Dicarboxylic acid concentration trends and sampling artifacts

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Atmospheric Environment 39 (2005) 7906–7919

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Dicarboxylic acid concentration trends and sampling artifacts

Joshua Raya,1, Stephen R. McDowb,�

aEnvironmental Science Program, Drexel University, Philadelphia, PA, USAbHuman Exposure and Atmospheric Sciences Division, National Exposure Research Lab, US Environmental Protection Agency,

Research Triangle Park, NC, USA

Received 3 April 2005; received in revised form 12 September 2005; accepted 12 September 2005

Abstract

Dicarboxylic acids associated with airborne particulate matter were measured during a summer period in Philadelphia

that included multiple air pollution episodes. Samples were collected for two 10 h periods each day using a high-volume

sampler with two quartz fiber filters in series, and analyzed by gas chromatography mass spectrometry (GCMS) with

diazomethane derivatization. Among the dicarboxylic acids investigated, phthalic acid and adipic acid exhibited the

greatest diurnal variations and the strongest linear relationship with maximum daily ozone concentration. Dicarboxylic

acids and ozone concentration exhibited a poor linear relationship with organic to elemental carbon ratio. All species

investigated were affected by significant sampling artifact errors at low concentrations, but sampling errors were negligible

at high concentrations observed during ozone episodes.

Published by Elsevier Ltd.

Keywords: Dicarboxylic acids; Secondary organic aerosol; Sampling artifacts

1. Introduction

Dicarboxylic acids are among the most abundantorganic constituents of ambient particulate matter(Rogge et al., 1993). Discussion of their potential astracers for secondary organic aerosol dates back tosome of the earliest mass spectral observations ofatmospheric aerosols (Schuetzle et al., 1975).Dicarboxylic acids are formed in the atmospherefrom gas phase photochemical reactions involving awide range of both anthropogenic and biogenicprecursors. They have been identified in smogchamber experiments as atmospheric oxidation

e front matter Published by Elsevier Ltd.

mosenv.2005.09.024

ing author.

dress. New Jersey Department of Environmental

nton, NJ, USA.

products of cyclic olefins (Grosjean et al., 1978;Hatakeyama et al., 1985), and proposed as atmo-spheric oxidation products of aromatic hydrocar-bons, fatty acids, and larger dicarboxylic acids(Kawamura and Ikushima, 1993). Their aqueousphase formation in cloud and fog water is alsoplausible (Blando and Turpin, 2000) and may belinked with photochemically generated radicals(Warneck, 2003). The relatively high concentrationsof dicarboxylic acids and their identification asatmospheric reaction products from a variety ofdifferent precursors make it useful to investigatetheir potential as indicators of secondary organicaerosol formation.

The use of atmospheric dicarboxylic acids asindicators of secondary formation is complicated bythe occurrence of both biogenic and anthropogenic

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ARTICLE IN PRESSJ. Ray, S.R. McDow / Atmospheric Environment 39 (2005) 7906–7919 7907

primary sources. Biogenic sources include plantemissions of metabolic products and soil particles.Anthropogenic sources include exhausts from gaso-line and diesel powered automobiles. An additionalcomplication is the potential for sampling artifacts.Recently, Limbeck et al. (2001) described seriousvapor adsorption artifact errors for a number ofcommon dicarboxylic acids not observed in the gasphase prior to their work. Their results demon-strated the need to consider the most abundantdicarboxylic acids in particulate matter as semi-volatile compounds and to address sampling arti-facts in particulate dicarboxylic acid measurements.

In this paper we report concentration trends fordicarboxylic acids during a summer period inPhiladelphia that included multiple air pollutionepisodes. We also identify relationships betweendicarboxylic acids with ozone and potential indica-tors of photochemical aerosol formation, anddiscuss their potential as secondary organic aerosolmarkers. Finally, we consider potential measure-ment bias due to sampling artifacts and practicalsolutions to address this problem.

2. Methods

Samples were collected following the proceduredescribed by Sihabut et al. (2005) with two quartzfiber filters arranged in series in an Anderson highvolume PM10 sampler at the northeast ozone andparticulate study (NEOPS) site in Philadelphiaduring the NEOPS summer 1999 intensive samplingperiod from 1 July to 18 August. There were twoseparate collection periods each day, from 10:00AM to 8:00 PM and from 10:00 PM to 8:00 AM.Meteorological and air pollution conditions for thisperiod have been described by Philbrick et al.(2002). Quartz fiber filters were baked out for 2 hat 600 1C before sampling. Blank levels for fivetravel blanks were below the equivalent of0.5 ngm�3 for malonic acid and below 0.2 ngm�3

for all other species. The average travel blankaccounted for less than 20% of average ambientconcentrations of suberic acid and less than 5% ofall other species. Concentrations are reported with-out blank correction.

Samples collected from 11 July through 3 Augustwere selected for dicarboxylic acid analysis. Thisperiod provided an excellent chance for assessingthe relationship between particulate dicarboxylicacids and ozone because it included three separateelevated ozone events, with a number of days of

significantly lower ozone concentrations before andafter each event. As described by Philbrick et al.(2002), a major ozone event occurred from 16 Julythrough 21 July, and a second major ozone eventoccurred from 27 July through 1 August. A morelimited regional ozone event occurred on 23 and 24July.

Samples were extracted and analyzed followingthe procedure of Li et al. (2005). Soxhlet extractionwas carried out for 3 h in 1:1 dichloromethane:ace-tone. Immediately prior to extraction, samples werespiked with deuterated n-tetracosane as an internalstandard. Extracts were reduced to 0.5ml with aKuderna–Danish apparatus followed by gentleevaporation under a stream of nitrogen. Theconcentrated extract was split into two fractions,one of which was derivatized with diazomethane toconvert carboxylic acids to their methyl esters. Bothfractions were refrigerated at 4 1C until analysis.

Samples were analyzed on a Varian 3800 gaschromatograph/Saturn 2000 ion trap GC/MS witha DB-1701 J&W Scientific column with 0.25 mm filmthickness. All standards and samples were analyzedusing 1 ml manual injections. The oven temperaturewas as follows: hold at 50 1C for 3min afterinjection, heat to 150 1C at 20 1Cmin�1, heat to280 1C at 4 1Cmin�1, hold at 280 1C for 17min. Acomposite standard of 11 commonly occurringdicarboxylic acids was prepared and derivatizedwith diazomethane. Five calibration standard solu-tions were prepared from this standard to establishfive-point calibration curves for each compound.The species analyzed corresponded to the mostabundant aliphatic dicarboxylic acids and phthalicacid isomers observed by Rogge et al. (1993) andSchauer et al. (1996). In each case base peak ionswere used for quantification.

Quantification was carried out as described byMazurek et al. (1987). Method recoveries weremonitored with a multi-component recovery mix-ture of perdeuterated internal controls representa-tive of the analyte polarity and volatility range (Li,2003), based on the approach of Mazurek et al.(1987). This approach has been widely applied tomeasurement of multiple classes of compounds oforganic aerosol components in a number of recentstudies (Rogge et al., 1993; Zheng et al., 2002;Fraser et al., 2002). Our extraction method wasmodified from that of Mazurek et al. (1987) to avoidusing benzene by replacing sonication in benzene/isopropanol mixture with soxhlet extraction inequal amounts of dichloromethane and acetone.

ARTICLE IN PRESSJ. Ray, S.R. McDow / Atmospheric Environment 39 (2005) 7906–79197908

Analysis of the recovery mixture verified similarrecoveries for representative perdeuterated com-pounds between the two methods (Li, 2003).

An alternative method of extraction with waterfollowed by butyl esterification has been widely usedfor analysis of low molecular weight dicarboxylicacids (Kawamura et al., 1985) but was not suitablefor meeting our objective of measuring both non-polar and polar organic compounds. Both puredichloromethane and mixtures of dichloromethanewith polar solvents have been frequently used forextraction of dicarboxylic acids from particulatematter, followed by esterification by bis(trimethyl-silyl)trifluoroacetamide (BSTFA) or diazomethane.Recently, lower concentrations of malonic andsuccinic acids have been reported for derivatizationwith BSTFA than with butyl esterification andevaporative loss during the BSTFA derivatizationprocess was suggested as a possible explanation(Simoneit et al., 2004). Similar problems for malonicacid with diazomethane derivatization have alsobeen suggested (Kawamura and Ikushima, 1993).However, Yokouchi and Ambe (1986) reportedrecoveries ranging from 76% to 86% for soxhletextraction of succinic acid and larger dicarboxylicacids in dichloromethane and methanol withdiazomethane derivatization. Based on these re-ports, some evaporative loss during derivatizationfor the of the methyl esters of malonic acid andsuccinic acid is also possible and needs furtherinvestigation.

Table 1

Dicarboxylic acid concentrations (ngm�3)

Acid species Total Day

Mean Range Mean

Malonic 14.2 0.11–95.6 13.0

Succinic 15.4 0.09–66.7 12.5

Methylsuccinic 1.1 0.02–3.2 1.0

Glutaric 2.3 0.05–5.7 2.2

Malic 9.4 NDa–71.0 9.9

Adipic 2.0 0.07–7.4 2.3

Suberic 0.50 0.03–1.4 0.41

Phthalic 3.5 0.07–16.3 4.9

Isophthalic 0.30 0.01–0.99 0.26

Azelaic 1.0 0.03–2.6 1.2

Total C3–C9 49.4 0.74–242 47.8

aAcid was not detected (ND).

3. Results

3.1. Concentrations and diurnal variation

Table 1 summarizes average total, daytime andnighttime concentrations for each individual com-pound and the sum of all compounds analyzed. Theaverage concentration for the sum of the dicar-boxylic acid species analyzed in this study was49.4 ngm�3. Of the species analyzed, succinic acidwas the most abundant dicarboxylic acid, with anaverage concentration of 15.4 ngm�3, followedclosely by malonic acid with an average concentra-tion of 14.2 ngm�3. Although oxalic acid is usuallythe most abundant atmospheric dicarboxylic acidspecies (Chebbi and Carlier, 1996), it was consid-ered too volatile to be quantified using methylesterification (Kawamura et al., 1985). Considerableday-to-day variation was observed, with totaldicarboxylic acid concentration ranging from 0.74to 242 ngm�3. Most average concentrations ofdicarboxylic acid species were lower than thosereported for Philadelphia (Lawrence and Koutrakis,1996), New York (Khwaja, 1995), and Los Angeles(Kawamura and Kaplan, 1987; Rogge et al., 1993).All of the dicarboxylic acids analyzed were presentin every sample, with the exception of malic acid.

In general, aliphatic dicarboxylic acid concentra-tions decreased with carbon number, a pattern alsoobserved by other investigators (Limbeck et al.,2001; Sempere and Kawamura, 1996; Kawamura

Night Day/night

Range Mean Range Mean

0.38–95.6 15.4 0.11–69.1 0.84

0.92–66.7 18.1 0.09–28.0 0.69

0.19–3.0 1.2 0.02–3.2 0.83

0.22–5.5 2.2 0.05–5.7 1.00

ND–71.0 8.5 ND–48.1 1.17

0.20–6.3 1.7 0.07–7.4 1.35

0.04–0.93 0.4 0.03–1.4 1.22

0.27–6.2 2.1 0.07–16.3 2.33

0.03–0.99 0.33 0.01–0.89 0.79

0.11–2.2 1.0 0.03–2.6 1.21

2.8–242 50.8 0.74–154 0.94


and Ikushima, 1993; Kawamura and Kaplan, 1987).The only major exception was succinic acid (C4),which has also been reported at higher concentra-tions than malonic acid (C3) in New York (Khwaja,1995), Tokyo (Satsumabayashi et al., 1990), andChina (Wang et al., 2002). The aromatic dicar-boxylic acid, phthalic acid, was present at higherconcentration than glutaric (C5) and adipic (C6)acids, but at lower concentrations than malonic andsuccinic acids, a pattern also seen by Sempere andKawamura (1996).

From Table 1, daytime and nighttime concentra-tions were within 25% for most species, but withsome important exceptions. Phthalic acid and adipicacid were considerably higher during the day than atnight. Average daytime concentration of phthalic

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Fig. 1. Concentration trends from 11 July to 3 August 1999 for (a) phth

circles and nighttime data are indicated by filled circles.

acid was more than twice as high and adipic acidwas 35% higher than average nighttime concentra-tion. The average daytime concentration of succinicacid was less than 70% of its average nighttimeconcentration. Fig. 1 shows that measured concen-trations of these compounds frequently exhibited analternating peak and valley structure, with peaksoccurring during the day for phthalic acid (Fig. 1a)and during the night for succinic acid (Fig. 1b).

Other researchers have observed similar diurnalconcentration patterns. Daytime concentrationmaxima have been observed by Cronn et al. (1977)for adipic acid and by Satsumabayashi et al. (1990)for phthalic acid. Satsumabayashi et al. (1990) alsoobserved daytime maxima for malonic, succinic,glutaric, and isophthalic acids, but Grosjean (1989)

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alic acid and (b) succinic acid. Daytime data are indicated by open


found maximum concentrations of acetic andformic acids during the evening and suggested thathigher humidity in the evening could favor thereaction of the Criegee intermediate with water,leading to increased production of organic acids atnight. Alternatively, Grosjean (1989) noted thatlower nighttime hydroxyl radical concentrationscould reduce competitive reactions such as hydroxylradical reactions with olefin precursors of car-boxylic acid formation. A similar explanationwould account for observations of clear nighttimemaxima for malonic and succinic acids.

3.2. Relationship with ozone

The relationship between dicarboxylic acids andother species formed through atmospheric oxidationprocesses was explored by comparing concentrationtrends between these compounds and ozone, or-ganic to elemental carbon ratio, and sulfate. Fig. 2shows the variation of total dicarboxylic acidconcentration with peak daily maximum ozoneconcentration. There were two major periods ofhigh dicarboxylic acid concentration, centeredaround 18 July and 31 July. These events corre-spond to a major ozone episode that lasted from 15to 19 July, and a single day of high ozone on 31July, respectively. There were also two smallerdicarboxylic acid events around 23 and 27 July,which also matched two weaker ozone events. Themeteorological aspects of ozone events during thisperiod were thoroughly characterized by Ramanlidar, wind profilers, tethered blimps, and aircraft

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Total Diacids Ozo

Fig. 2. Total C3–C9 diacid and ozone tr

measurements (Philbrick et al., 2002), and generallyoccurred in association with westerly flow withsignificant regional transport of ozone from theOhio Valley and western mid-Atlantic region (Clarket al., 2002).

Fig. 3 compares total daytime dicarboxylic acidconcentration with corresponding peak ozone con-centration for each sampling period. A clear linearincrease is observed (r2 ¼ 0:73). This result indicatesa moderately strong linear relationship betweenozone and total dicarboxylic acid concentration thatis consistent with the likelihood that dicarboxylicacids and ozone are often products of the sameatmospheric oxidation processes. Expressed as afraction of particulate organic carbon, the linearrelationship with ozone is only slightly weaker(r2 ¼ 0:65), and stronger than a similar relationshipin Tokyo reported by Kawamura and Ikushima(1993). When both daytime and nighttime dicar-boxylic acid data are combined to approximate a24 h sampling period a relatively strong relationshipis still observed (r2 ¼ 0:63).

The linear relationships with daily maximumozone for daytime concentrations of individualspecies are described in Table 2. These variedconsiderably between species, with the strongestlinear relationship between individual dicarboxylicacids and ozone observed for phthalic acid(r2 ¼ 0:79) and adipic acid (r2 ¼ 0:77). These arethe same two species which exhibited the greatestratio of daytime to nighttime concentrations inTable 1. Plots of their concentrations against ozoneconcentration are given in Fig. 4. Significant

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r2 = 0.73

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tal D

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Fig. 3. Relationship between maximum ozone concentration daytime total dicarboxylic acid concentration.

Table 2

Daytime dicarboxylic acid linear relationships and artifact thresholds

Acid species r2 vs. O3 max r2 vs. O3 avg r2 vs. OC/EC r2 vs. sulfate Threshold (ngm�3) Corrected r2 vs. O3

Malonic 0.51 0.41 0.037 0.34 10.0 0.49

Succinic 0.59 0.43 0.096 0.46 12.0 0.54

Methyl succinic 0.31 0.27 0.086 0.46 0.16

Glutaric 0.41 0.45 0.040 0.42 3.2 0.35

Malic 0.58 0.53 0.019 0.31 2.0 0.58

Adipic 0.77 0.67 0.057 0.63 2.5 0.75

Suberic 0.59 0.51 0.056 0.58 0.8 0.51

Phthalic 0.79 0.68 0.014 0.49 2.5 0.77

Isophthalic 0.52 0.34 0.10 0.45 0.5 0.46

Azelaic 0.51 0.46 0.078 0.66 1.2 0.50

Maximum ozone 3� 10�5

Total C3–C9 0.73 0.58 0.056 0.50 0.49

J. Ray, S.R. McDow / Atmospheric Environment 39 (2005) 7906–7919 7911

negative intercepts are apparent for both adipic acidand phthalic acid, indicating that on the cleanestdays ozone was still present at substantial back-ground levels when concentrations of the dicar-boxylic acid species were extremely low. The linearrelationships for daytime concentrations of allspecies with average ozone concentrations are alsolisted in Table 2. These were slightly weaker thanlinear relationships with maximum ozone, butfollowed the same general pattern.

Both phthalic acid and adipic acid have beenidentified as photochemical products in smogchamber experiments. Phthalic acid is a product ofnaphthalene photodegradation (Bunce et al., 1997)and possibly photodegradation of other polycyclic

aromatic hydrocarbons in atmospheric aerosols(Jang and McDow, 1997). Adipic acid was identifiedin early smog chamber experiments as a product ofcyclohexene photooxidation (Grosjean et al., 1978;Hatakeyama et al., 1985), and has since beenidentified as a product of other cycloalkene pre-cursor reactions also (Docherty and Ziemann,2001). Formation from other precursors cannot beruled out, especially given the abundance of thesetwo compounds relative to their known precursors.

Adipic and phthalic acid have been attributed toanthropogenic precursors, whereas azelaic acid hasbeen attributed to biogenic precursors. Therefore,the ratios of these have been used as indicators ofanthropogenic nature of the dicarboxylic acids.

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r2 = 0.79

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ipic

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Fig. 4. Relationship between maximum ozone concentration and (a) daytime phthalic acid concentration and (b) daytime adipic acid

concentration.

J. Ray, S.R. McDow / Atmospheric Environment 39 (2005) 7906–79197912

Kawamura and Ikushima (1993) and Kawamuraand Kaplan (1987) reported adipic to azelaic acidratios of 0.72 and 7.4 for Tokyo and Los Angeles,respectively. They also reported phthalic to azelaicacid ratios of 0.83 and 8.0 for Tokyo and LosAngeles, respectively. We observed an averageadipic to azelaic acid ratio of 1.8 and an averagephthalic to azelaic acid ratio of 3.2. Both of thesevalues fall between the values for Tokyo and LosAngeles, suggesting a somewhat higher anthropo-genic input for dicarboxylic acids than reported forTokyo, but lower than that Kawamura and Kaplanoriginally reported for Los Angeles. More recently,higher concentrations for azelaic acid than adipic

acid have been observed in southern California(Rogge et al., 1993), Houston (Fraser et al., 2002),and the southeastern US (Zheng et al., 2002). Thesenew results suggest that adipic to azelaic acid ratiosmay not be so different in the southern US thanin Tokyo, and that there could be a slightlygreater influence of anthropogenic precursors inPhiladelphia.

Automobile exhaust may also be a significantcontributor to the total dicarboxylic acid concen-tration (Kawamura and Kaplan, 1987). The ratiosof malonic acid to succinic acid concentrationsmeasured in automobile exhaust and in urbanTokyo air by Kawamura and Ikushima (1993) were


0.35 and 1.6, respectively. This difference indicatesthe presence of an important additional contribu-tion besides automobile exhaust. Our results in-dicated an overall malonic acid to succinic acidconcentration ratio of 0.85, greater than automobileexhaust, but less than that measured in Tokyo air.However, if the data collected during ozone eventsare excluded, the malonic to succinic acid ratio is0.37, which is similar to the ratio reported forautomobile exhaust by Kawamura and Kaplan(1987). During the ozone events the ratio increasessubstantially to 1.03, further suggesting significantphotochemical formation of dicarboxylic acidsduring ozone events. This suggests that during dayswith low ozone, the dicarboxylic acid concentrationcan largely be explained by primary emissions, buton days with elevated ozone, the potential forsecondary formation of the individual species has asignificant effect on the dicarboxylic acid concen-trations.

3.3. Relationship with OC/EC ratio and sulfate

There is a clear indication that organic toelemental carbon ratio should be applied cautiouslyas an indicator of secondary organic aerosol, andmay be more problematic for daily measurementsthan for hourly measurements. Table 2 compareslinear relationships between dicarboxylic acids andtwo other photochemically produced species, sec-ondary organic carbon and sulfate. Organic toelemental carbon ratios have been used to estimatesecondary organic contributions to total organicmatter on short time scales (Turpin and Huntzicker,1995). A surprisingly poor relationship betweendicarboxylic acids and organic to elemental carbonratio was also observed. Since strong linear relation-ships between dicarboxylic acids and ozone wereobserved, these results suggest that particulateorganic matter from a natural primary source thatis free of elemental carbon might have contributedto elevated organic to elemental carbon ratiosobserved in the 10 h samples at this site. Forexample, Sihabut et al. (2005) observed consider-ably higher concentrations of plant wax relatedhydrocarbons than other hydrocarbons at this site.Dicarboxylic acids may have some advantage overorganic to elemental carbon ratios as indicators ofsecondary organic aerosol for extended samplingperiods.

For sulfate a fair linear relationship with the totaldicarboxylic acids was observed, (r2 ¼ 0:50). The

strongest linear relationships with individual specieswere observed for azelaic acid (r2 ¼ 0:66) and adipicacid (r2 ¼ 0:63). In Table 2, linear relationships withsulfate are poorer than those reported by Yao et al.(2002) for malonic acid (r2 ¼ 0:85) and for succinicacid (r2 ¼ 0:88), but are consistent a relationshipbetween photochemical sulfate and dicarboxylicacid formation that would be expected for an airmass that picks up sulfate and organic aerosolprecursors from multiple locations during transport.The differences between our results and those ofYao et al. (2002) could be attributed to a greatercontribution from primary sources. If only daytimesamples are considered and samples are excluded ifmalonic to succinic acid ratios are less than 0.6, orroughly double the ratio for primary sources(awamura and Kaplan, 1987, Yao et al., 2004), theresults are very similar for the relationship of ozonewith both malonic acid (r2 ¼ 0:83) and succinic acid(r2 ¼ 0:87). Moreover, sulfate concentrations werehighest on 19 July, 24 July, and 31 July, when theyexceeded 20 mgm�3, and both total dicarboxylicacid and maximum ozone exhibited their twohighest concentrations on 19 and 31 July, and werehigher than average on 24 July.

3.4. Sampling artifacts

A considerable fraction of organic mass collectedby quartz fiber filters during ambient sampling ofparticulate matter is due to adsorption of organicvapor, and the extent of adsorption artifact hasbeen estimated by comparing organic vapor col-lected on backup quartz fiber filters immediatelydownstream of filters used for collection of parti-culate matter or downstream of Teflon membranefilters sampled in parallel (Turpin et al., 2000). Arecent summary of field observations reportedadsorption artifact ranged from 10% to more than50% of total particulate organic carbon (Turpinet al., 2000). We recently observed similar serioussampling artifacts for a number of classes ofindividual organic species, including semi-volatilen-alkanes, hopanes, and n-alkanoic acids (Sihabutet al., 2005).

Volatilization of semi-volatile species from aero-sol filters has also been reported in a number ofstudies by blowing clean air or nitrogen acrosscollected particulate matter (Commins, 1962; VanVaeck et al., 1984) or passing sampled air through adenuder (Coutant et al., 1988; Ding et al., 2002;Eatough et al., 1993). While loss of substantial


amounts of semi-volatile species from aerosol filtershave been observed in these investigations, removalof adsorptive vapors from the air stream is likely toalter gas-particle equilibrium, creating a net poten-tial for volatilization that may not be present duringsampling of ambient air through the filter, andmaking it difficult to devise experiments to under-stand the relative importance of the adsorption andvolatilization artifacts (Turpin et al., 2000; Sub-ramanian et al., 2004). Moreover, some of thematerial removed from the front filter in a clean airstream may be adsorbed to the filter rather thanassociated with the collected particulate matter(McDow, 1999).

Because numerous observations indicate a poten-tially serious measurement bias with conflictingindications of its magnitude and direction, anumber of different approaches have been imple-mented to determine the relative importance of thevapor adsorption and volatilization artifacts. Fitz(1990), Appel et al. (1989), and Subramanian et al.(2004) each used a different type of denuder toremove adsorptive vapors and concluded thatnearly all of the backup filter organic carbon canbe attributed to vapor adsorption. In contrast,Eatough et al. (1993) concluded that carboncollected on backup filters was primarily due tovolatilization in similar experiments. A variety ofother approaches also used to compare vaporadsorption and volatilization artifacts gave resultsconsistent with a more important adsorptionartifact. McDow and Huntzicker (1990) demon-strated that backup filter subtraction removedapproximately 90% of consistently observed facevelocity dependence in particulate organic carbon

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measurement, but that backup filter additioncreated an even stronger face velocity dependence.McMurry and Zhang (1989) observed a majority oforganic carbon on quartz fiber afterfilters in cascadeimpactor sampling that could not be accounted forby theoretical predictions of volatilization loss, andconcluded that vapor adsorption was the most likelyexplanation. Turpin et al. (1994) diluted unfilteredambient air with varying amounts of particle-freeambient air to demonstrate that backup filterorganic carbon was independent of upstream filterparticle loading.

Based on the number of studies and the variety ofapproaches that suggest a predominant role ofadsorption artifact, we have applied the recommen-dations of McDow and Huntzicker (1990) for usingbackup filters to estimate vapor adsorption artifactto individual semi-volatile organic compounds.However, we also recognize that volatilization fromupstream collected particulate matter could con-tribute to material collected on backup filters. Theobservations reported here are more important asindicators of a serious sampling problem than asevidence for a particular type of artifact. Regardlessof its source, the presence of a species in abundanceon a backup filter must be addressed in order toobtain reliable particulate concentrations.

Artifact errors were estimated by calculating thebackup fraction, defined as the ratio of the mass of acompound on the backup filter to the mass collectedon the front filter for a dual quartz fiber filter series(Sihabut et al., 2005). This approach provides alower limit estimate of artifact error due to vaporadsorption (McDow and Huntzicker, 1990). Fig. 5shows that substantial artifact errors were observed

adipi

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r as a function of front filter concentration.


for a number of dicarboxylic acids measured. Thehighest average backup fractions were 45% formalonic acid and 35% for glutaric acid. Theseresults are consistent with a relationship betweensampling artifacts and volatility, since malonic acidalso has the highest vapor pressure of the com-pounds analyzed, followed by glutaric acid. Recentmeasurements of straight chain dicarboxylic acidvolatility indicate higher vapor pressures for oddcarbon than for even carbon compounds probablybecause twisting of carboxylic acid groups in oddcarbon species leads to a greater energy required fordimer formation (Bilde et al., 2003). Vapor pressuremeasurements by two different methods clearlyshow that succinic acid has a considerably lowervapor pressure than glutaric acid (Bilde et al., 2003;Chattopadhyay and Ziemann, 2005).

Average backup fractions were greater than 20%for malonic acid, methylsuccinic acid, glutaric acid,and adipic acid. They were under 15% for succinicacid, malic acid, phthalic acid, isophthalic acid, andazelaic acid. These results can be compared to thoseof Limbeck et al. (2001), who also found backupfractions greater than 20% for adipic and glutaricacid, but not for malonic acid, which they describedas exhibiting an anomaly as compared to the otherdicarboxylic acids. In addition they found backupfractions of greater than 20% for succinic acid,which was higher than our average.

The extent of artifact error was strongly depen-dent on apparent particulate concentration from themass collected on the front filters. This is illustratedby a plot of backup fraction versus front filterconcentration for malonic acid in Fig. 6a. At thevery lowest concentrations, below about 5 ngm�3,backup fractions are often greater than 50% andoccasionally greater than 100%, indicating evenmore material collected on the backup filter than onthe front filter. However, at concentrations greaterthan 10 ngm�3 backup fractions never exceeded10%. The trend of decreasing backup fraction withapparent PM2.5 mass (Kim et al., 2001), particulateorganic carbon (Kim et al., 2001; McDow andHuntzicker, 1990) and individual species (Sihabutet al., 2005) concentrations is a characteristic featureof the vapor adsorption artifact. It is consistent withobservations of a nearly constant vapor adsorptionartifact with a more variable ambient particulateorganic carbon concentration (Kim et al., 2001;Subramanian et al., 2004). The low variability of thesampling artifact magnitude in comparison toparticulate organic carbon concentration has been

attributed to saturation or equilibrium with respectto gas-particle partitioning (Subramanian et al.,2004), which has been confirmed by laboratoryexperiments for both total organic carbon (Turpinet al., 1994) and individual species (Mader andPankow, 2001a).

These results for malonic acid suggest that in spiteof the high average backup fractions observed in theentire data set, artifact errors are of little concernwhen its primary emission sources and/or secondaryformation are most important, since high atmo-spheric concentrations occur at these times. Incontrast, at low concentrations atmospheric con-centrations appear to have too high a potential forbias to be useful at concentrations below 10 ngm�3,and either some artifact correction procedure or animproved sampling method that eliminates sam-pling artifact must be developed. No such accom-modation appears to be necessary and a single filterappears to be adequate for addressing artifactproblems for atmospheric concentrations greaterthan 10 ngm�3.

Similar relationships between backup fractionand concentration were observed for other dicar-boxylic acids. Figs. 6b and c show that phthalic andadipic acid, which correlated best with ozone, oftenexhibited backup fractions between 15% and 60%when concentrations were below about 2.5 ngm�3,but above this concentration backup fractions werebelow 15%. In Fig. 6d, the backup fraction forsuccinic acid also increased with concentration, butit was more difficult to clearly discern a concentra-tion above which artifact error was substantiallyreduced. However, backup fractions were under20% in well over 90% of the samples withconcentrations exceeding 12 ngm�3.

The other dicarboxylic acids all followed a similartrend of decreasing backup fraction with increasingconcentration and threshold concentration valuesabove which artifact errors are considerably lower.The results suggest that setting a minimum con-centration above which artifact error has littleprobability of exceeding some small fraction ofapparent concentration might be a plausible ap-proach to addressing sampling artifact errors. Thisthreshold concentration could function as aneffective quantitation limit below which bias fromartifact errors is too high and uncertain to supportassignment of a concentration value.

The approximate threshold concentrations foreach of the dicarboxylic acids are also listed inTable 2. For methylsuccinic acid high backup

ARTICLE IN PRESS

0

50

100

150

200

250

0 20 40 60 80 100 120

Malonic Acid Front Filter Concentration (ng m-3)

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Pthalic Acid Front Filter Concentration (ng m-3)

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Adipic Acid Front Filter Concentration (ng m-3)

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1

(a)

(b)

(c)

(d)

Fig. 6. Relationship between backup fraction and apparent particulate concentration for (a) malonic acid, (b) phthalic acid, (c) adipic

acid, and (d) succinic acid.

J. Ray, S.R. McDow / Atmospheric Environment 39 (2005) 7906–79197916

fractions were observed over the entire concentrationrange observed, suggesting that if such a concentra-tion threshold exists it was not reached. For all othercompounds except succinic acid no backup fractionsgreater than 20% were observed at concentrationshigher than the concentration threshold listed. Forsuccinic acid, one out of 18 concentrations was abovethis value, but a backup fraction of 26% wasobserved at a relatively high concentration. As aresult, a stricter criterion for the threshold concen-trations listed in Table 2 is that more than 90% ofsamples with concentrations above the thresholdexhibit backup fractions lower than 20%.

When apparent concentrations were corrected bysubtracting backup filter compound mass from

front filter compound mass to account for vaporadsorption artifact, the linear relationship betweenparticulate dicarboxylic acid concentration andozone was not improved, as illustrated in Table 2.A possible explanation for this is that over theconcentration range where backup fractions aregreatest they are also highly variable, as indicated inFig. 6. Also, since only the lowest concentrationsare strongly affected by artifact errors, the differ-ences on a mass or concentration basis are moresimilar across the concentration range than are thebackup fractions. Similar results are obtained ifsamples with high backup fractions are excludedrather than corrected. As a result, it may be stillpossible to detect relationships between dicarboxylic


acids and other pollutants during photochemicalsmog episodes when their concentrations are highestwithout correction for sampling artifacts.

Gas-particle partitioning theory (Pankow, 1987,1994) predicts that adsorption of semi-volatileorganic vapors on quartz fiber filters occurs becausevapors are abruptly exposed to substantially moreadsorptive surface area associated with the filterthan with the sampled aerosol (Mader and Pankow,2001b; McDow, 1999; Turpin et al., 2000). Theadsorption is likely to be influenced by a number offactors, including temperature and relative humid-ity. The fraction of vapor adsorbed to filters hasbeen predicted to decrease with increasing tempera-ture because partitioning to the filter is predicted todecrease with increasing sub-cooled liquid vaporpressure, which in turn increases with temperature(Mader and Pankow, 2001b). However, a similartemperature dependence for gas particle partition-ing is predicted to increase the total amount ofvapor available for filter adsorption (Pankow, 1987,1994). Vapor adsorption to filters has also beenpredicted to decrease with increasing relativehumidity due to competition with adsorption ofwater (McDow and Huntzicker, 1990) and recentexperiments have confirmed that this is the case fornon-polar species (Mader and Pankow, 2001b).However, considerably greater adsorption of non-polar compounds has been observed on filters withpreviously collected vapor than on clean filters(Cotham and Bidleman, 1992; Mader and Pankow,2001b). Since dicarboxylic acids likely have a higheraffinity for water than other ambient vapors, thepresence of adsorbed water could have the oppositeeffect of promoting rather than inhibiting theiradsorption. As a result the net effects of tempera-ture and relative humidity are difficult to predict,and more research is needed to understand thebehavior of species sampling artifacts in otherlocations and under wider variety of atmosphericconditions.

3.5. Use of dicarboxylic acid measurements

Our results support other research indicating thatdicarboxylic acids, in particular phthalic acid andadipic acid, are promising potential indicators ofsecondary organic carbon. Although significantartifact errors are evident, they appear to be greatlyreduced at the high concentrations expected whensecondary particulate matter is of the greatestconcern. Concentration thresholds above which

artifact errors become less important were approxi-mately 2.5 ngm�3 for both adipic acid and phthalicacid. Similar concentration threshold patterns areapparent for other dicarboxylic acids. Further workis needed to determine whether similar thresholdsapply in other locations and conditions, and tounderstand the relationship between observedthresholds to adsorptive vapor concentrations andfilter adsorption capacity. However, these observa-tions clearly demonstrate the potential for use ofartifact thresholds as effective quantitation limitsfor dicarboxylic acid concentrations that will help toreduce uncertainty in estimating the secondarycontribution to organic particulate matter.

Acknowledgements

We are indebted to Dr. Richard Clark of Mill-ersville University for provision of ozone measure-ment data, Dr. Petros Koutrakis of HarvardUniversity for organic and elemental carbon andsulfate data, and Dr. Russell Philbrick of Penn StateUniversity for provision of space, resources, andoverall direction of the NEOPS project.

Disclaimer: The United States EnvironmentalProtection Agency through its Office of Researchand Development collaborated in the researchdescribed here under EPA Agreement numberR82373. It has been subjected to Agency reviewand approved for publication.

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