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Yearly trend of dicarboxylic acids in organic aerosols from south of Sweden and source attribution
Atmospheric Environment 57 (2012) 197e204
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Yearly trend of dicarboxylic acids in organic aerosols from south of Sweden and source attribution Murtaza Hyder a, *, Johan Genberg b, Margareta Sandahl a, Erik Swietlicki b, Jan Åke Jönsson a a b
Center for Analysis and Synthesis, Department of Chemistry, Lund University, POB 124, 22100 Lund, Sweden Division of Nuclear Physics, Department of Physics, Lund University, POB 118, 22100 Lund, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2011 Received in revised form 4 April 2012 Accepted 11 April 2012
Seven aliphatic dicarboxylic acids (C3eC9) along with phthalic acid, pinic acid and pinonic acid were determined in 35 aerosol (PM10) samples collected over the year at Vavihill sampling station in south of Sweden. Mixture of dichloromethane and methanol (ratio 1:3) was preferred over water for extraction of samples and extraction was assisted by ultrasonic agitation. Analytes were derivatized using N,Obis(trimethylsilyl)trifluoroacetamide (BSTFA) containing 1% trimethylsilyl chloride and analyzed using gas chromatography/mass spectrometry. Among studied analytes, azelaic acid was found maximum with an average concentration of 6.0 3.6 ng m3 and minimum concentration was found for pimelic acid (1.06 0.63 ng m3). A correlation coefficients analysis was used for defining the possible sources of analytes. Higher dicarboxylic acids (C7eC9) showed a strong correlation with each other (correlation coefficients (r) range, 0.96e0.97). Pinic and pinonic acids showed an increase in concentration during summer. Lower carbon number dicarboxylic acids (C3eC6) and phthalic acid were found strongly correlated, but showed a poor correlation with higher carbon number dicarboxylic acids (C7eC9), suggesting a different source for them. Biomass burning, vehicle exhaust, photo-oxidation of volatile organic compounds (natural and anthropogenic emissions) were possible sources for dicarboxylic acids. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Dicarboxylic acids Ultrasonic assisted solvent extraction Correlation coefficient Biomass burning Photo-oxidation Phthalic acid Pinonic acid
1. Introduction The impact of aerosols on cloud macro- and microphysical properties, and precipitation has received a great deal of attention for over 50 years (Levin and Brenguier, 2008). For understanding the link between aerosols, clouds and climate, the chemical composition of aerosol is considered a key factor. Carbonaceous aerosols are a significant subgroup of atmospheric aerosols, which consist of elemental carbon (EC) (black/graphitic carbon) and organic compounds (Sun and Ariya, 2006). A substantial fraction of the organic component of atmospheric particles consists of polar (water soluble), possibly multifunctional compounds (Saxena and Hildemann, 1996). Organic acids are a prominent group of polar organics that have been detected in urban, rural, marine and polar aerosols in various regions around the world (Kawamura and Ikushima, 1993; Kawamura et al., 1996). Among organic acids, aliphatic dicarboxylic acids are an important compound class and are recognized as ubiquitous organic components. Aliphatic dicarboxylic acids have been extensively studied in atmospheric aerosols from urban (Ho * Corresponding author. E-mail address: [email protected] (M. Hyder). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2012.04.027
et al., 2007), rural/suburban (Khwaja, 1995; Legrand et al., 2007), remote marine (Mochida et al., 2003), coastal marine (Kawamura et al., 2003), Arctic (Narukawa et al., 2003) and Antarctic regions (Kawamura et al., 1996). Due to their polarity and low-vapor pressure, dicarboxylic acids partition very strongly to the particulate phase in the atmosphere, and in part, they are responsible for reduction in the visibility of the atmosphere (i.e., they may act as nuclei for aerosol formation) (Kawamura and Usukura, 1993; Saxena and Hildemann, 1996). Dicarboxylic acids in fact have strong cloud condensation capabilities due to strong hydrophilic and hygroscopic properties (Cruz and Pandis, 1998; Kerminen, 2001). Oxalic acid has been found as the most abundant dicarboxylic acid as compared to higher homologues and is probably most important dicarboxylic acid for cloud condensation nucleation (CCN) activity. However, higher dicarboxylic acids are considered more as tracers for different sources as they are found in very low concentrations and are therefore considered less important in CCN activity. Typical concentrations of dicarboxylic acids in ambient air are a few to several hundred nanograms per cubic meter and thus these compounds contribute to a considerable fraction of the total identifiable resolved organic mass in fine aerosols (Kawamura and Yasui, 2005; Kippenberger et al., 2008). Their contribution to the
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total particulate carbon ranges from about 1 to 3% in the urban and semi-urban areas, to values close to or even above 10% in the remote marine environment (Kawamura and Ikushima, 1993; Kawamura and Sakaguchi, 1999). Dicarboxylic acids originate from a wide range of sources. Primary sources include motor exhaust (Kawamura and Kaplan, 1987), biomass combustion (Kundu et al., 2010) and oceanic emissions (Mochida et al., 2003). One of the main sources of dicarboxylic acids in airborne particulate matter is atmospheric photo-oxidation of volatile and semi-volatile organic compounds (Zhang et al., 2010). It has been proposed that low molecular weight dicarboxylic acids (C2eC6) originate from anthropogenic as well as biogenic precursors (Mochida et al., 2003). Glutaric acid (C5) and adipic acid (C6) are the homologues of dicarboxylic acids that are formed by the oxidation of cyclic and aliphatic olefins (Hatakeyama et al., 1987; Koch et al., 2000), while the suberic acid (C8) and azelaic acid (C9) are formed by the photo-oxidation of unsaturated carboxylic acids such as oleic and linoleic acids (Stephanou and Stratigakis, 1993). Biogenic volatile organic compounds (BVOCs) including isoprene, alcohols, ketones, monoterpenes, and sesquiterpenes, emitted by terrestrial vegetation have been estimated to about 10-folds higher in amount than anthropogenic volatile organic compounds (VOC) globally (Cahill et al., 2006; Seinfeld and Pandis, 2006). The photooxidation of biogenic hydrocarbons is a major source of secondary organic aerosol (SOA) (Hoffmann et al., 1997), and SOA formation by oxidation of pinene and other terpenes has been studied extensively (Hallquist et al., 2009). Pinic acid as a dicarboxylic acid and pinonic acid as an oxocarboxylic acid are major products of the ozonolysis or OH-initiated oxidation of pinenes (Atkinson and Arey, 2003; Hatakeyama et al., 1991). In the present study we collected aerosol samples on weekly basis from southern part of Sweden for one complete year (from April 2008 to April 2009). Samples were extracted using a mixture of methanol and dichloromethane because it is well known that solubility of low molecular dicarboxylic acids (C2 to C6) is almost same in water as in methanol. However, higher homologues of dicarboxylic acid (C7eC9) have relatively lower solubility in water than in methanol (Saxena and Hildemann, 1996). Also methanol is a better solvent for other organic acids like phthalic, pinic and pinonic acids etc. Methanol is a suitable organic solvent for gas chromatographic analysis, easy to evaporate at low temperature, hence, less risks of losing analytes in this step and saves time as few mL of water extract would consume a lot of time in evaporation for pre-concentration of analytes. Polar compounds require a derivatization process to analyze them using gas chromatography mass spectrometery (GCeMS) technique. In this study we used trimethylsilylation of carboxylic acids with BSTFA containing 1% trimethylsilyl chloride. BSTFA is sensitive to water contents; therefore organic solvent used for extraction is better than extraction with water also for the derivatization step. All samples were analyzed for aliphatic straight chain dicarboxylic acids (C3eC9), phthalic acid, pinic acid, and pinonic acid using GCeMS. Samples collected over the year were analyzed to see seasonal trend of analytes at the sampling site.
1-phenyldodecane (purity, 97%) was provided by Acros Organics (Geel, Belgium). Acetone (HPLC grade) and N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA 98%, excluding trimethylsilyl chloride) containing 1% trimethylsilyl chloride were also purchased from SigmaeAldrich. Methanol (purity, 99.9% HPLC grade) was provided from Fisher Scientific (Waltham, MA, USA). Helium gas (purity, 99.9995%) was provided from Strandmøllen Lab Line (Denmark). Ultrapure reagent water purified by a Milli-Q gradient system (Millipore, Bedford, MA, USA) was used. Individual standard solutions of each dicarboxylic acid were prepared at concentrations of 100 mg mL1 in methanol. A mixture solution containing the ten acids was prepared at concentration of 2 mg mL1 in methanol from each individual solution. An internal standard solution of 500 mg mL1 of 1-phenyldodecane was prepared in n-hexane. From this a solution of 2 mg mL1 was prepared in n-hexane. The calibration curves were established from standard solutions in methanol at six different concentrations levels (0.005 mg mL1e1.000 mg mL1) by injecting them in the GCeMS after derivatization with BSTFA (see below). Each level of concentration was analyzed in triplicate. All the solutions were stored in the refrigerator at 4 C. 2.2. Sample collection The sampling site Vavihill is a EUSAAR and EMEP (European Monitoring and Evaluation Programme) background site located in Southern Sweden (56 010 N, 13 090 E). No local pollution sources are situated near the sampling station, although the distances to the densely populated areas of Malmö, Copenhagen and Helsingborg west to southwest of the station are only 45, 50 and 25 km, respectively. Samples were collected weekly on 47 mm quartz filters (Pall TissuquartzÔ, binder free) by airflow of 38 L min1 from a PM10 inlet. Samples collected between April 2008 and April 2009 were used in this study. Filters were baked at 900 C for 4 h prior to sampling. After sampling, the filters were stored in a Petri dish, wrapped in aluminum foil in the refrigerator (þ8 C) or freezer (30 C). 2.3. Ultrasonic assisted solvent extraction Each filter (sample) piece (0.5 cm2 area) was divided into further small pieces and it was placed in a pre-cleaned, dry 50 mL conical flask. An aliquot of 15 mL solvent mixture of dichloromethane and methanol (ratio 1:3) was added to each flask. Extraction was carried out by sonicating for 45 min. The extract from each flask was collected in a separate beaker for each sample and then another aliquot of 10 mL of mixture of dichloromethane and methanol (1:3) was added to each flask for re-extraction for 30 min. This extract was added to respective beaker of each sample. Another extraction was done using 10 mL of mixture of the same solvents for 15 min. The total extract for each sample was concentrated to dryness by evaporation at 60 C under a gentle stream of N2. The final volume of extract was made up to 1 mL using CH2Cl2.
2. Materials and methods 2.4. Derivatization 2.1. Reagents and standards The standards of dicarboxylic acids were purchased from SigmaeAldrich (St. Louis, MO, USA) and consisted of malonic acid (purity, 99%), succinic acid (purity, 99.5%), glutaric acid (purity, 99%), adipic acid (purity, 99%), pimelic acid (purity, 99%), suberic acid (purity, 98%), azelaic acid (purity, 98%), phthalic acid (purity, 99.5%) (99%), pinic acid (purity, 99%) and cis-pinonic acid (purity, 98%).
50 mL of each solution for calibration curves and extract solution for samples analysis were taken in an autosampler vial with a 300 mL glass insert. It was evaporated to dryness under a stream of nitrogen at 60 C. Then 15 mL of 1-phenyldodecane (internal standard) solution (2 mg mL1 in n-hexane) and 10 mL of N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) containing 1% trimethylsilyl chloride was added to each vial. The vials were air sealed with screw cap
M. Hyder et al. / Atmospheric Environment 57 (2012) 197e204
having Teflon septa. Derivatization was allowed by putting vials in an oven at a temperature of 80 C for 1 h. Then the vial was cooled to room temperature and the contents were injected to GCeMS for analysis. 2.5. GCeMS analysis All the analyses were performed using a 6890 Series GC equipped with a split/splitless 7683 Series injector, an autosampler and a 5973-N Mass Selective detector (Agilent Technologies, Palo Alto, USA). Analytes were separated using an Agilent HP-5ms capillary GC column (Agilent Technologies 19091S-433) of 30 m 0.25 mm with a film thickness of 0.25 mm. The temperature program was: 70 C, hold 2 min, increased at 2.5 C min1 to 120 C, increased at 10 C min1 to 220 C and increased at 20 C min1 to a final temperature of 300 C (total analysis time: 36 min). The carrier gas used was helium (99.9995%) with a constant flow rate of 1.5 mL min1. The injector temperature was 290 C, and the injection was made in splitless mode with split vent opening time of 1.75 min. The injection volume was 1 mL. The transfer line, quadrupole and ion source temperatures were 280 C, 180 C and 250 C, respectively. The MS was operated in the electron impact ionization mode (EI) at 70 eV. Scan mode was used for the standards and for the identification of each compound. The mass scan range was set to m/z ¼ 50e500. For the quantification of analytes in the samples selective ion monitoring (SIM) mode was used. The most abundant ion was used for quantification in each case, while the second most abundant ions were usually used for the identification and confirmation of the analytes. The coefficients of determination (R2) for the regression lines ranged 0.995e0.999 for the different analytes studied. For method validation precisions in term of repeatability and reproducibility, limits of detections and the extraction recoveries for all analytes were determined. For this purpose, 12 pieces of filter (similar to those used for aerosol sampling) each with an area of 0.5 cm2, were spiked with known amount of analytes (20 mL of mixture with concentration of 2 mg mL1), dried at room temperature and then kept in refrigerator for 24 h. Then 6 of the spiked filters were extracted using same procedure as described above in Section 2.3 and analyzed on GCeMS and other 6 filters extracted and analyzed on the next day. Recoveries of the analytes ranged 60.7e79.1% and are shown in Table 1. Repeatability was found in a range of 4.7e9.3% and reproducibility 6.2e11.8% in terms of RSD. Limits of detection (LODs) were calculated as the minimum concentration providing chromatographic signals 3 times higher than background noise. The LODs for different analytes were in a range of 0.5e8.5 ng mL1 corresponding to concentrations of 0.6e1 ng m3 in aerosols.
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3. Results and discussions 3.1. Molecular distributions of dicarboxylic acids Table 1 summarizes the observed concentration ranges, mean values and standard deviations of the linear dicarboxylic acids with 3e9 carbon atoms as well as phthalic acid and pinene oxidation products (pinic acid and pinonic acid) investigated in this study. A homologous series of normal saturated dicarboxylic acids (C3eC9) was detected in all the samples studied. We did not study the oxalic acid as the derivatization method was not successful for oxalic acid. Total aliphatic dicarboxylic acids concentration ranged from 7.7 to 40.7 ng m3 (average 19.9 ng m3). Total aliphatic dicarboxylic acids were found to constitute from 0.39 to 2.81% (average ¼ 1.37%), of organic carbon (OC). The trend in the concentration of aliphatic dicarboxylic acids over the year studied is shown in Fig. 1. Azelaic Acid (C9) was one with the highest concentration among aliphatic dicarboxylic acids detected. Its concentration ranged from 1.6 to 16.2 ng m3 (Average ¼ 6.0 ng m3), with a maximum during summer on 15th of July. Azelaic acid has been reported with considerable high concentrations in aerosol but it has never been found as the most abundant component among dicarboxylic acids, except for oxalic acid. To some extent this difference can be due to relatively less recovery of lower carbon numbered dicarboxylic acids. However, even after compensation of loss in recovery, azelaic acid concentration remains the highest among dicarboxylic acids studied. Succinic acid was found as the second most abundant species followed by adipic acid. Succinic acid concentration ranged from 0.8 to 12.5 ng m3 with an average of 3.5 ng m3. In some other studies succinic acid has been reported with the highest concentration or second highest concentration among aliphatic dicarboxylic acids. Adipic acid was another considerably abundant dicarboxylic acid with a concentration range of 1.1e7.4 ng m3 (average ¼ 3.2). In general, aliphatic dicarboxylic acid concentrations have been reported to decrease with carbon number (Kawamura and Ikushima, 1993; Kawamura and Kaplan, 1987; Limbeck et al., 2001). The mean concentrations of individual dicarboxylic acids in particulate matter observed in this study (PM10, 1.1e6.0 ng m3) were generally lower than the values reported from polluted urban areas (0.29e35.6 ng m3) (Ho et al., 2007). However, in most of the cited studies samples were PM2.5 (particulate matter with an aerodynamic diameter 2.5 mm) and TSP (total suspended particulate) (Ho et al., 2006; Zhang et al., 2010). Few of the studies considered (or sampled) PM10 (particulate matter with an aerodynamic diameter 10 mm) for dicarboxylic acids analysis but either the sampling was done just for a short period of the year, or only a few samples were used for the study (Ray and McDow, 2005).
Table 1 Concentration range, mean concentrations, and standard deviation and relative abundance of analytes for 35 analyzed samples. Dicarboxylic Acid (Carbon Number)
Minimum (ng m3)
Maximum (ng m3)
Average (ng m3)
SD (ng m3)
Relative Abundance Range (%)
Average Relative Abundance SD (%)
Extraction Recoveries %
Malonic Acid (C3) Succinic Acid (C4) Glutaric Acid (C5) Adipic Acid (C6) Pimelic Acid (C7) Suberic Acid (C8) Azelaic Acid (C9) C3eC5 (Sum) C6eC9 (Sum) Phthalic Acid Pinonic Acid Pinic Acid
0.95 0.78 0.75 1.14 BDL 1.08 1.6 3.8 4.35 1.18 0.94 1.18
3.18 12.5 4.27 7.4 2.8 6.1 16.2 23.9 32.5 6.24 8.61 8.56
1.61 3.54 1.83 3.24 1.06 2.66 5.9 10.2 12.9 2.4 3.57 3.82
0.52 2.42 0.76 1.64 0.63 1.14 3.65 4.4 6.9 1.37 2.01 2.07
3.64e16.7 8.15e33.8 5.76e12.1 9.94e21.5 3.11e7.81 9.97e17.6 20.7e40.9 17.6e516 48.4e82.4 e e e
9.09 3.17 17.43 6.90 9.46 1.61 16.2 3.01 5.22 1.22 13.7 2.07 28.8 5.07 36.0 8.37 64.0 8.37 e e e
60.7 62.5 65.7 67 68.2 70.9 79.1 e e 69.4 73 72.8
BDL ¼ Below Detection Limit.
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Fig. 1. Yearly trend in the concentrations of aliphatic dicarboxylic acids (C3eC9).
In this study, we collected weekly samples over a complete year from April 2008 to March 2009. However, only 35 selected samples were analyzed for dicarboxylic acids and some of them were also analyzed for polycyclic aromatic hydrocarbons (PAHs) (Hyder et al., 2011). So, making comparison of mean values for individual compounds with other studies is not suitable. Looking closely at the data obtained in present study, it can be observed that lower dicarboxylic acids (C3eC6) show mean values less than those found in other studies, while mean values for larger (C8eC9) dicarboxylic acids are considerably higher, see Table 2. A different behavior in the concentrations of dicarboxylic acids in this study can be due to different extraction solvent and relatively lower recoveries of C3eC6 than those of C8eC9 or different sampling location/time. Along with aliphatic dicarboxylic acids, an aromatic dicarboxylic acid (phthalic acid) and other secondary organic aerosol constituents, pinic acid and pinonic acid have also been analyzed in this study. Phthalic acid concentrations ranged from 1.2 to 6.2 ng m3 with an average of 2.4 ng m3. An increasing trend was observed in the
concentration of phthalic acid from summer to winter. Pinic acid and pinonic acid, arising from oxidation of biogenic volatile organic compounds (terpenes) ranged between 1.2 and 8.5 ng m3 (average ¼ 3.8), and 0.9e8.6 ng m3 (average ¼ 3.6), respectively. The concentration of pinonic acid and pinic acid boosts up during summer (Fig. 2). However, another increase (not as much as in summer) in pinic and pinonic acid was observed in winter that may be associated with higher emission of pinenes in biomass burning. For example, Simpson et al. (2011) have shown that roughly 0.5 TgC yr1 is released from boreal forest fires in the form of pinenes (Simpson et al., 2011). Among aliphatic dicarboxlic acids, azelaic acid is found to have highest relative abundance (20.7e40.9%, average 28.9%). Succinic acid stands at second position with regards to relative abundance (8.2e33.9%, average 17.4%). Adipic acid followed succinic acid with relative abundance from 9.9 to 21.4% (average 16.2%). Least contribution to total aliphatic dicarboxylic acids was from pimelic acid with relative abundance ranging from 3.1 to 7.8% (average 5.2%).
M. Hyder et al. / Atmospheric Environment 57 (2012) 197e204
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Table 2 Concentration ranges, average and standard deviation of analyts in aerosol compared with some of previous studies. Concentration ng m3
Particle Size
Location, Time
Reference
1.61 0.51 14.4 68.4 41.6
PM10 PM10 PM2.5
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14)
This Study Ray and McDow, 2005 Ho et al., 2006
0.77-12.5 0.09e66.7 13.1e121
3.54 2.42 15.4 52.5 31.8
PM10 PM10 PM2.5
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14)
This Study Ray and McDow, 2005 Ho et al., 2006
0.75e4.27 0.05e5.7 2.82e28.1 0.16e6.15 1.24e18.41
1.82 2.3 13.5 1.28 2.56
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
1.13e7.39 0.07e7.4 3.78e32.1 0.17e2.71 0.59e4.69
3.24 2 11.7 0.78 1.51
7.53 0.57 0.84
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
Pimelic Acid (C7)
BDL2.79 0.74e9.69 BDL*-1.77 BDL-2.05
1.050.63 2.30 2.33 0.32 0.31 0.58 0.39
PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
Suberic Acid (C8)
1.07-6.1 0.03e1.4 0.00e6.67 0.11e1.87 0.32e2.44
2.65 0.5 2.25 0.44 0.78
1.14
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
1.616.2 0.03e2.6 6.01e28.1 0.12e4.64 0.13e1.45
5.96 1 12.9 1.60 2.91
6.74 1.04 1.25
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
1.18e6.24 0.07e16.3 40.1e105 0.69e13.38 0.32e2.44
2.4 1.37 3.5 83.9 21.3 3.76 2.87 0.78 0.45
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
Dicarboxylic Acid (Carbon Number)
Range
Average SD
Malonic Acid (C3)
0.94e3.18 0.11e95.6 10.5e145
Succinic Acid (C4)
Glutaric Acid (C5)
Adipic Acid (C6)
Azelaic Acid (C9)
Phthalic Acid
0.75 8.17 0.99 1.27 1.63
1.88 0.35 0.45 3.64
BDL ¼ Below Detection Limit.
Fig. 2. Yearly trend in the concentration of phthalic acid, pinonic acid and pinic acid.
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Table 3 Correlation coefficients for dicarboxylic acids (C3-C9), phthalic acid, pinonic acid, pinic acid and organic carbon.
Melonic Acid (C3) Succinic Acid (C4) Glutaric Acid (C5) Adipic Acid (C6) Pimelic Acid (C7) Suberic Acid (C8) Azelaic Acid (C9) Pinonic Acid Pinic Acid Phthalic Acid (Ph)
C3
C4
C5
C6
C7
C8
C9
Pinonic Acid
Pinic Acid
Ph
1.000 0.591 0.653 0.701 0.164 0.272 0.241 0.179 0.141 0.760
1.000 0.906 0.667 0.184 0.268 0.249 0.179 0.349 0.686
1.000 0.834 0.352 0.431 0.367 0.249 0.406 0.647
1.000 0.438 0.510 0.417 0.396 0.348 0.534
1.000 0.973 0.961 0.738 0.533 0.055
1.000 0.969 0.735 0.518 0.071
1.000 0.748 0.592 0.055
1.000 0.735 0.030
1.000 0.205
1.000
3.2. Source attribution to the acidic contents of organic aerosol Organic acids are not only emitted directly to the environment contributing to the primary organic aerosols but are also produced from oxidation of primarily emitted volatile organic compounds to form secondary organic aerosol (SOA). Among primary sources, biomass burning and direct vehicle emissions not only add dicaboxylic acids directly to the environment but also introduce many volatile organic species that ultimately oxidize to give dicarboxylic acids, and also less volatile compounds with a great tendency to contribute to secondary particle formation. Table 3 shows a comprehensive picture of correlation coefficients among aliphatic dicarboxylic acids, phthalic acid, pinonic acid and pinic acid. It is very clear that the higher dicarboylic acids (C7eC9) are very nicely correlated, with correlation coefficients (r) ranging from 0.96 to 0.97, which strongly suggest similar source for these dicarboxylic acids. Azelaic acid (C9) and subaric acid (C8) have been intensively studied and are believed to be produced as photooxidation products of unsaturated carboxylic acids such as oleic and linoleic acids having unsaturation at carbon 9 position (Stephanou and Stratigakis, 1993). A strong correlation among C7 to C9 may help in drawing an inference that pimelic acids (C7) is produced either directly from the same sources as those of suberic acid (C8) and azelaic acid (C9) or they are produced by further oxidation of suberic acid and azelaic acid down to lower carbon numbered dicarboxylic acids. Similarly for lower carbon numbered dicarboxylic acids (C3 to C6) and phthalic acid we observe that they are correlated well to each other with correlation coefficients (r) ranging from 0.53 to 0.90 so we may infer that sources attributed to these dicarboxylic acid are similar or at least very closely related. Malonic acid (C3) is more strongly correlated with phthalic acid (r ¼ 0.76), which may suggest
that they have possibly same source. (C4eC6) dicarboxylic acids show correlation coefficients ranging from 0.667 to 0.906, which suggests a similar source for them as well. Photo-oxidation of anthropogenic precursors have been reported to be the main source of lower carbon numbered dicarboxylic acids (Kawamura and Yasui, 2005). For example, glutaric acid and adipic acid are believed to be produced by photo-oxidation of unsaturated cyclic alkenes (Narukawa et al., 2007). If we look at correlations coefficients we find that low carbon numbered dicarboxlic acids (C3eC6) are very poorly correlated to higher dicarboxlic acids (C7eC9) with correlation coefficients ranging from 0.16 to 0.34. It can be concluded that lower carbon numbered dicarboxylic acids have totally different origin than that of higher carbon numbered dicarboxylic acids (C6eC9). Phthalic acid is an aromatic dicarboxylic acid that has been reported to originate from multiple anthropogenic sources. It may have origin from primary sources like biomass burning or vehicle emission or photo-oxidation of polycyclic aromatic hydrocarbons and phthalates may produce it as a secondary oxidation product in the environment. If we look at its correlation with aliphatic dicarboxylic acids, we find that it is well correlated with lower carbon numbered dicarboxylic acids (C3eC6) with correlation coefficients (r) ranging from 0.65 to 0.76, while very poorly correlated to higher carbon dicarboxylic acids (C7eC9) with correlation coefficients ranging from 0.055 to 0.071. Also phthalic acid shows a very poor correlation with pinic acid and pinonic acid, which suggest anthropogenic secondary formation of phthalic acid. However, Fig. 3 shows a trend in the ratio of phthalic acid to adipic acid. It increases from summer to winter and it may be due to an increase in the primary emission of phthalic acid from gasoline and diesel engines and partially from biomass burning. Also phthalic acid is
Fig. 3. Phthalic Acid to Adipic Acid concentration ratio over the year.
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Fig. 4. Trend for sum of collective relative abundances of lower carbon dicarboxylic acids (C3 to C6) and higher carbon dicarboxylic acids (C7 to C9).
well correlated with malonic acid (C3) and succinic acid (C4) that supports the photo-oxidation of anthropogenic precursors as an origin of phthalic acid. Also the significant contribution of primary emissions vs. SOA can also be confirmed by the low correlation between C3 and C4 acids (0.591). Fig. 4 shows the trend in the relative abundance for the sum of (C3 to C6) and (C7 to C9). This clearly demonstrates the different trend for both sums over the year, so it supports the suggestion that lower carbon numbered dicarboxylic acids (C3 to C6) have different source than those of higher carbon dicarboxylic acids (C7 to C9). In order to verify the above-mentioned sources of dicarboxylic acids, we also performed a principal component (PC) analysis to the data set. The original PC solution was subjected to a VARIMAX rotation in order to facilitate the interpretation of the various PCs as sources. A solution with three PCs (sources) was chosen, in agreement with the previous discussion. These three PCs together account for about 87% of the cumulative variance in the original data. The first PC characterizes the secondary formation of dicarboxylic acids from anthropogenic precursors with high loadings of C3eC6 dicarboxylic acids. The second PC characterizes the photooxidation formation of C7eC9 originating from unsaturated fatty acids and third PC characterizes the photo-oxidation production of pinic acid and pinonic acid from biogenic precursors. In conclusion, the PC solution provides the same interpretation of the sources of dicarboxylic acids as given by the correlation matrix itself, which is hardly surprising since the PC analysis is based on this matrix. 3.3. Conclusion Seven aliphatic dicarboxylic acids (C3eC9) along with phthalic acid, pinic acid and pinonic acid were analyzed from 35 aerosol samples collected over the year at Vavihill sampling station in south of Sweden. Azelaic acid was found the most abundant and pimelic acid (C7) was found the least abundant component among analyzed aliphatic dicarboxylic acids. Pimelic acid (C7), suberic acid (C8) and azelaic acid (C9) show a maximum during summer and are considered as photo-oxidation products of biogenic precursors and hence are secondary organic compounds in aerosols. They showed strong correlation that may suggest a similar source for these compounds. Malonic acid (C3), succinic acid (C4) and phthalic acid are well
correlated and it indicates a common origin for them i.e. photooxidation of anthropogenic precursors. It is obvious from the results that lower carbon numbered dicarboxylic acids have different yearly trend than that of higher carbon dicarboxylic acids. Pinic acid and pinonic acid that are considered as biogenic secondary organic aerosols also have boosting up concentrations during summertime. References Atkinson, R., Arey, J., 2003. Gas-phase tropospheric chemistry of biogenic volatile organic compounds: a review. Atmos. Environ. 37, S197eS219. Cahill, M.T., Seaman, Y.V., Charles, M.J., Holzinger, R., Goldstein, H.A., 2006. Secondary organic aerosols formed from oxidation of biogenic volatile organic compounds in the Sierra Nevada Mountains of California. J. Geophys. Res. 110, 1e14. Cruz, C.N., Pandis, S.N., 1998. The effect of organic coatings on the cloud condensation nuclei activation of inorganic atmospheric aerosol. J. Geophys. Res. 103, 13111e13123. Hallquist, M., Wenger, C.J., Baltensperger, U., Rudich, Y., Simpson, D., Claeys, M., Dommen, J., Donahue, M.N., George, C., Goldstein, H.A., Hamilton, F.J., Herrmann, H., Hoffmann, T., Iinuma, Y., Jang, M., Jenkin, M., Jimenez, L.J., KiendlerScharr, A., Maenhaut, W., McFiggans, G., Mentel, F.T., Monod, A., Prévôt, H.S.A., Seinfeld, H.J., Surratt, D.J., Szmigielski, R., Wildt, J., 2009. The formation, properties and impact of secondary organic aerosol: current and emerging issues. Atmos. Chem. Phys. 9, 5155e5236. Hatakeyama, S., Ohno, M., Weng, J.H., Takagi, H., Akimoto, H., 1987. Mechanism for the formation of gaseous and particulate products from ozone-cycloalkene reactions in air. Environ. Sci. Technol. 21, 52e57. Hatakeyama, S., Izumi, K., Fukuyama, T., Akimoto, H., Washida, N., 1991. Reactions of OH with a-pinene and ß-pinene in air: estimate of global CO production from the atmospheric oxidation of terpenes. J. Geophys. Res. 96, 947e958. Ho, K.F., Lee, S.C., Cao, J.J., Kawamura, K., Watanabe, T., Cheng, Y., Chowd, J.C., 2006. Dicarboxylic acids, ketocarboxylic acids and dicarbonyls in the urban roadside area of Hong Kong. Atmos. Environ. 40, 3030e3040. Ho, K.F., Cao, J.J., Lee, S.C., Kawamura, K., Zhang, R.J., Chow, J.C., Watson, J.G., 2007. Dicarboxylic acids, ketocarboxylic acids, and dicarbonyls in the urban atmosphere of China. J. Geophys. Res. 112 Hoffmann, T., Odum, J.R., Bowman, F., Collins, D., Klockow, D., Flagan, R.C., Seinfeld, J.H., 1997. Formation of organic aerosols from the oxidation of biogenic hydrocarbons. J. Atmos. Chem. 26, 189e222. Hyder, M., Aguilar, L.L., Genberg, J., Sandahl, M., Wesén, C., Jönsson, J.Å., 2011. Determination of polycyclic aromatic hydrocarbons (PAHs) from organic aerosols using hollow fiber microeporous membrane liquideliquid extraction (HF-MMLLE) followed by gas chromatographyemass spectrometry analysis. Talanta 85, 919e926. Kawamura, K., Ikushima, K., 1993. Seasonal changes in the distribution of dicarboxylic acids in the urban atmosphere. Environ. Sci. Technol. 27, 2227e2235. Kawamura, K., Kaplan, I.R., 1987. Motor exhaust emissions as a primary source for dicarboxylic-acids in Los-Angeles ambient air. Environ. Sci. Technol. 21, 105e110.
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Kawamura, K., Sakaguchi, F., 1999. Molecular distributions of water soluble dicarboxylic acids in marine aerosols over the Pacific Ocean including tropics. J. Geophys. Res. 104, 3501e3509. Kawamura, K., Usukura, K., 1993. Distributions of low molecular weight dicarboxylic acids in the north pacific aerosol samples. J. Oceanogr. 49, 271e283. Kawamura, K., Yasui, O., 2005. Diurnal changes in the distribution of dicarboxylic acids, ketocarboxylic acids and dicarbonyls in the urban Tokyo atmosphere. Atmos. Environ. 39, 1945e1960. Kawamura, K., Semere, R., Imai, Y., Fujii, Y., Hayashi, M., 1996. Water soluble dicarboxylic acids and related compounds in Antarctic aerosols. J. Geophys. Res.-Atmos. 101, 18721e18728. Kawamura, K., Umemoto, N., Mochida, M., Bertram, T., Howell, S., Huebert, J.B., 2003. Water-soluble dicarboxylic acids in the tropospheric aerosols collected over East Asia and western North Pacific by ACE-Asia C-130 aircraft. J. Geophys. Res. 108, 8639. Kerminen, V.-M., 2001. Relative roles of secondary sulfate and organics in atmospheric cloud condensation nuclei production. J. Geophys. Res. 106, 17321e17333. Khwaja, H.A., 1995. Atmospheric concentrations of carboxylic acids and related compounds at a semiurban site. Atmos. Environ. 29, 127e139. Kippenberger, M., Winterhalter, R., Moortgat, G.K., 2008. Determination of higher carboxylic acids in snow samples using solid-phase extraction and LC/MS-TOF. Anal. Bioanal. Chem. 392, 1459. Koch, S., Winterhalter, R., Uherek, E., Kolloff, A., Neeb, P., Moortgat, K.G., 2000. Formation of new particles in the gas-phase ozonolysis of monoterpenes. Atmos. Environ. 34, 4031e4042. Kundu, S., Kawamura, K., Andreae, T.W., Hoffer, A., Andreae, M.O., 2010. Molecular distributions of dicarboxylic acids, ketocarboxylic acids and alpha-dicarbonyls in biomass burning aerosols: implications for photochemical production and degradation in smoke layers. Atmos. Chem. Phys. 10, 2209e2225. Legrand, M., Preunkert, S., Oliveira, T., Pio, C.A., Hammer, S., Gelencer, A., KasperGiebl, A., Laj, P., 2007. Origin of C2eC5 dicarboxylic acids in the European atmosphere inferred from year round aerosol study conducted at a westeeast transect. J. Geophys. Res. 112 Levin, Z., Brenguier, J.-L., 2008. Aerosol Pollution Impact on Precipatation, a Scientific Review: Effects of Pollution and Biomass Aerosols on Clouds and Precipitation: Observational Studies, first ed. Springer, Berlin.
Limbeck, A., Puxbaum, H., Otter, L., Scholes, M.C., 2001. Semivolatile behavior of dicarboxylic acids and other polar organic species at a rural background site (Nylsvley, RSA). Atmos. Environ. 35 Mochida, M., Kawabata, A., Kawamura, K., Hatsushika, H., Yamazaki, K., 2003. Seasonal variation and origins of dicarboxylic acids in the marine atmosphere over the western North Pacific. J. Geophys. Res.-Atmos. 108, 4193. Narukawa, M., Kawamura, K., Anlauf, K.G., Barrie, L.A., 2003. Fine and coarse modes of dicarboxylic acids in the arctic aerosols collected during the polar sunrise experiment 1997. J. Geophys. Res. 108, 4575. Narukawa, M., Matsumi, Y., Matsumoto, J., Takahashi, K., Yabushita, A., Sato, K., Imamura, T., 2007. Real-time analysis of secondary organic aerosol particles formed from cyclohexene ozonolysis using a laser-ionization single-particle aerosol mass spectrometer. Anal. Sci. 23, 507e512. Ray, J., McDow, S.R., 2005. Dicarboxylic acid concentration trends and sampling artifacts. Atmos. Environ. 39, 7906e7919. Saxena, P., Hildemann, L.M., 1996. Water-soluble organics in atmospheric particles: a critical review of the literature and application of thermodynamics to identify candidate compounds. J. Atmos. Chem. 24, 57e109. Seinfeld, J.H., Pandis, S., 2006. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, first ed. John Wiley & Sons, Hoboken, N. J. Simpson, I.J., Akagi, S.K., Barletta, B., Blake, N.J., Choi, Y., Diskin, G.S., Fried, A., Fuelberg, H.E., Meinardi, S., Rowland, F.S., Vay, S.A., Weinheimer, A.J., Wennberg, P.O., Wiebring, P., Wisthaler, A., Yang, M., Yokelson, R.J., Blake, D.R., 2011. Boreal forest fire emissions in fresh Canadian smoke plumes: C1eC10 volatile organic compounds (VOCs), CO2, CO, NO2, NO, HCN and CH3CN. Atmos. Chem. Phys. 11, 6445e6463. Stephanou, E.G., Stratigakis, N., 1993. Oxocarboxylic and a,u-dicarboxylic acids: photooxidation products of biogenic unsaturated fatty acids present in urban aerosols. Environ. Sci. Technol. 27, 1403. Sun, J., Ariya, A.P., 2006. Atmospheric organic and bio-aerosols as cloud condensation nuclei (CCN): a review. Atmos. Environ. 40, 795e820. Zhang, Y.Y., Müller, L., Winterhalter, R., Moortgat, G.K., Hoffmann, T., Pöschl, U., 2010. Seasonal cycle and temperature dependence of pinene oxidation products, dicarboxylic acids and nitrophenols in fine and coarse air particulate matter. Atmos. Chem. Phys. 10, 7859e7873.
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Yearly trend of dicarboxylic acids in organic aerosols from south of Sweden and source attribution Murtaza Hyder a, *, Johan Genberg b, Margareta Sandahl a, Erik Swietlicki b, Jan Åke Jönsson a a b
Center for Analysis and Synthesis, Department of Chemistry, Lund University, POB 124, 22100 Lund, Sweden Division of Nuclear Physics, Department of Physics, Lund University, POB 118, 22100 Lund, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2011 Received in revised form 4 April 2012 Accepted 11 April 2012
Seven aliphatic dicarboxylic acids (C3eC9) along with phthalic acid, pinic acid and pinonic acid were determined in 35 aerosol (PM10) samples collected over the year at Vavihill sampling station in south of Sweden. Mixture of dichloromethane and methanol (ratio 1:3) was preferred over water for extraction of samples and extraction was assisted by ultrasonic agitation. Analytes were derivatized using N,Obis(trimethylsilyl)trifluoroacetamide (BSTFA) containing 1% trimethylsilyl chloride and analyzed using gas chromatography/mass spectrometry. Among studied analytes, azelaic acid was found maximum with an average concentration of 6.0 3.6 ng m3 and minimum concentration was found for pimelic acid (1.06 0.63 ng m3). A correlation coefficients analysis was used for defining the possible sources of analytes. Higher dicarboxylic acids (C7eC9) showed a strong correlation with each other (correlation coefficients (r) range, 0.96e0.97). Pinic and pinonic acids showed an increase in concentration during summer. Lower carbon number dicarboxylic acids (C3eC6) and phthalic acid were found strongly correlated, but showed a poor correlation with higher carbon number dicarboxylic acids (C7eC9), suggesting a different source for them. Biomass burning, vehicle exhaust, photo-oxidation of volatile organic compounds (natural and anthropogenic emissions) were possible sources for dicarboxylic acids. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Dicarboxylic acids Ultrasonic assisted solvent extraction Correlation coefficient Biomass burning Photo-oxidation Phthalic acid Pinonic acid
1. Introduction The impact of aerosols on cloud macro- and microphysical properties, and precipitation has received a great deal of attention for over 50 years (Levin and Brenguier, 2008). For understanding the link between aerosols, clouds and climate, the chemical composition of aerosol is considered a key factor. Carbonaceous aerosols are a significant subgroup of atmospheric aerosols, which consist of elemental carbon (EC) (black/graphitic carbon) and organic compounds (Sun and Ariya, 2006). A substantial fraction of the organic component of atmospheric particles consists of polar (water soluble), possibly multifunctional compounds (Saxena and Hildemann, 1996). Organic acids are a prominent group of polar organics that have been detected in urban, rural, marine and polar aerosols in various regions around the world (Kawamura and Ikushima, 1993; Kawamura et al., 1996). Among organic acids, aliphatic dicarboxylic acids are an important compound class and are recognized as ubiquitous organic components. Aliphatic dicarboxylic acids have been extensively studied in atmospheric aerosols from urban (Ho * Corresponding author. E-mail address: [email protected] (M. Hyder). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2012.04.027
et al., 2007), rural/suburban (Khwaja, 1995; Legrand et al., 2007), remote marine (Mochida et al., 2003), coastal marine (Kawamura et al., 2003), Arctic (Narukawa et al., 2003) and Antarctic regions (Kawamura et al., 1996). Due to their polarity and low-vapor pressure, dicarboxylic acids partition very strongly to the particulate phase in the atmosphere, and in part, they are responsible for reduction in the visibility of the atmosphere (i.e., they may act as nuclei for aerosol formation) (Kawamura and Usukura, 1993; Saxena and Hildemann, 1996). Dicarboxylic acids in fact have strong cloud condensation capabilities due to strong hydrophilic and hygroscopic properties (Cruz and Pandis, 1998; Kerminen, 2001). Oxalic acid has been found as the most abundant dicarboxylic acid as compared to higher homologues and is probably most important dicarboxylic acid for cloud condensation nucleation (CCN) activity. However, higher dicarboxylic acids are considered more as tracers for different sources as they are found in very low concentrations and are therefore considered less important in CCN activity. Typical concentrations of dicarboxylic acids in ambient air are a few to several hundred nanograms per cubic meter and thus these compounds contribute to a considerable fraction of the total identifiable resolved organic mass in fine aerosols (Kawamura and Yasui, 2005; Kippenberger et al., 2008). Their contribution to the
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total particulate carbon ranges from about 1 to 3% in the urban and semi-urban areas, to values close to or even above 10% in the remote marine environment (Kawamura and Ikushima, 1993; Kawamura and Sakaguchi, 1999). Dicarboxylic acids originate from a wide range of sources. Primary sources include motor exhaust (Kawamura and Kaplan, 1987), biomass combustion (Kundu et al., 2010) and oceanic emissions (Mochida et al., 2003). One of the main sources of dicarboxylic acids in airborne particulate matter is atmospheric photo-oxidation of volatile and semi-volatile organic compounds (Zhang et al., 2010). It has been proposed that low molecular weight dicarboxylic acids (C2eC6) originate from anthropogenic as well as biogenic precursors (Mochida et al., 2003). Glutaric acid (C5) and adipic acid (C6) are the homologues of dicarboxylic acids that are formed by the oxidation of cyclic and aliphatic olefins (Hatakeyama et al., 1987; Koch et al., 2000), while the suberic acid (C8) and azelaic acid (C9) are formed by the photo-oxidation of unsaturated carboxylic acids such as oleic and linoleic acids (Stephanou and Stratigakis, 1993). Biogenic volatile organic compounds (BVOCs) including isoprene, alcohols, ketones, monoterpenes, and sesquiterpenes, emitted by terrestrial vegetation have been estimated to about 10-folds higher in amount than anthropogenic volatile organic compounds (VOC) globally (Cahill et al., 2006; Seinfeld and Pandis, 2006). The photooxidation of biogenic hydrocarbons is a major source of secondary organic aerosol (SOA) (Hoffmann et al., 1997), and SOA formation by oxidation of pinene and other terpenes has been studied extensively (Hallquist et al., 2009). Pinic acid as a dicarboxylic acid and pinonic acid as an oxocarboxylic acid are major products of the ozonolysis or OH-initiated oxidation of pinenes (Atkinson and Arey, 2003; Hatakeyama et al., 1991). In the present study we collected aerosol samples on weekly basis from southern part of Sweden for one complete year (from April 2008 to April 2009). Samples were extracted using a mixture of methanol and dichloromethane because it is well known that solubility of low molecular dicarboxylic acids (C2 to C6) is almost same in water as in methanol. However, higher homologues of dicarboxylic acid (C7eC9) have relatively lower solubility in water than in methanol (Saxena and Hildemann, 1996). Also methanol is a better solvent for other organic acids like phthalic, pinic and pinonic acids etc. Methanol is a suitable organic solvent for gas chromatographic analysis, easy to evaporate at low temperature, hence, less risks of losing analytes in this step and saves time as few mL of water extract would consume a lot of time in evaporation for pre-concentration of analytes. Polar compounds require a derivatization process to analyze them using gas chromatography mass spectrometery (GCeMS) technique. In this study we used trimethylsilylation of carboxylic acids with BSTFA containing 1% trimethylsilyl chloride. BSTFA is sensitive to water contents; therefore organic solvent used for extraction is better than extraction with water also for the derivatization step. All samples were analyzed for aliphatic straight chain dicarboxylic acids (C3eC9), phthalic acid, pinic acid, and pinonic acid using GCeMS. Samples collected over the year were analyzed to see seasonal trend of analytes at the sampling site.
1-phenyldodecane (purity, 97%) was provided by Acros Organics (Geel, Belgium). Acetone (HPLC grade) and N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA 98%, excluding trimethylsilyl chloride) containing 1% trimethylsilyl chloride were also purchased from SigmaeAldrich. Methanol (purity, 99.9% HPLC grade) was provided from Fisher Scientific (Waltham, MA, USA). Helium gas (purity, 99.9995%) was provided from Strandmøllen Lab Line (Denmark). Ultrapure reagent water purified by a Milli-Q gradient system (Millipore, Bedford, MA, USA) was used. Individual standard solutions of each dicarboxylic acid were prepared at concentrations of 100 mg mL1 in methanol. A mixture solution containing the ten acids was prepared at concentration of 2 mg mL1 in methanol from each individual solution. An internal standard solution of 500 mg mL1 of 1-phenyldodecane was prepared in n-hexane. From this a solution of 2 mg mL1 was prepared in n-hexane. The calibration curves were established from standard solutions in methanol at six different concentrations levels (0.005 mg mL1e1.000 mg mL1) by injecting them in the GCeMS after derivatization with BSTFA (see below). Each level of concentration was analyzed in triplicate. All the solutions were stored in the refrigerator at 4 C. 2.2. Sample collection The sampling site Vavihill is a EUSAAR and EMEP (European Monitoring and Evaluation Programme) background site located in Southern Sweden (56 010 N, 13 090 E). No local pollution sources are situated near the sampling station, although the distances to the densely populated areas of Malmö, Copenhagen and Helsingborg west to southwest of the station are only 45, 50 and 25 km, respectively. Samples were collected weekly on 47 mm quartz filters (Pall TissuquartzÔ, binder free) by airflow of 38 L min1 from a PM10 inlet. Samples collected between April 2008 and April 2009 were used in this study. Filters were baked at 900 C for 4 h prior to sampling. After sampling, the filters were stored in a Petri dish, wrapped in aluminum foil in the refrigerator (þ8 C) or freezer (30 C). 2.3. Ultrasonic assisted solvent extraction Each filter (sample) piece (0.5 cm2 area) was divided into further small pieces and it was placed in a pre-cleaned, dry 50 mL conical flask. An aliquot of 15 mL solvent mixture of dichloromethane and methanol (ratio 1:3) was added to each flask. Extraction was carried out by sonicating for 45 min. The extract from each flask was collected in a separate beaker for each sample and then another aliquot of 10 mL of mixture of dichloromethane and methanol (1:3) was added to each flask for re-extraction for 30 min. This extract was added to respective beaker of each sample. Another extraction was done using 10 mL of mixture of the same solvents for 15 min. The total extract for each sample was concentrated to dryness by evaporation at 60 C under a gentle stream of N2. The final volume of extract was made up to 1 mL using CH2Cl2.
2. Materials and methods 2.4. Derivatization 2.1. Reagents and standards The standards of dicarboxylic acids were purchased from SigmaeAldrich (St. Louis, MO, USA) and consisted of malonic acid (purity, 99%), succinic acid (purity, 99.5%), glutaric acid (purity, 99%), adipic acid (purity, 99%), pimelic acid (purity, 99%), suberic acid (purity, 98%), azelaic acid (purity, 98%), phthalic acid (purity, 99.5%) (99%), pinic acid (purity, 99%) and cis-pinonic acid (purity, 98%).
50 mL of each solution for calibration curves and extract solution for samples analysis were taken in an autosampler vial with a 300 mL glass insert. It was evaporated to dryness under a stream of nitrogen at 60 C. Then 15 mL of 1-phenyldodecane (internal standard) solution (2 mg mL1 in n-hexane) and 10 mL of N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) containing 1% trimethylsilyl chloride was added to each vial. The vials were air sealed with screw cap
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having Teflon septa. Derivatization was allowed by putting vials in an oven at a temperature of 80 C for 1 h. Then the vial was cooled to room temperature and the contents were injected to GCeMS for analysis. 2.5. GCeMS analysis All the analyses were performed using a 6890 Series GC equipped with a split/splitless 7683 Series injector, an autosampler and a 5973-N Mass Selective detector (Agilent Technologies, Palo Alto, USA). Analytes were separated using an Agilent HP-5ms capillary GC column (Agilent Technologies 19091S-433) of 30 m 0.25 mm with a film thickness of 0.25 mm. The temperature program was: 70 C, hold 2 min, increased at 2.5 C min1 to 120 C, increased at 10 C min1 to 220 C and increased at 20 C min1 to a final temperature of 300 C (total analysis time: 36 min). The carrier gas used was helium (99.9995%) with a constant flow rate of 1.5 mL min1. The injector temperature was 290 C, and the injection was made in splitless mode with split vent opening time of 1.75 min. The injection volume was 1 mL. The transfer line, quadrupole and ion source temperatures were 280 C, 180 C and 250 C, respectively. The MS was operated in the electron impact ionization mode (EI) at 70 eV. Scan mode was used for the standards and for the identification of each compound. The mass scan range was set to m/z ¼ 50e500. For the quantification of analytes in the samples selective ion monitoring (SIM) mode was used. The most abundant ion was used for quantification in each case, while the second most abundant ions were usually used for the identification and confirmation of the analytes. The coefficients of determination (R2) for the regression lines ranged 0.995e0.999 for the different analytes studied. For method validation precisions in term of repeatability and reproducibility, limits of detections and the extraction recoveries for all analytes were determined. For this purpose, 12 pieces of filter (similar to those used for aerosol sampling) each with an area of 0.5 cm2, were spiked with known amount of analytes (20 mL of mixture with concentration of 2 mg mL1), dried at room temperature and then kept in refrigerator for 24 h. Then 6 of the spiked filters were extracted using same procedure as described above in Section 2.3 and analyzed on GCeMS and other 6 filters extracted and analyzed on the next day. Recoveries of the analytes ranged 60.7e79.1% and are shown in Table 1. Repeatability was found in a range of 4.7e9.3% and reproducibility 6.2e11.8% in terms of RSD. Limits of detection (LODs) were calculated as the minimum concentration providing chromatographic signals 3 times higher than background noise. The LODs for different analytes were in a range of 0.5e8.5 ng mL1 corresponding to concentrations of 0.6e1 ng m3 in aerosols.
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3. Results and discussions 3.1. Molecular distributions of dicarboxylic acids Table 1 summarizes the observed concentration ranges, mean values and standard deviations of the linear dicarboxylic acids with 3e9 carbon atoms as well as phthalic acid and pinene oxidation products (pinic acid and pinonic acid) investigated in this study. A homologous series of normal saturated dicarboxylic acids (C3eC9) was detected in all the samples studied. We did not study the oxalic acid as the derivatization method was not successful for oxalic acid. Total aliphatic dicarboxylic acids concentration ranged from 7.7 to 40.7 ng m3 (average 19.9 ng m3). Total aliphatic dicarboxylic acids were found to constitute from 0.39 to 2.81% (average ¼ 1.37%), of organic carbon (OC). The trend in the concentration of aliphatic dicarboxylic acids over the year studied is shown in Fig. 1. Azelaic Acid (C9) was one with the highest concentration among aliphatic dicarboxylic acids detected. Its concentration ranged from 1.6 to 16.2 ng m3 (Average ¼ 6.0 ng m3), with a maximum during summer on 15th of July. Azelaic acid has been reported with considerable high concentrations in aerosol but it has never been found as the most abundant component among dicarboxylic acids, except for oxalic acid. To some extent this difference can be due to relatively less recovery of lower carbon numbered dicarboxylic acids. However, even after compensation of loss in recovery, azelaic acid concentration remains the highest among dicarboxylic acids studied. Succinic acid was found as the second most abundant species followed by adipic acid. Succinic acid concentration ranged from 0.8 to 12.5 ng m3 with an average of 3.5 ng m3. In some other studies succinic acid has been reported with the highest concentration or second highest concentration among aliphatic dicarboxylic acids. Adipic acid was another considerably abundant dicarboxylic acid with a concentration range of 1.1e7.4 ng m3 (average ¼ 3.2). In general, aliphatic dicarboxylic acid concentrations have been reported to decrease with carbon number (Kawamura and Ikushima, 1993; Kawamura and Kaplan, 1987; Limbeck et al., 2001). The mean concentrations of individual dicarboxylic acids in particulate matter observed in this study (PM10, 1.1e6.0 ng m3) were generally lower than the values reported from polluted urban areas (0.29e35.6 ng m3) (Ho et al., 2007). However, in most of the cited studies samples were PM2.5 (particulate matter with an aerodynamic diameter 2.5 mm) and TSP (total suspended particulate) (Ho et al., 2006; Zhang et al., 2010). Few of the studies considered (or sampled) PM10 (particulate matter with an aerodynamic diameter 10 mm) for dicarboxylic acids analysis but either the sampling was done just for a short period of the year, or only a few samples were used for the study (Ray and McDow, 2005).
Table 1 Concentration range, mean concentrations, and standard deviation and relative abundance of analytes for 35 analyzed samples. Dicarboxylic Acid (Carbon Number)
Minimum (ng m3)
Maximum (ng m3)
Average (ng m3)
SD (ng m3)
Relative Abundance Range (%)
Average Relative Abundance SD (%)
Extraction Recoveries %
Malonic Acid (C3) Succinic Acid (C4) Glutaric Acid (C5) Adipic Acid (C6) Pimelic Acid (C7) Suberic Acid (C8) Azelaic Acid (C9) C3eC5 (Sum) C6eC9 (Sum) Phthalic Acid Pinonic Acid Pinic Acid
0.95 0.78 0.75 1.14 BDL 1.08 1.6 3.8 4.35 1.18 0.94 1.18
3.18 12.5 4.27 7.4 2.8 6.1 16.2 23.9 32.5 6.24 8.61 8.56
1.61 3.54 1.83 3.24 1.06 2.66 5.9 10.2 12.9 2.4 3.57 3.82
0.52 2.42 0.76 1.64 0.63 1.14 3.65 4.4 6.9 1.37 2.01 2.07
3.64e16.7 8.15e33.8 5.76e12.1 9.94e21.5 3.11e7.81 9.97e17.6 20.7e40.9 17.6e516 48.4e82.4 e e e
9.09 3.17 17.43 6.90 9.46 1.61 16.2 3.01 5.22 1.22 13.7 2.07 28.8 5.07 36.0 8.37 64.0 8.37 e e e
60.7 62.5 65.7 67 68.2 70.9 79.1 e e 69.4 73 72.8
BDL ¼ Below Detection Limit.
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Fig. 1. Yearly trend in the concentrations of aliphatic dicarboxylic acids (C3eC9).
In this study, we collected weekly samples over a complete year from April 2008 to March 2009. However, only 35 selected samples were analyzed for dicarboxylic acids and some of them were also analyzed for polycyclic aromatic hydrocarbons (PAHs) (Hyder et al., 2011). So, making comparison of mean values for individual compounds with other studies is not suitable. Looking closely at the data obtained in present study, it can be observed that lower dicarboxylic acids (C3eC6) show mean values less than those found in other studies, while mean values for larger (C8eC9) dicarboxylic acids are considerably higher, see Table 2. A different behavior in the concentrations of dicarboxylic acids in this study can be due to different extraction solvent and relatively lower recoveries of C3eC6 than those of C8eC9 or different sampling location/time. Along with aliphatic dicarboxylic acids, an aromatic dicarboxylic acid (phthalic acid) and other secondary organic aerosol constituents, pinic acid and pinonic acid have also been analyzed in this study. Phthalic acid concentrations ranged from 1.2 to 6.2 ng m3 with an average of 2.4 ng m3. An increasing trend was observed in the
concentration of phthalic acid from summer to winter. Pinic acid and pinonic acid, arising from oxidation of biogenic volatile organic compounds (terpenes) ranged between 1.2 and 8.5 ng m3 (average ¼ 3.8), and 0.9e8.6 ng m3 (average ¼ 3.6), respectively. The concentration of pinonic acid and pinic acid boosts up during summer (Fig. 2). However, another increase (not as much as in summer) in pinic and pinonic acid was observed in winter that may be associated with higher emission of pinenes in biomass burning. For example, Simpson et al. (2011) have shown that roughly 0.5 TgC yr1 is released from boreal forest fires in the form of pinenes (Simpson et al., 2011). Among aliphatic dicarboxlic acids, azelaic acid is found to have highest relative abundance (20.7e40.9%, average 28.9%). Succinic acid stands at second position with regards to relative abundance (8.2e33.9%, average 17.4%). Adipic acid followed succinic acid with relative abundance from 9.9 to 21.4% (average 16.2%). Least contribution to total aliphatic dicarboxylic acids was from pimelic acid with relative abundance ranging from 3.1 to 7.8% (average 5.2%).
M. Hyder et al. / Atmospheric Environment 57 (2012) 197e204
201
Table 2 Concentration ranges, average and standard deviation of analyts in aerosol compared with some of previous studies. Concentration ng m3
Particle Size
Location, Time
Reference
1.61 0.51 14.4 68.4 41.6
PM10 PM10 PM2.5
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14)
This Study Ray and McDow, 2005 Ho et al., 2006
0.77-12.5 0.09e66.7 13.1e121
3.54 2.42 15.4 52.5 31.8
PM10 PM10 PM2.5
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14)
This Study Ray and McDow, 2005 Ho et al., 2006
0.75e4.27 0.05e5.7 2.82e28.1 0.16e6.15 1.24e18.41
1.82 2.3 13.5 1.28 2.56
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
1.13e7.39 0.07e7.4 3.78e32.1 0.17e2.71 0.59e4.69
3.24 2 11.7 0.78 1.51
7.53 0.57 0.84
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
Pimelic Acid (C7)
BDL2.79 0.74e9.69 BDL*-1.77 BDL-2.05
1.050.63 2.30 2.33 0.32 0.31 0.58 0.39
PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
Suberic Acid (C8)
1.07-6.1 0.03e1.4 0.00e6.67 0.11e1.87 0.32e2.44
2.65 0.5 2.25 0.44 0.78
1.14
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
1.616.2 0.03e2.6 6.01e28.1 0.12e4.64 0.13e1.45
5.96 1 12.9 1.60 2.91
6.74 1.04 1.25
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
1.18e6.24 0.07e16.3 40.1e105 0.69e13.38 0.32e2.44
2.4 1.37 3.5 83.9 21.3 3.76 2.87 0.78 0.45
PM10 PM10 PM2.5 PM3 TSP
Vavihill, Sweden, April 2008 to March 2009 (n ¼ 35) Philadelphia, USA, JulyeAugust 1999 Hong Kong, Roadside, 2003 (n ¼ 14) Mainz, Germany, May 2006eJune 2007 Mainz, Germany, May 2006eJune 2007
This Study Ray and McDow, 2005 Ho et al., 2006 Zhang et al., 2010 Zhang et al., 2010
Dicarboxylic Acid (Carbon Number)
Range
Average SD
Malonic Acid (C3)
0.94e3.18 0.11e95.6 10.5e145
Succinic Acid (C4)
Glutaric Acid (C5)
Adipic Acid (C6)
Azelaic Acid (C9)
Phthalic Acid
0.75 8.17 0.99 1.27 1.63
1.88 0.35 0.45 3.64
BDL ¼ Below Detection Limit.
Fig. 2. Yearly trend in the concentration of phthalic acid, pinonic acid and pinic acid.
202
M. Hyder et al. / Atmospheric Environment 57 (2012) 197e204
Table 3 Correlation coefficients for dicarboxylic acids (C3-C9), phthalic acid, pinonic acid, pinic acid and organic carbon.
Melonic Acid (C3) Succinic Acid (C4) Glutaric Acid (C5) Adipic Acid (C6) Pimelic Acid (C7) Suberic Acid (C8) Azelaic Acid (C9) Pinonic Acid Pinic Acid Phthalic Acid (Ph)
C3
C4
C5
C6
C7
C8
C9
Pinonic Acid
Pinic Acid
Ph
1.000 0.591 0.653 0.701 0.164 0.272 0.241 0.179 0.141 0.760
1.000 0.906 0.667 0.184 0.268 0.249 0.179 0.349 0.686
1.000 0.834 0.352 0.431 0.367 0.249 0.406 0.647
1.000 0.438 0.510 0.417 0.396 0.348 0.534
1.000 0.973 0.961 0.738 0.533 0.055
1.000 0.969 0.735 0.518 0.071
1.000 0.748 0.592 0.055
1.000 0.735 0.030
1.000 0.205
1.000
3.2. Source attribution to the acidic contents of organic aerosol Organic acids are not only emitted directly to the environment contributing to the primary organic aerosols but are also produced from oxidation of primarily emitted volatile organic compounds to form secondary organic aerosol (SOA). Among primary sources, biomass burning and direct vehicle emissions not only add dicaboxylic acids directly to the environment but also introduce many volatile organic species that ultimately oxidize to give dicarboxylic acids, and also less volatile compounds with a great tendency to contribute to secondary particle formation. Table 3 shows a comprehensive picture of correlation coefficients among aliphatic dicarboxylic acids, phthalic acid, pinonic acid and pinic acid. It is very clear that the higher dicarboylic acids (C7eC9) are very nicely correlated, with correlation coefficients (r) ranging from 0.96 to 0.97, which strongly suggest similar source for these dicarboxylic acids. Azelaic acid (C9) and subaric acid (C8) have been intensively studied and are believed to be produced as photooxidation products of unsaturated carboxylic acids such as oleic and linoleic acids having unsaturation at carbon 9 position (Stephanou and Stratigakis, 1993). A strong correlation among C7 to C9 may help in drawing an inference that pimelic acids (C7) is produced either directly from the same sources as those of suberic acid (C8) and azelaic acid (C9) or they are produced by further oxidation of suberic acid and azelaic acid down to lower carbon numbered dicarboxylic acids. Similarly for lower carbon numbered dicarboxylic acids (C3 to C6) and phthalic acid we observe that they are correlated well to each other with correlation coefficients (r) ranging from 0.53 to 0.90 so we may infer that sources attributed to these dicarboxylic acid are similar or at least very closely related. Malonic acid (C3) is more strongly correlated with phthalic acid (r ¼ 0.76), which may suggest
that they have possibly same source. (C4eC6) dicarboxylic acids show correlation coefficients ranging from 0.667 to 0.906, which suggests a similar source for them as well. Photo-oxidation of anthropogenic precursors have been reported to be the main source of lower carbon numbered dicarboxylic acids (Kawamura and Yasui, 2005). For example, glutaric acid and adipic acid are believed to be produced by photo-oxidation of unsaturated cyclic alkenes (Narukawa et al., 2007). If we look at correlations coefficients we find that low carbon numbered dicarboxlic acids (C3eC6) are very poorly correlated to higher dicarboxlic acids (C7eC9) with correlation coefficients ranging from 0.16 to 0.34. It can be concluded that lower carbon numbered dicarboxylic acids have totally different origin than that of higher carbon numbered dicarboxylic acids (C6eC9). Phthalic acid is an aromatic dicarboxylic acid that has been reported to originate from multiple anthropogenic sources. It may have origin from primary sources like biomass burning or vehicle emission or photo-oxidation of polycyclic aromatic hydrocarbons and phthalates may produce it as a secondary oxidation product in the environment. If we look at its correlation with aliphatic dicarboxylic acids, we find that it is well correlated with lower carbon numbered dicarboxylic acids (C3eC6) with correlation coefficients (r) ranging from 0.65 to 0.76, while very poorly correlated to higher carbon dicarboxylic acids (C7eC9) with correlation coefficients ranging from 0.055 to 0.071. Also phthalic acid shows a very poor correlation with pinic acid and pinonic acid, which suggest anthropogenic secondary formation of phthalic acid. However, Fig. 3 shows a trend in the ratio of phthalic acid to adipic acid. It increases from summer to winter and it may be due to an increase in the primary emission of phthalic acid from gasoline and diesel engines and partially from biomass burning. Also phthalic acid is
Fig. 3. Phthalic Acid to Adipic Acid concentration ratio over the year.
M. Hyder et al. / Atmospheric Environment 57 (2012) 197e204
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Fig. 4. Trend for sum of collective relative abundances of lower carbon dicarboxylic acids (C3 to C6) and higher carbon dicarboxylic acids (C7 to C9).
well correlated with malonic acid (C3) and succinic acid (C4) that supports the photo-oxidation of anthropogenic precursors as an origin of phthalic acid. Also the significant contribution of primary emissions vs. SOA can also be confirmed by the low correlation between C3 and C4 acids (0.591). Fig. 4 shows the trend in the relative abundance for the sum of (C3 to C6) and (C7 to C9). This clearly demonstrates the different trend for both sums over the year, so it supports the suggestion that lower carbon numbered dicarboxylic acids (C3 to C6) have different source than those of higher carbon dicarboxylic acids (C7 to C9). In order to verify the above-mentioned sources of dicarboxylic acids, we also performed a principal component (PC) analysis to the data set. The original PC solution was subjected to a VARIMAX rotation in order to facilitate the interpretation of the various PCs as sources. A solution with three PCs (sources) was chosen, in agreement with the previous discussion. These three PCs together account for about 87% of the cumulative variance in the original data. The first PC characterizes the secondary formation of dicarboxylic acids from anthropogenic precursors with high loadings of C3eC6 dicarboxylic acids. The second PC characterizes the photooxidation formation of C7eC9 originating from unsaturated fatty acids and third PC characterizes the photo-oxidation production of pinic acid and pinonic acid from biogenic precursors. In conclusion, the PC solution provides the same interpretation of the sources of dicarboxylic acids as given by the correlation matrix itself, which is hardly surprising since the PC analysis is based on this matrix. 3.3. Conclusion Seven aliphatic dicarboxylic acids (C3eC9) along with phthalic acid, pinic acid and pinonic acid were analyzed from 35 aerosol samples collected over the year at Vavihill sampling station in south of Sweden. Azelaic acid was found the most abundant and pimelic acid (C7) was found the least abundant component among analyzed aliphatic dicarboxylic acids. Pimelic acid (C7), suberic acid (C8) and azelaic acid (C9) show a maximum during summer and are considered as photo-oxidation products of biogenic precursors and hence are secondary organic compounds in aerosols. They showed strong correlation that may suggest a similar source for these compounds. Malonic acid (C3), succinic acid (C4) and phthalic acid are well
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