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Inhibition of efflorescence in mixed organic-inorganic particles at temperatures less than 250 K. Bodsworth, A.; Zobrist, B.; Bertram, Allan K. 2010

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This journal iscthe Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12259–12266 12259Inhibition of efflorescence in mixed organic–inorganic particlesat temperatures less than 250 KA. Bodsworth,aB. Zobristband A. K. Bertram*aReceived 14th May 2010, Accepted 12th July 2010DOI: 10.1039/c0cp00572jIt is now well recognized that mixed organic–inorganic particles are abundant in the atmosphere.While there have been numerous studies of efflorescence of mixed organic–inorganic particlesclose to 293 K, there are only a few at temperatures less than 273 K. Understanding theefflorescence properties of these particles at temperatures less than 273 K could be especiallyimportant for predicting ice nucleation in the upper troposphere. We studied the efflorescenceproperties of mixed citric acid–ammonium sulfate particles as a function of temperature to betterunderstand the efflorescence properties of mixed organic–inorganic particles in the middle andupper troposphere. Our data for 293 K illustrate that the addition of citric acid decreases theERH of ammonium sulfate, which is consistent with the trends observed with other systemscontaining highly oxygenated organic compounds. At low temperatures the trend is qualitativelythe same, but efflorescence can be inhibited by smaller concentrations of citric acid. For exampleat temperatures o250 K an organic mass/(organic mass + sulfate mass) of only 0.33 isneeded to inhibit efflorescence of ammonium sulfate. In the upper troposphere the organicmass/(organic mass + sulfate mass) can often be larger than this value. As a result, particles inthe upper troposphere may be more likely to remain in the liquid state than previously thoughtand solid ammonium sulfate may be less likely to participate in heterogeneous ice nucleation inthe upper troposphere. Additional studies are required on other model organic systems.1. IntroductionMeasurements showthatbothorganicandinorganicmaterialsare abundant in atmospheric aerosols,1with the ratio oforganic species to inorganic material depending on location,aerosol source and season. In addition, single particlemeasurements suggest that the dry organic mass fraction,organic/(organic + sulfate), in the upper troposphere rangesfrom 0.3to 0.8 with more variation below 5 km. There are alsoabundant data from single particle measurements that showthat organic and inorganic materials are often internally mixed inthesameparticles.2–4These internally mixed organic–inorganicparticles can undergo a range of phase transitions includingdeliquescence and efflorescence.Efflorescence occurs when an aqueous aerosol is exposed toa low relative humidity and the inorganic and/or organiccomponents crystallize. The reverse process is deliquescence,where a crystalline particle exposed to a high relative humiditytakes up water to form an aqueous droplet. Deliquescence isthought to be a thermodynamically controlled process andoccurs at a higher relative humidity than efflorescence.Between the deliquescence relative humidity (DRH) andefflorescence relative humidity (ERH) is a metastable regionwhere particles can be crystalline, partially crystalline, oraqueous droplets, depending on their history. Recent workhas shown that in most cases the organic component in themixed organic–inorganic particles will not effloresce sincethe concentration of any one organic species is small.5Theinorganic component, however, can deliquesce and effloresce.Understanding and predicting the deliquescence andefflorescence properties of mixed organic–inorganic particlesmay be important for several reasons. For example laboratorystudies have shown that N2O5hydrolysis is more efficient onaqueous deliquesced particles compared to efflorescedparticles.6,7In addition effloresced particles are smaller thanthe corresponding deliquesced particles and have differentoptical properties.4,8Modelling studies suggest that thehysteresis effect of sulfate can change the direct effect by asmuch as 16%.9Recently several studies have focused on the deliquescenceand efflorescence of mixed organic–inorganic particles.5,10–24To date, however, most of these studies have focused ontemperatures around 293 K, and there has only been onestudy of the efflorescence properties of mixed organic–inorganicparticles at temperatures less than 273 K.25While roomtemperature studies are useful for understanding the phaseproperties of atmospheric aerosols in the lower troposphere,information on the temperature dependence of these phasetransitions are stillneeded forpredicting the deliquescence andefflorescence properties of aerosol particles in the middle andupper troposphere. In the following we have investigated theefflorescence properties of mixed organic–inorganic particlesat temperatures down to 233 K.Low temperature ERH measurements may be especiallyimportant for predicting ice nucleation in the troposphere.aDepartment of Chemistry, University of British Columbia,Vancouver, British Columbia, Canada.E-mail: bertram@chem.ubc.ca.bInstitute for Atmospheric and Climate Science, ETH Zurich, Zurich,SwitzerlandPAPER www.rsc.org/pccp | Physical Chemistry Chemical PhysicsDownloaded by The University of British Columbia Library on 18 April 2011Published on 16 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00572JView Online12260 Phys. Chem. Chem. Phys., 2010, 12, 12259–12266 This journal iscthe Owner Societies 2010Deliquesced particles (free of foreign nuclei such as mineraldust) can only form ice by homogeneous nucleation, whereaseffloresced particles can potentially act as heterogeneous icenuclei.25–30It has been shown that effloresced ammoniumsulfate particles can act as good heterogeneous ice nucleiunder certain conditions. These effloresced ammonium sulfateparticles may compete with other effective ice nuclei in theatmosphere, and as a result, they could play an important rolein climate and the aerosol indirect effect.28Closely related tothe above, Jensen et al.31examined the properties of cirrusclouds at low temperatures in the tropical tropopauselayer (TTL) and concluded that ice number concentrations,ice crystal size distributions and cloud extinctions wereinconsistent with homogeneous nucleation. The authorssuggested heterogeneous ice nucleation on efflorescedammonium sulfate particles as a possible mechanism to explainthe in situ and remote-sensing measurements. Froyd et al.32also suggested heterogeneous ice nucleation on efflorescedammonium sulfate particles as a possible mechanism toexplain the composition of residual particles from evaporatedcirrus ice crystals near the TTL. This mechanism assumesthat mixed organic–inorganic particles will effloresce in theTTL or upper troposphere.There are some data that suggest that the efflorescenceproperties of mixed organic–inorganic particles may bedifferent at low temperatures compared to 293 K. Studies byMullin and Leci33,34more than 40 years ago have shown thatthe rate of nucleation in concentrated citric acid aqueoussolutions decreases at temperatures less than 273 K,33possiblydue to an increase in viscosity at lower temperatures. The rateof efflorescence of mixed organic–inorganic particles maydecrease at lower temperatures due to an increase in viscosityas well. In addition recent studies have shown that aqueousorganic components35,36,37and aqueous organic–inorganic36particles can form glasses under atmospherically relevantconditions.38The formation of glasses should limit efflorescence.On the other hand, Wise et al.25studied mixed palmiticacid–ammonium sulfate particles at temperatures down to245 K and found that insoluble palmitic acid had little effecton the efflorescence properties of ammonium sulfate.In the following we study the efflorescence properties of mixedcitricacid–ammoniumsulfateparticlesasafunctionof temperature.Citric acid (COOH–CH2–COH(COOH)–CH2–COOH, molarweight: 192.12 g molC01) contains three carboxyl groups(–COOH) and an alcohol group (–OH) and exists in theatmosphere in small quantities.39We also investigated theglass transition temperatures of mixed citric acid–ammoniumsulfate solutions using a Differential Scanning Calorimeter(DSC) in order to relate efflorescence limitation to glassformation of the particles. Citric acid has recently been usedas a model system to represent oxygenated organics in theatmosphere.35,38Also aqueous solutions of citric acid oftenform glasses at low temperatures.40,41Ammonium sulfate waschosen for these studies since it is an important inorganicspecies in the atmosphere. Also, for the conditions studiedby Jensen et al. discussed above, the sulfate was fullyneutralized to ammonium sulfate, which has recently beensupported by single particle mass spectrometer data taken atthe tropical tropopause.322. ExperimentalThe technique used to study efflorescence has been describedin detail elsewhere.17,19,42A brief overview is provided herewith a focus on details specific to the current experiments. Thesystemconsistsofanopticalmicroscope(usingpolarizedlight)coupled to a temperature controlled flow cell. The bottomsurface of the flow cell is a hydrophobic glass slide upon whichthe particles of interest are deposited and observed. Relativehumidity in the cell was controlled by a continuous flow of amixture of humid and dry N2. Typical flow rates wereapproximately 1.5 L minC01.Efflorescence experiments were conducted at five temperaturesranging from 293 K to 233 K. During experiments theparticles were first deliquesced by exposing them to an RHclose to 100%. Next the humidity was reduced to approximately50% RH in one step and then decreased by approximately0.1% RH per minute for the remainder of the experiments.During the experiments images were taken every 15 s by acamera coupled to the microscope. Particles ranged in sizefrom 5–30 mm in diameter. For each particle, the efflorescencerelative humidity (ERH) was considered to be the firstappearance of solid in the particle, even if the particlesappeared only partially crystalline.Ammonium sulfate (Fisher, 99.8%) and citric acid(Sigma-Aldrich, 99+%) were used as supplied. Bulk mixtureswere prepared gravimetrically and dissolved in milliporefiltered water (18.2 MO). To prepare the particles the solutionwas passed through a nebulizer to produce submicronparticles. These particles were directed towards a hydrophobicglass slide where they coagulated into supermicron droplets.The compositions of particles and/or solutions are typicallyreported in dry mole fraction citric acid. This dry molefraction is calculated by the following equation:XCA;dry¼nCAnCAþnASð1Þwhere XCA,dryis the mole fraction of citric acid in a dry(containing no water) particle or solution, nCAis the molesof citric acid and nASis the moles of ammonium sulfate.Keep in mind, however, that this does not imply the particlesand solutions are completely dry. This nomenclature is usedsince it is a convenient method for representing the citricacid-to-ammonium sulfate ratio in particles and solutions.Glass transition temperatures of different citric acid–ammoniumsulfate–water solutions were performed in a commercialDSC (TA instruments Q10). In one set of experiments, themass fractions of the total solutes (citric acid and ammoniumsulfate) varied between 0.603 and 0.7916, whereas XCA,drystayed constant at 0.7. In the second set of experiments, thetotal mass fraction of solutes was kept at roughly 0.6, whereasthe XCA,drywaschanged. All experiments wereperformed withbulk samples. Ammonium sulfate (Fluka, >99.5%) and citricacid (Fluka, >99.5%) were used as supplied. The glasstransition temperatures were determined as the onset of theheat signal in the heating cycle, and have accuracy in theabsolute temperature of C60.9 K.36Downloaded by The University of British Columbia Library on 18 April 2011Published on 16 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00572JView OnlineThis journal iscthe Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12259–12266 122613. Results3.1 ERH of pure ammonium sulfate (XCA,dry= 0.0) vs.temperaturePrior to studying mixed organic–inorganic particles, we studiedthe efflorescence properties of pure ammonium sulfateparticles as a function of temperature. This provided a referencepoint for the mixed organic–inorganic particles, as well as away to validate our system for low temperature studies.Measurements shown in Fig. 1 (solid symbols) are ourresultsforpureammoniumsulfateparticles.Thesolidsymbolsrefer to the average RH. Since efflorescence is a stochasticprocess, all the particles did not effloresce at the same RH evenfor the same temperature. The error bars represent a combinationof the range over which 95% of the particles efflorescence (2s)as well as the uncertainty from measuring the relative humidity.The data suggest that efflorescence of ammonium sulfatebetween 293 K and 233 K is relatively insensitive to temperature;this trend follows closely the temperature trend for deliquescenceof ammonium sulfate, which only varies by roughly 4% RHover 50 K.43Also included in Fig. 1 are results from othergroups that have studied the same temperature range. Theprevious studies include a range of different techniques includingan electrodynamic balance (EDB)44where a particle issuspended in an electric field and aerosol flow tubes45,46wherethe particles are suspended in a gas flow. The good agreementbetween our measurements and previous measurements whereparticles were not in contact with a surface suggests that thehydrophobic support in our experiments do not significantlyaffect our efflorescence results, a conclusion that is consistentwith previous studies from our laboratory18,19,47and othergroups.21,483.2 ERH of mixed ammonium sulfate–citric acid particles atroom temperatureShown in Fig. 2 are our results for mixed ammonium sulfate–citric acid particles at room temperature. Similar to Fig. 1, thesymbols represent the average efflorescence relativehumidity. For experiments where more than 20 particles wereobserved at a given concentration, the error bar correspondsto C62s for the results as well as the uncertainty of thehygrometer (B1.1%). In cases where less than 20 nucleationevents were observed the error bars correspond to C67.8%.This is a conservative estimate based on the maximum 2sobserved in our experiments where more than 20 nucleationeventswereobserved.Ourresultsillustratethattheaddition ofcitric acid slowly decreases the ERH of ammonium sulfate inthe particles. At 0.25 mole fraction the decrease is approximately10% RH. At 0.3 mole fraction the particles did not effloresce atall, even when exposed to our system’s minimum (RHo1%)for longer than an hour. Also shown in Fig. 2 are results fromother groups who studied the efflorescence of mixed citricacid–ammonium sulfate particles at room temperature.Choi and Chan used an EDB to study a citric acid molefraction of B0.59. In these studies they did not observecrystallization, consistent with our measurements.16Zardiniet al. used both an EDB and hygroscopicity tandemdifferential mobility analyzer (HTDMA) to study efflorescence.All the data from Zardini et al. are in excellent agreement withour data except at XCA,dry= 0.2, where they see a slightlyhigher ERH than observed in our studies. When comparingthe data sets a relevant parameter is the change in ERH whengoing from pure ammonium sulfate to XCA,dry= 0.2. Zardiniobserved a decrease in ERH between 0–3%; whereas, weobserved a decrease between 2–15%. Considering theuncertainties in the measurements the differences between datasets appear to be relatively small.The general trend illustrated in Fig. 2 (decrease in efflorescencewith addition of organic compounds) for citric acid–ammoniumsulfate is consistent with the trends observed with systemsFig. 1 Measurements of the efflorescence of pure ammonium sulfateas a function of temperature. Filled squares are this study, unfilledsymbols represent ref. 44–46, 49, 63–66.Fig. 2 Efflorescence of citric acid–ammonium sulfate particles atroom temperature. Filled squares represent this study, unfilled circlesrepresent Choi and Chan,16unfilled triangles are Zardini et al.20Forcompositions of 0.33 and 0.5 Zardini et al.20did not observeefflorescence at RH values greater than or equal to an RH of 10%.Experiments were not carried out at RH values less than 10%, andthe symbols and error bars at these compositions are used to indicatethis fact.Downloaded by The University of British Columbia Library on 18 April 2011Published on 16 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00572JView Online12262 Phys. Chem. Chem. Phys., 2010, 12, 12259–12266 This journal iscthe Owner Societies 2010containing other highly oxygenated organic species. To illustratethis point, we compare in Fig. 3 the results from our citric acidstudies with previous measurements from our group whichutilized a similar apparatus. Included in the figure are resultsfor malonic acid, glycerol and levoglucosan.19The resultsillustrate that all systems have a similar trend, but there aresignificant quantitative differences between the systems. Forexample at a mole fraction of 0.3 the citric acid system doesnot effloresce whereas the malonic acid system effloresces atroughly 20.7% RH.The differences between systems shown in Fig. 4 may beexplained by classical nucleation theory. In our experimentsthe efflorescence relative humidity is expected to be limited bythe rate of homogeneous nucleation of ammonium sulfate inthe particles since the rate of crystal growth is almostinstantaneous based on observations of particles duringefflorescence events. According to classical nucleation theorythe rate of homogeneous nucleation of crystalline ammoniumsulfate in the particles can be described by the followingequation, where the nucleation rate, J, is the number of nucleiformed for a unit volume and time.50J ¼ Aexp C016pg3v23k3T3ðlnSÞ2þDG0kT"#ð2Þwhere A is a pre-exponential factor, k is Boltzmann’s constant,v is the molecular volume of ammonium sulfate, T is temperature,S is the supersaturation of crystalline ammonium sulfate, g istheinterfacial energybetween theembryoandthesurroundingliquid and DG0is a molecular rearrangement term (which isstrongly correlated to viscosity). As discussed previously, onepossible explanation for the variation in efflorescence fromsystem to system is that g varies significantly from system tosystem. This would lead to considerably different nucleationrates at a similar relative humidity since the surface tension iscubed. Another possibility is that at low RH, viscosity maybecome significant and vary from system to system at highmole fractions of organics. In this case, viscosity can limit thenucleation rate (through DG0). A final possibility is that thedegree of supersaturation at a given RH varies significantlyfrom system to system due to non-ideal behavior.3.3 ERH as a function of temperatureFig. 4 shows the efflorescence results for the five temperaturesstudied. Qualitatively, the trend observed at lower temperaturesremains the same, where increasing amounts of organic causea slight reduction in ERH followed by a complete inhibition.However, at 248 K and 233 K the efflorescence is inhibitedat lower mole fractions, at 0.25 and 0.2, respectively.This suggests that at low temperatures efflorescence can beinhibited by smaller concentrations of citric acid.Fig. 3 Room temperature comparison between citric acid (this study)and several organics studied previously in our laboratory.19Fig. 4 Efflorescence data for different dry organic mole fractions ofcitric acid–ammonium sulfate particles. Different panels representdifferent temperatures, solid points represent observed efflorescenceevents and unfilled points are non-efflorescing mixtures. The dashedline is included to help guide the eye.Downloaded by The University of British Columbia Library on 18 April 2011Published on 16 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00572JView OnlineThis journal iscthe Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12259–12266 12263Fig. 5 shows the same data as Fig. 4, replotted in RH–temperature space. This figure more clearly illustrates that theefflorescence for XCA,dry= 0.0, 0.1 and 0.15 is relativelyinsensitive to temperature. A linear fit for these three datasets shows slopes between 0.03 and 0.07% RH per Kelvin.Fig. 5 also illustrates that at XCA,dry= 0.2 and 0.25 the ERHis a strong function of temperature, with a sudden change inERH at 233 K and 248 K for XCA,dry= 0.2 and 0.25respectively. Similar to Fig. 4, this shows that at low temperatures(o250 K) and high organic concentrations (XCA,dry= 0.2 and0.25) efflorescence appears to be inhibited.Recentlyin anotableset ofexperiments Wise et al.25studiedthe efflorescence properties of ammonium sulfate particlescoatedwithpalmitic aciddowntoapproximately245K.Theseauthors observed efflorescence at approximately 35% RH fortheentiretemperaturerangestudiedandnotedthattheorganiccoating hadlittleeffectontheefflorescence ofammoniumsulfate.Onelikelyreasonforthedifferencebetweenourresultsandtheresults by Wise et al. is the properties of the organic materialstudied. Palmitic acid is insoluble in water. On the other hand,citric acid is water soluble and does not crystallize under theconditions we explored.3.4 Explanation of temperature dependenceAgain, the temperature dependence of the ERH may berationalized with classical nucleation theory and eqn (2).One possible explanation for the efflorescence inhibitionillustrated in Fig. 4 and 5 is that the particles which were richin organics (0.20 and 0.25 mole fractions) and at lowertemperatures do not reach a large supersaturation with respectto ammonium sulfate (S(NH4)2SO4), possibly due to non-idealsolution behaviour. As a result the driving force for efflorescence(i.e. S variable in eqn (2)) is smaller at the lower temperaturesand in the most concentrated particles. To explore this wecalculated the S(NH4)2SO4reached in all experimentswhere efflorescence was observed using the e-AIM model.51These results are represented by solid symbols in Fig. 6.Although e-AIM assumes that ion–organic component inter-actions are minor, it has been used to represent this system in1 : 1 mole ratio previously with some success.52In general,supersaturations with respect to ammonium sulfate rangedfrom 30 to approximately 75. Next we calculated lower limitsto the S(NH4)2SO4reached in the experiments where efflorescencewas not observed. The results from these calculations arerepresented by the open symbols in Fig. 6. To calculate theselower limits we used the e-AIM model and an RH of 10%. Inthe experiments where no efflorescence was observed S(NH4)2SO4-values greater than 150 were reached which is clearly largerthan the S(NH4)2SO4-values reached in the experiments whereefflorescence was observed. We conclude that low S(NH4)2SO4-valuesatthelowesttemperaturesandinparticleswithgreaterorganiccomponent loadings cannot explain our observations.53–56The inhibition trend at low temperatures observed in ourexperiments may be related to crystallization studies inaqueous citric acid solutions carried out 40 years ago.33Theauthors argued that the decrease in nucleation events at lowertemperatures was due to an increase in viscosity which wouldincrease DG0in eqn (2). An increase in viscosity could alsoexplain the observed inhibition of efflorescence of mixedammonium sulfate–citric acid particles at the lowest temperaturesin our experiments.3.5 Possible connections with glass transition temperaturesAs mentioned in the Introduction, recent work has shown thatboth organic particles and mixed organic–inorganic particlescan form glasses at low temperatures of atmosphericrelevance.35,36Prior to our work there have not been anyreports of glass transition temperatures of citric acid–ammoniumsulfate–water solutions. Below we investigated the glass transitiontemperatures of these solutions using a DSC and relate thesevalues to the observed efflorescence conditions.Shown in Fig. 7 (solid symbols) are the glass transitiontemperatures, Tg, we determined for mixed citric acid–ammoniumsulfate–water solutions as a function of total mass fractionFig. 5 Efflorescence data from this study combined with calculatedglass transition data (see text for further details). The dotted linerepresents the Tgof a binary mixture of water and citric acid(XCA,dry= 1.0). The dash-dot line and open symbols correspond toternary mixtures with XCA,dry= 0.7.Fig. 6 The supersaturation required to induce efflorescence, Scritical,observed in our experiments. The solid points correspond to observedefflorescence events while unfilled points represent the lower limitassociated with non-efflorescing mixtures.Downloaded by The University of British Columbia Library on 18 April 2011Published on 16 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00572JView Online12264 Phys. Chem. Chem. Phys., 2010, 12, 12259–12266 This journal iscthe Owner Societies 2010solutes. Each Tgis a mean value of two single measurements.Note we were unable to measure glass transition temperaturesfor a wider range of solute mass fractions or for differentXCA,dry-values since in these solutions either ice or ammoniumsulfate crystallization was observed before reaching the glassstate. The solid line through the square symbols is aparameterization of the experimental data for XCA,dry= 0.7using the Gordon–Taylor equation.57Gordon and Taylor firstpostulated an equation which allows computing the glasstransition temperature of aqueous solutions, Tg, as a functionof the mass fraction of the solute, w2:TgðwÞ¼w1Tg1þ1kw2Tg2w1þ1kw2; ð3Þwhere w1is the mass fraction of water, Tg1and Tg2are theglass transition temperatures of pure water and of the solute,respectively. For XCA,dryvalues of 0.7 we fit the experimentaldata to this equation using Tg1as 136 K.58,59The parametersfrom the fit are Tg2= 273.6 K and k = 4.383.Also shown in Fig. 7 (dashed line) is a parameterization ofthe glass transition temperatures of citric acid–water solutionstaken from the literature.41The citric acid–water datacorrespond to XCA,dry= 1.0.In Fig. 5, we have plotted the glass transition temperaturesfor XCA,dry= 1.0 and 0.7 from Fig. 7 in RH–temperaturespace. For both the binary and ternary systems we used theextended-AIMmodeltorelatethecompositionofthemixturesto RH. Note that the Tg-values for XCA,dry= 0.5 and 0.3shown in Fig. 7 have not been included in Fig. 5, since theextended-AIM model is limited to temperatures above 180 K.The Tg-values for XCA,dry= 1.0 and 0.7 shown in Fig. 5shouldbeupperlimitstothe Tg-valuesforXCA,dry=0.0to0.3(which is the composition range used in the efflorescenceexperiments). This is because the addition of ammoniumsulfate decreases the TgofthesystemasshowninFig.5and7.As a result the Tg-values shown in Fig. 5 are insufficient toargue that glass formation is responsible for the inhibition ofefflorescence observed in our studies with XCA,dry= 0.0 to 0.3.Nevertheless it is interesting to note that the glass transitiontemperatures shown are very close to the temperatures andRH values where we observed inhibition of efflorescence.3.6 Atmospheric implicationsThis study has shown that relatively small quantities of citricacid can completely suppress efflorescence of ammoniumsulfate at low temperatures. For example, Fig. 4 suggests thatat temperatures below 233 K, mixed citric acid–ammoniumsulfate particles will not effloresce if XCA,dryZ 0.2. Thiscorresponds to a dry organic mass fraction (organic mass/(organic mass + sulfate mass)) of Z0.33. Recent field studiesusing single particle mass spectrometry suggests that the dryorganic mass fraction in the upper troposphere is 0.3 to 0.8.60If we assume that 60% to 90% of the total organics mass iswater soluble61,62in the free troposphere, we obtained a watersoluble organic mass fraction (organic mass/(organic mass +sulfate mass)) of 0.18 to 0.72. It is interesting to notethat the dry organic mass fraction where we see inhibition ofefflorescence (Z0.33) is within the composition range ofimportance inthe atmosphere(0.18to 0.72). Thecurrent studyis only a first step toward understanding the ERH propertiesof mixed organic–inorganic particles in the middle andupper troposphere. Additional studies are required on othermodel organic systems. Nevertheless our data do point outthat the efflorescence properties of mixed organic–inorganicparticles can be different at low temperatures comparedto room temperature and only small amounts of water solubleorganic material may be needed to inhibit efflorescenceof ammonium sulfate. As a result, particles in the uppertroposphere may be more likely to remain in the liquid statethan previously thought and solid ammonium sulfate may beless likely to participate in heterogeneous ice nucleation in theupper troposphere.As mentioned above, we see a different trend to thatobserved by Wise et al.25who used particles composed ofpalmitic acid and ammonium sulfate. The palmitic acid resultsmay be more relevant to conditions where the organics areinsoluble in water. Our citric acid results may bemore relevantfor conditions where the organics are water soluble and do noteffloresce (i.e. remain in the liquid state or glass state) foratmospheric conditions.4. ConclusionsWe studied the efflorescence properties of mixed citricacid–ammonium sulfate particles as a function of temperatureto better understand the efflorescence properties of mixedorganic–inorganic particles in the middle and upper troposphere.Our data for 293 K illustrate that the addition of citric aciddecreases the ERH of ammonium sulfate in the particles,which is consistent with the trends observed with other highlyoxygenated organic systems. At low temperatures the trend isqualitatively the same, but at these low temperatures efflorescencecan be inhibited by smaller concentrations of citric acid.Fig. 7 Glass transition temperatures of citric acid–ammonium sulfatesolutions as a function of the total mass fraction of the solutes. Squares:XCA,dry= 0.7; circle: XCA,dry= 0.5 and triangle: XCA,dry=0.3.Thedash-dotlineisafittothedataforXCA,dry= 0.7 using eqn (3).The dashed line is a parameterization of the glass transition temperaturesof citric acid–water solutions taken from the literature (see the text formore details).Downloaded by The University of British Columbia Library on 18 April 2011Published on 16 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00572JView OnlineThis journal iscthe Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12259–12266 12265In other words, this study shows that at low temperaturesrelatively small quantities of citric acid can completelysuppress the efflorescence of ammonium sulfate. For exampleat temperatureso250 K a citric acid dry mole fraction of only0.2 is needed to inhibit efflorescence ammonium sulfate in theparticles. This corresponds to a dry organic mass fraction ofonly 0.33. In the upper troposphere the dry mass fractionof water soluble organics is often larger than this value.Additional studies are required on other model organicsystems. Nevertheless our data do point out that theefflorescence properties of mixed organic–inorganic particlescan be different at low temperatures compared to roomtemperature and only small amounts of water soluble organicmaterial may be needed to inhibit efflorescence of ammoniumsulfate. As a result, particles in the upper troposphere may bemore likely to remain in the liquid state than previouslythought and solid ammonium sulfate may be less likely toparticipate in heterogeneous ice nucleation in the uppertroposphere.AcknowledgementsThis research was supported by the Canadian Foundation forClimate and Atmospheric Science (CFCAS), the NationalSciences and Engineering Research Council of Canada(NSERC), and the Canada Research Chair Program. 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