Insights into the nature of secondary organic aerosol in Mexico City during the MILAGRO experiment 2006

Atmospheric Environment 44 (2010) 312e319

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Insights into the nature of secondary organic aerosol in Mexico City during the MILAGRO experiment 2006 Elizabeth A. Stone a, Curtis J. Hedman b, Jiabin Zhou a, c, Mark Mieritz b, James J. Schauer a, b, * a

Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 N. Park St, Madison, WI 53706, USA Wisconsin State Laboratory of Hygiene, University of Wisconsin-Madison, Madison, WI 53718, USA c School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 August 2009 Received in revised form 21 October 2009 Accepted 22 October 2009

This study targets understanding the secondary sources of organic aerosol in Mexico City during the Megacities Impact on Regional and Global Environment (MIRAGE) 2006 field campaign. Ambient PM2.5 was collected daily at urban and peripheral locations. Particle-phase secondary organic aerosol (SOA) products of anthropogenic and biogenic precursor gases were measured by gas chromatography mass spectrometry. Ambient concentrations of SOA tracers were used to estimate organic carbon (OC) from secondary origins (SOC). Anthropogenic SOC was estimated as 20e25% of ambient OC at both sites, while biogenic SOC was less abundant, but was relatively twice as important at the peripheral site. The OC that was not attributed secondary sources or to primary sources in a previous study showed temporal consistency with biomass-burning events, suggesting the importance of secondary processing of biomass-burning emissions in the region. The best estimate of biomass-burning-related SOC was in the range of 20e30% of ambient OC during peak biomass burning events. Low-molecular weight (MW) alkanoic and alkenoic dicarboxylic acids (C2eC5) were also measured, of which oxalic acid was the most abundant. The spatial and temporal trends of oxalic acid differed from tracers for primary and secondary sources, suggesting that it had different and/or multiple sources in the atmosphere. Ó 2009 Published by Elsevier Ltd.

Keywords: Secondary organic aerosol Source apportionment Water-soluble organic carbon Mexico City

1. Introduction The Megacities Impact on Regional and Global Environment (MIRAGE) field campaign occurred in Mexico City and the surrounding region in March 2006. MIRAGE was part of the Megacity Initiative: Local and Global Research Observations (MILAGRO) study which sought to understand the role of megacities in regional and global air quality. Of particular interest was chemical transformation, including secondary organic aerosol (SOA) formation by chemical reaction of gas-phase precursors and gas-to-particle partitioning of reaction products. The formation of SOA is impacted by the abundance and reactivity of gas-phase precursors, the availability of oxidants in the atmosphere, incoming solar radiation, the chemical composition of existing particles, and meteorological parameters including relative humidity and temperature (Poschl, 2005). Volatile organic compounds (VOC) of high atmospheric abundance and reactivity

* Corresponding author at: Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 N. Park St, Madison, WI 53706, USA. E-mail address: [email protected] (J.J. Schauer). 1352-2310/$ e see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.atmosenv.2009.10.036

are expected to be important precursors to SOA formation. These include gaseous biogenic emissions, such as isoprene, monoterpenes (e.g. a-pinene), and sesquiterpenes (e.g. b-caryophyllene), and anthropogenic emissions of aromatic compounds (e.g. toluene) and other solvents (Tsigaridis and Kanakidou, 2002, 2003). Smog chamber studies have investigated the SOA formation of these gases, by identifying reaction products (SOA tracers), measuring yields, and characterizing conditions favorable to SOA formation (Claeys et al., 2004a,b; Kleindienst et al., 2004; Edney et al., 2005; Jaoui et al., 2007; Ng et al., 2007). Further studies have applied these empirical measurements to estimate SOA yields in ambient continental atmospheres (Kleindienst et al., 2007; Lewandowski et al., 2008). This approach to SOA attribution is a simplification of a truly complex system of the real atmosphere. It is not currently known if there is a linear relationship between tracer species and yields, yet this technique was the best tool available at the time of this study. The uncertainties associated with this estimation method range from 20 to 48%, based on the standard deviation of the SOA tracer-toorganic carbon (OC) ratios observed across multiple smog chamber experiments with different initial conditions (Kleindienst et al., 2007). Much still needs to be elucidated about SOA sources, including the

E.A. Stone et al. / Atmospheric Environment 44 (2010) 312e319

importance of in-cloud production. Further studies are needed to improve estimations and to understand the relative importance of anthropogenic versus natural aerosol precursors. The atmosphere in Mexico City has been characterized as highly photochemically active (Shirley et al., 2006). Yet, it is not clear how Mexico City's unique atmosphere, with high actinic flux and oxidant concentrations, but lower number density of pollutants due to altitude, affects SOA production. Secondary organic carbon (SOC) has been suggested to be a major component of organic aerosol mass in the metropolitan and surrounding areas. Stone et al. (2008) estimated that SOA accounted for up to 60% of carbon in the urban center and sub-urban locations, based on molecular marker based chemical mass balance (MM-CMB) source apportionment of primary organic aerosol. This study focuses on understanding the secondary OC sources in Mexico City during the MIRAGE 2006 field campaign. Biogenic and anthropogenic SOA tracer species were quantified in ambient PM2.5 and were used to estimate secondary contributions to OC at urban and peripheral locations. Low-MW organic diacids were measured and compared to measurements of water-soluble organic carbon (WSOC). Correlation analysis was applied to investigate the relationships between known primary and secondary sources, low-MW diacids, and WSOC that was not explained by defined sources. 2. Methods 2.1. Sample collection Samples discussed in this paper were collected from 17 to 30 March 2006 during the MIRAGE field campaign at an urban site (T0, 19.488N, 99.147W, 2250 m asl) and a peripheral site located on northeastern edge of the metropolitan area (T1, 19.703N, 98.982W, 2273 m asl). Fine particulate matter (PM2.5) was collected on quartz fiber filters (QFF, 90 mm, Pallflex, Pall Life Sciences) using a medium-volume sampling apparatus (URG). Prior to sample collection, filters were pre-cleaned in the laboratory by baking at 550
C for 18 h. Particles in the designated size range were selected by a Teflon-coated aluminum cyclone operating at a flow rate of 104 L per minute. Two samples were collected each day, from 06:00 to 18:00 and vice versa. The measurements discussed in this paper

were made on composites of two samples from each day, corresponding to a 24-h period, except for 19 March when only the daytime filter sample was available. One field blank was collected for every five samples. 2.2. Carbonaceous aerosol measurements OC and elemental carbon (EC) were measured on QFF using thermaleoptical analysis (Sunset Laboratories) following the ACEAsia base case protocol (Schauer et al., 2003). Ambient OC and EC concentrations were presented and discussed in a preceding study (Stone et al., 2008). WSOC was measured in a single extract of 1.5 cm2 punches of QFF sonicated in 12 mL ultrapure water (MilliQ, >18 Megohm cm) (Sannigrahi et al., 2006) using a total organic carbon analyzer (Sievers 9000) (Sullivan et al., 2004). All measurements were field blank subtracted. 2.3. Organic molecular markers and primary source apportionment The samples discussed in this paper were previously analyzed by Stone et al. (2008) for solvent-extractable organic species, in which QFF were spiked with isotopically labeled internal standards and ketopinic acid prior to sequential extraction with dichloromethane and methanol. Sonication extracts were used for measurement of levoglucosan and SOA tracers (discussed below) and soxhlet extracts were used for measurement of all other compounds. Extracts were derivatized by methylation and/or silylation and analyzed by electron impact (EI) gas chromatography mass spectrometry (GC-6890, MS5973, Agilent Technologies) with an HP5-MS column (30 m  0.25 mm  0.25 mm, Agilent Technologies). Analyte concentrations were interpolated from five-point calibration curves of authentic standards and normalized to internal standards. Primary aerosol source contributions to OC were estimated using chemical mass balance (EPA CMB-8.2) modeling based on molecular marker species; these results are shown in Fig. 1. 2.4. SOA tracers and secondary source apportionment SOA tracers were measured following the analysis protocol described by Kleindienst et al. (2007). Positive chemical ionization (PCI) GCeMS (GC-6890, MS-5973, Agilent Technologies) was used

Urban Site

10

Primary Source Contribution to OC (μgC m-3)

8 6 4 2 0

Secondary 8 6 4 2

Peripheral Site

Primary

Vegetative Detritus Diesel Engines Smoking Vehicles Gasoline Vehicles Biomass Burning

8 6 4 2 0

Secondary

Anthropogenic SOC Isoprene SOC α-Pinene SOC β-Caryophyllene SOC CMB Estimated SOC

8 6 4 2

30 Mar

29 Mar

28 Mar

27 Mar

26 Mar

25 Mar

24 Mar

22 Mar

21 Mar

20 Mar

19 Mar

18 Mar

23 Mar

*

*

0 17 Mar

29 Mar

28 Mar

27 Mar

25 Mar

24 Mar

23 Mar

22 Mar

21 Mar

20 Mar

19 Mar

18 Mar

17 Mar

30 Mar

*

*

0

26 Mar

Source Contribution to OC (μgC m-3)

10

313

* no SOC tracer data

Fig. 1. Primary and secondary source contributions for urban (T0) and peripheral (T1) sites in Mexico City in March 2006. Primary source data and chemical mass balance (CMB) estimates of secondary organic carbon (SOC) were taken from Stone et al. (2008).

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for analysis, where methane was the reagent gas and an HP5-MS column was used (30 m  0.25 mm  0.25 mm, Agilent Technologies). SOA tracers were quantified using pinonic acid as a surrogate for quantification, which was normalized to the internal standard ketopinic acid. This approach builds upon that of Kleindienst et al. (2007) who used ketopinic acid as both internal standard and for quantification. Samples for 26 and 30 March at the urban site and 20 and 24 March at the peripheral site were used for a different analytical purpose, prohibiting analysis of SOA tracers. The observed SOA tracer compounds included: 2-methylthreitol, 2-methylerythritol, pinonic acid, 3-methyl-1,2,3-butanetricarboxylic acid, 3-acetyl hexanedioic acid, 3-hydroxyglutaric acid, b-caryophyllinic acid, 2,3-dihydroxy-4-oxopentanoic acid. These tracers represent eight of the most abundant compounds reported in preceding studies, in which up to fifteen tracers were observed (Kleindienst et al., 2007; Lewandowski et al., 2008). SOC was estimated using SOA tracer-to-OC ratios measured in chamber studies (Kleindienst et al., 2006, 2007). For example, the a-pinene was characterized in fifteen smog chamber experiments where precursor gas and NOx concentrations ranged from 1.0 to 5.3 ppmC and 0.19e4.9 ppm, respectively. The concentrations of identified particle-phase tracer species ranged from 0.9 to 79 mg m3, while observed SOA concentrations ranged from 10 to 330 mg m3. The observed mass fraction of OC attributable to the observed tracer species or SOA tracer-to-OC ratios, associated uncertainties, and percent relative uncertainties were 0.155  0.038 (25%) for isoprene, 0.231  0.111 (48%) for a-pinene, 0.0230  0.0046 (20%) for b-caryophyllene, and 0.0079  0.0026 (33%) for toluene. Uncertainties represent the variations in SOA yields in chamber studies under different chamber parameters, including concentration of precursor gases and oxidants, the presence of seed aerosol, temperature, relative humidity, and others (Claeys et al., 2004a,b; Kleindienst et al., 2004; Edney et al., 2005; Jaoui et al., 2007; Ng et al., 2007). Additional details on smog chamber experiments, characterization of precursors, and application to ambient SOC estimation are described elsewhere (Kleindienst et al., 2007). The utilized method of SOC estimation assumed that SOA tracers were stable in the atmosphere, unique to their precursor gas, and formed only by secondary reaction.

2.5. Low-molecular weight organic acids A series of six low-MW organic acids (oxalic, malonic, succinic, glutaric, maleic, and fumaric acids) were measured using high performance liquid chromatography (HPLC) tandem mass spectrometry (MS/MS) following a method adapted from a previous publication (Kakola and Alen, 2006). A stock standard solution containing the analytes of interest was prepared by obtaining a known weight of pure material, dissolving in water with the aid of sonication, and diluting in a volumetric flask. Chloroform was added to the stock solution as a preservative at a volume of 1 mL mL1 (Dabek-Zlotorzynska and McGrath, 2000), which was stored in amber ampoules at 4
C in dark conditions. Immediately prior to analysis, working standards were prepared by appropriate dilution of the stock standard in ultrapure water. An aliquot of the sample extract used for analysis of WSOC and working standards were spiked with an internal standard solution containing isotopically labeled phthalic acid-D4 prior to analysis. Samples and standards were analyzed by HPLC using a C18 column (Agilent Zorbax SB-C18, 5 mm, 150 mm  4.6 mm) and gradient elution chromatography. The mobile phase consisted of ultrapure water with 0.1% formic acid (A, reagent grade) and methanol (B, HPLC grade) and followed the solvent program listed in Supplemental Table S1. Analytes were detected using Turbo Ion Spray triple quadrupole mass spectrometry (MS/MS) in the negative ion mode. Parent ions and up to three fragment ions for each compound were monitored. Five- to eight-point calibration curves normalized to phthalic acid-D4 were used for quantification. Measurement uncertainty was propagated from the standard deviation of five field blank values and the standard error of the slope of calibration curves for each analyte. 3. Results and discussion 3.1. Water-soluble organic carbon (WSOC) WSOC accounted for an average (standard deviation) of 61  14% of OC at the urban site and 55  5% at the peripheral site, as summarized in Table 1. In the preceding study by Stone et al. (2008),

Table 1 Summary of carbon measurements, secondary organic aerosol (SOA) tracers, and estimated secondary organic carbon (SOC) yields based on a 24-h sampling time. Urban site Average Carbonaceous aerosol (mgC mL3) Organic carbona (OC) Elemental carbona (EC) Water-soluble OC SOA tracersb (ng mL3) 2-Methylthreitol 2-Methylerythritol Pinonic acid 3-Methyl-1,2,3-butanetricarboxylic acid 3-Acetyl hexanedioic acid 3-Hydroxyglutaric acid b-Caryophyllinic acid 2,3-Dihydroxy-4-oxopentanoic acid SOC yieldsc (mgC mL3) Isoprene-derived a-Pinene-derived b-Caryophyllene-derived Anthropogenically-derived

8.68 3.78 5.92 9.6 24.3 8.3 7.6 5.6 26.9 3.4 14.7 0.22 0.21 0.15 1.86

Peripheral site Std 2.43 1.67 1.76 4.4 12.6 3.3 4.6 2.6 16.6 3.4 9.3 0.11 0.07 0.15 1.17

Min 5.46 2.09 3.39 3.8 5.7 4.0 2.8 1.6 3.9 <1.2 <2.5 0.06 0.07 <0.05 <0.32

Max 14.40 8.08 9.86 15.9 45.2 15.4 18.2 9.6 50.1 11.0 29.9 0.39 0.31 0.48 3.78

Average 5.04 1.61 3.22 9.2 22.4 5.5 11.0 5.7 32.9 5.7 9.9 0.20 0.24 0.25 1.26

Std 1.56 0.53 1.15 3.8 9.8 1.3 6.3 3.8 18.1 3.3 6.2 0.08 0.10 0.14 0.79

Min 2.39 0.62 1.20 2.7 6.2 4.0 2.8 2.0 1.8 <0.6 <1.3 0.07 0.11 <0.03 <0.16

Std ¼ standard deviation. a From Stone et al. (2008). b Identified as described in Kleindienst et al. (2007) and Szmigielski et al. (2007). c The uncertainties associated with SOC yields were 25% for isoprene, 48% for a-pinene, 20% for b-caryophyllene, and 33% for toluene (Kleindienst et al., 2007).

Max 8.28 2.46 5.72 15.1 34.9 8.1 23.9 15.4 66.2 12.0 21.1 0.32 0.40 0.52 2.68

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real time WSOC concentrations at the peripheral site correlated with CMB-modeled WSOC, equivalent to the water-soluble fraction of biomass-burning emissions and SOC. The use of non-biomass WSOC as a proxy for SOC in Mexico City has been supported by diurnal correlations of WSOC with secondary inorganic species (Hennigan et al., 2008) and oxygenated organic species (de Gouw et al., 2009). Secondary sources were consequently considered to be a major contributor to ambient aerosol in Mexico City in March 2006. The remainder of this paper presents an investigation of the nature of these secondary sources. 3.2. Biogenic SOA estimates Eight SOA tracer species were observed at the two sites in Mexico City; concentration averages, standard deviations, minimum, and maximum values are summarized by site in Table 1. The SOA tracers included major products of isoprene oxidation: two compounds tentatively identified as 2-methylthreitol and 2-methylerythritol (Claeys et al., 2004a,b; Edney et al., 2005). These isoprene tracers were observed in every sample concurrently and average concentrations at both sites were roughly equal. The observed concentrations of 2-methylerythritol were consistently 2.5 times more abundant than the other isomer at both sites. This isomeric preference has been documented in smog chamber experiments (Edney et al., 2005) and in the Southeastern and Midwestern United States (US) (Kleindienst et al., 2007; Lewandowski et al., 2008). Isoprene-derived SOC was estimated to be 0.06e0.39 mgC m3 at the urban site and 0.07e0.32 mgC m3 at the peripheral site, as shown in Fig. 1. Four a-pinene SOA products, pinonic acid and 3-methyl-1,2,3butanetricarboxylic acid and compounds tentatively identified as 3-acetyl hexanedioic acid and 3-hydroxyglutaric acid (Jaoui et al., 2005; Szmigielski et al., 2007), were observed in every sample. In the majority of samples, 3-hydroxyglutaric acid was the most abundant a-pinene tracer. Exceptions included 20e21 March at the urban site when 3-methyl-1,2,3-butanetricarboxylic acid was the most abundant and 19 March at the peripheral site when 3-acetyl hexanedioic acid had the highest concentrations. In the Southeastern US, the most abundant a-pinene tracers varied across seasons and days and was usually either 3-hydroxyglutaric acid or 3-acetyl hexanedioic acid (Kleindienst et al., 2007). The Mexico City SOC contributions from a-pinene were estimated at 0.07e0.31 mgC m3 for the urban site and 0.11e0.40 mgC m3 for the peripheral site. The b-caryophyllene product tentatively identified as bcaryophyllinic acid (Jaoui et al., 2007) was observed in 75% of the samples at the urban site and all but one sample at the peripheral site. This tracer was not observed at either site on 19 March. On average, bcaryophyllinic acid was more abundant at the peripheral site than the urban site. The estimated contributions of b-caryophyllene-derived SOC averaged 0.15 mgC m3 at the urban site and 0.25 mgC m3at the peripheral site. The average concentrations of biogenic SOA tracers observed in Mexico City were similar in magnitude to a co-located modeling study (Hodzic et al., 2009); the agreement between observations and modeling gave credence to the assumption that the measured SOA tracers were relatively stable in Mexico City's atmosphere. Biogenic SOA estimates were relatively similar across the two sites, while there was a tendency for average concentrations to be greater at the peripheral site. The overall similarity in average concentrations indicated consistency in the magnitude of biogenic SOA across two sites. Yet, the temporal analysis of biogenic SOA tracers suggested urban and peripheral sites may have been influenced by different air masses. For transport of the urban plume to the peripheral site, southwesterly winds must occur. Such transport

315

was considered to be likely on 18e22, 24e25, and 30 March and possible on other days. When the urban plume did not impact the peripheral site, background air masses, pollutants from other sources, or aged urban pollutants were expected to preside (Fast et al., 2007). The March 2006 secondary source contributions from biogenic sources in Mexico City were comparable to previous studies in the United States. Kleindienst et al. (2007) reported concentrations of these tracers for Research Triangle Park in North Carolina (NC) over an annual cycle and Lewandowski et al. (2008) reported monthlyaverage SOA yields for five Midwestern sites over an annual cycle. These data from the US came from samples collected in 2003 and 2004, respectively. The Mexico City concentrations of biogenic SOA from isoprene, a-pinene, b-caryophyllene were on par with late springtime months in the US (AprileMay) and were nearly 10 times greater than low wintertime concentrations and 10 times lower than highest summertime concentrations. Another difference between measurements in Mexico City and the United States was the number of SOA species observed. One additional isoprene tracer and five additional a-pinene SOA products were observed at the NC and the Midwestern US sites; these compounds were typically of lower concentrations than those observed in this study. This difference may have stemmed from differences in sample preparation or instrumental analysis (Stone et al., 2009). 3.3. Anthropogenic SOA estimates High levels of aromatic VOC were observed in Mexico City during 2002e2003 in Mexico City (Velasco et al., 2007). Among the most prevalent aromatic VOC was toluene, also observed at elevated levels during March 2006 (Fortner et al., 2009). Earlier studies identified diurnal trends where toluene concentrations peaked during morning rush hour and sporadically during the nighttime to implicate motor vehicles (Velasco et al., 2007) and industrial solvent use (Fortner et al., 2009), respectively, as major sources. The anthropogenic fraction of SOC was estimated using 2,3dihydroxy-4-oxopentanoic acid, formed in smog chamber experiments of toluene oxidation (Kleindienst et al., 2004). Observed concentrations are summarized in Table 1 and averaged 15 ng m3 at the urban site and 10 ng m3 at the peripheral site. This tracer was observed in every sample at both sites, except on Sunday 19 March, when other biogenic SOA tracers were also not detected. The levels at the peripheral site were similar to 4 and 5 day average concentrations in Cleveland, OH and Detroit, MI during the summer of 2007, whereas urban site concentrations were similar to those observed in the Los Angeles (LA) air basin in the summer of 2005 (Stone et al., 2009). The observed spatial trends were consistent with the expectation that SOA of anthropogenic origin was more abundant in the urban area. The estimated anthropogenic SOC contribution averaged 1.9 mgC m3 in the urban area and 1.3 mgC m3 at the peripheral site, as shown in Fig. 1. Limitations of quantifying anthropogenic SOA from a single compound include potential biases that may arise if this tracer was not representative of anthropogenic SOA chemistry in Mexico City. Another shortfall of this technique is that it cannot capture other anthropogenic precursors that may or may not have aromatic structures. The purpose of this data is not to suggest that toluene contributes all of the anthropogenic SOC, but to provide an estimate of SOA from anthropogenic VOC. 3.4. Unexplained WSOC The OC that was not explained by CMB-modeled primary or estimated secondary sources is referred to as “unexplained OC.”

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Unexplained OC is shown in Fig. 1 as the difference between the CMB-estimated SOC and the sum of biogenic and anthropogenic SOC contributions. For approximately half of sampling days at urban and peripheral sites, this difference was negative, indicating an over-apportionment by defined sources. In these cases, the negative value was not greater than the estimate's propagated uncertainty and was not considered significant. Unexplained OC was greatest 20e21 March at the urban site at maximum contributions of 3.7e5.3 mg m3 corresponding to 32e34% of total OC. Maximum contributions at the peripheral site occurred 18e22 March when unexplained OC levels averaged 1.7 mg m3 corresponding to 22e28% of ambient OC. Days with high levels of unexplained OC were corresponded to high contributions from biomass burning. The linear correlation of unexplained OC with levoglucosan, a tracer for biomass burning with well-established atmospheric stability (Simoneit et al., 1999; Fraser and Lakshmanan, 2000), is shown in Fig. 2. A positive linear correlation was observed at urban (R2 ¼ 0.72) and peripheral (R2 ¼ 0.73) sites. This correlation could be interpreted in two ways: 1) an underestimation of primary biomass emissions or 2) SOC production from biomass-burningrelated precursors. The first possibility was tested with a sensitivity test examining different biomass-burning profiles (Sheesley et al., 2007). The source profile used in CMB modeling had a levoglucosan-to-OC ratio of 0.15  0.03 mg mg1 compared to an openburning biomass profile with a ratio of 0.09  0.04 mg mg1 (Lee et al., 2005). The latter profile is known to represent an upper estimate (Sheesley et al., 2007) and when used for biomass-burning estimation in Mexico City, the amount of OC apportioned to biomass burning increased by 54%. Meanwhile, this upper estimate of primary biomass-burning OC could still not account for 57% of the unexplained OC at the urban site or 61% at the peripheral site, giving credibility to the second interpretation. The concept of SOC generation by processing of biomassburning VOC has been suggested by previous studies. Field studies characterizing emissions from prescribed burns on open lands demonstrated that biomass fires release large amounts of VOC in smoke plumes (Lee et al., 2005) and laboratory experiments show that aged woodsmoke VOC produce SOA (Grieshop et al., 2009). Evidence of atmospheric processing of biomass-burning-derived VOC has also been observed with high-time resolution, singleparticle mass spectrometry in Fresno, CA, where when primary biomass-burning tracers co-varied with aged organic compounds, particularly those with high molecular weights. The diurnal trends

600 Urban Site Data Peripheral Site Data Urban Site Peripheral Site

Levoglucosan (ng m-3)

500

y = 88x R2 = 0.77

400 y = 120x R2 = 0.67

300

observed in Fresno were such that biomass emissions and aged organic compounds peaked at night, leading to the hypothesis that biomass-related VOC partitioned to aqueous particle phases at night and underwent cloud processing to form humic-like substances (HULIS) (Qin and Prather, 2006). The secondary processing of primary biomass emissions was suggested to be important in Mexico City in March 2006, based on observations of the chemical transformation of biomass-burning organic aerosol by aircraft measurements in biomass-burning plumes (Aiken et al., 2008). Such atmospheric transformation of primary biomass emissions was also expected to be relevant to the unexplained OC at urban and peripheral sites discussed in this study. Estimates of SOC from biomass-burning-precursors could be improved by identification and quantification of tracer species and yields for different types of biomasses and burn conditions.

3.5. Low-molecular weight organic diacids Low-MW organic diacids are an important contributor to WSOC in atmospheric aerosols and are of interest because of their high cloud condensation nuclei (CCN) activity (Sun and Ariya, 2006). Their origins are varied and include both primary and secondary sources (Decesari et al., 2006). In this study, a homologous series of C2eC5 n-alkanoic diacids, oxalic, malonic, succinic, and glutaric acids and isomers of cis- and trans-C4 n-alkenoic diacids, maleic and fumaric acids, were measured. The concentrations of these acids are summarized in Table 2 and shown in Fig. 3 as mass contributions to WSOC for urban and peripheral sites. Oxalic, the C2 diacid, was by far the most abundant of the measured species; its concentrations ranged from 0.2 to 2.4 mgC m3 at the urban site and 1.3e3.8 mgC m3 at the peripheral site. Malonic acid was the next most abundant at average levels of 0.10 mgC m3 at the urban site and 0.08 mgC m3 at the peripheral site. The very high oxalic levels made this compound one of the most abundant single organic species measured in PM2.5 in this and the precursor study (Stone et al., 2008). High concentrations of oxalic acid in comparison to the other measured diacids have been previously documented in large urban and remote locations (Limbeck and Puxbaum, 1999). The levels of oxalic acid observed at the urban site accounted for an average of 6.4% of WSOC. Such levels were comparable to Nanjing, China (Wang et al., 2002) and Tokyo, Japan (Kawamura and Ikushima, 1993). The dominant spatial trend in diacid concentrations was a higher average concentration of oxalic acid at the peripheral site (2.5 mg m3) compared to the urban site (1.3 mg m3). Furthermore, low-MW acids made a larger contribution to WSOC at the peripheral site (25  5%) compared to the urban site (8.2  3.4%). These trends were consistent with the idea that oxalic acid was formed by secondary reactions in the atmosphere. An earlier study in Tokyo suggested photochemical origins of oxalic acid based on temporal

Table 2 Summary of low-molecular weight (MW) carboxylic acid concentrations, based on a 24-h sampling time.

200

Urban site Average

100 0 0

2

4

6

8

Unexplained OC (μgC m-3) Fig. 2. Correlation of OC that was unexplained by defined primary and secondary sources with levoglucosan at urban and peripheral sites.

Peripheral site Min

Max

Average

Std

Min

Max

Low-MW carboxylic acids (mg mL3) Oxalic acid 1.33 0.62 0.19 Malonic acid 0.10 0.03 0.07 Maleic acid 0.06 0.02 0.02 Fumaric acid 0.01 0.01 0.01 Succinic acid 0.07 0.02 0.04 Glutaric acid 0.02 0.01 0.02

2.39 0.15 0.09 0.03 0.10 0.03

2.47 0.08 0.06 0.01 0.04 0.01

1.02 0.03 0.02 0.002 0.01 0.005

1.25 0.04 0.02 0.003 0.03 0.01

3.79 0.16 0.09 0.01 0.07 0.03

Std ¼ standard deviation.

Std

E.A. Stone et al. / Atmospheric Environment 44 (2010) 312e319

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40%

1500

Urban Site

oxalic acid malonic acid maleic acid fumaric acid succinic acid glutaric acid see right axis

1200

900

30%

20%

10% 300

0%

0

Peripheral Site 1200

30%

Contribution to WSOC

Concentration (ngC m-3)

600

900 20% 600 10% 300

0% 30 Mar

29 Mar

28 Mar

27 Mar

26 Mar

25 Mar

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23 Mar

22 Mar

21 Mar

20 Mar

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0

Fig. 3. Concentrations of organic diacids in PM2.5 observed at two locations in Mexico City and their contribution to water-soluble organic carbon (WSOC).

trends showing peak oxalic acid concentrations occurring in the summertime with atmospheric oxidant concentrations (Kawamura and Ikushima, 1993). Similarly, a modeling study proposed that oxalic acid may be formed when larger organic molecules breakdown by photochemical aging (Ervens et al., 2004). In Mexico City, temporal trends in oxalic acid concentrations showed consistency with meteorological changes in Mexico City. Partly cloudy conditions were observed during the afternoons of 17 and 21e30 March and precipitation events occurred during the occurred during the last week of the month, corresponding to the samples from 23 to 30 March (Fast et al., 2007). In this study,

low-MW diacids accounted for nearly twice as much WSOC during the second meteorological regime compared to the first. Correlation analysis was used to investigate the correlations between low-MW diacids and major primary and secondary OC source categories; these results are summarized in Table 3 and select oxalic acid correlations are shown in Fig. 4. The strongest correlations were observed between various low-MW diacid species, which suggested that portions of these compounds may have come from similar sources. The correlations of low-MW diacids with primary source tracers (Stone et al., 2008) for biomass burning (Simoneit et al., 1999) and motor vehicles (Simoneit, 1999)

Table 3 Correlation coefficients (r) for low-molecular weight acids, primary source tracers, and secondary organic carbon (SOC) contributions. Primary source tracers and chemical mass balance (CMB) estimates of SOC are presented elsewhere (Stone et al., 2008) and isoprene, a-pinene, and toluene SOC were estimated using methods discussed in this paper. Correlations where r > 0.6 are shown in bold and 0.6 > r > 0.4 are underlined. Aliphatic diacids Oxalic acid Urban site Oxalic acid Malonic acid Maleic acid Fumaric acid Succinic acid Glutaric acid

0.82 0.52 0.13 0.22 0.38

Peripheral site Oxalic acid Malonic acid Maleic acid Fumaric acid Succinic acid Glutaric acid

0.75 0.82 0.21 0.68 0.68

Malonic acid

0.76 0.21 0.26 0.40

0.70 0.51 0.89 0.87

Maleic acid

0.21 0.42 0.19

0.41 0.64 0.67

Fumaric acid

0.47 0.36

0.37 0.60

Isoprene SOC

a-Pinene SOC

Toluene SOC

CMB estimate of SOC

0.21 0.28 0.18 0.35 0.37 0.27

0.39 0.58 0.54 0.40 0.14 0.21

0.57 0.59 0.31 0.34 0.32 0.20

0.24 0.06 0.02 0.33 0.24 0.17

0.12 0.05 0.38 0.54 0.39 0.76

0.01 0.06 0.25 0.30 0.19 0.07

0.50 0.25 0.28 0.32 0.54 0.17

0.58 0.37 0.24 0.30 0.31 0.42

0.07 0.03 0.29 0.50 0.15 0.07

0.38 0.70 0.46 0.63 0.70 0.81

Biomass-burning tracer

Fossil fuel marker

Levoglucosan

Norhopane

0.57

0.07 0.02 0.22 0.44 0.62 0.55

0.77

0.26 0.27 0.01 0.48 0.24 0.17

Succinic acid

318

E.A. Stone et al. / Atmospheric Environment 44 (2010) 312e319

b

700

17β(H)-21α(H)-30-Norhopane (ng m-3)

a Levoglucosan (ng m-3)

600 500 Urban ρ = -0.07

400

Peripheral ρ = -0.26

300 200 100 0 0

1

2

3

2.0 Urban Site Data Peripheral Site Data

1.5

Urban ρ = 0.21

1.0 Peripheral ρ = -0.01

0.5

0.0

4

0

1

d

7 6

Unexplained Other (μgC m-3)

CMB-estimated SOC (μgC m-3)

c

Oxalic acid (μg m )

Urban ρ = 0.12

5 4 3 2

Peripheral ρ = 0.38

1

2

3

4

Oxalic acid (μg m-3)

-3

6 5

Urban ρ = 0.32

4 3 2

Peripheral ρ = 0.38

1 0 -1 -2

0 0

1

2

3

4

0

1

-3

2

3

4

Oxalic acid (μg m-3)

Oxalic Acid (μg m )

Fig. 4. Correlations coefficients (r) of oxalic acid with a) levoglucosan, a biomass-burning marker, b) norhopane, a motor vehicle marker, c) CMB estimates of SOC, and d) OC unexplained by primary or secondary sources.

were relatively weaker. Weak to moderate correlations were found for SOC yields from isoprene, a-pinene, and toluene. Some positive, strong correlations were observed between select diacids and the CMB estimate of SOC, with a preference towards higher MW compounds. Overall, these results did not point towards any of the identified sources as likely origins for low-MW diacids, but eliminated the possibility of them being dominated by biomass burning, motor vehicles, or known SOA sources. Diacids likely have complex origins that may include mixtures of many sources and/or the aqueous processing of WSOC. 4. Conclusions This study builds upon the understanding of organic aerosol sources in Mexico City during the MIRAGE 2006 field campaign. The water-soluble component of ambient aerosol in Mexico City was investigated by measurement of SOA tracers and estimations of biogenic and anthropogenic SOC contributions. Anthropogenic SOA was found to be an important contributor to ambient OC at both urban and peripheral sites and accounted for a large fraction of non-primary OC. Biogenic SOA was less abundant at both sites, but made up a larger fraction of OC at the peripheral site. The OC that was not explained by defined primary and secondary sources was found to have a positive linear correlation with levoglucosan concentrations at urban and peripheral sites and suggested that SOC was formed in relation to biomass-burning events. During peak biomass periods, biomass-burning-related SOA may have contributed a major fraction of ambient OC, but was not considered to be a significant source on other days. Low-MW organic diacid concentrations supported the hypothesis that the atmosphere of Mexico City was highly oxidative and conducive to atmospheric processing. The spatial trends between the two sites suggested that

more atmospheric processing occurred at the peripheral site compared to the urban site, due to increased aerosol residence time. Estimates of SOA contributions to SOC from various sources could be improved by further characterization of tracer compounds for a larger number of precursor gases. A focus on VOC associated with anthropogenic activity is particularly needed in order to provide a more robust estimation SOC in urban locations. Acknowledgements This research was funded by the National Science Foundation (NSF award number 0514280). We thank Andrew Rutter, David Snyder, Luz Padro, Rodney Weber, Rodrigo Gonzales Abraham, and Gustavo Sosa for assistance with sample collection. We also thank Martin Shafer, Julie Hahm, and Jeff DeMinter for their contributions to filter preparation and chemical analysis. Appendix. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.atmosenv.2009.10.036. References Aiken, A.C., Decarlo, P.F., Kroll, J.H., Worsnop, D.R., Huffman, J.A., Docherty, K.S., Ulbrich, I.M., Mohr, C., Kimmel, J.R., Sueper, D., Sun, Y., Zhang, Q., Trimborn, A., Northway, M., Ziemann, P.J., Canagaratna, M.R., Onasch, T.B., Alfarra, M.R., Prevot, A.S.H., Dommen, J., Duplissy, J., Metzger, A., Baltensperger, U., Jimenez, J. L., 2008. O/C and OM/OC ratios of primary, secondary, and ambient organic aerosols with high-resolution time-of-flight aerosol mass spectrometry. Environmental Science & Technology 42 (12), 4478e4485. Claeys, M., Graham, B., Vas, G., Wang, W., Vermeylen, R., Pashynska, V., Cafmeyer, J., Guyon, P., Andreae, M.O., Artaxo, P., Maenhaut, W., 2004a. Formation of

E.A. Stone et al. / Atmospheric Environment 44 (2010) 312e319 secondary organic aerosols through photooxidation of isoprene. Science 303 (5661), 1173e1176. Claeys, M., Wang, W., Ion, A.C., Kourtchev, I., Gelencser, A., Maenhaut, W., 2004b. Formation of secondary organic aerosols from isoprene and its gas-phase oxidation products through reaction with hydrogen peroxide. Atmospheric Environment 38 (25), 4093e4098. Dabek-Zlotorzynska, E., McGrath, M., 2000. Determination of low-molecular-weight carboxylic acids in the ambient air and vehicle emissions: a review. Fresenius' Journal of Analytical Chemistry 367 (6), 507e518. Decesari, S., Fuzzi, S., Facchini, M.C., Mircea, M., Emblico, L., Cavalli, F., Maenhaut, W., Chi, X., Schkolnik, G., Falkovich, A., Rudich, Y., Claeys, M., Pashynska, V., Vas, G., Kourtchev, I., Vermeylen, R., Hoffer, A., Andreae, M.O., Tagliavini, E., Moretti, F., Artaxo, P., 2006. Characterization of the organic composition of aerosols from Rondonia, Brazil, during the LBA-SMOCC 2002 experiment and its representation through model compounds. Atmospheric Chemistry and Physics 6, 375e402. Edney, E.O., Kleindienst, T.E., Jaoui, M., Lewandowski, M., Offenberg, J.H., Wang, W., Claeys, M., 2005. Formation of 2-methyl tetrols and 2-methylglyceric acid in secondary organic aerosol from laboratory irradiated isoprene/NOX/SO2/air mixtures and their detection in ambient PM2.5 samples collected in the eastern United States. Atmospheric Environment 39 (29), 5281e5289. Ervens, B., Feingold, G., Frost, G.J., Kreidenweis, S.M., 2004. A modeling study of aqueous production of dicarboxylic acids: 1. Chemical pathways and speciated organic mass production. Journal of Geophysical Research e Atmospheres 109 (D15). Fast, J.D., De Foy, B., Acevedo Rosas, F., Caetano, E., Carmichael, G., Emmons, L., McKenna, D., Mena, M., Skamarock, W., Tie, X., Coulter, R.L., Barnard, J.C., Wiedinmyer, C., Madronich, S., 2007. A meteorological overview of the MILAGRO field campaigns. Atmospheric Chemistry and Physics Discussions 7, 2233e2257. Fortner, E.C., Zheng, J., Zhang, R., Knighton, W.B., Volkamer, R.M., Sheehy, P., Molina, L., Andre, M., 2009. Measurements of volatile organic compounds using proton transfer reactionemass spectrometry during the MILAGRO 2006 Campaign. Atmospheric Chemistry and Physics 9 (2), 467e481. Fraser, M.P., Lakshmanan, K., 2000. Using levoglucosan as a molecular marker for the long-range transport of biomass combustion aerosols. Environmental Science & Technology 34 (21), 4560e4564. de Gouw, J.A., Welsh-Bon, D., Warneke, C., Kuster, W.C., Alexander, L., Baker, A.K., Beyersdorf, A.J., Blake, D.R., Canagaratna, M., Celada, A.T., Huey, L.G., Junkermann, W., Onasch, T.B., Salcido, A., Sjostedt, S.J., Sullivan, A.P., Tanner, D.J., Vargas, O., Weber, R.J., Worsnop, D.R., Yu, X.Y., Zaveri, R., 2009. Emission and chemistry of organic carbon in the gas and aerosol phase at a sub-urban site near Mexico City in March 2006 during the MILAGRO study. Atmospheric Chemistry and Physics 9 (10), 3425e3442. Grieshop, A.P., Logue, J.M., Donahue, N.M., Robinson, A.L., 2009. Laboratory investigation of photochemical oxidation of organic aerosol from wood fires 1: measurement and simulation of organic aerosol evolution. Atmospheric Chemistry and Physics 9 (4), 1263e1277. Hennigan, C.J., Sullivan, A.P., Fountoukis, C.I., Nenes, A., Hecobian, A., Vargas, O., Peltier, R.E., Hanks, A.T.C., Huey, L.G., Lefer, B.L., Russell, A.G., Weber, R.J., 2008. On the volatility and production mechanisms of newly formed nitrate and water soluble organic aerosol in Mexico City. Atmospheric Chemistry and Physics 8 (14), 3761e3768. Hodzic, A., Jimenez, J.L., Madronich, S., Aiken, A.C., Bessagnet, B., Curci, G., Fast, J., Lamarque, J.F., Onasch, T.B., Roux, G., Schauer, J.J., Stone, E.A., Ulbrich, I.M., 2009. Modeling organic aerosols during MILAGRO: importance of biogenic secondary organic aerosols. Atmospheric Chemistry and Physics Discussions. Jaoui, M., Kleindienst, T.E., Lewandowski, M., Offenberg, J.H., Edney, E.O., 2005. Identification and quantification of aerosol polar oxygenated compounds bearing carboxylic or hydroxyl groups. 2. Organic tracer compounds from monoterpenes. Environmental Science & Technology 39 (15), 5661e5673. Jaoui, M., Lewandowski, M., Kleindienst, T.E., Offenberg, J.H., Edney, E.O., 2007. Betacaryophyllinic acid: an atmospheric tracer for beta-caryophyllene secondary organic aerosol. Geophysical Research Letters 34 (5). Kakola, J., Alen, R., 2006. A fast method for determining low-molecular-mass aliphatic carboxylic acids by high-performance liquid chromatographyeatmospheric pressure chemical ionization mass spectrometry. Journal of Separation Science 29 (13), 1996e2003. Kawamura, K., Ikushima, K., 1993. Seasonal-changes in the distribution of dicarboxylic-acids in the urban atmosphere. Environmental Science & Technology 27 (10), 2227e2235. Kleindienst, T.E., Conver, T.S., McIver, C.D., Edney, E.O., 2004. Determination of secondary organic aerosol products from the photooxidation of toluene and their implications in ambient PM2.5. Journal of Atmospheric Chemistry 47 (1), 79e100. Kleindienst, T.E., Edney, E.O., Lewandowski, M., Offenberg, J.H., Jaoui, M., 2006. Secondary organic carbon and aerosol yields from the irradiations of isoprene and alpha-pinene in the presence of NOx and SO2. Environmental Science & Technology 40 (12), 3807e3812.

319

Kleindienst, T.E., Jaoui, M., Lewandowski, M., Offenberg, J.H., Lewis, C.W., Bhave, P.V., Edney, E.O., 2007. Estimates of the contributions of biogenic and anthropogenic hydrocarbons to secondary organic aerosol at a southeastern US location. Atmospheric Environment 41 (37), 8288e8300. Lee, S., Baumann, K., Schauer, J.J., Sheesley, R.J., Naeher, L.P., Meinardi, S., Blake, D.R., Edgerton, E.S., Russell, A.G., Clements, M., 2005. Gaseous and particulate emissions from prescribed burning in Georgia. Environmental Science and Technology 39 (23), 9049e9056. Lewandowski, M., Jaoui, M., Offenberg, J.H., Edney, E.O., Sheesley, R.J., Schauer, J.J., 2008. Primary and secondary contributions to ambient PM in the Midwestern United States. Environmental Science & Technology 42 (9), 3303e3309. Limbeck, A., Puxbaum, H., 1999. Organic acids in continental background aerosols. Atmospheric Environment 33 (12), 1847e1852. Ng, N.L., Chhabra, P.S., Chan, A.W.H., Surratt, J.D., Kroll, J.H., Kwan, A.J., McCabe, D.C., Wennberg, P.O., Sorooshian, A., Murphy, S.M., Dalleska, N.F., Flagan, R.C., Seinfeld, J.H., 2007. Effect of NOx level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes. Atmospheric Chemistry and Physics 7 (19), 5159e5174. Poschl, U., 2005. Atmospheric aerosols: composition, transformation, climate and health effects. Angewandte Chemie e International Edition 44 (46), 7520e7540. Qin, X.Y., Prather, K.A., 2006. Impact of biomass emissions on particle chemistry during the California Regional Particulate Air Quality Study. International Journal of Mass Spectrometry 258 (1e3), 142e150. Sannigrahi, P., Sullivan, A.P., Weber, R.J., Ingall, E.D., 2006. Characterization of watersoluble organic carbon in urban atmospheric aerosols using solid-state C-13 NMR spectroscopy. Environmental Science & Technology 40 (3), 666e672. Schauer, J.J., Mader, B.T., Deminter, J.T., Heidemann, G., Bae, M.S., Seinfeld, J.H., Flagan, R.C., Cary, R.A., Smith, D., Huebert, B.J., Bertram, T., Howell, S., Kline, J.T., Quinn, P., Bates, T., Turpin, B., Lim, H.J., Yu, J.Z., Yang, H., Keywood, M.D., 2003. ACE-Asia intercomparison of a thermaleoptical method for the determination of particle-phase organic and elemental carbon. Environmental Science & Technology 37 (5), 993e1001. Sheesley, R.J., Schauer, J.J., Zheng, M., Wang, B., 2007. Sensitivity of molecular marker-based CMB models to biomass burning source profiles. Atmospheric Environment 41 (39), 9050e9063. Shirley, T.R., Brune, W.H., Ren, X., Mao, J., Lesher, R., Cardenas, B., Volkamer, R., Molina, L.T., Molina, M.J., Lamb, B., Velasco, E., Jobson, T., Alexander, M., 2006. Atmospheric oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003. Atmospheric Chemistry and Physics 6, 2753e2765. Simoneit, B.R.T., 1999. A review of biomarker compounds as source indicators and tracers for air pollution. Environmental Science and Pollution Research 6 (3), 159e169. Simoneit, B.R.T., Schauer, J.J., Nolte, C.G., Oros, D.R., Elias, V.O., Fraser, M.P., Rogge, W. F., Cass, G.R., 1999. Levoglucosan, a tracer for cellulose in biomass burning and atmospheric particles. Atmospheric Environment 33 (2), 173e182. Stone, E.A., Snyder, D.C., Sheesley, R.J., Sullivan, A.P., Weber, R.J., Schauer, J.J., 2008. Source apportionment of fine organic aerosol in Mexico City during the MILAGRO experiment 2006. Atmospheric Chemistry and Physics 8, 1249e1259. Stone, E.A., Zhou, J.B., Snyder, D.C., Rutter, A.P., Mieritz, M., Schauer, J.J., 2009. A comparison of summertime secondary organic aerosol source contributions at contrasting urban locations. Environmental Science & Technology 43 (10), 3448e3454. Sullivan, A.P., Weber, R.J., Clements, A.L., Turner, J.R., Bae, M.S., Schauer, J.J., 2004. A method for on-line measurement of water-soluble organic carbon in ambient aerosol particles: results from an urban site. Geophysical Research Letters 31 (13). Sun, J.M., Ariya, P.A., 2006. Atmospheric organic and bio-aerosols as cloud condensation nuclei (CCN): a review. Atmospheric Environment 40 (5), 795e820. Szmigielski, R., Surratt, J.D., Gomez-Gonzalez, Y., Van der Veken, P., Kourtchev, I., Vermeylen, R., Blockhuys, F., Jaoui, M., Kleindienst, T.E., Lewandowski, M., Offenberg, J.H., Edney, E.O., Seinfeld, J.H., Maenhaut, W., Claeys, M., 2007. 3Methyl-1,2,3-butanetricarboxylic acid: an atmospheric tracer for terpene secondary organic aerosol. Geophysical Research Letters 34 (24). Tsigaridis, K., Kanakidou, M., 2002. Importance of volatile organic compounds photochemistry over a forested area in central Greece. Atmospheric Environment 36 (19), 3137e3146. Tsigaridis, K., Kanakidou, M., 2003. Global modelling of secondary organic aerosol in the troposphere: a sensitivity analysis. Atmospheric Chemistry and Physics 3, 1849e1869. Velasco, E., Lamb, B., Westberg, H., Allwine, E., Sosa, G., Arriaga-Colina, J.L., Jobson, B. T., Alexander, M.L., Prazeller, P., Knighton, W.B., Rogers, T.M., Grutter, M., Herndon, S.C., Kolb, C.E., Zavala, M., de Foy, B., Volkamer, R., Molina, L.T., Molina, M.J., 2007. Distribution, magnitudes, reactivities, ratios and diurnal patterns of volatile organic compounds in the Valley of Mexico during the MCMA 2002 & 2003 field campaigns. Atmospheric Chemistry and Physics 7, 329e353. Wang, G.H., Niu, S.L., Liu, C., Wang, L.S., 2002. Identification of dicarboxylic acids and aldehyde of PM10 and PM2.5 aerosols in Nanjing, China. Atmospheric Environment 36 (12), 1941e1950.