the particles containing ammonium sulfate and one organic species was present. Liquid-liquid phase separation was regularly observed for O:C < 0.7. The occurrence of liquid-liquid phase separation in particles containing toluene-derived secondary organic material (O:C = 0.71 ± 0.2) and ammonium sulfate in this study is consistent with the correlation shown in Chapter 2.    50  Chapter 5. Liquid-liquid phase separation in particles containing organics mixed with ammonium sulfate, ammonium bisulfate, ammonium nitrate or sodium chloride 5.1 Introduction A large fraction of atmospheric particles contain both organic material and inorganic salts  The number of possible inorganic salts is relatively small with ammonium sulfate, ammonium bisulfate, ammonium nitrate and sodium chloride thought to be important (Chapter 1 Section 1.2). As the relative humidity cycles in the atmosphere, particles containing a mixture of organic species and inorganic salts can undergo a range of phase transitions including deliquescence, efflorescence and liquid-liquid phase separation (Chapter 1 Section 1.5). Recent studies on phase transitions of mixed organic-inorganic salt particles of atmospheric relevance have focused on liquid-liquid phase separations (Chapter 1 Section 1.5). The majority of the laboratory work on this subject has used ammonium sulfate as the inorganic salt.  In the following we studied liquid-liquid phase separation in particles containing organics mixed with the following inorganic salts: ammonium sulfate, ammonium bisulfate, ammonium nitrate and sodium chloride. In each experiment one organic was mixed with one inorganic salt and the liquid-liquid phase separation relative humidity (SRH) was determined. Since we studied twenty-three different organic species mixed with four different salts, a total of ninety-two different particle types were investigated. These studies provide insight into the effect of salt type on liquid-liquid phase separation in atmospheric particles. 51  5.2 Experimental 5.2.1 Preparation of particles Shown in Table 5.1 is the list of the organics studied. The organic species investigated had a wide range of oxygen-to-carbon elemental ratio (O:C) (from 0.29 to 1.33), covering the most of the range of O:C often observed in the atmospheric particles (approximately 0.1 to 1.0) (Chen et al., 2009; Aiken et al., 2008; DeCarlo et al., 2008; Jimenez et al., 2009; Hawkins et al., 2010; Heald et al., 2010; Ng et al., 2010; Takahama et al., 2011). The organic species studied herein also included several functional groups observed in the atmospheric samples (e.g. carboxylic acids, alcohols, esters, ethers, and aromatics) (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006; Decesari et al., 2006; Hallquist et al., 2009; Takahama et al., 2011; Rogge et al., 1993; Saxena and Hildemann, 1996; Finlayson-Pitts and Pitts, 1997; Day et al., 2009; Gilardoni et al., 2009; Liu et al., 2009; Russell et al., 2009; Russell et al., 2011; Fu et al., 2011; Fuzzi et al., 2001).   52  Table 5.1 Summary of different organics used in the liquid-liquid phase separation experiments.  Compounds  Formula Molecular weight (Da) O:C H:C Functional group (s) Diethyl sebacate C14H26O4 258.4 0.29 1.86 ester 2,5-hexanediol C6H14O2 118.2 0.33 2.33 alcohol Poly (propylene glycol) C3nH6n+2On+1 425 0.38 2.10 alcohol, ether Suberic acid monomethyl ester C9H16O4 188.2 0.44 1.78 carboxylic acid, ester  Poly (ethylene glycol) diacrylate C2n+6H4n+6On+3 575 0.50 1.77 ester, ether, C-C double bond 1,2,6-hexanetriol C6H14O3 134.2 0.50 2.33 alcohol α,4-dihydroxy-3-methoxybenzeneacetic acid C9H10O5 198.2 0.56 1.11 alcohol, aromatic, carboxylic acid, ether 2,5-hydroxybenzoic acid C7H6O4 154.2 0.57 0.86 alcohol, aromatic, carboxylic acid Diethylmalonic acid  C7H12O4 160.2 0.57 1.71 carboxylic acid 3,3-dimethylglutaric acid  C7H12O4 160.2 0.57 1.71 carboxylic acid Poly (ethylene glycol) 300 C2nH4n+2On+1 300 0.58 2.17 alcohol, ether Poly (ethylene glycol) 200 C2nH4n+2On+1 200 0.63 2.25 alcohol, ether Poly (ethylene glycol) bis (carboxymethyl) ether C2n+2H4n+6On+5 600 0.63 1.92 ester, ether, carboxylic acid 2,2-dimethylsuccinic acid C6H10O4 146.2 0.67 1.67 carboxylic acid 2-methylglutaric acid  C6H10O4 146.1 0.67 1.67 carboxylic acid Diethyl-L-tartrate C8H14O6 206.2 0.75 1.75 alcohol, ether Glutaric acid C5H8O4 132.1 0.80 1.6 carboxylic acid Levoglucosan C6H10O5 162.1 0.83 1.67 alcohol, ether Maleic acid C4H4O4 116.1 1.00 1 carboxylic acid, C-C double bound Glycerol C3H8O3 92.1 1.00 2.67 alcohol Citric acid C6H8O7 192.1 1.17 1.33 alcohol, carboxylic acid Malic acid C4H6O5 134.9 1.25 1.5 alcohol, carboxylic acid  Malonic acid C3H4O4 104.1 1.33 1.33 carboxylic acid  53  All organic compounds studied herein were purchased from Sigma-Aldrich with purities ≥ 98 %, with the exception of suberic acid monomethyl ester and 1,2,6-hexanetriol, which were purchased from Sigma Aldrich with a purity of 97 %, and glycerol, which was obtained from Thermo Fisher Scientific with a purity of 99.9 %. All organics were used without further purification. The organic-to-inorganic mass ratio (OIR) in the particle was fixed at 2.0 ± 0.1 for most of the experiments. This value is in the range of OIR values observed in many field studies (Zhang et al., 2007; Jimenez et al., 2009; Chen et al., 2009). In addition, previous research using solutions or particles containing organics mixed with ammonium sulfate suggest that SRH often is not dependent on the OIR for a wide range of OIR values (Ciobanu et al., 2009; Bertram et al., 2011; Song et al., 2012b, a). As an example, Bertram et al. (2011) investigated the effect of OIR on SRH. For eight out of the eleven organic species studied, the SRH varied by less than 6% for OIR values ranging from 0.1 to 10. As another example, Song et al. (2012) measured SRH in the particles containing ammonium sulfate and up to ten organic species with OIR ranging from 0.17 to 2. The results of that study showed that nine out of the fourteen systems that underwent liquid-liquid phase separation, the SRH varied by less than 15% for OIR values from 0.17 to 2.  Using the same setup and procedures shown and discussed in Chapter 2 Section 2.2.1, particles were generated by nebulizing a solution of one organic species and one salt, prepared in high purity water (Millipore, 18.2 MΩ cm) or in a mixture of water and methanol if the water solubility of the organic compound was less than 1 weight/weight %. The particle stream from the nebulizer was directed at a hydrophobic slide surface. As the droplets impacted on the slide surface, they coagulated into super-micron droplets. The water or the water/methanol mixture 54  was then evaporated to generate organic-inorganic salt particles with lateral dimensions ranging from 10 to 35 µm. The only two organic species that had a solubility of less than 1% (weight/weight) in water were liquid diethyl sebacate and liquid suberic acid monomethyl ester. Based on this solubility data, liquid-liquid phase separation is expected in the particles containing these two organic species even without the present of a salt at roughly ≥ 99% RH. 5.2.2 Optical microscopy experiments with a flow cell system As described in Chapter 2 Section 2.2.2, the hydrophobic glass slide was mounted to a temperature and relative humidity controlled flow cell, which was coupled to an optical reflectance microscope (Zeiss Axiotech; 50× objective). The temperature of the cell was held constant at 290 ± 1 K in all the experiments in this study.  We used similar experimental procedures as described in Chapter 2 Section 2.2.2. At the beginning of an experiment, the RH in the flow cell was set to roughly 95% for about 15 min. The RH was then ramped down at a rate of 0.4-0.6 % min-1, and images of particles were captured approximately every 10 s until one of the following conditions occurred: liquid-liquid phase separation was observed, the particles effloresced, or the RH reached ≤ 0.5 %. Roughly five particles were monitored in each experiment. For particles containing diethyl sebacate or suberic acid monomethyl ester, experiments started at RH = 100 ± 2.5 % since the RH of liquid-liquid phase separation was greater than 95 %. While the RH was decreased, liquid-liquid phase separation could be identified from the recorded images. To illustrate this point we have included images and movies of particles containing 2-methylglutaric acid mixed with different inorganic salts as the relative humidity 55  was decreased (Figure 5.1 a-d and Supplementary Materials Movie 5.1-5.4). The images and movies show that all particle types containing 2-methylglutaric acid at an OIR of 2.0 ± 0.1 underwent liquid-liquid phase separation.  Figure 5.1 Optical microscopy images and illustrations of particles containing 2-methylglutaric acid and one of the inorganic salts with OIR = 2.0 ± 0.1. Shown in the images and illustrations are relative humidities at which the images were recorded. The inorganic salts studied were ammonium sulfate in row (a), ammonium bisulfate in row (b), sodium chloride in row (c), and ammonium nitrate in row (d). The diameter of the particles shown ranged from 28 to 34 µm. 56  5.3 Results and discussion 5.3.1 Effect of H:C (hydrogen-to-carbon elemental ratio), O:C, and inorganic salt type on liquid–liquid phase separation Listed in Table 5.2 are the measured SRH values for the different particle types investigated. Out of 92 particle types, 49 underwent liquid–liquid phase separation between 100% and ≤ 0.5% RH. These results are summarized in Figure 5.2, which shows the dependence of observed liquid-liquid phase separation on the O:C and H:C (hydrogen-to-carbon elemental ratios) of the organic components of the particles. No trend with H: C is apparent for any of the salts studied. However, a trend with O: C is apparent: for all salts, liquid-liquid phase separation was never observed for O:C ≥ 0.8 and was always observed for O: C < 0.5. For 0.5 ≤ O:C < 0.8, the results depended on the salt type. For ammonium sulfate, phase separation was always observed for 0.5 ≤ O:C < 0.57 and phase separation was frequently observed for 0.57 ≤ O:C < 0.8. For the other three salts, phase separation was frequently observed for 0.5 ≤ O:C < 0.8. To further investigate the effect of O: C on liquid-liquid phase separation, the measured SRH values are plotted as a function of O: C in Figure 5.3. The solid curves in the figure are fits to all the data using a Sigmoidal-Boltzmann function. Table 5.2 Summary of SRH results for an organic-to-inorganic mass ratio (OIR) of 2.0 ± 0.1. Uncertainties represent 2  of multiple SRH measurements and the uncertainty from the calibration. Organics (NH4)2SO4 NH4HSO4 NaCl NH4NO3 OIR SRH (%) OIR SRH (%) OIR SRH (%) OIR SRH (%) Diethyl sebacate  2.0 100.0 ± 2.5 2.0 100.0 ± 2.5 2.0 100.0 ± 2.5 2.0 100.0 ± 2.5 2,5-hexanediol  2.0 88.8 ± 3.7 2.1 81.0 ± 3.7 2.0 72.4 ± 2.6 2.1 63.9 ± 4.1 Poly (propylene glycol)  2.0 94.1 ± 3.2 2.0 90.3 ± 3.1 2.0 89.6 ± 2.7 2.0 77.6 ± 2.5 Suberic acid monomethyl ester 2.0 100.0 ± 2.5 2.0 100.0 ± 2.5 1.9 100.0 ± 2.5 2.1 100.0 ± 2.5 Poly (ethylene glycol) diacrylate   2.0 94.7 ± 2.5 2.0 91.0 ± 2.9 2.0 87.0 ± 2.7 2.0 69.4 ± 4.4 1,2,6-hexanetriol  2.1 76.7 ± 2.5 2.0 Not observeda 2.0 Not observed 2.1 Not observed 57  Organics (NH4)2SO4 NH4HSO4 NaCl NH4NO3 OIR SRH (%) OIR SRH (%) OIR SRH (%) OIR SRH (%) α,4-dihydroxy-3-methoxybenzeneacetic acid  2.0 72.6 ± 2.6 2.0 38.2 ± 2.7 1.9 63.1 ± 2.9 1.9 Not observed 2,5-dihydroxybenzoic acid  2.0 Not observed 1.9 Not observed 2.0 65.5 ± 3.1 2.0 Not observed Diethylmalonic acid  2.0 89.2 ± 3.0 2.0 88.1 ± 2.6 1.9 87.4 ± 3.0 2.1 74.1 ± 3.7 3,3-dimethylglutaric acid  2.0 89.1 ± 3.4 2.0 88.7 ± 5.0 2.1 85.6 ± 2.6 2.0 60.5 ± 2.6 Poly (ethylene glycol) 300   2.0 86.7 ± 2.8 1.9 Not observed 2.0 Not observed 2.0 Not observed Poly (ethylene glycol) 200  2.0 79.8 ± 4.1 2.0 Not observed 2.0 Not observed 2.0 Not observed Poly (ethylene glycol) bis (carboxymethyl) ether  2.0 92.0 ± 2.7 2.0 53.6 ± 3.1 2.0 49.0 ± 2.6 2.0 Not observed 2,2-dimethylsuccinic acid  2.0 Not observed 2.0 61.4 ± 2.5 2.1 58.9 ± 2.6 1.9 40.0 ± 3.2 2-methylglutaric acid  2.0 75.3 ± 2.8 2.0 64.5 ± 4.4 2.1 60.1 ± 2.5 2.0 34.5 ± 3.0 Diethyl-L-tartrate  2.1 90.2 ± 3.0 2.1 65.2 ± 4.1 2.0 52.5 ± 2.5 2.0 28.7 ± 5.6 Glutaric  acid  2.0 Not observed 1.9 Not observed 2.0 Not observed 2.0 Not observed Levoglucosan  2.0 Not observed 1.9 Not observed 1.9 Not observed 1.9 Not observed Maleic acid  2.0 Not observed 2.0 Not observed 2.0 Not observed 1.9 Not observed Glycerol   2.0 Not observed 1.9 Not observed 2.0 Not observed 2.1 Not observed Citric acid   2.0 Not observed 2.0 Not observed 1.9 Not observed 1.9 Not observed Malic acid   2.1 Not observed 1.9 Not observed 1.9 Not observed 1.9 Not observed Malonic acid  2.0 Not observed 2.0 Not observed 2.0 Not observed 2.0 Not observed  a Not observed means liquid-liquid phase separation was not observed for the range of relative humidities probed. In some cases SRH was not probed below 20-40% RH since at RH values less than this value, efflorescence of the salts occurred.      58   Figure 5.2 Van Krevelen Diagram for the different mixed organic-inorganic salt particles (OIR = 2.0 ± 0.1) studied: (a) organic-ammonium sulfate particles, (b) organic-ammonium bisulfate particles, (c) organic-sodium chloride and (d) organic-ammonium nitrate particles. Open circles indicate that liquid-liquid phase separation was observed, while stars indicated that liquid-liquid phase separation was not observed. The vertical hatched regions correspond to the H:C and O:C conditions when liquid-liquid phase separation was always observed and the horizontal hatched regions correspond to the H:C and O:C conditions when liquid-liquid phase separation was never observed. Following Song et al., the Sigmoidal-Boltzmann function was chosen to avoid physically unrealistic values at both low and high O:C values (Song et al., 2012b). Many of the systems in which phase separation was observed (i.e. SRH values > 0 %) lie above the fit line since the 59  curve was fit to both zero and non-zero SRH values. Alternatively we could have fit only the non-zero SRH values, but this would give extra weight to cases where phase separation was observed. The results of the fits are given in Table 5.3. The fit to the ammonium sulfate SRH data is qualitatively consistent with fits previously reported in the literature for ammonium sulfate SRH data (Bertram et al., 2011; Song et al., 2012b). Table 5.3 Parameterizations of SRH results as a function of the oxygen-to-carbon elemental ratio (O:C) of the organic material.   Inorganic salt Parameterizations of SRH (NH4)2SO4  NH4HSO4  NaCl  NH4NO3   Although the results for each inorganic salt do not fall perfectly on the fit curves in Figure 5.3, a correlation between O:C and SRH is observed. This suggests that O:C is a useful parameter for estimating, to a first approximation, the relative humidity for liquid-liquid phase separation, as shown previously for particles containing organics with ammonium sulfate (Bertram et al., 2011; Song et al., 2012b). For high accuracy predictions, additional information such as the organic functional groups is required (Song et al., 2012b).   60   Figure 5.3 Summary of SRH results (OIR = 2.0 ±0.1) as a function of oxygen-to-carbon elemental ratio (O:C): (a) organic-ammonium sulfate particles, (b) organic-ammonium bisulfate particles, (c) organic-sodium chloride particles and (d) organic-ammonium nitrate particles. Circles represent the relative humidity at which separation occurred. Error bars associated with the circles represent 2σ of multiple SRH measurements and the uncertainty from the calibration. Stars indicate that liquid-liquid phase separation was not observed. The error bars corresponding to the stars indicate that liquid-liquid phase separation could potentially occur within the range indicated by the error bars, but could not be detected due to the occurrence of efflorescence in the particles. The curves in the panels are Sigmoidal-Boltzmann fits to the data. Red triangles represent the results of liquid-liquid phase separation in bulk solution from Marcolli and Krieger 2006). 61  Marcolli and Krieger recently measured SRH in bulk solutions containing one organic mixed with ammonium nitrate or sodium chloride. Organics studied were 1,2-hexanediol (O:C = 0.33), 1,4-butanediol (O:C = 0.5), polyethylene glycol (MW = 400 Da and O:C = 0.56) and glycerol (O:C = 1). Only one of these organics, glycerol, was investigated in the current study (Table 5.1). The SRH data from Marcolli and Krieger (2006) is included in Figure 5.3 as a function of O:C (red triangles), together with the results of this study. The overlap between the current data and the data from Marcolli and Krieger (2006) suggest good agreement between the particle studies and the bulk studies. SRH results shown in Figure 5.3 do not vary drastically with the types of inorganic salt. However, out of the 23 organics investigated, the SRH of 20 organics followed the SRH trend: (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3 (Figure 5.4a). In other words, the SRH-value measured with (NH4)2SO4 was greater than or equal to the SRH value measured with NH4HSO4, and so on. Early in the last century, Randall and Failey showed the following trends for the salting out efficiencies of ions relevant to our work: Na+ > NH4+ > H+ and SO42- > Cl− > NO3- (Randall and Failey, 1927). In addition, the Hofmeister series, which consists of a ranking of cations and anions in terms of their ability to salt out proteins follows the same trend (i.e. Na+ > NH4+ and SO42- > Cl− > NO3-) (Hofmeister, 1887; Hofmeister, 1888; Kunz et al., 2004). These trends allow one to compare the salting out efficiency (or SRH) of two salts if they have a common cation or anion. Based on these early salting out studies, we would expect (NH4)2SO4 to have a greater salting out efficiency (or higher SRH) compared to NH4HSO4 since the salting out efficiency follows the trend NH4+ > H+. In addition, we would expect (NH4)2SO4 to have a greater salting out efficiency (or higher SRH) than NH4NO3, since the salting out efficiency 62  follows the trend SO42- > NO3-. These expectations are consistent with the trends observed for 20 out of the 23 organics investigated (Figure 5.4a).  Figure 5.4 Summary of trends of SRH of particles (OIR=2.0 ± 0.1) as a function of inorganic salt type: (a) the SRH of the organics (20 in total) that followed the trend (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3 and (b) the SRH for 2,5-dihydroxybenzonic acid, α,4- dihydroxy-3-methoxybenzeneacetic acid, and 2,2-dimethylsuccinic acid. In panel a, colors represent the O:C of individual organics. The organics shown in panel b didn’t follow the trend (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3. Uncertainties in the SRH measurements have been left off for clarity. Three organics (2,5-dihydroxybenzonic acid, α,4-dihydroxy-3-methoxybenzeneacetic acid, and 2,2- dimethylsuccinic acid) were inconsistent with the SRH trend (NH4)2SO4 ≥ 63  NH4HSO4 ≥ NaCl ≥ NH4NO3 when using an OIR of 2.0 ± 0.1. The results from these organics are illustrated in Figure 5.4b. Interestingly, Bertram et al. (2011) also measured SRH values for these organics mixed with ammonium sulfate and found that the SRH values for these organics varied by more than 6 % with the OIR of the particles. In fact, the SRH of these three organics were the only ones observed to vary by more than 6 % with OIR out of the 13 organics studied by Bertram et al. (2011). To investigate this further we have measured SRH for these three organics mixed with the different salts using OIR values lower than 2.0 ± 0.1. The results from these measurements as well as the results from Bertram et al. (2011) are shown in Figure 5.5 and are summarized in Table 5.4. Table 5.4 Summary of SRH results as a function of the organic-to-inorganic (OIR) mass ratio for following organics: 2, 2-dimethylsuccinic acid; α, 4-dihydroxy-3-methoxybenzeneacetic acid; and 2, 5-dihydroxybenzonic acid. Included are results from both the current studies and results from Bertram et al. (2011). Uncertainties represent 2σ of multiple SRH measurements and the uncertainty from the calibration. Organics (NH4)2SO4 NH4HSO4 NaCl NH4NO3 OIR SRH (%) OIR SRH (%) OIR SRH (%) OIR SRH (%) 2,2-dimethylsuccinic acid 2.0 Not observeda 2.0 61.4 ± 2.5 2.1 58.9 ± 2.6 1.9 40.0 ± 3.2 0.3b 63.8 ± 2.5b 0.5 61.2 ± 2.5 0.5 60.0 ± 3.1 0.5 41.4 ± 2.7 0.5b 61.5 ± 2.5b       1.0b Not observedb       1.2 b Not observedb       1.5 b Not observedb       α,4-dihydroxy-3-methoxybenzeneacetic acid 2.0 72.6 ± 2.6 2.0 38.2 ± 2.7 1.9 63.1 ± 2.9 1.9 Not observed 0.4b 80.1 ± 2.5b 0.5 46.3 ± 3.0 0.5 62.6 ± 2.6 0.5 Not observed 1.0b 81.3 ± 2.5b       6.0b Not observedb       2,5-dihydroxybenzoic acid 2.0 Not observed  1.9 Not observed 2.0 65.5 ± 3.1 2.0 Not observed 0.2b 61.6 ± 2.5b 0.5 Not observed 0.5 65.2 ± 2.6 0.5 Not observed 0.3b 64.2 ± 2.5b       0.5b 62.9 ± 2.5b       0.8b Not observedb       1.0b Not observedb       1.5b Not observedb        64  a Not observed means liquid-liquid phase separation was not observed for the range of relative humidities probed. In some cases SRH was not probed below 20-40% RH since at RH values less than these values efflorescence of the inorganic salt occurred.  b Data from  previous study of Bertram et al. (2011).  Figure 5.5 Summary of SRH results as a function of OIR for the following types of particles: (a) 2,2-dimethylsuccinic acid and inorganic salts, (b) σ,4-dihydroxy-3-methoxybenzeneacetic acid and inorganic salts, and (c) 2,5-dihydroxybenzoic acid and inorganic salts. Closed symbols represent results from current study; while open squares represent results from previous studies by Bertram et al. (2011). Uncertainties in the SRH measurements have been left off for clarity. 65  Figure 5.5a shows that for OIR ≤ 0.5, SRH of 2,2-dimethylsuccinic acid followed the trend of (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3, consistent with the 20 organics shown in Figure 4a. Hence the anomaly at OIR = 2 for 2,2-dimethylsuccinic acid is absent at an OIR of ≤ 0.5. In other words, the trend for 2,2-dimethylsuccinic acid is consistent with the other 20 organics as long as the OIR is in a range where particles undergo liquid-liquid phase separation. Figure 5.5b and 5.5c show that α, 4-dihydroxy-3-methoxybenzeneacetic acid and 2,5-dihydroxybenzoic acid do not follow the trend of (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3 regardless of the OIR studied. Note out of all organics studied, these two organics are the only two organics which contained aromatic functional groups. One possible reason for the differences in trends of SRH observed for these organics may be due to strong cation-π interactions (Kumpf and Dougherty, 1993; Ma and Dougherty, 1997; Song et al., 2012b). Previous work by Song et al. has suggested that cation-π interactions may decrease the salting out effect of the ammonium cations, thereby influencing SRH values in particles containing ammonium salts and aromatic compounds (Song et al., 2012b). SRH was only investigated as a function of OIR for three organics (2,5-dihydroxybenzoic acid, α,4-dihydroxy-3-methoxybenzeneacetic acid, and 2,2-dimethylsuccinic acid). For the organics that were only investigated at OIR = 2.0 ± 0.1, we don’t expect that SRH will be a strong function of OIR in most cases, based on previous studies with organics and ammonium sulfate (see Section 5.2.1). However, experiments are needed to confirm this expectation. Recent work has shown that chloride anions may react with organic acids in the particle phase to form organic salts (Laskin et al., 2012). This type of reaction could potentially occur in our studies. However, if such reactions are occurring in our studies, they don’t appear to drastically affect the occurrence of liquid-liquid phase separation. 66  5.3.2 Atmospheric implications A large fraction of submicron particles in the atmosphere contain organics mixed with some combination of ammonium (NH4+), protons (H+), sulfate (SO42-) and nitrate (NO3-) (Adams et al., 1999; Lee et al., 2003; Martin et al., 2004; Tolocka et al., 2005; Murphy et al., 2006; Seinfeld and Pandis, 2006; Zhang et al., 2007; Pratt and Prather, 2010). As a result the inorganic salts ammonium sulfate, ammonium bisulfate and ammonium nitrate are thought to be important in atmospheric particles. As mentioned in the introduction to this chapter, most of the previous laboratory work on liquid-liquid phase transitions of atmospheric importance have used (NH4)2SO4 as the inorganic salt, even though sulfate is not always fully neutralized in atmospheric particles and NO3- can make up a large fraction of the inorganic anions under certain conditions (Dibb et al., 1996; Huebert et al., 1998; Tolocka et al., 2005; Murphy et al., 2006; Zhang et al., 2007; Pratt and Prather, 2010). To address this disconnect, we have carried out liquid-liquid phase separation experiments with particles containing organics mixed with (NH4)2SO4, NH4HSO4, NH4NO3. The results from these studies show that in all cases, liquid-liquid phase separation is a common occurrence when O:C < 0.8 and always observed when O:C < 0.5. These ranges of O:C values are frequently observed in the atmosphere, suggesting that liquid-liquid phase separation is a common process in atmospheric particles, regardless of the identity of the salt. In the marine boundary layer, super-micron particles containing NaCl make up a large fraction of the particulate mass (Quinn and Bates, 2005; Seinfeld and Pandis, 2006). These particles, which are produced from a bubble bursting mechanism (Woodcock et al., 1953; Blanchard and Woodcock, 1957), can often contain relatively low O:C organics such as sterols, fatty acids and fatty alcohols (Schneider and Gagosian, 1985; Peltzer and Gagosian, 1987; Sicre 67  et al., 1990; Kawamura et al., 2003). Based on the liquid-liquid phase separation results for NaCl containing particles presented here and the O:C of sterols, fatty acids and fatty alcohols (O:C less than approximately 0.5) thought to be present in the marine boundary layer, liquid-liquid phase separation is also expected to be a common occurrence in marine environments. 5.4 Conclusion Out of 92 types of particles studied, 49 underwent liquid-liquid phase separation. For all the inorganic salts, liquid-liquid phase separation was never observed when O:C ≥ 0.8 and was always observed for O:C < 0.5. For 0.5 ≤ O:C < 0.8, the results depended on the salt type. In addition, a correlation between the SRH and O:C was observed for all inorganic salts, suggesting that O:C is a useful parameter for estimating, to a first approximation, the relative humidity for liquid-liquid phase separation, although additional information will be required for predictions with high accuracy.  Out of the 23 organics investigated, the SRH of 20 organics had the following trend: (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ NH4NO3. The trend is consistent with previous salting out studies and the Hofmeister series. Based on the range of O:C values found in the atmosphere and the current results, liquid-liquid phase separation is likely a common occurrence in marine and non-marine environments. 68  Chapter 6. Effects of molecular weight and temperature on liquid-liquid phase separation in particles containing organics and ammonium sulfate 6.1 Introduction  In previous chapters we have investigated different parameters that influence liquid-liquid phase transitions in particles containing mixtures of organic species and inorganic salts.  Understanding the parameters that affect these transitions is necessary for predicting these phase transitions in atmospheric particles.   In the following chapter, we investigate if two additional parameters, molecular weight of the organic material and temperature, influence liquid-liquid phase separation in particles containing organic species and ammonium sulfate.  The effect of molecular weight of the organic material on the occurrence of liquid-liquid phase separation in mixed organic-inorganic particles has not been explored. However, recent studies have shown that molecular weight of the organic material is important for predicting glass transitions in organic particles (Koop et al., 2011; Saukko et al., 2012; Zobrist et al., 2008). Here we carried out a systematic study of the effect of molecular weight on liquid-liquid phase separation.  We first studied liquid-liquid phase separation in particles containing ammonium sulfate mixed with one of ten organic species, with molecular weights ranging from 180 to 1153 Da. The data from these studies were combined with recent studies of liquid-liquid phase separation in particles reported in the literature (You et al., 2013, Chapter 5) to assess if molecular weight is a useful parameter to predict the occurrence of liquid-liquid phase separation and liquid-liquid phase separation relative humidity (SRH).  Molecular weight of organic material in atmospheric particles ranges from less than 100 Da to 69  around 1000 Da (Gao et al., 2004; Kalberer et al., 2004; Tolocka et al., 2004; Nguyen et al., 2010). In the troposphere, the temperature ranges from approximately 220 to 300 K. However, only two studies have investigated liquid-liquid phase separation in mixed organic-inorganic salt particles at temperatures below 290 K. Bertram et al. reported that the SRH results were similar at 273 K and 290 K for particles containing ammonium sulfate and 1,2,6-hexanetriol (Chapter 2). Schill and Tolbert reported that SRH results were similar for temperatures from 240 to 265 K for particles containing ammonium sulfate and 1,2,6-hexanetriol and particles containing ammonium sulfate, 1,2,6-hexanetriol, and 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol (Schill and Tolbert, 2013). To gain a better understanding of the effect of temperature on liquid-liquid phase separation in mixed organic-inorganic salt particles, we investigated liquid-liquid phase separation in particles containing ammonium sulfate mixed with one of twenty different organic species at 244 ± 1 K, 263 ± 1 K, and 278 ± 1 K, respectively. Some of these particle types were also studied at 290 ± 1 K. This new data were combined with previous measurements of liquid-liquid phase separation at 290 ± 1 K in Chapter 5 to assess the effect of temperature on liquid-liquid phase separation in mixed organic-inorganic salt particles. 6.2 Experimental 6.2.1 Sample preparation and apparatus Solutions of ammonium sulfate and one organic species were prepared in high purity water (Millipore, 18.2 MΩ cm). The solutions were then nebulized to produce submicron particles, which impacted onto a hydrophobic glass slide (Hampton Research) and coalesced into 70  super-micron droplets. Water was then evaporated to generate organic-ammonium sulfate particles with lateral dimensions ranging from 10 to 35 µm. The glass slide was mounted to a temperature and relative humidity controlled flow cell, which was coupled to an optical reflectance microscope (Zeiss Axiotech; 50× objective) (Chapter 2 Section 2.2.2 and 2.2.3). While the RH was decreased, liquid-liquid phase separation was identified by monitoring the change of morphology. The relative humidity of the gas was determined by a chilled mirror hygrometer (General Eastern, Model 1311DR), which was calibrated by measuring the deliquescence relative humidity of ammonium sulfate particles.  6.2.2 Molecular weight dependent studies Particles containing ammonium sulfate mixed with one of ten organic species were studied at 290 ± 1 K (see Table 6.1).  Most of previous laboratory studies of liquid-liquid phase separation in particles containing organic species mixed with ammonium sulfate used organic species with molecular weight less than 200 Da (Bertram et al., 2011; Song et al., 2012a, b; You et al., 2013). To complement these previous studies, in the current study we studied particles containing ammonium sulfate and organic species with molecular weight ranging from 180 to 1153 Da (see Table 6.1). The specific organic species selected for these studies (Table 6.1) also had a relatively wide range of O:C values. The organic-to-inorganic salt mass ratio (OIR) was 2.0 ± 0.1 in all the studies.  In a typical experiment, the RH was then ramped down at a rate of 0.4-0.6% RH min-1, while the temperature of the cell was held constantly at 290 ± 1 K. At the same time, images of the particles were captured continuously until one of the following conditions occurred: liquid-liquid phase separation was observed, the particles effloresced, or the RH reached ≤ 0.5%. For each type of particle, experiments were repeated at least three times. All of the organic species 71  were purchased from Sigma-Aldrich and had purities ≥ 95%, except for maltoheptaose, which had a purity ≥ 90%.  Table 6.1 List of the ten organic species used in molecular weight dependent measurements. Each organic species was separately mixed with ammonium sulfate to make particles, and liquid-liquid phase separation was studied in these particles at 290 ± 1 K.  Compounds Formula Molecular weight (Da) O:C Glucose C6H12O6 180.2 1 Poly (ethylene glycol) bis (carboxymethyl) ether C2n+4H4n+6On+5 250 0.83 Sucrose C12H22O11 342.3 0.92 Poly (ethylene glycol) C2nH4n+2On+1 400 0.56 Ouabain  C29H44O12  584 0.41 Raffinose  C18H32O16 594.5 0.89 Poly (ethylene glycol) C2nH4n+2On+1 600 0.54 Maltopentaose C30H52O26 829 0.87 Poly (ethylene glycol) C2nH4n+2On+1 900 0.53 Maltoheptaose C42H72O36 1153 0.86  6.2.3 Temperature dependent studies   Particles consisting of ammonium sulfate mixed with one of sixteen organic species were studies (see Table 6.2) at 244 ± 1 K, 263 ± 1 K and 278 ± 1 K, as well as 290 ± 1 K for a few of these particle types. The organic species studied (see Table 6.2) were selected because they had a wide range of O:C values and included functional groups found in atmospheric particles. In addition, most of these organic species were studied previously at 290 K (Chapter 5), so comparisons could be made between the current studies and the previous studies in Chapter 5. 72  The OIR was 2.0 ± 0.1 in all the studies. The RH was ramped down at a rate of around 0.1- 0.5 % RH min-1. Images were recorded in the same way as the molecular weight dependent studies. The organic species studied were purchased from Sigma-Aldrich with purities ≥ 98%. All organics were used without further purification. Table 6.2  Summary of the twenty organic species used in temperature dependent experiments. Each organic species was separately mixed with ammonium sulfate to make particles, and liquid-liquid phase separation was studied in these particles at 244 ± 1 K, 263 ± 1 K and 278 ± 1 K. Two of the organic species (poly (ethylene glycol) diacrylate and raffinose) mixed with ammonium sulfate were also studied at 290 ± 1 K. Compounds Formula O:C Molecular weight (Da) Functional group (s) 2,5-hexanediol C6H14O2 0.33 118.2 alcohol Poly (propylene glycol) C3nH6n+2On+1 0.38 425 alcohol, ether Poly (ethylene glycol) diacrylate C2n+6H4n+6On+3 0.5 575 ester, ether, C-C double bond Poly (ethylene glycol) 900 C2nH4n+2On+1 0.53 900 alcohol, ether α,4-dihydroxy-3-methoxybenzeneacetic acid C9H10O5 0.56 198.2 alcohol, aromatic, carboxylic acid, ether Diethylmalonic acid C7H12O4 0.57 160.2 carboxylic acid 3,3-dimethylglutaric acid C7H12O4 0.57 160.2 carboxylic acid 2,5-hydroxybenzoic acid C7H6O4 0.57 154.2 carboxylic acid, aromatic Poly (ethylene glycol) 300 C2nH4n+2On+1 0.58 300 alcohol, ether Poly (ethylene glycol) 200 C2nH4n+2On+1 0.63 200 alcohol, ether Poly (ethylene glycol) bis (carboxymethyl) ether C2n+4H4n+6On+5 0.63 600 alcohol, ether 2-methylglutaric acid C6H10O4 0.67 146.1 carboxylic acid 2,2-dimethylsuccinic acid C6H10O4 0.67 146.2 carboxylic acid Diethyl-L-tartrate C8H14O6 0.75 206.2 alcohol, ether Glycerol C3H8O3 1.00 92.1 alcohol 73  Compounds Formula O:C Molecular weight (Da) Functional group (s) Glutaric acid C5H8O4 0.8 132.1 carboxylic acid Levoglucosan C6H10O5 0.83 162.1 alcohol, ether Raffinose C18H32O16 0.89 594.5 alcohol, ether Citric acid C6H8O7 1.17 192.1 carboxylic acid Malonic acid C3H4O4 1.33 104.1 carboxylic acid  6.3 Results and Discussion 6.3.1 Effect of molecular weight on liquid-liquid phase separation Particles containing ammonium sulfate mixed with one of ten organic species were studied at 290 ± 1 K. In these studies organic species with large molecular weights (180 to 1153Da) were used. The organic-to-inorganic mass ratio (OIR) was 2.0 ± 0.1 in all the studies. The results from these studies are listed in Table 6.3 as well as the data from Chapter 5, which showed results of particles with an OIR of 2.0 ± 0.1 at 290 ± 1 K. The combined data set in Table 6.3, which includes results on liquid-liquid phase separation for 33 different particle types and includes results for organic species with molecular weights ranging from 92 to 1153 Da, was used to determine the importance of molecular weight of the organic species on liquid-liquid phase separation.     In Figure 6.1, the data from Table 6.3 were plotted as a function of O:C and molecular weight. Open circles indicate liquid-liquid phase separation was observed and stars indicate liquid-liquid phase separation was not observed. No clear relationship between molecular weight and the occurrence of liquid-liquid phase separation was observed, however a relationship between occurrence of liquid-liquid phase separation and O:C was clear: liquid-liquid phase 74  separation was always observed when O:C < 0.57 (orange hatched region), was never observed when O:C > 0.83 (green hatched region), and was frequently observed when O:C ranged from 0.57 to 0.83.   Figure 6.1 The effect of molecular weight and O:C of the organic species on the occurrence of liquid-liquid phase separation in  mixed organic-ammonium sulfate particles (OIR = 2.0 ± 0.1).  Data plotted are from the current study and Chapter 5 and are summarized in Table 6.3. Open circles indicate liquid-liquid phase separation was observed, while stars indicate liquid-liquid phase separation was not observed. The orange hatched region corresponds to the molecular weight and O:C of the organic species when liquid-liquid phase separation was always observed, and the green hatched region corresponds to the molecular weight and O:C of the organic species when liquid-liquid phase separation was never observed. In Figure 6.2, the SRH data from Table 6.3 were plotted as a function of molecular weight of the organic species. The colors of the symbols indicate the O:C of organic species in the mixed particles. Data at SRH = 0 % indicate liquid-liquid phase separation was not observed 75  even at the lowest relative humidity studied. Similar to Figure 6.1, no correlation with molecular weight was apparent. For contrast, in Figure 6.3, we show the same SRH data plotted as a function of O:C of organic species with the color of the symbols representing the molecular weight of organic species. The solid curve is a Sigmoidal-Boltzmann fit to all the SRH data. Following Song et al. (Song et al., 2012b), a Sigmoidal-Boltzmann function was chosen to avoid unphysical values at high and low SRH values (negative values or values greater than 100%). The fit to the current SRH data set is qualitatively consistent with previous fits for SRH values of mixed organic-ammonium sulfate particles (Bertram et al., 2011; Song et al., 2012b; You et al., 2013). Consistent with previous results (Bertram et al., 2011; Song et al., 2012b), a correlation between SRH and O:C is apparent. These results suggest that O:C is more important for predicting the occurrence of liquid-liquid phase separation in atmospherically relevant mixed organic-ammonium sulfate particles compared with molecular weight.  76  Figure 6.2 SRH as a function of molecular weight of organic species in the particles at 290 ± 1 K. The SRH results are from the current study and Chapter 5 (see Table 6.3). The colors represent the O:C of different organic species. Squares represent SRH of particles in which liquid-liquid phase separation was observed. Bars for the squares include 2σ of multiple measurements and the uncertainty from the calibration. Stars indicate that liquid-liquid phase separation was not observed. OIR = 2.0 ± 0.1 in all the experiments.    Figure 6.3 SRH as a function of O:C of the organic species at 290 ± 1 K. The SRH results are from the current study and Chapter 5 (see Table 6.3). The colors represent the molecular weight of the different organic species. Squares represent the SRH of particles in which liquid-liquid phase separation was observed. Bars for the squares include 2σ of multiple measurements and the uncertainty from the calibration. Stars indicate liquid-liquid phase separation was not observed. OIR = 2.0 ± 0.1 in all the experiments. The black curve is a Sigmoidal-Boltzmann fit to the data.  77   Table 6.3  Combined data set used to assess the effect of molecular weight on SRH in mixed organic-ammonium sulfate particles.  This data set includes the ten types of particles studied here (see Table 6.1) and the SRH results of twenty-three types of particles containing single organic species and ammonium sulfate studied in Chapter 5. OIR = 2.0 ± 0.1 in all the experiments. Uncertainties include 2σ of multiple SRH measurements and the uncertainty from the calibration.  Compounds Formula Molecular weight (Da) O:C SRH (%) References Glycerol C3H8O3 92.1 1 Not observed You et al. (2013) Malonic acid C3H4O4 104.1 1.33 Not observed You et al. (2013) Maleic acid C4H4O4 116.1 1 Not observed You et al. (2013) 2,5-hexanediol C6H14O2 118.2 0.33 88.8 ± 3.7 You et al. (2013) Glutaric acid C5H8O4 132.1 0.8 Not observed You et al. (2013) Malic acid C4H6O5 134.1 1.25 Not observed You et al. (2013) 1,2,6-hexanetriol C6H14O3 134.2 0.5 76.7 ± 2.5 You et al. (2013) 2-methylglutaric acid C6H10O4 146.1 0.67 75.3 ± 2.8 You et al. (2013) 2,2-dimethylsuccinic acid C6H10O4 146.2 0.67 Not observed You et al. (2013) 2,5-hydroxybenzoic acid C7H6O4 154.2 0.57 Not observed You et al. (2013) Diethylmalonic acid C7H12O4 160.2 0.57 89.2 ± 3.0 You et al. (2013) 3,3-dimethylglutaric acid C7H12O4 160.2 0.57 89.1 ± 3.4 You et al. (2013) Levoglucosan C6H10O5 162.1 0.83 Not observed You et al. (2013) Glucose C6H12O6 180.2 1 Not observed Current study Suberic acid monomethyl ester C9H16O4 188.2 0.44 100 ± 2.5 You et al. (2013) Citric acid C6H8O7 192.1 1.17 Not observed You et al. (2013) α,4-dihydroxy-3-methoxybenzeneacetic acid C9H10O5 198.2 0.56 72.6 ± 2.6 You et al. (2013) Poly (ethylene glycol) C2nH4n+2On+1 200 0.63 79.8 ± 4.1 You et al. (2013) Diethyl-L-tartrate C8H14O6 206.2 0.75 90.2 ± 3.0 You et al. (2013) 78  Compounds Formula Molecular weight (Da) O:C SRH (%) References Poly (ethylene glycol) bis (carboxymethyl) ether C2n+4H4n+6On+5 250 0.83 67.6 ± 2.5 Current study Diethyl sabacate C14H26O4 258.4 0.29 100 ± 2.5 You et al. (2013) Poly (ethylene glycol) C2nH4n+2On+1 300 0.58 86.7 ± 2.8 You et al. (2013) Sucrose C12H22O11 342.3 0.92 Not observed Current study Poly (ethylene glycol) C2nH4n+2On+1 400 0.56 88.3 ± 3.0 Current study Poly (propylene glycol) C3nH6n+2On+1 425 0.38 94.1 ± 3.2 You et al. (2013) Poly (ethylene glycol) diacrylate C2n+6H4n+6On+3 575 0.5 94.7 ± 2.5 You et al. (2013) Ouabain  C29H44O12  584 0.41 90.1 ± 3.3 Current study Raffinose  C18H32O16 594.5 0.89 Not observed Current study Poly (ethylene glycol) C2nH4n+2On+1 600 0.54 89.5 ± 2.7 Current study Poly (ethylene glycol) bis (carboxymethyl) ether C2n+4H4n+6On+5 600 0.63 92.0 ± 2.7 You et al. (2013) Maltopentaose C30H52O26 829 0.87 Not observed Current study Poly (ethylene glycol) C2nH4n+2On+1 900 0.53 92.9 ± 2.8 Current study Maltoheptaose C42H72O36 1153 0.86 Not observed Current study  6.3.2 Effect of temperature on liquid-liquid phase separation In the studies that explored the effect of temperature on liquid-liquid phase separation, particles containing ammonium sulfate mixed with one of twenty organic species were studied at 244 ± 1 K, 263 ± 1 K, and 278 ± 1 K.  Some of these particle types were also studied at 290 ± 1 K. These new results were included in Table 6.4 as well as results from Chapter 5, which studied most of the same types of particles but only at 290 ± 1 K. The combined SRH data from Table 79  6.4, which cover the temperature range of 290-244 K, were plotted in Figure 6.4A as a function of O:C of the organic species. Data at SRH = 0 % indicate liquid-liquid phase separation was not observed even at the lowest relative humidity studied. The results show that for all the particle types studied and at all the temperatures studied liquid-liquid phase separation was always observed when O:C < 0.57, frequently observed when 0.57 ≤ O:C < 0.8, and never observed when O:C ≥ 0.8. We conclude that the O:C range at which liquid-liquid phase separation was observed at 290-293 K in previous studies (Ciobanu et al., 2009; Bertram et al., 2011; Song et al., 2012b; You et al., 2013), is consistent with the range observed at temperatures down to 244 ± 1 K.     Table 6.4 Combined data set used to determine the effect of temperature on SRH. This includes the measurements at 244 ± 1 K, 263 ± 1 K and 278 ± 1 K in the current studies (Table 6.2) and results from Chapter 5 at 290 ± 1 K. OIR = 2.0 ± 0.1 in all the experiments. Uncertainties include 2σ of multiple SRH measurements and the uncertainty from the calibration.  Compound O:C SRH (%) at different temperatures 244 ± 1 K 263 ± 1 K 278 ± 1 K 290 ± 1 K 2,5-hexanediol 0.33 84.0 ± 5.1 84.6 ± 2.9 88.0 ± 2.5 88.8 ± 3.7a Poly (propylene glycol) 0.38 87.7 ± 3.0 89.4 ± 3.2 92.5 ± 2.6 94.1 ± 3.2a Poly (ethylene glycol) diacrylate 0.5 88.5 ± 3.6 90.4 ± 3.8 95.0 ± 2.5 94.7 ± 2.5 Poly (ethylene glycol) 900 0.53 89.2 ± 6.0 88.7 ± 2.5 91.8 ± 2.7 92.9 ± 2.8 a α,4-dihydroxy-3-methoxybenzeneacetic acid 0.56 76.0 ± 2.7 67.3 ± 10.1 72.3 ± 2.9 72.6 ± 2.6 a Diethyl malonic acid 0.57 89.8 ± 13.8 87.0 ± 3.3 88.6 ± 2.7 89.2 ± 3.0a 2,5-hydroxybenzoic acid 0.57 Not observed Not observed Not observed Not observeda 3,3-dimethylglutaric acid 0.57 98.2 ± 3.1 88.5 ± 2.7 88.5 ± 2.7 89.1 ± 3.4a Poly (ethylene glycol) 300 0.58 85.6 ± 6.5 83.6 ± 2.9 85.6 ± 2.8 87.6 ± 2.8a Poly (ethylene glycol) 200 0.63 71.3 ± 2.5 77.2 ± 2.7 79.7 ± 2.5 79.8 ± 4.1a Poly (ethylene glycol) bis (carboxymethyl) ether 600 0.63 87.8 ± 3.8 90.4 ± 2.5 90.4 ± 3.1 92.0 ± 2.7a 2-methylglutaric acid 0.67 76.7 ± 3.1 76.2 ± 2.6 76.6 ± 2.5 75.3 ± 2.8a 2,2-dimethylsuccinic acid 0.67 Not observed Not observed Not observed Not observeda 80  Compound O:C SRH (%) at different temperatures 244 ± 1 K 263 ± 1 K 278 ± 1 K 290 ± 1 K Diethyl-L-tartrate 0.75 86.9 ± 3.9 85.0 ± 2.9 87.4 ± 2.9 90.2 ± 3.0a Glutaric acid 0.8 Not observed Not observed Not observed Not observeda Levoglucosan 0.83 Not observed Not observed Not observed Not observeda Raffinose  0.89 Not observed Not observed Not observed Not observed Glycerol 1 Not observed Not observed Not observed Not observeda Citric acid 1.17 Not observed Not observed Not observed Not observeda Malonic acid 1.33 Not observed Not observed Not observed Not observeda a Data taken from Chapter 5   81   Figure 6.4 (A) SRH of mixed organic-ammonium sulfate particles as a function of O:C measured at four different temperatures. Different symbols represent the different temperatures. Bars for the data include 2σ of multiple measurements and the uncertainty from the calibration. Data at SRH= 0% indicate liquid-liquid phase separation was not observed. Data plotted are summarized in Table 6.4. The OIR = 2.0 ± 0.1 in all the experiments. (B) Range of the average O:C of organic material in particles from measurements at many locations in the Northern Hemisphere and the Amazon. Figure 6.5 shows the same data as in Figure 6.4A, but displayed in a slightly different way. The SRH results for the twelve types of mixed organic-ammonium sulfate particles that underwent liquid-liquid phase separation in Figure 6.4A, are shown as a function of temperature, with the colors of the symbols representing the O:C values of the organic species in the particles. For all the particle types included in Figure 6.5, the SRH varied by less than 9.7 % RH as the temperature varied from 244 to 290 K. Figure 6.4A and 6.5 illustrate that SRH is not a strong function of temperature for the particle types investigated. These results are consistent with earlier studies by Bertram et al. and Schill and Tolbert discussed in the Section 6.1.  82   Figure 6.5 Summary of SRH as a function of temperature for mixed organic-ammonium sulfate particles that underwent liquid-liquid phase separation. Data plotted were taken from Table 6.4. Bars for the data include 2σ of multiple measurements and the uncertainty from the calibration. Colors represent the O:C values of different organic species in the particles. OIR = 2.0 ± 0.1 in all the experiments. The average O:C of organic material in atmospheric particles has been measured at many locations in the Northern Hemisphere and in the Amazon and has been shown to range from 0.1 to 1.0 (Chen et al., 2009; Aiken et al., 2008; DeCarlo et al., 2008; Jimenez et al., 2009; Hawkins et al., 2010; Heald et al., 2010; Ng et al., 2010; Takahama et al., 2011). This range of O:C values is indicated in Figure 4B. The range of average O:C values measured in the atmosphere overlaps with the range of O:C values where liquid-liquid phase separation was observed at temperatures ranging from 244 to 290 K. This overlap suggests that liquid-liquid phase separation is likely a common occurrence in the atmosphere over this temperature range. 83  Although SRH does not appear to be a strong function of the temperature for the particle types investigated over the temperature range of 244 to 290 K, SRH may be a strong function of temperature at temperatures lower than 244 K. Some mixtures of organic species and inorganic salts can become highly viscous at lower temperatures and low relative humidities (Tong et al., 2011; Zobrist et al., 2008; Koop et al., 2011; Zobrist et al., 2011; Murray, 2008; Mikhailov et al., 2009; Saukko et al., 2012). At these low temperatures and relative humidities, liquid-liquid phase separation may not occur on typical atmospheric time scales due to diffusion limitations. Related, previous research has shown that efflorescence is inhibited in particles containing organic species and inorganic salts at low temperatures due to diffusion limitations (Bodsworth et al., 2010; Schill and Tolbert, 2013). Additional studies of SRH at temperatures lower than 244 K and for other particle types are still needed. Possible particle types that may show a stronger dependence on temperature are particles containing organic species with high molecular weights and particles that undergo liquid-liquid phase separation at low relative humidities, since viscosity is a strong function of relative humidity (Koop et al., 2011; Renbaum-Wolff et al., 2013; Power et al., 2013; Kidd et al., 2014). 6.4 Conclusions  Studies with particles containing ammonium sulfate mixed with one organic species illustrated that the occurrence of liquid-liquid phase separation and SRH do not depend strongly on the molecular weight of the organic species, at least for the particle types studied. These current studies also illustrate that the occurrence of liquid-liquid phase separation and SRH do not depend strongly on temperature over the range of 290- 244 K. For all the particle types studied and at all the temperatures studied (290 ± 1 K to 244 ± 1 K) liquid-liquid phase separation was always observed when the O:C < 0.57, frequently observed when 0.57 ≤ O:C < 84  0.8,  and never observed when O:C ≥ 0.8. The SRH varied by at most 9.7 % RH as the temperature varied from 290 K to 244 K for the particle types studied. The combined results suggest that liquid-liquid phase separation is likely a common occurrence in the atmospheric particles at 244-290 K. At the same time, additional studies of SRH at temperatures lower than 244 K and for other particle types are still needed.   85  Chapter 7. Conclusions 7.1 Conclusions This dissertation focuses on liquid-liquid phase separation in atmospherically relevant mixed particles consisting of organic species and inorganic salts. Liquid-liquid phase separation was monitored using an optical microscope coupled to a relative humidity and temperature controlled flow cell. To further characterize the spatial distribution of different species, fluorescence microscopy and Raman microscopy were also used. In Chapter 2, the occurrence of liquid-liquid phase separation and the relative humidity at which liquid-liquid phase separation occurred (SRH) were studied in particles consisting of one organic species and ammonium sulfate. Eleven organic species were studied and the organic-to-sulfate mass ratio (org:sulf) was varied in different experiments. The new laboratory data were combined with data reported previously in the literature to develop a parameterization of separation relative humidity (SRH) in terms of oxygen-to-carbon elemental ratio (O:C) and org:sulf. SRH was independent of org:sulf in most systems studied.  A rough correlation between SRH and O:C was observed.  Liquid-liquid phase separation was always observed with O:C < 0.7 and was never observed with O:C > 0.7. These results suggest that the O:C of organic material is a useful parameter for predicting the occurrence of liquid-liquid phase separation and SRH. In Chapter 3, the occurrence of liquid-liquid phase separation in particles containing materials collected from the atmosphere in Atlanta was studied. This work provided the first visual evidence that liquid-liquid phase separation can occur in atmospheric samples. Liquid-liquid phase separation in particles containing ammonium sulfate and secondary organic material  86  produced in the laboratory from dark ozonolysis of α-pinene and from photo-oxidation of 1,2,4-trimethylbenzene was also studied. These particles also underwent liquid-liquid phase separation at RH values relevant to the atmosphere.  The O:C values of the organic species in this study were roughly ≤ 0.5. Therefore, liquid-liquid phase separation is likely to occur in atmospheric particles when the O:C of organic material is less than roughly 0.5. The observation of liquid-liquid phase separation in these particles is consistent with predictions from Chapter 2.  In Chapter 4, liquid-liquid phase separation in particles consisting of ammonium sulfate and toluene-derived secondary organic material was investigated. At a low organic-to-sulfate mass ratio (org:sulf = 0.1), liquid-liquid phase separation was observed and SRH was 85 ± 5 %. The efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) of these particles were close to the literature values of pure ammonium sulfate particles, consisting with the observation that phase separation occurred in those particles. At a high organic-to-sulfate mass ratio (org:sulf = 3.0), a minor phase formed between 100 % and 70 % RH. The O:C of the secondary organic material in this study was 0.71 ± 0.2. In Chapter 5, the effect of different inorganic salts on liquid-liquid phase separation was studied.  The salts used were ammonium sulfate, ammonium bisulfate, ammonium nitrate, and sodium chloride. Forty-nine out of ninety-two types of particles studied underwent liquid-liquid phase separation, and liquid-liquid phase separation was never observed with O:C ≥ 0.8 and was always observed with O:C < 0.5, regardless of the salt type. For 0.5 ≤ O:C < 0.8, the occurrence depended on the type of salt and organic species. These results suggest that O:C is a useful parameter for estimating, to a first approximation, the relative humidity for liquid-liquid phase separation, although additional information will be required for predictions of high accuracy. The SRH of twenty organic species had the following trend: (NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥ 87  NH4NO3. Based on the range of O:C values found in the atmosphere and these results, liquid-liquid phase separation is likely a common occurrence in marine and non-marine environments. In Chapter 6, the effects of temperature and molecular weight of the organic material on liquid-liquid phase separation were studied. The occurrence of liquid-liquid phase separation and SRH did not depend strongly on the molecular weight of the organic species. In addition, the occurrence of liquid-liquid phase separation and SRH did not depend strongly on temperature over the range of 244 to 290 K. The combined results suggest that liquid-liquid phase separation is likely a common occurrence in atmospheric particles at 244-290 K. The results presented in this dissertation have improved our understanding of liquid-liquid phase separation in atmospherically relevant particles, and the results can be applied to atmospheric modeling studies to improve predictions of atmospheric chemistry and climate. 7.2 Outlook of future work Listed below are possible directions for further studies. (a) Prediction of SRH. Results in Chapter 2 and Chapter 5 show that O:C is a good parameter for predicting the occurrence of liquid-liquid phase separation and the SRH in mixed organic-inorganic salt particles. However, O:C is not a fundamental parameter related to liquid-liquid phase separation. Based on the electrostatic theories mentioned in Chapter 2, the dipole moment of the organic material should be a fundamental parameter related to liquid-liquid phase separation. Correlations between the dipole moment of the organic material and liquid-liquid phase separation should be explored in future research. (b) Studies of particles containing both oxidized and un-oxidized organic species. In the studies in Chapter 3 and Chapter 4, materials collected on the filters were extracted into high 88  purity water, therefore the particles in those experiments only contained the water soluble fraction of the material collected on the filters. The water insoluble organic material, mainly less oxidized or un-oxidized species such as hydrocarbons (Lunde and Bjorseth, 1977; Zhang et al., 2007; Hallquist et al., 2009), can be present in atmospheric samples and may influence the phase separation behavior in particles. Mixed organic-inorganic salt particles containing inorganic salts mixed with both oxidized and un-oxidized organic materials may have three liquid phases after liquid-liquid phase separation (Knickerbocker et al., 1982), and studies on these particles are needed.  (c) Studies of particles containing water insoluble organic species alone. In Chapter 2 and 5, most of the selected organic species studied were water soluble, while two organic species have water solubility less than 1% by weight. These two organic species may be hydrophobic enough to separate from water even in the absence of inorganic salts and form a second phase. Studies are needed to investigate liquid-liquid phase separation in particles containing just organic species and water. (d) Morphology of particles after liquid-liquid phase separation. Phase separation can decrease the reactive uptake of N2O5 onto atmospheric particles if the organic-rich phase forms a complete coating around the inorganic-rich phase (Anttila et al., 2006; Folkers et al., 2003; Zuend et al., 2010). Some recent experiments using super-micron levitated particles illustrate that a partially engulfed structure can sometimes occur for mixed particles composed of aqueous salt solution and organic species of low O:C values (Buajarern et al., 2007; Kwamena et al., 2010; Reid et al., 2011). Atmospheric particles that form a partially engulfed structure rather than a core-shell structure might have a considerably higher N2O5 uptake. Additional studies focusing on the factors influencing morphology after liquid-liquid phase separation are needed. 89   (e) Secondary organic materials from multiple precursors. In Chapter 3 and 4, secondary organic materials produced in the laboratory were from single volatile organic compound (VOC) precursors. In the atmosphere, secondary organic material is formed through different pathways and precursors forming more complicated secondary organic material. There is no experimental study focusing on liquid-liquid phase separation in particles containing secondary organic material produced from multiple VOC precursors. Liquid-liquid phase separation in particles containing inorganic salts and secondary organic material produced from mixed precursors are needed to better simulate atmospheric conditions. (f) Temperature dependent studies. In Chapter 6, the effect of temperature on liquid-liquid phase separation was studied at temperatures from 290 to 244 K. In the troposphere, the temperature could be as low as approximately 200 K. SRH may be a strong function of temperature, when temperature is lower than 244 K. Some mixed organic-inorganic salt aqueous systems could become highly viscous at lower temperatures or low relative humidities (Tong et al., 2011; Zobrist et al., 2008; Koop et al., 2011; Zobrist et al., 2011; Murray, 2008; Mikhailov et al., 2009; Saukko et al., 2012). Because of diffusion limitations, liquid-liquid phase separation may not occur on typical atmospheric time scales at low temperatures or low relative humidities. Therefore, additional studies of SRH at temperatures less than 244 K are still needed.  Possible systems that may show a stronger dependence on temperature are particles containing organics with high molecular weights and particles that undergo liquid-liquid phase separation at low relative humdities, since viscosity is a strong function of relative humidity (Renbaum-Wolff et al., 2013; Power et al., 2013).    90  (g) Studies on ambient samples. Chapter 3 involved samples collected from the atmosphere in urban Atlanta. There are a very limited number of studies on liquid-liquid phase separation in particles from ambient samples. More studies on samples from ambient environments at different locations are needed to verify our conclusions and improve our understanding of liquid-liquid phase separation in atmospheric particles. In summary, there are many possible directions in which to further explore liquid-liquid phase separation in atmospheric particles. 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