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Atmospheric Environment 68 (2013) 326e332

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Atmospheric Environment

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Characterization of atmospheric organic matter using size-exclusionchromatography with inline organic carbon detection

Youliang Wang a, Chao-An Chiu b, Paul Westerhoff b, Kalliat T. Valsaraj c, Pierre Herckes a,*

aDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USAb School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, USAcCain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA, USA

h i g h l i g h t s

< Size-exclusion chromatography with inline carbon detection was applied to atmospheric samples.< A substantial fraction of atmospheric organic matter is macromolecular.< There is little difference in molecular weight distributions between different events at a particular site.< There are small differences in molecular weight distributions between sites.< The results were confirmed with a second independent size-exclusion organic matter detection set-up.

a r t i c l e i n f o

Article history:Received 18 July 2012Received in revised form15 November 2012Accepted 19 November 2012

Keywords:Cloud chemistryFog chemistryWSOCHULISDissolved organic matter

* Corresponding author. Tel.: þ1 480 965 4497; faxE-mail address: pierre.herckes@asu.edu (P. Hercke

1352-2310/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.atmosenv.2012.11.049

a b s t r a c t

The atmosphere contains a substantial amount of water-soluble organic material in aerosols, clouds andfogs. Despite years of efforts, little is known on the structure, composition and properties of this organicmatter with most studies focusing on individual species while the bulk of the organic matter remainspoorly characterized.

In this work high-performance size-exclusion chromatography coupled with inline organic carbondetection (SEC-DOC) is used to characterize organic matter in fogs, clouds and aerosols collected inFresno (CA), Whistler (BC), Davis (CA) and Selinsgrove (PA). The molecular weight distributions showeda fractional overlap of atmospheric samples and terrestrial fulvic acids although for clouds and aerosolsthe smaller molecular weight (MW) material is dominant. This smaller MW material is clearly resolved.Cloud and fog samples showed a larger fraction of small molecular weight organic species compared tothe water-soluble fraction of aerosols, consistent with the partitioning of small molecular weight volatilespecies into the atmospheric aqueous phase. There are overall little differences between different sitesfor a same type of sample. These results obtained by one analytical set-up were confirmed with a secondsize-exclusion chromatography set-up using a different column and detection system. Size distributionsfor the same sampling location showed little inter-event variability and water-soluble organic carbon(WSOC) samples were slightly shifted toward larger sizes compared to clouds and fogs, consistent withan important contribution of volatile species to the latter ones. Cloud and aerosol samples contributed toa significant fraction (up to 21% of dissolved organic carbon (DOC)) of the macromolecular scale material.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Clouds and fogs play an important role in the processing ofatmospheric trace gases and particles. They integrate gas andparticulate species and might transform them into novel materialsor deposit them through precipitation processes. Clouds and fogs

: þ1 480 965 2747.s).

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can contain substantial concentrations (1e200 mgC L�1) of organiccarbon and up to 77% of the total organic carbon (TOC) in cloudand fog water is dissolved organic carbon (DOC), operationallydefined as passing through a filter (Herckes et al., 2002a). Severalstudies have characterized DOC in fog and cloud water to elucidateits chemical composition. A variety of analytical approaches havebeen used to improve our understanding of cloud and fog DOCmaterial as well as the WSOC present in atmospheric particulate(Kiss et al., 2001; Fuzzi et al., 2002; Herckes et al., 2002b; Collettet al., 2008; Raja et al., 2009; Mazzoleni et al., 2010). Ion-

Y. Wang et al. / Atmospheric Environment 68 (2013) 326e332 327

exchange chromatography has been proposed by Decesari andcoworkers (Decesari et al., 2000) as a means to characterize DOCand WSOC by separating it into different categories based on themolecular charge of the species present. While the method hasbeen widely applied (Fuzzi et al., 2001; Mayol-Bracero et al., 2002)and it provided interesting insights into the variability of atmo-spheric organic matter, the interpretation of the results had beenamatter of discussion (Chang et al., 2005; Decesari et al., 2005). Theseparated fractions were quantified or analyzed using nuclearmagnetic resonance (NMR) spectroscopy and useful insights weregained. Size-exclusion chromatography (SEC) combined witha XAD-8 resin column was applied by Sullivan and coworkers toseparate the WSOC component of ambient aerosols into acid,neutral and basic functional groups and quantified by TOC detec-tion (Sullivan and Weber, 2006). Other approaches to characterizeDOC andWSOC included an elaborate isolation scheme followed byFourier Transform Infrared Spectroscopy and NMR spectroscopy(Herckes et al., 2007). A different set of isolation/characterizationtechniques aimed at separating and hence classifying the organicmatter using molecular weight rather than chemical properties.These works included ultrafiltration (Herckes et al., 2002a) orelectrospray mass spectrometry (Kiss et al., 2003). Each techniquehaving its own challenges, all studies suggest that a part of theorganic matter has substantial molecular weight (>500 Da).

Similar challenges to characterize atmospheric organic matterexist for surface waters where the analytical approach of SEC isused (Pelekani et al., 1999; Lepane et al., 2004; Allpike et al., 2005;Her et al., 2008; Haberkamp et al., 2011). For atmospheric materials,Krivácsy et al. (2000) applied SEC coupled with ultraviolet (UV) andfluorescence detection and concluded that atmospheric humic-likesubstances (HULIS) has similar retention and migration behaviorwith reference humic materials. Samburova et al. (2005) used SECand laser desorption/ionization mass spectrometry (LDI-MS) toconfirm that the upper mass limit of HULIS reached 700 Da. Despiteall these efforts the organic matter remains unknown and theinterpretation challenging and confusing.

The present manuscript aims to contribute to the characteriza-tion of atmospheric organic matter by using awell established size-exclusion chromatography method coupled to inline organiccarbon characterization (SEC-DOC). SEC is a separation methodbased on the ability of an organic compound to enter the pores ofthe stationary phase via molecular diffusion (Her et al., 2002a). Intheory larger molecules have a shorter retention time because theyenter fewer pores than smaller molecules. However, in practicenonideal hydrophobic and electrostatic interactions exists betweenthe solutes and gel matrix which makes the separation not entirelydependent on the molecular size but also influenced by eluentcompositions and columns (Sullivan and Weber, 2006). Eventhough interactions can be minimized by optimizing factors likepH, ionic strength of the eluent, packing materials of columns, theystill can’t be totally excluded (Pelekani et al., 1999). Cautious cali-brations have been conducted to test SEC’s ability of separatingfunctional groups (Sullivan and Weber, 2006). The results showedSEC is not an ideal method to resolve individual species due to peakoverlapping but still can serve as a powerful tool in terms ofproviding overview sample compositions. The method has beenapplied to aerosol, cloud and fog samples and new insights oncomposition variability will be discussed.

2. Experimental

2.1. Sampling

Fog and aerosol samples were obtained from three fieldcampaigns conducted in Fresno, CA; Whistler, BC; and Davis, CA.

Also a few fog samples were collected near Selinsgrove, PA andprovided by Susquehanna University (Straub et al., 2012). Fresno,fog and aerosol samples were collected in January 2010 at theexperimental farm of California State University, Fresno. Thesampling site was situated in a large agricultural plot but relativelyclose (hundreds of meters) to major highways and residential areas(Ehrenhauser et al., 2012). Whistler, cloud water samples werecollected during the Whistler Aerosol and Cloud Study (WACS2010) in the summer of 2010 (Macdonald et al., in preparation; Leeet al., 2011, 2012). Samples were collected at two locations onWhistler Mountain: Raven’s Nest (RN), which is located withina clearing in a coniferous forest on themid-mountain (1300m), andon the summit ofWhistler Mountain (2182m). It is noteworthy thaton both Whistler locations, samples were intercepted cloudsamples and could also be considered high elevation fog samples.Davis, fog and aerosol samples were collected in January 2011. Thesampling site was located in a field close to the University of Cal-ifornia, Davis campus. The Susquehanna University sampling site islocated in the ridge and valley region of Pennsylvania near theconfluence of the north and west branches of the SusquehannaRiver and characterized as open grassy fields surrounded by activefarms.

Fog and cloud water were collected using Caltech Active StrandCloud Collectors (CASCC) (Demoz et al., 1996) or a stainless extralarge CASCC collector (XL-CASCC) (Herckes et al., 2007). Collectorswere triggered automatically on an event basis when liquid watercontent passed a threshold value, typically higher than 60 mg m�3.All fog samples were filtered with a 0.22 mm glass fiber filter in thefield and refrigerated until analysis.

Aerosol samples were also collected on 24 h basis (Whistlerstudy) as well as 4e24 h basis depending on the presence andabsence of fog (Fresno study). Only samples in fog/cloud freeperiods are included here. A high volume sampler (1.13 m3 min�1,Tisch Environmental, Village of Cleves, OH) with a PM2.5 impactorinlet (TE 231, Tisch Environmental, Village of Cleves, OH) was usedto obtain PM2.5 and PM>2.5 aerosol samples. Aerosol samples werecollected onto quartz fiber filters (Whatman, UK) that were pre-fired 650 �C overnight before use.

2.2. Analysis

2.2.1. Sample preparationFog and cloud water samples were filtered using 0.22 mm glass

fiber filters (Whatman, UK). The conductivity of the samples wereadjusted to 4.57 mS cm�1 with a phosphate buffer eluent (Sodiumdi hydrogen phosphate (NaH2PO4) and 0.0016 M disodiumhydrogen phosphate (Na2HPO4), pH ¼ 6.8). A section of eachaerosol sample (PM2.5 and PM > 2.5, section area of 9.81 cm2) wasextracted with 15 mL deionized (DI) water (>18 MU) then filteredusing a 0.22 mm glass fiber filter to obtain the WSOC aliquot.

2.2.2. SEC-DOC analysisThe SEC-DOC system is composed of a High-Performance Liquid

Chromatography system (Waters 2695 Separation Module, Mil-ford, MA) followed by an online TOC detector (Sievers TotalOrganic Carbon Analyzer 800 Turbo) adapted to inline detectionusing an inorganic carbon remover (900 ICR, GE). The column usedfor separation is a TSK 50S column (250 mm � 20 mm ToshoToyopearl HW-50S resin, Japan). Elution is isocratic using a phos-phate buffer eluent (NaH2PO4 and 0.0016 M Na2HPO4, pH ¼ 6.8)containing 0.025 M Na2SO4 which has ionic strength of 0.1 M(conductivity of 4.57 mS cm�1.). The set-up and its performanceare discussed in more detail in earlier work (Her et al., 2002b,2003; Allpike et al., 2005). The eluent reservoir is purged bynitrogen gas to reduce the inference on the stationary phase from

Y. Wang et al. / Atmospheric Environment 68 (2013) 326e332328

dissolved inorganic carbon and oxygen. The void volume of theSEC column was determined to be 27 ml (approximate corre-sponding MW ¼ 240,000 Da) by using dextran blue. The perme-ation volume was determined to be 67 ml (approximatelycorresponding to an MW ¼ 120 Da) by using a deionized waterinjection and electrical conductivity monitoring. The sampleinjection volume is 1 mL and the flow rate is set to 1 mL min�1.10 mgC L�1 polyethylene glycol (PEG) standards (Sigma Aldrich)with the MW of 600, 1000, 1450, 3350, 4600, 8000 and 10,000 Daare used to generate the MWD calibration. Suwannee River naturalorganic matter (SRNOM) stock solution (8 mgC L�1) and nanopurewater samples are run for each sample set as quality control.

A second size-exclusion separation scheme with an inlineorganic carbon detection system (LC-OCD from DOC Labor, Karls-ruhe, Germany) was used for comparison purposes on a limitedsubset of samples. The chromatographic column is a weak ion-exchange polymethacrylate column (250 mm � 20 mm, TSK HW50S, 3000 theoretical plates, Toso, Japan) and operated at a flowrate of 1.1 ml min�1. The mobile phase is a phosphate buffer of pH6.85 (2.5 g potassium dihydrogen phosphate (KH2PO4) þ 1.5 gNa2HPO4 � 2H2O to 1 L). This method is described in detail else-where (Huber et al., 2011).

3. Results and discussion

3.1. Organic matter in fogs

Fig. 1 shows SEC-DOC chromatograms recorded for fog samplescollected in Fresno. Each sample is from a different fog event andcontains different DOC concentrations, therefore chromatogramshave been normalized by DOC response signal (mV). A SEC-DOCchromatogram of an SRNOM is shown for comparison purposes.Integrated areas below the entire chromatogram correspond to theDOC (mg C/L) of the sample. To quantitatively compare the data, theinset shows integrated areas corresponding to “small, medium andlarge MW material”. Areas are subjectively defined by threediscrete peaks (S ¼ MW < 170 Da, M ¼ MW between 170 and600 Da, L¼MWbetween 600 and 100 kDa). The fog organic matterappears significantly different from SRNOM. In particular comparedto SRNOM or other surface waters (Song et al., 2010), the fogorganic matter elutes substantially later in what corresponds tosmaller molecular weight regions. The small size (<500 Da) ofatmospheric organic matter has been documented in other studiesusing liquid chromatography coupled to electrospray mass spec-trometry (Kiss et al., 2003) or ultrafiltration (Herckes et al., 2002a).It is also consistent with observations that small molecular weight

Fig. 1. SEC-TOC chromatogram of Fresno (CA) fog samples as well as SRNOM. Insetpresents integrated peak areas. S ¼ MW < 170 Da, M ¼ MW between 170 and 600 Da,L ¼ MW between 600 and 100 kDa.

compounds like small carboxylic acids (formic, acetic) andcarbonyls (formaldehyde, glyoxal) account for a substantial fractionof the organic matter in radiation fogs (Collett et al., 2008).

Chromatograms from different events all show a similar patternand there is little variability in shape and distribution of the organicmatter as observed by the SEC-DOC method. Most samples exhibit2 or 3 distinct fractions in the resulting DOC response peaks, whichdiffers significantly from SRNOM which exhibits a major responsepeak in the high MW area and a relatively small peak in the lowMW area. A similar pattern is also observed for fog samples fromother locations as shown in Fig. 2 except that the Selinsgrovesample has a peak in the area of MW > 10 kDa. We attribute it tobiogenic nanoscale material and will discuss it extensively later inthe text.

Discrete distributions have been observed by Decesari andcoworkers using an anion-exchange chromatography techniqueand attributed to 3 fractions: neutral/basic compounds, mono- anddicarboxylic acids and polyacidic compounds (Decesari et al.,2000). This interpretation has been debated (Chang et al., 2005;Decesari et al., 2005). In the present case, checking individualorganic species elution times against the calibrated molecularweight was inconclusive. It is obvious that the calibration based onPEG does not account for properties like the electrostatic effect,eluent chemical composition which affect size exclusion separa-tions beyond pure size. Therefore the scale of the x-axis in molec-ular weight should be considered an indication of molecular sizeand not as an absolute standard. This is consistent with specieseluting after the permeation volume (corresponding to <120 Da)which is an indication that the separation is not based solely on sizebut additional interactions with the stationary phase increasecompound retention.

3.2. Organic matter in clouds

Fig. 3 shows cloud organic matter molecular weight distribu-tions from samples collected in Whistler during different cloudevents. SRNOM is being plotted for comparison purposes. TOCcontents were substantially lower in Whistler (2e11 mgC L�1)compared to Fresno (12e35 mgC L�1).

The molecular weight distribution for cloud organic matter issimilar to fog samples (Fig. 2), with a predominance of late elutingmaterial, indicative of “small molecular weight” species. The clas-sification scale is not uniform from site to site, therefore areassubjectively defined by discrete peaks cannot be directly compared.The organic matter is resolved in distinct peaks although less peaksare apparent compared to Fresno fog samples. The higher molec-ular weight material (MW between 500 and 3 kDa) appears partlyin what is seen for SRNOM, an indication consistent with a number

Fig. 2. SEC-TOC chromatograms of fog samples collected at 3 locations and the SRNOMstandard.

Fig. 3. SEC-TOC chromatograms of Whistler (BC) cloud samples as well as SRNOM. Inset presents integrated peak areas. S ¼ MW < 500 Da, M ¼ MW between 500 and 3 kDa,L ¼ MW between 3 k and 100 kDa.

Y. Wang et al. / Atmospheric Environment 68 (2013) 326e332 329

of studies that attributed similar molecular properties to atmo-spheric organic matter and humic substances (Krivácsy et al., 2000;Kiss et al., 2003; Samburova et al., 2005) although, here it is onlya fraction of the total organic matter present. For these cloudsamples, there is again little variability in the molecular weightdistribution between different cloud events.

It can be seen from Fig. 3 that there is a relative peak around>10 kDa in all samples. This peak is barely visible in Fresno fogsamples (Fig. 1; 1e3% of Fresno DOC) but its proportion becomeslarger in Whistler cloud samples (12e17% DOC) and Selinsgrovefog samples (5e21% DOC). The observation of such high molecularweight material (several 10 kDa) is surprising but not an artifact ofthe analytical method as studies using the same analyticalapproach to surface waters observed peaks in the same region. Infact, Song et al. (2010) showed that organic matter can have suchMW and size, which can be classified as biogenic nanoscalematerial (BONM) or macromolecules. BONM is likely composed oforganic nitrogen, proteinaceous compounds and polysaccharides,which is consistent with previous research conducted by Zhangand Anastasio (2001). Their results show that proteins andpeptides account for approximately 13% and 16% of the organiccarbon and nitrogen in fog water. Birdwell and Valsaraj (2010)

Fig. 4. Comparison of WSOC in ambient aerosol and DOC in fog water. Inset presents integra650 and 100 kDa.

used excitationeemission matrix fluorescence spectroscopy tocharacterize organic compounds that cannot be speciated in fogwater, and further confirmed that a protein-like substance ispresent. Also a significant amount of biological materials, such asyeast and bacteria, was observed in fog water (Fuzzi et al., 1997).Some of the biological materials are referred to as transparentexopolymer particles (TEP) and they are gel-like polysaccharidematerials released from algae and bacteria that are ubiquitous inmarine and fresh waters (Grossart et al., 1997; Verdugo et al.,2004). In addition, Orellana and coworkers (2011) showed thatmarine microgels can serve as an important source of cloudcondensation nuclei in high arctic summer. These studies indicatethat biological materials are an important contribution to organicmatter in fog/cloud water.

3.3. Water-soluble fraction of carbonaceous particles

Besides clouds and fog droplets, a substantial part of theatmospheric carbonaceous particulate matter is water-soluble. Thelatter can be analyzed after aqueous extraction of aerosols collectedon filters and is commonly referred to as WSOC. Fig. 4 showsWSOCmolecular weight distributions for samples collected in Fresno

ted peak areas. S ¼ MW < 170 Da, M ¼ MW between 170 and 650 Da, L ¼ MW between

Fig. 5. Comparison of WSOC in PM2.5 (particulate matter with aerodynamic diameter<2.5 mm) and large (PM > 2.5 or larger than >2.5 mm aerodynamic diameter) aerosolcollected simultaneously in Whistler.

Y. Wang et al. / Atmospheric Environment 68 (2013) 326e332330

compared to one local fog sample. The aerosol samples are 24 hsamples collected on different days. The WSOC samples show littlevariability from one day to the next.

The WSOC similar to fog and cloud organic matter showsdiscrete peaks and material shifted toward smaller molecularweight material. This is not surprising as the fog droplets grow onexisting aerosol particles and hence the water-soluble material ispassed from the aerosol to the aqueous droplets. Still the fogdroplet distribution is shifted a little toward smaller molecularweight material which would be consistent with the presence oflow molecular weight volatile species that partition into theaqueous phase and are absent from aerosol particles.

While there is little variability at a single sampling site betweendifferent daily samples (Fig. 4), there is a substantial difference withaerosol particle size. Fig. 5 presents SEC-DOC chromatograms for 2WSOC samples collected in Whistler, one of fine (PM<2.5) and onefor large (PM>2.5) particulate matter. The results show that theorganic matter in the larger particulates is shifted toward largersizes. This could be the result of a stronger contribution of soilorganic matter and biological materials which tend to have highermolecular weight organics.

3.4. Comparison to other SEC-DOC setups

Size-exclusion chromatographymight be strongly dependent onthe column and analytical set-up. We therefore analyzed a subset

Fig. 6. LC-OCD chromatograms of WSOC in ambient aerosol and DOC in fog water as well asshow smaller molecules on the left and larger to the right to be consistent with the other

of the different atmospheric samples using a second set-up withsize-exclusion chromatography coupled to inline organic carbonand organic nitrogen detection (LC-OCD) (Huber et al., 2011). Theresults for select samples are shown in Fig. 6.

The results (Fig. 6) observed for a subset of atmospheric samplesare completely consistent with the SEC-DOC results detailed above.Again discrete peaks are observed with similar chromatographicpatterns between all atmospheric organic matters analyzed. LC-OCD analysis concluded that the air samples contained signifi-cantly higher hydrophobic organics (18e31% of DOC) comparedagainst SRNOM (3%). Hydrophobic organics are retained on the SECcolumn during the run. The hydrophobic organic content is calcu-lated as the difference between the total DOC, analyzed by the LC-OCD instrument during the sample run by by-passing the SECcolumn, minus the cumulative DOC response over time in the SECeffluent. The fraction of hydrophobic organic material is then takenas a ratio of the material retained on the SEC media divided by thetotal DOC. Between runs, the column is flushed with sodiumhydroxide which removes the hydrophobic organics (Huber et al.,2011).

WSOC in Whistler ambient aerosol has the most hydrophobicfraction (31% of DOC) and DOC in Whistler clouds had the leasthydrophobic fraction (18% of DOC). LC-OCD confirmed significantdifferences between an aquatic organic matter from theSuwannee River and atmospheric organic matter. Whereas theSuwannee River isolates contain a major single peak response,the atmospheric samples show two or three grouped peakresponses at longer retention times (i.e., lower molecularweights). This again is consistent with the results of SEC-DOCand the differences observed between atmospheric samplesand SRNOM. The detailed analysis of the LC-OCD set-up (Huberet al., 2011) indicates that small molecular weight neutralmolecules elute at longer retention time (w75 min) compared tosmall molecular weight acids (w55 min). For the atmosphericsamples analyzed, the fog and cloud samples show a moreimportant fraction of low molecular weight acids compared tothe WSOC samples, in which small molecular weight neutrals aremore abundant. This agrees with what is known about thecomposition of fog and cloud organic matter which tends to havea substantial fraction (24%) (Herckes et al., 2007) of smallmolecular weight acids like formic and acetic acids, species thatare hardly found in particulate matter due to their volatility.Finally a small but differentiable peak is being detected in thebiopolymer region eluting at 30 min (especially for the Whistlercloud sample). This is consistent with the SEC-DOC observation

Suwannee River natural organic matter (SRNOM). Time scale on the x-axis is inverted tofigures although no MW calibration was available.

Y. Wang et al. / Atmospheric Environment 68 (2013) 326e332 331

of BNOM confirming the presence of large molecular weightmaterial of biological origin.

4. Conclusions

High-Performance Size-Exclusion Chromatography coupledwith inline organic carbon detection (SEC-DOC) was applied to thecharacterization of water-soluble atmospheric organic matter inclouds, fogs and aerosol particles. The results show little variabilitybetween samples collected at a specific site and limited variabilitybetween sites and types of atmospheric samples. Results from twodifferent instrument setups drew comparable conclusions sug-gesting the method is reproducible.

The results show a distribution of organic matter across a widerange of nominal molecular weight (120e10 kDa) consistent withwork performed using different analytical techniques. The atmo-spheric organic matter differs substantially from aquatic organicmatter (e.g. Suwannee river natural organic matter) in terms ofmolecular weight (smaller) and distribution (several clearly definedpeaks). The clearly resolved peaks were observed also with a secondsize-exclusion chromatography set-up comprised of a differentcolumn and detection system. The comparison results also confirmedhigher concentrations of atmospheric organic matter in smallermolecular weight regions compared to surface water natural organicmatter. A major difference between types of samples is a largerfraction of small molecular weight organic species in fog and cloudsamples compared to the water-soluble fraction of aerosols, consis-tent with the partitioning of small molecular weight volatile speciesinto the atmospheric aqueous phase. With Aerosol samples, there isa noticeable difference between the molecular weight distributionorganic carbon in fine (PM2.5) and larger (PM>2.5) aerosol samples.

Samples from select environments presented a substantialfraction of organic matter in a very high molecular weight region(>10,000 Da) corresponding to biogenic nanoscale or macromo-lecular materials.

The low sample volumes (<5 mL) required by SEC-DOC wouldrender it a suitable tool to investigate processing of atmosphericorganic matter at high time resolution for aerosols and clouds.Inline TOC detection allow a more comprehensive characterizationcompared to optical detection techniques (e.g. UV absorbance)because all organic matter, not only the light absorbing fraction isdetected.

Acknowledgments

We are grateful to Prof. Yunho Lee from Gwangju Institute ofScience and Technology (GIST) who assisted us at EAWAG(Dubendorf, Switzerland) in operating the LC-OCD system in theresearch group of Urs von Gunten.

We are grateful to James Hutchings and Aurelie Marcotte forassistance with sampling during the fog field studies. We aregrateful to Shawn Ashkan for help with the Fresno field study, MikeMata and Cort Anastasio for help with the Davis field work andAnne Marie Macdonald, Richard Leaitch and Juniper Buller for helpwith the Whistler field work. We are thankful to Derek Straub fromSusquehanna University for the collection of fog samples in ruralPennsylvania.

Funding for the research was provided by the National ScienceFoundation (grants AGS0847710, AGS 0907261) and EnvironmentCanada. The statements and conclusions in this paper are those ofthe authors and not necessarily those of NSF or EnvironmentCanada. The mention of commercial products, their source, or theiruse in connection with material reported herein is not to beconstrued as actual or implied endorsement of such products.

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