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Deliquescence of malonic, succinic, glutaric, and adipic acid particles. Lipetz, Sarah R.; Mak, Jackson; Parsons, Matthew T.; Bertram, Allan K. 2004

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Deliquescence of malonic, succinic, glutaric, and adipic acid particlesMatthew T. Parsons, Jackson Mak, Sarah R. Lipetz, and Allan K. BertramDepartment of Chemistry, University of British Columbia, Vancouver, British Columbia, CanadaReceived 14 August 2003; revised 13 November 2003; accepted 13 January 2004; published 27 March 2004.[1] In order to understand and predict the role of organic particles in the atmosphere theirdeliquescence behavior must be understood. Using an optical microscope coupled to aflow cell, we investigated the deliquescence of malonic, succinic, glutaric, and adipic acidparticles with sizes ranging from 2 to 40 mm. Deliquescence relative humidities weredetermined for temperatures ranging from 293 to 243 K. Over this temperature range bothsuccinic acid and adipic acid deliquesced at approximately 100% relative humidity,whereas malonic acid and glutaric acid deliquesced at significantly lower relativehumidities. These results are generally in good agreement with previous studies and arewithin 3% of calculations based on the UNIQUAC (universal quasi-chemical) FunctionalGroup Activity Coefficients (UNIFAC) model and recently published interactionparameters. Our studies also include measurements at temperatures below the eutectictemperatures. At these temperatures, ice did not nucleate; rather the particles underwentdeliquescence to form metastable solution droplets. This indicates that solid dicarboxylicacids are not good ice nuclei above 243 K and hence will probably not play a role in icecloud formation at these temperatures. INDEX TERMS: 0305 Atmospheric Composition andStructure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physicsand chemistry; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1610 Global Change:Atmosphere (0315, 0325); KEYWORDS: deliquescence, optical microscopy, aerosol, atmospheric chemistry, icenucleation, dicarboxylic acidCitation: Parsons, M. T., J. Mak, S. R. Lipetz, and A. K. Bertram (2004), Deliquescence of malonic, succinic, glutaric, and adipicacid particles, J. Geophys. Res., 109, D06212, doi:10.1029/2003JD004075.1. Introduction[2] Condensed phase organic material is abundant in theatmosphere. In urban areas of the U.S., for example, organicmaterial typically accounts for 10–40% of the fine particlemass, and in rural and remote areas of the U.S., organicmaterial typically accounts for 30–50% of the fine partic-ulate mass [Environmental Protection Agency, 1996]. Fur-thermore, in certain areas, such as the Amazon basin, theorganic fraction can approach 90% of the total aerosol mass[Artaxo et al., 1988; Talbot et al., 1988]. The total amountof condensed phase organic material produced from thesesources is estimated to be 175 Tg yrC01[Kanakidou et al.,2000; Liousse et al., 1996].[3] Despite the abundance of condensed-phase organicmaterial in the atmosphere, relatively little is known aboutthe possible phase transitions of organic particles. In orderto understand and predict the role of organic particles inthe atmosphere, these phase transitions must be understood.For example, recent laboratory studies have shown thatthe hydrolysis of N2O5to form HNO3will vary dramati-cally depending on whether or not particles are solid oraqueous solution droplets [Hanson and Ravishankara,1993; Mozurkewich and Calvert, 1988]. Radiative forcingby atmospheric particles and the mechanism of ice nucle-ation on or in these particles will also depend strongly onthe phase and water content [DeMott, 2002; Martin, 1998;Tabazadeh and Toon, 1998; Zuberi et al., 2001].[4] We have carried out a series of experiments todetermine the deliquescence properties of pure dicarboxylicacid particles. Deliquescence is an atmospherically relevantphase transition that involves the uptake of water by solidparticles to form solution droplets. These studies shouldlead to a better understanding of the more complex organicparticles found in the atmosphere. Specifically, we focusedon malonic, succinic, glutaric, and adipic acid particles.These organic compounds were chosen since field measure-ments have shown that these acids are a significant com-ponent of fine particulate matter in the troposphere [Chebbiand Carlier, 1996; Kawamura et al., 1996b; Saxena andHildemann, 1996; Yao et al., 2002]. Sources of thesedicarboxylic acids include biomass burning, fossil fuelcombustion, and photochemical oxidation of gas-phasehydrocarbons [Chebbi and Carlier, 1996; Hatakeyama etal., 1985; Kawamura and Kaplan, 1987; Kawamura et al.,1996a].[5] The deliquescence of dicarboxylic acid particles havepreviously been studied using a tandem differential mobilityanalyzer [Cruz and Pandis, 2000; Prenni et al., 2001], anelectrodynamic balance [Peng et al., 2001], an aerosol flowtube-FTIR system [Braban et al., 2003], a static modeJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D06212, doi:10.1029/2003JD004075, 2004Copyright 2004 by the American Geophysical Union.0148-0227/04/2003JD004075$09.00D06212 1of8chamber-FTIR system [Braban et al., 2003], and bulkmethods [Braban et al., 2003; Brooks et al., 2002; Penget al., 2001; Wise et al., 2003]. In contrast, we used anoptical microscope to study particles with sizes rangingfrom 2 to 40 mm. We investigated deliquescence at temper-atures ranging from 293 to 243 K, which extends thetemperature range covered in most previous studies. Thesestudies include measurements below the eutectic temper-atures. At such temperatures the vapor is supersaturatedwith respect to ice prior to deliquescence, and hence, icecould nucleate. Currently, there is a dearth of experimentaldata on deliquescence at temperatures below the eutectic. Inthe following we present the deliquescence measurementsand compare these measurements with previous studies andtheoretical calculations.2. Experimental Setup[6] The apparatus used in these studies is illustrated inFigures 1a and 1b. The particles of interest are deposited onthe bottom surface of the flow cell, the relative humidity inthe cell is controlled by the continuous flow of a mixture ofdry and humidified N2, and the phase of the particles ismonitored with an optical microscope. This approach issimilar to the approach recently used to study the micro-physics of NaCl-H2O and HNO3-H2O particles [Koop etal., 2000; Salcedo et al., 2000, 2001]. This methodologyenables long observation times, temperature cycling, controlof the relative humidity, and statistically significantresults (typically 30–80 particles are monitored in a singleexperiment).[7] The cell body and the inlet and outlet were con-structed of stainless steel. The top window was sealed to thecell body with high vacuum grease (Dupont, Krytox LVP,vapor pressure less than 10C013torr) and the bottom surfacewas sealed with a Viton O-ring. Two different bottomsurfaces were used in these experiments: the first surfaceconsisted of a thin glass cover slide treated with an organo-silane to form a monomolecular hydrophobic layer, and thesecond surface consisted of a 0.03 mm polytetrafluoro-ethylene (PTFE) film annealed to a plain glass cover slide.The annealing process significantly reduced the number ofdefects on the surface of the PTFE film and providedadhesion between the glass substrate and the film. Thesehydrophobic surfaces were chosen to prevent ice nucleationdirectly on the bottom surface at sub-eutectic temperatures.We have also performed measurements at room temperatureon several different substrates, ranging from hydrophobic(PTFE film) to hydrophilic (bare glass). In all cases, weobserved the same result, indicating that the surface did notinfluence deliquescence.[8] The relative humidity (RH) over the particles wascontrolled by continuously flowing a mixture of dry andhumidified N2through the cell (total flow rate of 100 to300 cm3minC01standard temperature and pressure). Ultra-high purity nitrogen (Praxair, 99.999%) was first passedthrough a hydrocarbon filter (Supelco, Supelcarb HC 24449)and then subsequently split into two flows. One flow waspassed through a water bubbler situated inside a refrigeratingcirculator (ThermoNeslab, RTE-140) to generate humidifiedN2, and the second flow served as the dry N2line. Therelative humidity in the cell was varied by adjusting therelative flows of the dry and humidified N2or adjusting thetemperature of the circulator, while maintaining a constanttotal flow. A dew point hygrometer (General Eastern,Hygro M4) was used to determine the RH of the combinedflows. This instrument measured the dew point or ice frostpoint of the gas, from which the relative humidity wascalculated with the Goff and Gratch equations [Goff andGratch, 1946] and knowledge of the substrate temperature.[9] The sample cell was mounted on a cooling stage fortemperature control. The temperature of the cooling stageand hence the sample cell was regulated with a refrigeratingcirculator (Thermo Neslab RTE-740). A type T thermocou-ple located in the cooling stage just below the flow cell wasused to determine the temperature of the bottom surface ofthe flow cell. This thermocouple was calibrated against thedew point or ice frost point, as done previously in theliterature [Middlebrook et al., 1993]. Briefly, calibrationinvolved the following steps: first a constant relative hu-midity was established in the flow cell. Next the tempera-ture of the flow cell was decreased until liquid water or iceparticles condensed on the bottom surface of the cell due toheterogeneous nucleation on the PTFE film. The tempera-ture of the flow cell was then slowly increased whilevisually monitoring the size of the particles with areflected-light microscope (see below). From these obser-vations we determined the temperature at which waterparticles neither grew nor shrunk in size. Under theseconditions the particles were in equilibrium with the gas-phase water vapor, and the temperature of the bottomsurface of the cell was equal to the dew point or ice frostpoint, which was determined with the hygrometer. Thedifference between the temperature measured with thethermocouple and the dew point or ice frost point wasused to construct a calibration curve for the thermocouple.Figure 1. Diagram of the flow cell and experimentalapparatus: (a) side view of the flow cell and (b) side view ofthe assembled apparatus.D06212 PARSONS ET AL.: DELIQUESCENCE OF ORGANIC ACID PARTICLES2of8D06212At 273 K the correction was 0.1 K, and at 243 K thecorrection was 0.7 K.[10] Deliquescence of the particles was monitored with areflected-light microscope (Zeiss, Axiotech 100) equippedwith a 20 times and a 50 times objective. Images of theparticles were recorded with a CCD camera attached to amonitor and video recorder, and the temperature and dewpoint/ice frost point were recorded simultaneously. From theimages we determined the size and morphology of theparticles and the deliquescence relative humidities. In mostof the experiments we used polarized light to enhance thecontrast between solid and liquid particles; however, phasetransitions were also discernable with unpolarized light asdemonstrated previously [Bertram et al., 2000; Koop et al.,2000, 1998]. Shown in Figures 2a, 2b, and 2c are imagesrecorded during a typical experiment. Figure 2a showsimages of ammonium sulfate particles prior to deliques-cence; Figure 2b shows images during deliquescence; andFigure 2c shows images after deliquescence.[11] Particles ranging in size from 2 to 40 mmwereproduced by two methods. The first method consisted ofgrinding crystals and placing the resulting particles directlyon the bottom surface prior to assembling the flow cell. Thesecond method involved directing a stream of submicronparticles from an atomizer (TSI 7660), at the PTFE substratefor 1–5 s. During this time the submicron particles impactedon the surface and coagulated resulting in supermicronparticles. Solutions used in the atomizer were made witheither deionized ultrafiltered water (Fisher) or HPLC-gradewater (Fisher), both of which have been passed through asubmicron filter. The results were independent of the gradeof water used. Ammonium sulfate (Fisher, 99.8%), malonicacid (Aldrich, 99%), succinic acid (Fisher, 99.8%), glutaricacid (Aldrich, 99%), and adipic acid (Fisher, 99.9% mini-mum) were all used as supplied without further purification.Ammonium sulfate was used to validate our experimentalsetup as discussed below.[12] The size of atmospheric particles range from approx-imately 0.002 to 10 mm[Finlayson-Pitts and Pitts, 2000]. Asmentioned, the size of the particles used in our experimentsranged from 2 to 40 mm. Hence the smallest particles studiesin our experiments fall within the size range that is importantfor atmospheric chemistry and physics. Furthermore, ourdeliquescence results were independent of particle size,suggesting that our results could be applicable to smallerparticles.[13] During a deliquescence experiment the relativehumidity was first held close to 0% to ensure all theparticles were solid. Then the relative humidity wasincreased by either adjusting the dry and humidified N2flows or adjusting the temperature of the humidifyingbubbler. Close to deliquescence the relative humidity wasincreased at a rate of approximately 0.05% per minute. Theuncertainty in our deliquescence measurements (±2s) wasapproximately ±2.1% relative humidity, based on repeatedmeasurements at a fixed temperature.[14] For measurements below the eutectic temperatureswe modified our setup slightly. In these experiments a 2 mmthick PTFE spacer was inserted between the stainless steelcell body and the bottom surface. The PTFE spacer main-tained a large temperature differential (at least 10 K)between the cell body and the bottom substrate. Thisensured that ice did not nucleate directly on the cell bodyand ice supersaturation was maintained above the organicparticles.3. Results and Discussion3.1. Deliquescence of Ammonium Sulfate Particles[15] In order to evaluate the performance of our experi-mental apparatus and approach, we first studied ammoniumsulfate particles. The deliquescence of these particles hasFigure 2. Images of ammonium sulfate particles recordedduring a deliquescence experiment: (a) solid particlesprior to deliquescence, (b) solid-liquid particles duringdeliquescence, and (c) liquid particles just after completedeliquescence.D06212 PARSONS ET AL.: DELIQUESCENCE OF ORGANIC ACID PARTICLES3of8D06212been studied extensively and is well understood [Martin,2000]. The deliquescence results are shown in Figure 3. Alsoshown are experimental results from other groups andpredictions based on thermodynamic considerations. Theresults from Braban et al. [2001] and Tang and Munkelwitz[1993] were obtained with supermicron particles; the datafrom both Cziczo and Abbatt [1999] and Onasch et al.[1999] were obtained using submicron aerosol particles;and the data from Wise et al. [2003] were obtained usingbulk solutions. The solid line was calculated using a ther-modynamic model by Clegg et al. [1998]. Our results arein excellent agreement with the previous experimentalmeasurements and the theoretical predictions.3.2. Deliquescence of Dicarboxylic Acid Particles as aFunction of Temperature[16] The deliquescence relative humidities (DRH) for thefour organic acids studied are shown in Figures 4–7.Succinic acid and adipic acid deliquesce close to 100%RH; whereas malonic acid and glutaric acid deliquesce atlower relative humidities, consistent with the solubilities ofthese organics. The high DRH values for both adipic acidand succinic acid suggest that these organics may exist assolids in atmospheric particles if these organics are asignificant component of the aerosol mass. For comparison,the solubilities for malonic, succinic, and adipic acid at298.15 K are 0.2176, 0.01337, and 0.00307 mole fractionacid, respectively [Apelblat and Manzurola, 1987]; and thesolubility for glutaric acid at 297.05 K is 0.1506 molefraction acid [Stephen and Stephen, 1963].[17] As done previously in the literature for inorganicsalts [Tabazadeh and Toon, 1998; Onasch et al., 1999], wefit the temperature-dependent DRH data for malonic andglutaric acid to the following equation:lnðDRHÞ¼AþBTþCT2þDT3ð1Þwhere DRH is the deliquescence relative humidity (%) andT is the temperature (K). The solid thick lines shown inFigure 3. Deliquescence of ammonium sulfate particles asa function of temperature. Our data were obtained withparticles ranging in size from 2 to 40 mm. The uncertainty inour deliquescence measurements (±2s) was approximately±2.1% relative humidity, based on repeated measurementsat a fixed temperature. The results from Braban et al. [2001]and Tang and Munkelwitz [1993] were also obtained withsupermicron particles, and the data from both Cziczo andAbbatt [1999] and Onasch et al. [1999] were obtained usingsubmicron particles. The results from Wise et al. [2003]were obtained with bulk solutions. The solid line wascalculated using a thermodynamic model by Clegg et al.[1998].Figure 4. Deliquescence of malonic acid as a function oftemperature. Our data were obtained with particles rangingin size from 2 to 40 mm. The results from Braban et al.[2003] were obtained using submicron and supermicronparticles, and the data from Brooks et al. [2002], Wise et al.[2003], and Peng et al. [2001] were obtained using bulksolutions. Details of the theoretical calculations are given inthe text.Figure 5. Deliquescence of succinic acid as a function oftemperature. Our data were obtained with supermicronparticles (2–40 mm). The results from Peng et al. [2001]were obtained using supermicron particles and bulksolutions. The data from Prenni et al. [2001] were obtainedusing submicron particles, and the data from Brooks et al.[2002] and Wise et al. [2003] were obtained using bulkmethods. The overlapping theoretical calculations aredescribed in the text.D06212 PARSONS ET AL.: DELIQUESCENCE OF ORGANIC ACID PARTICLES4of8D06212Figures 4 and 6 are the results from this least squaresanalysis, and the parameters that describe these curves aregiven in Table 1. We did not perform a similar analysisfor succinic and adipic acid since within experimentaluncertainty the DRH for these particles was 100% andindependent of temperature.[18] Also shown in Figures 4–7 are results from othergroups. Cruz and Pandis [2000] as well as Prenni et al.[2001] investigated the deliquescence of submicron par-ticles using a tandem differential mobility analyzer system.Peng et al. [2001] investigated the deliquescence using anelectrodynamic balance and bulk methods, and Brooks etal. [2002] and Wise et al. [2003] determined the deliques-cence relative humidity of these acids from bulk measure-ments. Braban et al. [2003] measured the deliquescenceproperties of submicron and supermicron particles usingan aerosol flow tube-FTIR system and a static modechamber-FTIR system. In several of the measurementsmentioned above only lower limits to deliquescence weredetermined. These limits are represented by the hatchedbars in Figures 5 and 7.[19] In general, our results are in good agreement with theresults from previous studies. Our malonic acid results areconsistent with the measurements from Braban et al.[2003], Brooks et al. [2002], and Wise et al. [2003]. Theresults from Peng et al. [2001] appear to be 5% lower thanour measurements. Our succinic acid results are consistentwith the measurements by Peng et al. [2001], Prenni et al.[2001], and Wise et al. [2003]; however, our results areapproximately 7% above the measurement at 298 K byBrooks et al. [2002]. For glutaric acid, our results areconsistent with the results reported by Brooks et al.[2002], and our results when extrapolated to warmer tem-peratures are consistent with the results reported by Cruzand Pandis [2000], Peng et al. [2001], and Wise et al.[2003]. Finally, our results for adipic acid are consistentwith the lower limits reported by Prenni et al. [2001] andBrooks et al. [2002]. As mentioned above, the experimentalresults presented in Figures 4–7 were obtained usingsubmicron and supermicron particles as well as bulk sol-utions. The good agreement between these results illustratesthat there is no significant kinetic barrier to the deliques-cence of dicarboxylic acid particles and that bulk thermo-dynamics can be used to predict the DRH of these particles.Braban et al. [2003] recently reached a similar conclusionfor the malonic acid system.3.3. Theoretical Calculations of DRH[20] We first calculated the deliquescence relative humid-ities of the dicarboxylic acid particles using solubility dataand by assuming the saturated solutions obey Raoult’s law(ideal solution). Solubility data for malonic, succinic, andadipic acid where taken from Apelblat and Manzurola[1987], and solubility data for glutaric acid were taken fromStephen and Stephen [1963]. Apelblat and Manzurola[1987] reported solubility data from 298.15 to 278.15 K,and Stephen and Stephen [1963] reported solubility datafrom 318.95 to 276.55 K. The solubility data were fit to thefollowing equation:lnðxÞ¼AþBTþCT2ð2Þwhere x is the solubility (mole fraction) and T is thetemperature (Kelvin). The parameters from the least squaresFigure 6. Deliquescence of glutaric acid as a function oftemperature. Our data were obtained with particles rangingin size from 2 to 40 mm. The results from Cruz and Pandis[2000] were obtained using submicron aerosol particles.The results from Peng et al. [2001] were obtained usingsupermicron particles and bulk solutions, and the data fromBrooks et al. [2002] and Wise et al. [2003] were obtainedusing bulk methods.Figure 7. Deliquescence of adipic acid as a function oftemperature. Our data were obtained with particles rangingin size from 2 to 40 mm. The results from Prenni et al.[2001] were obtained with submicron aerosol particles, andthe data from Brooks et al. [2002] were obtained using bulkmethods. The overlapping theoretical calculations aredescribed in the text.Table 1. Parameters Describing the Deliquescence ResultsaAcid A B C DMalonic acid 2.5930 463.17 7.1176 C2 104C01.7740 C2 107Glutaric acid 3.1912 384.68 4.6458 C2 104C01.3706 C2 107aDeliquescence relative humidity can be calculated from ln(DRH) = A +B/T + C/T2+D/T3, where DRH is the deliquescence relative humidity (inpercent RH) and T is the temperature (in K).D06212 PARSONS ET AL.: DELIQUESCENCE OF ORGANIC ACID PARTICLES5of8D06212analysis were used when calculating deliquescence. Equa-tion (2) was chosen for this study because it fit the solubilitydata well (R2ranged from 0.9963 to 0.9999) and because itgave realistic solubilities when extrapolated to lowtemperatures. In contrast, other equations we tried gaveeither negative solubilities when extrapolated to 243 K orsolubilities that increased with decreasing temperature atlow temperatures. The relative uncertainty of the solubilitydata for malonic, succinic, glutaric, and adipic acid areapproximately 1%, 1%, 4%, and 3%, respectively, based onthe scatter in the data.[21] Calculations based on solubility data and Raoult’slaw are shown in Figures 4–7 (labeled ideal solutioncalculation). The uncertainties in the calculations due tothe uncertainties in the solubility data are estimated to beless than 1%. These calculations accurately describe thedeliquescence of succinic acid and adipic acid, which is notsurprising since these particles are dilute solutions at deli-quescence. The calculation also appears to agree reasonablywell with the glutaric acid measurements (the calculationsfall within our error bars except for two data points);however, for malonic acid the calculations are significantlyabove the measurements at temperatures greater than 270 K,indicating that saturated solutions of malonic acid do notobey Raoult’s law.[22] We have also calculated the DRH of these dicarbox-ylic acid particles as a function of temperature using thesolubility data described above and the original UNIQUAC(universal quasi-chemical) Functional Group Activity Coef-ficients (UNIFAC) model [Fredenslund et al., 1975]. TheUNIFAC model is a group contribution method that is usedfor predicting thermodynamic properties of nonideal solu-tions. Here we use the UNIFAC model to calculate the wateractivity of saturated solutions of dicarboxylic acids. Notethat the water activity at saturation is equal to DRH dividedby 100. The use of UNIFAC has been described in detail[Fredenslund et al., 1977, 1975; Fredenslund and Sorensen,1994].[23] In order to calculate the water activity of an organic-water solution using UNIFAC, group volume parameters,group area parameters, and group interaction parameters arerequired. For the first set of UNIFAC calculations shown inFigures 4–7 (UNIFAC calculation 1) we used group vol-ume, group area, and group interaction parameters fromReid et al. [1987]. For the second set of UNIFAC calcu-lations shown in Figures 4–7 (UNIFAC calculation 2) weused group volume and group area parameters from Reid etal. and group interaction parameters from Peng et al.[2001]. The calculations based on the interaction parametersfrom Reid et al. [1987] deviate from our glutaric acid resultsat temperatures greater than 285 K, and deviate significantlyfrom our malonic acid results over the entire temperaturerange studied. This is not surprising since recently it wasshown that water activity predictions for malonic andglutaric acid based on parameters from Reid et al. deviatefrom water activity measurements [Peng et al., 2001]. Incontrast, the calculations based on the parameters from Penget al. are in good agreement with the experimental resultsfor glutaric acid, and the calculations are within 3% of theexperimental results for malonic acid.[24] We also calculated the DRH for these acids usinginteraction parameters reported by Ming and Russell [2002],group volume and group area parameters from Reid et al.[1987], and the solubility data described above. The resultsfrom this third set of UNIFAC calculations (not shown) fellbetween the results from the first two sets of UNIFACcalculations (UNIFAC calculations 1 and 2).[25] The UNIFAC model combined with interactionparameters from Peng et al. [2001] gave the best agreementwith our experimental results. The reasonable agreementsuggests that the UNIFAC model combined with appropri-ate interaction parameters should be a useful tool forcalculating the deliquescence properties of multicomponentorganic particles found in the atmosphere.3.4. DRH Values Below the Eutectic Temperature[26] As mentioned above, at temperatures below theeutectic the vapor is supersaturated with respect to ice priorto deliquescence. At these temperatures ice may nucleatedirectly on the solid organic particles prior to deliquescence.Furthermore, if these solid dicarboxylic acids are good icenuclei then they may play an important role in ice cloudformation in the atmosphere.[27] The eutectic temperature for each system was iden-tified by determining the temperature at which the vaporpressure of ice equals the partial pressure of water over thesaturated organic solution. The vapor pressure of ice as afunction of temperature was calculated with the Goff andGratch equation [Goff and Gratch, 1946]. The partialpressure of water over the saturated aqueous solutions asa function of temperature was determined from our deli-quescence results. On the basis of these results, the eutectictemperatures for malonic, succinic, glutaric, and adipic acidare 255.7, 273.2, 269.0, and 273.2 K, respectively. Alsoshown in Figures 4–7 is the ice saturation line from theGoff and Gratch equation. This line illustrates the condi-tions at which the vapor is supersaturated with respect toice.[28] At temperatures below the eutectic temperature icewas not observed in any of our experiments rather theparticles underwent deliquescence to form metastable solu-tion droplets. Furthermore, the measured DRH was inagreement with the DRH predicted with the UNIFAC modeland interaction parameters from Peng et al. [2001] (seeabove). Deliquescence below the eutectic has also beenobserved for the malonic acid-water system [Braban et al.,2003], the ammonium sulfate-water system [Braban et al.,2001; Fortin et al., 2002], and the sodium chloride-watersystem [Koop et al., 2000].[29] Our measurements at temperatures below the eutecticindicate that solid dicarboxylic acids (with surface struc-tures similar to the surface structures employed in ourstudies) will not play an important role in ice cloudformation at temperatures above 243 K. These solid acidsmay be important in ice cloud formation at temperaturesbelow 243 K, or if the particles are preactivated or havesignificantly more defects than the particles in our studies[Zuberi et al., 2001].4. Summary and Conclusions[30] Deliquescence relative humidities for malonic, glu-taric, succinic, and adipic acid particles were determined fortemperatures ranging from 293 to 243 K. Over this temper-D06212 PARSONS ET AL.: DELIQUESCENCE OF ORGANIC ACID PARTICLES6of8D06212ature range both succinic acid and adipic acid deliquesced atapproximately 100% relative humidity. In contrast, theDRH for malonic acid at 293 K and 243 K was 73.7%and 86.6%, respectively, and the DRH for glutaric acid at293 K and 243 K was 91.0% and 100%, respectively. Ourresults are in good agreement with previous studies whichused bulk solutions, supermicron particles and submicronparticles. This agreement suggests that there is no signifi-cant kinetic barrier to the deliquescence of these dicarbox-ylic acid particles. The deliquescence measurements werecompared with a series of theoretical calculations. TheUNIFAC model combined with interaction parameters fromPeng et al. [2001] gave the best agreement with theexperimental results. The reasonable agreement suggeststhat the UNIFAC model combined with appropriate inter-action parameters should be a useful tool for calculating thedeliquescence properties of multicomponent organic par-ticles found in the atmosphere.[31] At temperatures below the eutectic, ice was notobserved in any of our experiments, rather the particlesunderwent deliquescence to form metastable solution drop-lets. This indicates that the solid organics studied are notgood ice nuclei above 243 K and hence will probably notplay a role in ice cloud formation at these temperatures.[32] Acknowledgments. We thank M. Eastwood and L. Sun for theirassistance in equipment design and substrate selection and A. Fok for helpwith some of the deliquescence measurements. We also thank C. F. Braban,J. P. D. Abbatt, M. E. Wise, and M. A. Tolbert for providing us withmanuscripts prior to publication and L. M. 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