chemically-resolved volatility measurements of organic aerosol...

Chemically-Resolved VolatilityMeasurements of Organic Aerosolfrom Different SourcesJ . A . H U F F M A N , † , ‡ K . S . D O C H E R T Y , †

C . M O H R , † , § M . J . C U B I S O N , †

I . M . U L B R I C H , † , ‡ P . J . Z I E M A N N , |

T . B . O N A S C H , ⊥ A N D J . L . J I M E N E Z * , † , ‡

Cooperative Institute for Research in Environmental Sciences(CIRES), Boulder, Colorado; Department of Chemistry andBiochemistry, University of Colorado, Boulder, Colorado; AirPollution Research Center, University of California-Riverside,Riverside, California; and Aerodyne Research, Inc.,Billerica, Massachusetts

Received December 13, 2008. Revised manuscript receivedMay 14, 2009. Accepted May 23, 2009.

A newly modified fast temperature-stepping thermodenuder(TD) was coupled to a High Resolution Time-of-Flight AerosolMass Spectrometer for rapid determination of chemicallyresolved volatility of organic aerosols (OA) emitted from individualsources. The TD-AMS system was used to characterizeprimary OA (POA) from biomass burning, trash burning surrogates(paper and plastic), and meat cooking as well as chamber-generated secondary OA (SOA) from R-pinene and gasolinevapor. Almost all atmospheric models represent POA asnonvolatile, with no allowance for evaporation upon heatingor dilution, or condensation upon cooling. Our resultsindicate that all OAs observed show semivolatile behaviorand that most POAs characterized here were at least as volatileas SOA measured in urban environments. Biomass-burningOA (BBOA) exhibited a wide range of volatilities, but more oftenshowed volatility similar to urban OA. Paper-burningresembles some types of BBOA because of its relativelyhigh volatility and intermediate atomic oxygen-to-carbon (O/C) ratio, while meat-cooking OAs (MCOA) have consistently lowervolatility than ambient OA. Chamber-generated SOA underthe relatively high concentrations used in traditional experimentswas significantly more volatile than urban SOA, challengingextrapolation of traditional laboratory volatility measurementsto the atmosphere. Most OAs sampled show increasingO/C ratio and decreasing H/C (hydrogen-to-carbon) ratio withtemperature, further indicating that more oxygenated OAcomponents are typically less volatile. Future experimentsshould systematically explore a wider range of massconcentrations to more fully characterize the volatilitydistributions of these OAs.

1. IntroductionOrganic aerosol (OA) comprises an important fraction of thefine particle mass at most locations (1). The volatility of OAdetermines the likelihood that a compound will exist in thegas or particle phase, and thus strongly influences theconcentration and lifetimes of OA through governing primaryOA (POA) gas-particle partitioning and secondary OA (SOA)formation. Deposition rates of chemical species due to bothwet and dry processes are greatly influenced by the phaseof the species (2), and chemical mechanisms and rates ofgas, liquid, and heterogeneous reactions of the same moleculecan be very different. Many instruments can have samplingbiases as a result of particle evaporation or condensation (3)and aircraft inlets are particularly susceptible to losses fromheating (4). An accurate understanding of OA volatility istherefore important to properly predict the chemistry andphase-partitioning of both natural and anthropogenic OAspecies, and to properly interpret OA measurements.

POA, emitted into the atmosphere directly in the particlephase, has long been assumed to be nonvolatile in atmo-spheric models. Thus, the POA mass in models does notchange when air masses are cooled, heated, or diluted, andPOA is not considered a source of gas-phase species availablefor atmospheric oxidation and potential condensation as SOA.Recently, this assumption has been challenged (5-7), mostnotably by Robinson et al. (5), who suggested that evaporationof semivolatile POA is an important source of gas-phasematerial for SOA production. Meat-cooking emissions of POA(MCOA) can be a significant contributor to ambient OA bysome estimates, accounting for as much fine POA mass asvehicles and up to 21% of the fine primary organic mass (8)in some locations. Biomass burning POA emitted from wildand agricultural fires and from wood-burning stoves (BBOA)can also be an important fraction of the total OA under manyconditions. For example, biomass burning accounted forapproximately 16% of the organic carbon in Mexico Cityduring the MILAGRO 2006 campaign (9). POA from trashburning (TBOA) may be an important source in developingcountries. The volatilities of these different types of POA arepoorly constrained at present. SOA can comprise a largefraction of the ambient OA even in urban areas (10) and issignificantly underestimated by current models in manyenvironments (11, 12). While the semivolatility of SOA hasbeen included in models for more than a decade, the relevantproperties remain poorly constrained. Recently, Biswas etal. (13) showed that the production of reactive oxygen species,a surrogate for particle toxicity, is greatly reduced when thesemivolatile fraction was removed from combustion exhaustparticles, suggesting that this fraction may be more directlyassociated with human health effects.

Publications on aerosol volatility date back to at leastthe 1960s (14) and several groups have utilized heated tube(“thermodenuder”) volatility measurements to infer particlecomposition by, for example, measuring changes in sizedistributions (e.g., ref 15). Real-time chemically resolvedmeasurements of OA volatility, however, have not beenavailable until recently (16). This study applies this recentlydeveloped methodology to direct measurements of chemi-cally resolved source volatility from BBOA, MCOA, TBOA,and chamber SOA.

2. Experimental SectionA recently developed thermodenuder (TD) (16) was placedupstream of a High-Resolution Time-of-Flight Aerosol MassSpectrometer (AMS, Aerodyne Research) (17, 18) for several

* Corresponding author phone +1-303-492-3557; e-mail:[email protected].

† Cooperative Institute for Research in Environmental Sciences(CIRES).

‡ Department of Chemistry and Biochemistry, University ofColorado.

§ Now at Paul Scherrer Institute (PSI), Villigen, Switzerland.| Air Pollution Research Center, University of California-Riverside.⊥ Aerodyne Research, Inc.

Environ. Sci. Technol. 2009, 43, 5351–5357

10.1021/es803539d CCC: $40.75 2009 American Chemical Society VOL. 43, NO. 14, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5351

Published on Web 06/12/2009

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studies of aerosol sources. The TD consists of a 50-cm long,2.5-cm OD stainless steel tube heated with a 3-zone tem-perature controller through which the sample air flows,leading to evaporation of some of the particle mass. A 50-cmcharcoal diffusion denuder follows the heating section toadsorb vapors and prevent recondensation. A rapid-switchingvalve system toggles the AMS between sampling undenudedambient air and TD-processed air every 1-10 min in orderto acquire a rapidly interleaved data set. The TD-AMS systemcan measure from ambient temperature to ∼230 °C in ∼1 h.Detailed descriptions of both TD characterization andoperation (7, 16, 19) and the effect of residence time onobserved evaporation (7, 20) are available elsewhere. In eachstudy described herein, air was sampled through a PM2.5

cyclone and then through both the TD and a 1/4 inch bypassline at 0.6 lpm each. The TD was operated with the same setof instrumental settings (e.g., sample line configuration, flowrate, valve switching pattern) during all experiments toprovide self-consistent data. All data shown here werecorrected for experimentally determined particle numberlosses (16), due mostly to diffusion and thermophoresis,which increase with temperature from 5% at ambient to 20%at 230 °C. As a result, the decrease in MFR as a function oftemperature is due primarily to particle mass evaporation.Comparisons of ammonium sulfate thermograms fromlaboratory and field experiments (7) showed similar results,giving confidence that the instrument behaves similarly inboth environments. Changes in particle phase can in principlelead to collection efficiency (CE) changes as a function oftemperature, although this issue appears to be small forambient particles other than ammonium sulfate as discussedin previous publications (7, 16). It is also possible that particlesize changes have an effect on the reported thermograms ifsignificant mass enters or leaves the AMS size transmissionwindow after evaporation (7), however this effect is estimatedto be small for the experiments reported here based on themeasured size distributions.

The TD-AMS was utilized during three aerosol sourcemeasurement campaigns. First, volatility of BBOA wasmeasured during FLAME-1 (Fire Lab At Missoula Experiment,phase 1), which took place June 2006 at the US Forest ServiceFire Sciences Lab (FSL) in Missoula, MT. The study analyzedhighly diluted (∼× 104) smoke from burns of ∼18 differentbiomass specimens over one week, attempting to simulatewildfires. The smoke was confined to a large chamber andOA from flaming and smoldering phases of combustion ofeach biomass were allowed to mix and stabilize before startingthe TD-AMS cycle. FLAME-1 and the FSL chamber aredescribed in detail elsewhere (e.g. 21).

Second, measurements of OA from meat-cooking andtrash-burning emission sources were performed in a 240-m3

aluminum shed located outside of Boulder, CO (22). Allinstruments were located inside the shed and sampled froman inlet placed near the top of the enclosed space. Emissionswere sampled at dilution factors of ∼100-1000, but atconcentrations that were at least 10× the low rural back-ground levels (∼1 µg m-3 OA). Meats were cooked on apropane grill under conditions similar to home-cookingprocedures, and the air in the shed was purged betweenexperiments by opening the door and using exhaust fans toexchange the air until the concentrations returned tobackground levels. A number of meats were cooked indi-vidually (22), of which four with the highest fat content andemissions are shown here because they are expected to bemore important contributors in the real world, and becausetheir signals were sufficiently high to provide good signal-to-noise and stay higher than ten times the ambientbackground OA during the TD cycle. Additionally, severaltypes of plastic and paper were burned as examples ofcomponents of trash-burning. POA generated by burning

plastic No. 2 (high-density polyethylene) and brown papersupermarket-style bags are shown.

Finally, SOA was generated from two well-studied VOCoxidation reactions in an environmental chamber (23) locatedin the Ziemann laboratory at UC-Riverside. In the firstreactionR-pinene reacted with O3, and in the second gasolinevapor was photooxidized in the presence of NOx (producingSOA that should be dominated by reaction products fromaromatic precursors (24)). These reactions produced typicalbiogenic and anthropogenic SOA and had total aerosolconcentrations of 500 and 60 µg/m3, respectively. Recentstudies have shown that SOA formed with lower concentra-tions is less volatile (e.g., ref 25), however, the conditionsused here are typical of those used for most SOA modeldevelopment until recently (e.g., ref 24). Characterization ofthis SOA is therefore still of interest, since these models arewidely used at present and will likely continue to be used foryears.

The relationship of the relative volatility determined witha TD and the mass concentration in the experiment has beenstudied by Faulhaber et al. (19), who observed 5-10 °C shiftsin the thermograms of pure-component particles afterchanging mass concentration over a large range (∼0-600µg/m3). TD thermograms are partially a measure of relativevolatility, and since a larger OA concentration also carrieswith it a larger concentration of semivolatile VOCs (SVOCs),the ways in which thermograms change as mass concentra-tion is reduced can become complex. This effect is experi-mentally characterized below and found to be smaller thanthe differences caused by chemical composition in the fewcases for which data is available. We also note that thevolatility of POA at concentrations higher than ambient is ofdirect interest since (a) POA emission factors have often beendetermined at elevated concentrations similar to those usedhere, and such factors may be too high when dilution toambient levels occurs with volatility being the key propertythat determines the reduction in emission factor; and (b)during the equilibration to ambient levels, the evaporatedmaterial is a source of SVOCs with saturation conditions thatare only 1-2 orders of magnitude above ambient OA levels,and which should be very efficient SOA precursors (5).

3. Results and Discussion3.1. Biomass-Burning Organic Aerosol Volatility. PrimaryBBOA emissions were measured as part of FLAME-1, andthermograms (mass fraction remaining, MFR, vs calibratedcenterline TD temperature) of total OA mass for two examplesare shown in Figure 1a. These two biomasses (dried lodgepolepine needles and sticks, and a mix of Utah sage andrabbitbrush sticks and foliage) bracket the range of behaviorsthat have been measured by the TD-AMS system for any OA,with evaporation rates within the TD of 1.6 and 0.2% K-1

near ambient temperature, respectively (measured betweenambient and 50 °C). Figure 1b shows the same biomasses,but using the high resolution marker C2H4O2

+ (m/z 60) forlevoglucosan and similar species such as galactosan andmannosan (21) (totaling ∼3 × levoglucosan in Mexico City(26), which is often used as an AMS tracer for biomassburning. Because the shapes of most thermograms aresmooth, we use the temperature at which 50% of the masshas evaporated (T50) as a concise metric for comparingexperiments. Vapor pressure distributions or lumped sur-rogate enthalpies of vaporization can be estimated from theseresults with additional assumptions, which however intro-duce several complications (19, 27, 28). In this first work, wefocus on a qualitative survey of volatility trends withcomparisons to those for ambient OA in Mexico City (7). TheTD-AMS was also deployed to Riverside, CA with results verysimilar to those in Mexico City (7). We will explore thequantitative interpretation of the results herein in future work.

5352 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 14, 2009

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Figure 1c summarizes results for the 18 different BBOAsin a histogram of T50 values. While BBOA produced by burninglodgepole pine (T50 ) 61 °C) exhibits the lowest MFR as afunction of temperature for all OAs analyzed, many BBOAsfrom FLAME-1 also show relatively high volatility (T50 < 100°C). Conversely, BBOA produced by burning sage andrabbitbrush (T50 ) 248 °C) shows the highest MFR of any OAmeasured to date with the TD-AMS. A large variability inBBOA volatility has also been observed in ambient air, withMILAGRO BBOA being close to the high volatility end (Figure1a and ref 7). Grieshop et al. (29) also recently reported therelatively high volatility of woodstove BBOA emissions, whichhad T50 values of ∼42 °C in a TD setup similar to ours. Theyalso showed that unit resolution m/z 60 was one of the morevolatile components in the MS from both fresh and agedwood-burning emissions. Aircraft measurements duringICARTT reported a BBOA with much lower volatility (MFR∼25% at 400 °C (30)), though the much shorter RT (∼1s)used for this study would lead to higher MFR and complicatesdirect comparison.

The more volatile BBOAs were generally dominated bysmoldering combustion, while the less volatile ones were

more influenced by flaming combustion (characterized byboth visual inspection and modified combustion efficiency).T50 values are plotted versus the average OA concentrationin Figure 1d to illustrate the correlation of high OA-emittingsmoldering-dominated burns with relatively high volatilityBBOA, and less OA-emitting flaming-dominated burns withlower volatility BBOA. Note that the higher concentrationsenhance gas-to-particle partitioning, thus accentuating theobserved trend. However, the main cause for the observedvolatility differences are the differences in chemical com-position, as repeat measurements at some temperatures atdifferent concentration levels show small differences in MFR,and the large differences in MFR are apparent for differentbiomasses when sampling similar concentrations (FigureS1-S3 of the Supporting Information). The volatility of mostobserved BBOA under the conditions of FLAME-1 is similarto or higher than that of urban OA, although much less volatileBBOAs are possible.

Oja and Suuberg (31) report the vapor pressure oflevoglucosan at 68 °C as 1 × 10-4 mbar, which correspondsto a saturation concentration of 600 µg m-3, indicating thatlevoglucosan should appear as relatively volatile within the

FIGURE 1. (a) Total primary BBOA thermograms showing median mass fraction remaining (MFR) vs TD temperature for BBOA fromthe FLAME-1 experiments. Spline fits are shown as guides to the eye. Also shown are the average thermograms for total OA inMexico City during MILAGRO, and the thermogram of ambient OA during a 3-h morning period in that campaign when BBOA was thedominant OA type due to heavy impact from a forest fire plume. (b) Thermograms of BBOA marker ion C2H4O2

+ (m/z 60) for samesamples as in Part (a). The C2H4O2

+ signal was 6% the total OA signal for the Lodgepole Pine BBOA and 1% for Sage/RabbitbrushBBOA. (c) Histogram of T50 values for 18 different BBOAs measured during FLAME-1. (d) Log-log plot of OA volatility (T50) vs averageOA mass concentration for each burn. Spline curve is added as a guide to the eye.

VOL. 43, NO. 14, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5353

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TD-AMS. As expected, therefore, the marker for levoglucosan-like species (C2H4O2

+) is consistently more volatile than thetotal OA, but the T50 for each are highly correlated forindividual sources (Figure 2). The composition of the speciesthat give rise to this marker signal at m/z 60 does not changegreatly between biomasses (21), thus it is likely that thedifferences in T50 of m/z 60 for different biomasses are dueto reduced evaporation of the same levoglucosan-like species

when embedded in a less-volatile matrix. The high correlationalso suggests that the C2H4O2

+ ion can be used as a markerin ambient measurements to estimate the total BBOAvolatility when the signal at this ion is dominated by BBOA.

Atomic oxygen- and hydrogen-to-carbon (O/C, H/C)ratios were estimated directly from the high-resolution massspectra using the technique developed by Aiken et al. (32, 33).The evolution of these ratios with TD temperature after partialevaporation of the two BBOAs discussed here are shown inFigure 3a,b. Both O/C curves show an average increase fromambient temperature to the hottest set-point, although thecurve for the lodgepole pine dips between ambient tem-peratures and ∼65 °C. H/C decreases with temperature inboth cases. This indicates that the most oxidized species ineach BBOA are less volatile than the more reduced species.With increasing TD temperature the more volatile speciesevaporate, leaving less volatile, more oxidized species in theparticles. The overall lower volatility of the sage andrabbitbrush BBOA is consistent with its higher O/C ratio.The increase in O/C and decrease in H/C with temperaturefor both BBOAs is similar to ambient observations madeduring two megacity studies, which showed that POA (fromBBOA and urban sources) was of similar or higher volatilityto SOA, and that the most oxygenated OA (aged SOA) wasthe least volatile (7). While additional factors (i.e., molecularweight) affect the volatility of a given OA mixture, our resultsshow the degree of oxidation (i.e., O/C) is an important factorin determining aerosol volatility. We note that we cannotrule out the possibility that chemical reactions induced by

FIGURE 3. Elemental composition of the OAs studied here as a function of TD temperature. Left panels show oxygen-to-carbon (O/C)ratios and right panels show hydrogen-to-carbon (H/C) ratios. (a,b) Primary BBOAs and TBOAs; (c,d) chamber SOAs and primaryMCOAs. The absolute accuracies of the O/C and H/C estimates are estimated at 31% and 10%, respectively, while the relativeprecision is ( 3% in each case (33).

FIGURE 2. Relationship of T50 for the C2H4O2+ BBOA marker ion

(m/z 60) and total BBOA for 18 different primary BBOAsmeasured during FLAME-1.


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TD heating, especially at the highest temperatures, couldalter the particle composition, and thereby affect the observedtrends.

3.2. Meat-Cooking and Trash-Burning OA Volatility. Asdescribed by Mohr et al. (22), the MCOA mass spectra arehydrocarbon-like and dominated by reduced ions (CxHy).C3H7

+ (m/z 43) and C4H7+ (m/z 55) are consistently the

most abundant ions in the MS, while the relative contri-bution from oxygenated ions is considerably lower. TheO/C ratios of ∼0.11-0.14 are similar to those of typicalfatty acids (e.g. oleic acids) and much lower than forambient urban OOA. The C3H3O+ ion (m/z 55) was themost abundant oxygenated ion present in each ofthe MCOAs shown here, with 40-50% of the signal of thehighest reduced ion peak. The MS of plastic TBOA weresimilar to those of MCOA, with even lower relativeabundance of oxygenated ions and lower O/C thanthe meats (0.08) (22). In contrast to the MCOA, however,in the MS of TBOA produced by burning plastic No. 2(high-density polyethylene, HDPE) C2H3O+ (m/z 43) wasthe most abundant oxygenated peak, with approximately16% of the intensity of the C3H7

+ peak. Results from plasticNo. 4 (low-density polyethylene, LDPE) are almost identical(not shown). Paper burning emissions were markedlydifferent from these, with an MS similar to BBOA producedfrom pine burning in FLAME-1 in which both reduced andoxygenated ions make significant contributions, the O/C(0.31) is an intermediate value, and C2H4O2

+ (m/z 60) wasa dominant peak in the spectrum. None of the sourcessampled here had significant emissions of nonrefractoryinorganic aerosol.

Figure 4a-d shows thermograms of total OA from eachsource as well as of major individual high-resolution (HR)ions, and compare them to the average urban aerosol inMexico City during MILAGRO (7, 16). The three MCOAsshow complex volatility behavior, with MFRs initiallysimilar to urban OA (bratwurst MCOA evaporates at rateof 0.7%K-1 near ambient T) but then lower as the TDtemperature was increased. This suggests a bimodalvolatility distribution with the majority of the mass havinglow volatility. This is not unexpected, since compositionof MCOA particles are thought to be dominated by low-volatility fatty acids that evaporate at high cookingtemperatures and then condense (22, 34).

Consistent with the ambient urban OA observations(7), the differences in volatility between CxHy

+ ions aresmaller than for CxHyOz

+ ions. The C4H7+ ion is shown in

each of the MCOA thermograms to represent the reducedions, because it is the most abundant ion in each of theMCOA spectra and its thermogram has a similar profile tothat of the total MCOA. Most of the important CxHy

+ andCxHyOz

+ ions have similar thermograms and slopes asC4H7

+, with positive or negative offsets. The C2H4O2+ ion

is also shown in MCOA thermograms, where both it andC3H5O2

+ (not shown) exhibit unique profiles when com-pared to those of other CxHyOz

+ ions. This differenceindicates that these ions arise from a different set ofcompounds in the MCOA. The dependence of MCOA O/Cand H/C ratios on TD temperature are shown in Figure3c,d, and are very similar to each other, consistent withthe high similarity in their mass spectra (22). This suggeststhat the major components of MCOA produced by cookingdifferent meats are similar. The atomic ratios of MCOAsalso show little change with increasing temperature, exceptat the hottest values (>175 °C), where again there is anincrease in O/C and a decrease in H/C.

TBOA produced by burning paper is considerably morevolatile than the average ambient urban OA, consistent withthe similarity of its MS to that of pine-burning OA fromFLAME-1 (22). TBOA from plastic shows approximately the

same volatility (0.8% K-1 near ambient T) as average urbanOA, but with a smaller residual mass fraction at the highesttemperatures. The thermograms of the reduced ions in TBOAMS (Figure 4d) again show a narrow spread, whereas thoseof the oxygenated ions are more varied. The C2H4O2

+ ion inthe MS from paper-burning OA (m/z 60, and similarlyC3H5O2

+, m/z 73, also a BBOA tracer ion, not shown) decrea-ses faster with increasing temperature than the total OA,consistent with the observations from BBOA (Figure 2). Thepaper burning experiment was unique because the ther-mograms of the oxygenated ions (excluding CO2

+), althoughmore variable with respect to one another than the CxHy

+

ions also showed consistently higher volatility than the CxHy+

ions. The H/C ratios for both TBOAs shows a slight decreasewith temperature, while the O/C ratio of plastic TBOA behavessimilar to those of the MCOAs, being nearly constant witha small increase at the highest temperatures (Figure 3a,b).The O/C ratio of the paper-burning TBOA initially decreases,again similar to the behavior of the pine BBOA. Unfortunately,data at higher temperatures are not available.

3.3. Volatility of Chamber SOA. Figure 4e shows thethermograms of SOA formed from both chamber reactions,which appear to be significantly more volatile than theaverage OA from MILAGRO and the ambient oxygenated OA(OOA, an SOA surrogate: refs 1,10,12,18,22,33 and 35) fromMexico City and Riverside, CA (7). Both types of SOA had T50

values of 84 °C or less, similar to ambient urban OA duringperiods strongly influenced by urban POA, but much lowerthan the 106 °C value measured for the average urban totalOA and 110-130 °C for urban SOA (7). This indicates,therefore, that SOA formed from these two chamber reactionsunder conditions typical of those used to develop traditionalSOA models (24) is more volatile than the SOA measured inpolluted urban environments. Part of this difference may bedue to higher OA concentrations in the chamber, as discussedabove. However, ambient submicrometer OA concentrationin Mexico City was ∼1/3 of that present during our gasolinephotooxidation experiment. It is thus unlikely that this effectcan explain the magnitude of the differences observed.Rather, additional aging or the influence of other precursorsand particle-phase reactions (e.g., ref 35) may be responsiblefor the lower volatility of ambient SOA. Stanier et al. (28)have recently studied the effects of temperature on SOAformed from R-pinene ozonolysis, observing evaporation of0.4 to 3.6% K-1 near ambient temperature, which encom-passes the value of 1% K-1 reported here. Our observedevaporation of 0.9% K-1 near ambient temperatures for SOAformed from the photooxidation of gasoline vapor is lowerthan the 1.9 to 2.4% K-1 range of values calculated for tolueneSOA from observations by Offenberg et al. (27). However,the difference may be partially due to the longer residencetime used in that study, which was about twice that of ourstudy.

Figure 4f shows that the species associated with theCO2

+ ion in the chamber-generated SOA were considerablymore volatile than in the MILAGRO study, consistent withtrends in total OA. This again supports the conclusion thatambient urban SOA is significantly less volatile than SOAgenerated under conditions of many traditional experi-ments. Using SOA yields and vapor pressures obtainedfrom fits to these chamber data in atmospheric modelswill thus likely overestimate ambient SOA volatility, aswill the use of volatilities obtained from direct measure-mentsonchamber-generatedSOAunderhighconcentrations.

Figure 3c,d shows the estimated average O/C and H/Cratios as a function of temperature for the two chamber-generated SOAs. Both show a slight increase in O/C anddecrease in H/C with increasing temperature, with theSOA formed from gasoline photooxidation SOA being about


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0.1 higher in O/C and lower in H/C compared to thatformed from R-pinene ozonolysis. The O/C ratios ofchamber-generated SOA at ambient temperature are lowerthan those of ambient OOA/SOA (0.5-1 (33)), which is

consistent with the higher volatility of chamber-generatedSOA, given the inverse correlation between oxygenationand volatility observed for most ambient OA and OAsources.

FIGURE 4. Thermograms of total OA and individual high-resolution ions from meat-cooking and trash-burning POA and chamber SOAexperiments. Total OA from MILAGRO is also plotted on each graph for visual comparison. (a) bratwurst (b) ground beef, fatty, (c)chicken breast with skin, (d) paper and plastic burning, (e) total OAs of two chamber-generated SOA reactions compared to ambienturban OA and total ambient OOA (SOA), separated from total OA by positive matrix factorization (PMF) analysis (f) individual ions formore-oxygenated and more-reduced species for chamber-generated SOA, compared to urban OA and CO2

+. The MFR of C2H4O2+ (a)

increases from ∼75-160 °C, possibly due to chemistry or matrix effects in the TD, or changes in AMS collection efficiency.


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AcknowledgmentsThe authors would also like to thank J. Kimmel, E. Dunlea,M. Northway, D. Salcedo, and A. Trimborn for assistance incollecting data and J. Jayne and D. Worsnop for usefuldiscussions. We also thank the organizers of FLAME-1 (S.Kreidenweis, C. Wold, W. Hao, and W. Malm) and the NiwotSource Emissions Study (M. Hannigan and M. Lechner).These studies were partially supported by Grant Nos. EPASTAR R831080, RD-83216101-0, and R833747, DOE (BER, ASPProgram) DE-FG02-05ER63981, NSF ATM-0449815, andATM-0528634, by NASA fellowships NGT5-30516 andNNG05GQ50H. Neither agency has reviewed these resultsand thus no endorsement should be inferred.

Supporting Information AvailableThis material is available free of charge via the Internet athttp://pubs.acs.org.

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Supplementary Information for 1

2

Chemically-Resolved Volatility Measurements of Organic Aerosol from Different 3

Sources 4 5

J.A. Huffman, K.S. Docherty, C. Mohr, M.J. Cubison, I.M. Ulbrich, P.J. Ziemann, T.B. 6

Onasch, and J.L. Jimenez 7

8

9 All Supplementary figures colored by total OA concentration in undenuded state. 10

11

Figure Captions: 12 13

Figure S1: Total primary BBOA thermograms showing median mass fraction remaining 14

(MFR) vs. TD temperature for BBOA from the FLAME-1 experiments. Spline fits are 15

shown as guides to the eye. Also shown are the average thermograms for total OA in 16

Mexico City during MILAGRO, and the thermogram of ambient OA during a 3-hour 17

morning period in that campaign when BBOA was the dominant OA type due to heavy 18

impact from a forest fire plume. 19

20

Figure S2: Total primary BBOA thermograms for emissions from burning additional 21

wood types. (a) Ponderosa pine (duff), (b) Alaskan core tundra (duff), (c) Southern 22

California manzanita, (d) Utah juniper (foliage and sticks). The plots are colored by 23

undenuded mass concentration. In all cases some temperatures are studied at different 24

concentrations, and modest differences in evaporation are observed. 25

26 Figure S3: Thermograms of total OA and individual high-resolution ions from meat-27

cooking and trash-burning POA and chamber SOA experiments, colored by the 28

undenuded mass concentration. Total OA from MILAGRO is also plotted on each graph 29

for visual comparison. (a) bratwurst, (b) paper and plastic burning, (c) chicken breast 30

with skin, (d) total OAs of two chamber-generated SOA reactions compared to ambient 31

urban OA and total ambient OOA (SOA), separated from total OA by positive matrix 32

factorization (PMF) analysis. Some temperatures are studied at different concentrations, 33

and modest differences in evaporation are observed. 34

35

Figure S1 36

1.0

0.8

0.6

0.4

0.2

0.0

Ma

ss F

ractio

n R

em

ain

ing

(M

FR

)

25020015010050TD Temperature (

oC)

Total OA Sage, Rabbitbrush

11 - 19 µg / m3

MILAGRO Average MILAGRO, High BBOA Period Lodgepole Pine

24 - 236 µg / m3

20

18

16

14

12

10

Sage / R

abbitb

rush O

A C

onc.

25020015010050

Lodgepole Pine OA Conc.

37

Figure S2 38

39

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction

Rem

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Utah Juniper (Foliage & Sticks)

5 - 9 µg / m3

9

8

7

6

5

Tota

l OA

Concen

tratio

n (µ

g/m

3)

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Ponderosa Pine Duff

15 - 37 µg / m3

40

35

30

25

20

15

To

tal O

A C

oncentra

tion

(µg/m

3)

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Alaskan Core Tundra Duff

16 - 33 µg / m3

35

30

25

20

15

Tota

l OA

Con

cen

tratio

n (µ

g/m

3)

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Southern CA Manzanita

3.5 - 7.2 µg / m3

7

6

5

4

3

To

tal O

A C

oncen

tratio

n (µ

g/m

3)

(a)

(d)(c)

(b)

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction

Rem

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Utah Juniper (Foliage & Sticks)

5 - 9 µg / m3

9

8

7

6

5

Tota

l OA

Concen

tratio

n (µ

g/m

3)

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Ponderosa Pine Duff

15 - 37 µg / m3

40

35

30

25

20

15

To

tal O

A C

oncentra

tion

(µg/m

3)

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Alaskan Core Tundra Duff

16 - 33 µg / m3

35

30

25

20

15

Tota

l OA

Con

cen

tratio

n (µ

g/m

3)

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

ain

ing (

MF

R)

25020015010050

TD Temperature (oC)

Southern CA Manzanita

3.5 - 7.2 µg / m3

7

6

5

4

3

To

tal O

A C

oncen

tratio

n (µ

g/m

3)

(a)

(d)(c)

(b)

40

Figure S3 41 42

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

ain

ing (

MF

R)

20015010050

Total OA 48 - 375 µg/m3

MILAGRO Total OA

(a): Bratwurst

30025020015010050

Total OA Concentration (µg/m3)

1.0

0.8

0.6

0.4

0.2

0.0

20015010050

TD Temperature (oC)

MILAGRO OOA MILAGRO OA Average

Gasoline + OH, NOx OA 52 - 64 µg / m3

α-pinene + O3 OA 428 - 691 µg / m3

(d): Chamber SOA Total OA

700650600550500450400

µ-Pinene Reaction: Total OA Conc (µg / m3)

6462605856545250

Gasoline Reaction: Total OA Conc (µg / m3)

1.0

0.8

0.6

0.4

0.2

0.0

20015010050

Plastic No. 2 Total OA

1.8 - 25 µg/m3

Paper Total OA

22 - 94 µg/m3

MILAGRO Total OA

(b): Paper, Plastic No. 2

Pla

stic

No. 2

Paper

10080604020

Plastic No. 2 OA Conc.

20151050

Paper OA Conc.

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

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MF

R)

20015010050


(c): Chicken Breast, with Skin

400300200100


1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

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MF

R)

20015010050


MILAGRO Total OA

(a): Bratwurst

30025020015010050


1.0

0.8

0.6

0.4

0.2

0.0

20015010050

TD Temperature (oC)

MILAGRO OOA MILAGRO OA Average

Gasoline + OH, NOx OA 52 - 64 µg / m3

α-pinene + O3 OA 428 - 691 µg / m3

(d): Chamber SOA Total OA

700650600550500450400

µ-Pinene Reaction: Total OA Conc (µg / m3)

6462605856545250

Gasoline Reaction: Total OA Conc (µg / m3)

1.0

0.8

0.6

0.4

0.2

0.0

20015010050

Plastic No. 2 Total OA

1.8 - 25 µg/m3

Paper Total OA

22 - 94 µg/m3

MILAGRO Total OA

(b): Paper, Plastic No. 2

Pla

stic

No. 2

Paper

10080604020

Plastic No. 2 OA Conc.

20151050

Paper OA Conc.

1.0

0.8

0.6

0.4

0.2

0.0

Mass F

raction R

em

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MF

R)

20015010050


(c): Chicken Breast, with Skin

400300200100


chemically-resolved volatility measurements of organic aerosol...

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