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Deliquescence and crystallization of ammonium sulfate-glutaric acid and sodium chloride-glutaric acid.. Parsons, Matthew T.; Fok, Abel; Pant, Atul; Bertram, Allan K.; Mak, Jackson 2004

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Deliquescence and crystallization of ammonium sulfate-glutaric acidand sodium chloride-glutaric acid particlesAtul Pant, Abel Fok, Matthew T. Parsons, Jackson Mak, and Allan K. BertramDept. of Chemistry, the Univ. of British Columbia, Vancouver, B. C., CanadaReceived 18 March 2004; revised 10 May 2004; accepted 26 May 2004; published 25 June 2004.[1] In the following, we report the deliquescence relativehumidities (DRH) and crystallization relative humidities(CRH) of mixed inorganic-organic particles, specificallyammonium sulfate-glutaric acid and sodium chloride-glutaric acid particles. Knowledge of the DRH and CRHof mixed inorganic-organic particles is crucial for predictingthe role of aerosol particles in the atmosphere. Our DRHresults are in good agreement with previous measurements,but our CRH results are significantly lower than some of theprevious measurements reported in the literature. Ourstudies show that the DRH and CRH of ammoniumsulfate and sodium chloride only decreased slightly whenthe mole fraction of the acid was less than 0.4. If otherorganics in the atmosphere behave in a similar manner, thenthe DRH and CRH of mixed inorganic-organic atmosphericparticles will only be slightly less than the DRH and CRHof pure inorganic particles when the organic mole fraction isless than 0.4. Our results also show that if the particlescontain a significant amount of organics (mole fraction >0.5) the crystallization relative humidity decreasessignificantly and the particles are more likely to remain inthe liquid state. Further work is needed to determine if otherorganics species of atmospheric importance have a similareffect. INDEX TERMS: 0305 Atmospheric Composition andStructure: Aerosols and particles (0345, 4801); 0345 AtmosphericComposition and Structure: Pollution—urban and regional (0305);0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry. Citation: Pant, A., A. Fok, M. T.Parsons, J. Mak, and A. K. Betram (2004), Deliquescence andcrystallization of ammonium sulfate-glutaric acid and sodiumchloride-glutaric acid particles, Geophys. Res. Lett., 31, L12111,doi:10.1029/2004GL020025.1. Introduction[2] Aerosol particles play several key roles in the atmo-sphere. For example they influence the chemistry of theatmosphere by providing a medium for heterogeneousreactions, they decrease visibility in both urban and ruralareas, and they affect climate by scattering and absorbingsolar and terrestrial radiation. Before the role of particles inthese processes can be quantified, however, the phase andhygroscopic properties of atmospheric particles must beunderstood and accurately represented. This is because thephase and water content govern the total mass of airborneparticles, the amount of light they scatter and absorb, andtheir reactivity.[3] It is now well established that atmospheric aerosolsconsist of both organic and inorganic material. For example,an average composition of urban fine particles, based onmeasurements at several sites, is 37% sulfate, 24% organiccarbon, 11% ammonium, 5% elementary carbon, and4% nitrate by weight [Heintzenberg, 1989]. Single particlemeasurements havealso shownthat mixedorganic-inorganicparticles are abundant in the atmosphere [Middlebrook et al.,1998; Murphy et al., 1998].[4] The phase and hygroscopic properties of pure inor-ganic particles, such as ammonium sulfate, have beenstudied extensively and are relatively well understood[Martin, 2000]. In contrast, very little is known about thephase and hygroscopic properties of organic and mixedorganic-inorganic particles. Significantly more research isneeded on this topic in order to predict accurately the role ofaerosol particles in the atmosphere.[5] In the following, we report measurements of thedeliquescence and crystallization of ammonium sulfate-glutaric acid and sodium chloride-glutaric acid particles.Glutaric acid, which is a dicarboxlyic acid, has been iden-tified as a component of atmospheric particles [Kawamura etal.,1996; Kerminen et al., 1999], and ammonium sulfate andsodium chloride are two of the major inorganic speciesfound in atmospheric particulate material [Finlayson-Pittsand Pitts, 2000]. This study offers insight into the phase andhygroscopic properties of mixed organic-inorganic particlesfound in the atmosphere.2. Experimental[6] The apparatus has been described in detail elsewhere[Parsons et al., 2004]. Briefly, the particles of interest weredeposited on the bottom surface of the flow cell using aglass nebulizer. The bottom surface, which supported theparticles, consisted of a hydrophobic polytetrafluoroethylene(PTFE) film annealed to a plain glass cover slide. Therelative humidity in the cell, which was measured with adew point hygrometer, was controlled by the continuousflow of a mixture of dry and humidified N2. Flow ratesranged from 150 to 500 standard cm3minC01.[7] The size of particles monitored in the deliquescenceexperiments ranged from 2–20 mm, and the size of theparticles monitored in the crystallization experiments werelimited to diameters between 10–20 mm. Deliquescence andcrystallization of the particles was monitored with areflected-light microscope. Polarized light was used toenhance the contrast between solid and liquid particles.This technique is sensitive to small amounts of crystallinematerial present in the aqueous droplets. As an example, inthe deliquescence experiments, we could identify solidGEOPHYSICAL RESEARCH LETTERS, VOL. 31, L12111, doi:10.1029/2004GL020025, 2004Copyright 2004 by the American Geophysical Union.0094-8276/04/2004GL020025$05.00L12111 1of4inclusions present in the liquid droplets when the diameterof the solid inclusions was less than 1 mm in size. A 1 mmsolid inclusion corresponds to less than 0.1% of the totalmass of a 20 mm liquid droplet.[8] During a deliquescence experiment the relativehumidity was increased at a rate of 0.1–0.3% minuteC01,and during a crystallization experiment the relative humiditywas reduced at a rate of 0.2–0.4% minuteC01. The temper-ature of the cell was maintained at 20.0 ± 0.1C176C for allexperiments. Typically 10–20 particles were monitored in asingle experiment.3. Results and Discussion[9] Deliquescence. Shown in Figures 1 and 2 are therelative humidities at which ammonium sulfate-glutaric acidand sodium chloride-glutaric acid particles deliquesced. Theresults we report refer to when the particles fully deli-quesced (and no solid remains) rather than the onset ofwater uptake. This is an important point as a solid canremain in equilibrium with an aqueous solution over a rangeof relative humidities in a multicomponent particle.[10] The minimum deliquescence relative humidity forthe ammonium sulfate-glutaric acid system and the sodiumchloride-glutartic acid system occurred at 0.5 ± 0.1 molefraction and 0.3 ± 0.1 mole fraction, respectively. Thesecompositions correspond to the isothermal invariant points,which are also called the eutonic compositions [Kirgintsevand Trushnikova, 1968]. In Figure 1, the results to the left ofthe eutonic composition corresponds to full deliquescenceof ammonium sulfate and the results to the right corre-sponds to full deliquescence of glutaric acid. In Figure 2, theresults to the left of the eutonic composition corresponds tofull deliquescence of sodium chloride; whereas the results tothe right of the eutonic corresponds to full deliquescence ofglutaric acid.[11] Also included in Figures 1 and 2 are results fromother groups. The work by Wise et al. [2003] and Brooks etal. [2002] were carried out with bulk solutions and theresults from Choi and Chan [2002] were carried out with asingle particle suspended in an electrodynamic trap. In allcases our results agree within experimental uncertainty withthe previous measurements.[12] The results shown in Figures 1 and 2 indicate that thedeliquescence relative humidity of pure glutaric acid issuppressed when either ammonium sulfate or sodium chlo-ride is added. Similarly the deliquescence relative humidityof pure ammonium sulfate and pure sodium chloridedecreases when glutaric acid is added. The deliquescencerelative humidity of ammonium sulfate and sodium chlo-ride, however, only decreased slightly (within the uncer-tainty of the measurements) when the mole fraction of theacid was less than 0.4. Brooks et al. [2002] observed asimilar trend for a series of dicarboxylic acids (malonicacid, glutaric acid, maleic acid, and L-malic acid) withammonium sulfate.[13] Crystallization. Shown in Figures 3 and 4 are therelative humidities at which the inorganic-organic liquiddroplets crystallized. Due to the stochastic nature of nucle-ation, all the particles did not crystallize at the same relativehumidity. The open circles correspond to where 50% of theFigure 1. DRH of ammonium sulfate-glutaric acidparticles. The data correspond to full deliquescence. Seecolor version of this figure in the HTML.Figure 2. DRH of sodium chloride-glutaric acid particles.The data correspond to full deliquescence. See color versionof this figure in the HTML.Figure 3. CRH of ammonium sulfate-glutaric acid parti-cles. See text for details.L12111 PANT ET AL.: DRH AND CRH OF ATMOSPHERIC PARTICLES L121112of4particles crystallized, and the vertical bars associated withthe open circles correspond to the range over which weobserved crystallization. For the remainder of the documentwe will refer to the relative humidity at which 50% of theparticles crystallized as the crystallization relative humidity(CRH). We were unable to determine from the images of theparticles if they were completely or partially solid aftercrystallization had occurred.[14] The CRH results for pure ammonium sulfate andpure sodium chloride (mole fraction = 0.0 in Figures 3and 4) are within 3% RH of most previous measurements(see for example Martin [2000] and Tang and Munkelwitz[1994]). This good agreement suggests that our technique iscapable of measuring homogeneous nucleation of(NH4)2SO4and NaCl from aqueous solutions. In otherwords, the PTFE substrate supporting the particles doesnot influence the nucleation of these salts.[15] Our crystallization results for pure glutaric acidparticles (mole fraction = 1.0) are compared with measure-ments by other groups in Table 1. Also included in Table 1are the observation times and particle sizes used in theexperiments. Our results are in excellent agreement withmeasurements by Peng et al. [2001] but do not agree withthe results from Prenni et al. [2001] and Braban [2004].The difference between our results and the results presentedby Prenni et al. and Braban may be due to differences inparticle volume. Classical nucleation theory predictsth crystallization relative humidity decreases with the vol-ume of the particle, and the particle volume in the experi-ments by Prenni et al. and Braban was approximately6 orders of magnitude less than the particle volume in ourstudies.[16] We have also included in Figures 3 and 4 resultsfrom other groups that have studied the CRH of ammoniumsulfate-glutaric acid and sodium chloride-glutaric acid par-ticles. The hatched bars represents the range over whichChoi and Chan observed crystallization [Choi and Chan,2002]. Clearly, Choi and Chan observed crystallization atsignificantly higher relative humidities than observed in ourexperiments. The difference cannot be explained by particlesize as similar sizes were used in both experiments. Oneexplanation is that the particles in the Choi and Chanexperiments contained trace amounts of contamination thatacted as a heterogeneous nucleus for crystallization. Re-gardless, our experiments indicate that the CRH of theseparticles is significantly lower than previously suggested byChoi and Chan.[17] Using an aerosol flow tube, Braban investigated theCRH of 0.5 mole fraction ammonium sulfate-glutaric acidparticles [Braban, 2004]. The square in Figure 3 representswhere ammonium sulfate crystallized in approximately 50%of the particles in their experiments. (From the IR spectrumthey were able to conclude that only ammonium sulfatecrystallized in the particles in their experiments.) Theassociated vertical line represents the range over which theyobserved crystallization. Our results are higher than themeasurements by Braban; however, this difference may bedue to a difference in particle size, as Braban studiedsubmicron particles.[18] Our results in Figures 3 and 4 show that the additionof small amounts of glutaric acid (mole fraction < 0.4) topure ammonium sulfate or pure sodium chloride only low-ers the CRH by less than 2% RH, which is within theuncertainty of our experiments. However, if the molefraction is greater than this value, the CRH decreasessignificantly.[19] Recently it was shown that the CRH of H2SO4–NH3–H2O aerosol particles can be estimated by subtractionof a constant relative humidity (DRH) from the deliques-cence relative humidity (DRH) curves [Colberg et al.,2003]. This was motivated by the procedure by Koop etal. [2000] who suggested that homogeneous nucleation oficefromaqueoussolutionsoccursataconcentrationdifferingby a constant RH from the melting point curve. In contrast,Martin et al. [2003] studied the CRH of H2SO4–NH3–HNO3–H2O particles, and they observed that the differencebetween the DRH and CRH varied from 40–55%. Here weinvestigate whether or not the CRH in our experiments canbe predicted by subtraction of a constant relative humidityfrom the DRH curves. The dashed line in Figure 3 wascalculated by subtracting 46% RH from the (NH4)2SO4deliquescence curve; the dashed line in Figure 4 wascalculated by subtracting 28.5% RH from the NaCldeliquescence curve, and the dotted lines in Figures 3and 4 were calculated by subtracting 62% RH fromthe glutaric acid deliquescence curves. The dashed linesoverlap the measurements suggesting that this method isvalid for predicting the crystallization relative humidity ofFigure 4. CRH of sodium chloride-glutaric acid particles.See text for details.Table 1. CRH of Pure Glutaric Acid ParticlesReference Temperature Observation Time Diameter CRHCurrent data 293 K 60 minutes to scan from 45–20% RH 10–20 mm 36–22.5%Peng et al. [2001] 298 K 40 minutes to scan from 93–5% RH 10–20 mm 33–29%Prenni et al. [2001] 303 K <1 minute at fixed RH 0.1–0.05 mmnoaBraban [2004] 293 K <1 minute at fixed RH submicron noaaPhase transition not observed when the particles were dried to less than 5% RH.L12111 PANT ET AL.: DRH AND CRH OF ATMOSPHERIC PARTICLES L121113of4(NH4)2SO4and NaCl in these aqueous solutions (at least upto a mole fraction of 0.4). The dotted lines, however, do notreproduce the CRH values, suggesting that this procedure isnot appropriate for predicting the CRH of glutaric acid inthese aqueous solutions. One possible explanation is that atlow RH and high mole fractions the solutions becomehighly non-ideal, and hence the thermodynamic propertiesof glutaric acid, such as supersaturation, are a strongfunction of both DRH and composition. Another possibleexplanation is that at low RH and high mole fractionsviscosity becomes important and limits the rate of nucle-ation. This may also explain why crystallization occurredover a wide range when the composition was greater than0.6 mole fraction. Further work is needed to understand thisbehavior.[20] As mentioned above, the CRH results shown inFigures 3 and 4 correspond to when we first saw crystal-lization in the liquid droplets. We also observed a secondphase transition at relative humidities below the CRHvalues when the composition was between approximately0.2 and 0.6 mole fraction. This suggests that the individualparticles did not completely crystallize at the CRH valueswhen the composition was between 0.2 and 0.6 molefraction. The second phase transition was clearly discern-able from the images of the particles as this phase transitionresulted in a sudden increase in the intensity of the particleswhen polarized light was used. The second phase transitionwill be the focus of a future publication.4. Conclusions and Atmospheric Importance[21] Our measurements of full deliquescence are in goodagreement with previous measurements. These studies showthat the addition of glutaric acid to ammonium sulfate orsodium chloride only decreases the DRH slightly if the molefraction of the acid is less than 0.4. Our measurements ofCRH are significantly lower than previous measurements byChoi and Chan [2002], and our results show that theaddition of glutaric acid to either ammonium sulfate orsodium chloride only decreases the crystallization relativehumidity slightly if the mole fraction is less than 0.4. Ifother organics in the atmosphere behave in a similarmanner, then the DRH and CRH of mixed inorganic-organicatmospheric particles will only be slightly less than theDRH and CRH of pure inorganic particles when the organicmole fraction is less than 0.4. This conclusion is similar toprevious conclusions concerning the effects of organics oninorganic phase transitions [Braban, 2004; Brooks et al.,2002; Martin et al., 2004].[22] A large fraction of the particles in the atmosphere aresub-micron in size. Since the relative humidity at whichparticles crystallize decreases with particle volume, ourCRH results provide an upper limit to the crystallizationrelative humidities of sub-micron particles with similarchemical compositions. Our results also show that if theparticle contains a significant amount of organics (molefraction > 0.5) the crystallization relative humiditydecreases significantly and is more likely to remain in theliquid state. This conclusion, however, is based on oneorganic species, glutaric acid. Hundreds of different organicspecies have been identified in the atmosphere, and thesespecies may have a range of chemical and physical proper-ties [Finlayson-Pitts and Pitts, 2000]. Research on otherorganic species typically found in the atmosphere is needed.[23] Acknowledgments. The authors would like to thank D. A.Knopf for helpful discussions regarding the manuscript, and S. T. Martinfor fruitful discussions about the effects of organic molecules on the phasetransitions of inorganic particles. This research was supported by theNatural Sciences and Engineering Research Council of Canada, theCanadian Research Chair Program, and the Canadian Foundation forInnovation.ReferencesBraban, C. F. (2004), Laboratory studies of model tropospheric aerosolphase transitions, Ph.D. thesis, Univ. of Toronto, Toronto.Brooks, S., M. Wise, M. Cushing, and M. Tolbert (2002), Deliquescencebehavior of organic/ammonium sulfate aerosol, Geophys. Res. Lett.,29(19), 1917, doi:10.1029/2002GL014733.Choi, M. Y., and C. K. Chan (2002), The effects of organic species on thehygroscopic behaviors of inorganic aerosols, Environ. Sci. Technol.,36(11), 2422–2428.Colberg, C. A., B. P. Luo, H. Wernli, T. Koop, and T. Peter (2003), A novelmodel to predict the physical state of atmospheric H2SO4/NH3/H2Oaerosol particles, Atmos. Chem. Phys., 3, 909–924.Finlayson-Pitts, B. J., and J. N. Pitts Jr. (2000), Chemistry of the Upper andLower Atmosphere, Academic, Boston.Heintzenberg, J. (1989), Fine particles in the global troposphere, Tellus,41B, 149–160.Kawamura, K., H. Kasukabe, and L. A. Barrie (1996), Source and reactionpathways of dicarboxylic acids, ketoacids and dicarbonyls in arctic aero-sols: One year of observations, Atmos. Environ., 30(10–11), 1709–1722.Kerminen, V. M., K. Teinila, R. Hillamo, and T. Makela (1999), Size-segregated chemistry of particulate dicarboxylic acids in the Arctic atmo-sphere, Atmos. Environ., 33(13), 2089–2100.Kirgintsev, A. V., and L. N. Trushnikova (1968), Isopiestic method ofdetermining the composition of solid phases in three-component systems,Russ. J. Inorg. Chem., 13(4), 600–601.Koop, T., B. P. Luo, A. Tsias, and T. Peter (2000), Water activity as thedeterminant for homogeneous ice nucleation in aqueous solutions,Nature, 406(6796), 611–614.Martin, S. T. (2000), Phase transitions of aqueous atmospheric particles,Chem. Rev., 100(9), 3403–3453.Martin, S. T., H. M. Hung, R. J. Park, D. J. Jacob, R. J. D. Spurr, K. V.Chance, and M. Chin (2004), Effects of the physical state of troposphericammonium-sulfate-nitrate particles on global aerosol direct radiative for-cing, Atmos. Chem. Phys., 4, 183–214.Martin, S. T., J. C. Schlenker, A. Malinowski, H. M. Hung, and Y. Rudich(2003), Crystallization of atmospheric sulfate-nitrate-ammonium parti-cles, Geophys. Res. Lett., 30(21), 2102, doi:10.1029/2003GL017930.Middlebrook, A., D. Murphy, and D. Thomson (1998), Observations oforganic material in individual marine particles at Cape Grim during theFirst Aerosol Characterization Experiment (ACE 1), J. Geophys. Res.,103(D13), 16,475–16,484.Murphy, D. M., D. S. Thomson, and T. M. J. Mahoney (1998), In situmeasurements of organics, meteoritic material, mercury, and other ele-ments in aerosols at 5 to 19 kilometers, Science, 282(5394), 1664–1669.Parsons, M. T., J. Mak, S. R. Lipetz, and A. K. Bertram (2004), Deliques-cence of malonic, succinic, glutaric, and adipic acid particles, J. Geophys.Res., 109, D06212, doi:10.1029/2003JD004075.Peng, C., M. N. Chan, and C. K. Chan (2001), The hygroscopic propertiesof dicarboxylic and multifunctional acids: Measurements and UNIFACpredictions, Environ. Sci. Technol., 35(22), 4495–4501.Prenni, A. J., P. J. DeMott, S. M. Kreidenweis, D. E. Sherman, L. M.Russell, and Y. Ming (2001), The effects of low molecular weight dicar-boxylic acids on cloud formation, J. Phys. Chem. A, 105(50), 1240–1248.Tang, I., and H. Munkelwitz (1994), Water activities, densities, and refrac-tive indices of aqueous sulfates and sodium nitrate droplets of atmo-spheric importance, J. Geophys. Res., 99(D9), 18,801–18,808.Wise, M. E., J. D. Surratt, D. B. Curtis, J. E. Shilling, and M. A. Tolbert(2003), The hygroscopic growth of ammonium sulfate/dicarboxylic acids,J. Geophys. Res., 108(D20), 4638, doi:10.1029/2003JD003775.C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0A. K. Bertram, A. Fok, J. Mak, A. Pant, and M. T. Parsons, Dept. ofChemistry, the Univ. of British Columbia, 2036 Main Mall, Vancouver,B. C., Canada V6T 1Z1. ( PANT ET AL.: DRH AND CRH OF ATMOSPHERIC PARTICLES L121114of4


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