Modal characteristics of carbonaceous aerosol size distribution in an urban atmosphere of South China

Atmospheric Research 100 (2011) 51–60

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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s

Modal characteristics of carbonaceous aerosol size distribution in an urban atmosphere of South China Zi-Juan Lan a, Dong-Lei Chen a, Xiang Li a, Xiao-Feng Huang a, Ling-Yan He a,⁎, Yan-Ge Deng a, Ning Feng a, Min Hu b a Key Laboratory for Urban Habitat Environmental Science and Technology, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China b State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 25 October 2010 Received in revised form 16 December 2010 Accepted 17 December 2010 Keywords: Carbonaceous aerosol Size distribution OC/EC Oxalate PAHs

a b s t r a c t Size distributions can provide important information about aerosol sources, formation, and growth mechanisms. However, compared to size distributions of inorganic aerosols, size distributions of carbonaceous aerosols have been much less studied and reported in the literature. In this paper, we systematically measured size distributions of elemental carbon (EC), organic carbon (OC), oxalate, polycyclic aromatic hydrocarbons (PAHs), as well as major inorganic ions in urban aerosols in Shenzhen, China. Totally 24 sets of samples were collected using a ten-stage micro orifice uniform deposit impactor (MOUDI) during October 2009 to February 2010. Three lognormal modes contained in the size distributions of species were resolved based on positive matrix factorization (PMF) analysis of the measured dataset, corresponding to the condensation (peak = 0.34 μm), droplet (peak = 0.84 μm), and coarse (peak = 5.4 μm) modes, respectively. The mean concentrations of EC in the condensation, droplet, and coarse modes were 2.20, 1.18, and 0.64 μg m−3, respectively, and the modal characteristics of EC indicate that fresher local combustion emissions contributed mostly to aerosol EC in the urban atmosphere of Shenzhen. The mean concentrations of OC in the condensation, droplet, and coarse modes were 2.29, 3.34, and 3.51 μg m−3, respectively, and the modal characteristics of OC indicate that its sources were more primary in the condensation mode while more secondary in the droplet mode. The modal characteristics of aerosol oxalate and PAHs suggest that they were predominantly from in-cloud secondary formation and local emissions, respectively. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Carbonaceous aerosols, including both organic matter (OM) and elemental carbon (EC), are a key part of atmospheric aerosols on local to global scales and play a crucial role in understanding regional air pollution, climate change, and atmospheric chemistry. As aerosol size distribu-

⁎ Corresponding author. Tel.: +86 755 26035008; fax: +86 755 26035332. E-mail address: [email protected] (L.-Y. He). 0169-8095/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2010.12.022

tions can provide important information about the sources, formation and growth mechanisms (Hinds, 1999), knowledge of the size-resolved chemical composition of carbonaceous aerosols is essential when studying their sources and environmental impacts (e.g., Venkataraman and Friedlander, 1994; Hitzenberger and Tohno, 2001; Huang et al., 2006a; Hwang et al., 2008). Measurements of ambient carbonaceous aerosol size distributions have been mostly made in developed countries in North America and Europe, while few relevant studies in China have been conducted and reported, where air pollution has become a severe environmental issue, especially in urban environments. Size distribution

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characteristics of carbonaceous aerosols reported in the literature vary significantly among different sampling sites due to the different air pollution sources and levels. For example, the size distributions of aerosol EC in urban atmospheres in the US and European countries were ever reported to be bimodal in the submicron range with the peaks at 0.05–0.15 μm and 0.39–1.0 μm, respectively (e.g., Venkataraman and Friedlander, 1994; Hitzenberger and Tohno, 2001), while in an urban atmosphere in China the EC size distributions showed a broad peak across the submicron range (Huang et al., 2006a; Huang and Yu, 2008). In this paper, we systematically measured aerosol size distributions of EC, OC (organic carbon), and several important organic species, i.e., oxalate and PAHs (polycyclic aromatic hydrocarbons), in the urban atmosphere of Shenzhen, a coastal mega-city in the Pearl River Delta (PRD) region in South China. Many previous aerosol studies in PRD have indicated the key role of carbonaceous aerosol in fine particle pollution (e.g., Louie et al., 2005; Hagler et al., 2006; Huang et al., 2006b). Source apportionment analysis has indicated that vehicle emissions, biomass burning, and secondary formation are the major sources of organic matter in fine particles in this region (e.g., Yuan et al., 2006a, 2006b; Huang et al., 2010). Most of previous carbonaceous aerosol studies in PRD, however, were based on bulk PM2.5 or PM10 samples, which cannot provide further size-resolved information on aerosol sources and formation mechanisms. Therefore, it is necessary to explore the size distribution characteristics of carbonaceous aerosol in this region. Size distributions of major inorganic ions, such as sulfate, nitrate and potassium, were also measured simultaneously in this study to aid the interpretation of the size distribution characteristics of carbonaceous aerosols. Different modes contained in the measured aerosol size distributions were resolved by factor analysis technique in this study to investigate their respective sources and formation mechanisms. The modal characteristics of carbonaceous aerosol size distributions revealed in this study can help understand the causes of urban aerosol pollution in this region and provide useful implications for other regions. 2. Experimental methods 2.1. Sample collection Shenzhen (113.9° E, 22.6° N), a mega-city in South China, is located in the subtropics along the southeast coast of China. It is in the southeast corner of PRD, neighboring Hong Kong to the south. The sampling site in this study was located on the rooftop of a 4-floor building (~18 m above ground) on the campus of Peking University Shenzhen Graduate School, with no significant local pollution sources nearby. The campus is located in the western urban area of Shenzhen. A ten-stage micro orifice uniform deposit impactor (MOUDI, Model 110, MSP Co., USA) was used to collect size-segregated aerosol samples in the size range of 0.056–18 μm onto quartz fiber filters that were slowly rotating at a constant speed during sampling. The MOUDI sampling flow rate was 30 L min−1, and the aerodynamic size cutoffs were 0.056, 0.10, 0.18, 0.32, 0.56, 1.0, 1.8, 3.2, 5.6, 10 and 18 μm, respectively. All the quartz fiber filters were baked at 550 °C for 5 h before sampling to

reduce organic impurities. The MOUDI sampling were conducted inconsecutively during the fall and winter from October 2009 to February 2010, when Shenzhen was in the dry seasons and under the influence of prevailing northeasterly continental wind. Many previous studies have revealed that the fall and winter seasons are the most polluted period in this region during the whole year due to lower boundary layer heights, less wet removal, and the input of the polluted continental air mass from the north (e.g., Louie et al., 2005; Hagler et al., 2006; Yuan et al., 2006a). Totally 24 sets of samples were collected and each sampling event lasted for about 24 h. All filter samples were stored at −18 °C in a refrigerator before the chemical analysis. A SO2 analyzer and a NOx analyzer (Thermo Co., USA) were used for the on-line monitoring at this site. 2.2. Chemical analysis The MOUDI samples were analyzed for OC and EC, ionic − + − 2− + species (i.e., SO2− 4 , NO3 , Cl , C2O4 , NH4 , and K ), as well as PAHs. The OC/EC contents were analyzed using a thermal/ optical carbon analyzer (Desert Research Institute, Model 2001A, USA) and the temperature program used in this study followed the EPA standard method, i.e., the IMPROVE protocol (Chow et al., 1993, 2001). Four temperature steps (140, 280, 480, and 580 °C) were used in the first stage of analysis in pure helium, followed by another three temperature steps (580, 740, and 840 °C) in the second stage of analysis in 2% O2/98% He. The charred organic carbon generated during the thermal carbon analysis was corrected by utilizing the reflectance laser signal (Chow et al., 1993). Following Turpin and Lim (2001), OC mass was converted to OM mass by multiplying a factor of 1.6 in this study. To determine ionic species, part of each filter was extracted with 10 mL ultrapure water in an ultrasonic bath for 20 min and then filtered with a 0.45 μm Teflon filter (Millipore, USA). The resulting solutions were analyzed using an ion chromatography system (ICS-2500, Dionex, USA) equipped with an electro− − 2− chemical detector. The anions (SO2− 4 , NO3 , Cl , and C2O4 ) were determined using an AS-11 column and a gradient + elution solution of NaOH. The cations (NH+ 4 and K ) were determined using a CS-12A column and an isocratic elution solution of methanesulfonic acid. Due to significant filter blanks, the data for Ca2+, Na+, Mg2+ ions were not used in this study. The 16 sets of samples collected in 2009 were also analyzed for major PAHs in the six MOUDI stages in the accumulation mode (0.1–3.2 μm), which is mostly related to adverse health effects of aerosols. The analysis of aerosol PAHs was done by Curie-point pyrolysis gas chromatography–mass spectrometry (CPP-GC/MS). This method has been described previously (Neusüss et al., 2000; Gnauk et al., 2008) and some modifications were made in this study for optimization. A brief introduction of the analytical procedures is given below. A JHP-5 Curie-point pyrolyzer (CPP, Analytical Industry Company, Japan) was connected to an Agilent GC/ MS (Model 7890/5975 N). The temperatures of the CPP oven and the needle between the CPP and GC were set at 330 °C and 300 °C, respectively. A Fe/Ni alloy foil (Pyrofoil) with a Curie point of 445 °C was used to wrap up the quartz filter in CPP. This temperature was achieved in less than 0.1 s and

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held constant for 120 s, during which PAHs were evaporated and introduced into the GC column by helium. The GC was equipped with a 30 m length × 0.25 mm diameter DB-5MS capillary column (0.25 μm film thickness, J&W Scientific). The temperature program for the GC oven consisted of a 5 min isothermal hold at 50 °C, followed by a ramp to 100 °C at 15 °C min−1 and to 300 °C at 6 °C min−1, and then an isothermal hold at 300 °C for 15 min. The MS detection was conducted by electron impact ionization of 70 eV, and the mass fragment scan was from 50 to 550 amu. The PAHs in the samples were identified by comparing the sample mass spectra with those in the National Institute of Standards and Technology mass spectral reference library (NIST05a) and further confirmed by comparison with standards for retention time and mass spectra. Compound quantification was based on the calibration curves of the corresponding PAH standards.

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3. Results and discussion 3.1. Size-resolved bulk aerosol compositions Fig. 1a shows the variation of the bulk chemical composition (~PM18) of the 24 sets of samples collected between October 2009 and February 2010 in Shenzhen. The reconstructed total mass concentration (i.e., the sum of the measured species mass) varied from 18 to 71 μg/m3. Apparently in Fig. 1a, organic matter and sulfate were the two most abundant aerosol species. Fig. 1b shows the mean size-resolved chemical composition of the 24 sets of samples. In this work, we use 1.8 μm as the split diameter between fine and coarse particles according to the MOUDI cutoff sizes. It is − seen that the total mass of the measured species (SO2− 4 , NO3 , − + + Cl , NH4 , K , oxalate, EC and OM) mostly existed in the fine mode, peaking at the size range of 0.56–1.0 μm. It should be

Fig. 1. The bulk composition variation (a) and average size-segregated composition of the MOUDI samples collected in Shenzhen.

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noted that, however, the reconstructed mass in the coarse mode calculated in this study should be much less than the actual particulate mass due to the missing measurements of Ca2+, Na+, Mg2+ and other crustal and sea salt elements, which are popular materials of coarse particles; while the sum of the measured species could account for over three quarters of the actual particulate mass in PM2.5 as indicated by previous studies (e.g., Louie et al., 2005; Hagler et al., 2006). In the fine mode, carbonaceous aerosol (OM + EC) averagely contributed 44.7% of the total measured mass in this study, indicating the key role of carbonaceous aerosol in determining fine particle loading in this region. The OC/EC

ratio in the fine mode was averagely 1.8, much higher than ~ 0.5 observed for primary vehicular emissions in this region (He et al., 2008), implying that there was much contribution from secondary organic aerosol. The mean ten-stage size distributions of the measured species are shown in Fig. 2. Most of the species showed a prominent peak in the fine mode except nitrate and chloride, which also had a comparable coarse mode. The mass size distributions of ambient aerosols are known to typically have three modes, i.e., the nucleation, accumulation, and coarse modes, and each mode is associated with a specific size range, source, and formation mechanism (Seinfeld and Pandis,

Fig. 2. The average size distributions of the eight measured aerosol species.

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1998). In the ambient atmosphere, the different aerosol modes may partially overlap with each other (Ondov and Wexler, 1998). The overlapping of the different aerosol modes is analogous to the overlapping spectroscopic peaks arising from a mixture of similar chemical compounds. In most of the previous studies, the overlapping aerosol modes were not resolved, which was unfavorable for identifying the sources and formation mechanisms of different aerosol modes. In the next section, mathematical methods are used to resolve the overlapping modes contained in the aerosol size distributions measured by MOUDI in this study. 3.2. Size distribution mode resolving Positive matrix factorization (PMF) analysis has been applied to resolve different modes contained in aerosol size distributions (Kim et al., 2004; Zhou et al., 2005; Huang et al., 2006b). Huang et al. (2006b) successfully used the PMF method to resolve the condensation, droplet, and coarse modes in the size distributions of water-soluble aerosol species in Shenzhen. In that study, however, the size distributions of EC and OC were not included. Here, we used the PMF model to resolve different aerosol modes for the + − 2− − eight measured species (NH+ 4 , K , SO4 , NO3 , Cl , oxalate, EC and OC). The detailed principle of the PMF model can be found elsewhere in the literature (Paatero and Tapper, 1994). A total of 192 sets of mass size distribution data were generated, with 24 sets of mass distribution data for each of the eight measured species. The PMF model was then performed on these 192 sets of data with the number of factors fixed at three. Three factor profiles were resolved to correspond to the condensation, droplet, and coarse modes, respectively, as shown in Fig. 3, the sum of which explained 95% of the original reconstructed mass in the whole size range. The continuous size distribution of each mode was then retrieved by utilizing the pre-determined kernel function following the method described by Dong et al. (2004). In result, each mode resolved by PMF was excellently fitted by a lognormal distribution, also shown in Fig. 3, indicating the effectiveness of the PMF modeling. The peak diameters for the condensation, droplet, and coarse modes were 0.34, 0.84, and 5.4 μm, respectively. It is noted that the droplet mode showed a narrower distribution (standard deviation σ = 1.39) than that of the condensation mode (standard deviation σ = 1.56), consistent with the fact that the droplet mode mostly corresponded to aged aerosols which tend to become narrowly distributed in size distribution due to addition of secondary materials on existing particles (Ondov and Wexler, 1998). The mode contributions to the total size distributions of the measured species are also shown in Fig. 2, and the mean mode concentrations of different species are tabulated in Table 1. In the following sections, the atmospheric behaviors of the measured species will be discussed based on their modal characteristics in size distributions. 3.3. Size distribution modes of inorganic ions Substantial knowledge has been gained on the size distributions of aerosol inorganic ions during the past decades (Seinfeld and Pandis, 1998), and their size distribution patterns

Fig. 3. The coarse, droplet, and condensation mode distributions resolved by PMF (black lines) and the fitted lognormal curves (color lines).

Table 1 The list of average mode concentrations of the measured aerosol species.

SO2− 4 NO− 3 − Cl NH+ 4 K+ EC OC Oxalate

Condensation mode, μg m−3

Droplet mode, μg m−3

Coarse mode, μg m−3

1.49 ± 1.09 0.41 ± 0.22 0.14 ± 0.08 0.49 ± 0.35 0.15 ± 0.11 2.20 ± 1.04 2.29 ± 1.27 0.05 ± 0.04

7.16 ± 3.81 1.69 ± 1.13 0.60 ± 0.85 2.44 ± 1.70 0.59 ± 0.91 1.18 ± 0.60 3.34 ± 1.73 0.14 ± 0.08

3.36 ± 1.15 2.38 ± 2.08 1.04 ± 0.73 0.14 ± 0.18 0.13 ± 0.07 0.64 ± 0.38 3.51 ± 1.82 0.05 ± 0.03

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can provide convenient references for understanding the size distribution characteristics of carbonaceous aerosols. We describe below the size distribution characteristics of the inorganic species measured in this study before the analysis of carbonaceous aerosol size distributions. On average, sulfate had a predominant droplet mode, a small condensation mode, and a small coarse mode (Fig. 2a) in this study. The size distributions of sulfate have been the subject of numerous studies in the past few decades because of sulfate's link with acid deposition, and thus the size distribution characteristics of sulfate are relatively well understood: the condensation mode SO2− arises from homogeneous gas phase photochemical 4 oxidation of SO2 followed by gas-to-particle conversion (John et al., 1990; Seinfeld and Pandis, 1998); the droplet mode SO2− 4 has been revealed to be formed through aqueous oxidation of SO2 in clouds/fogs (Meng and Seinfeld, 1994; Kerminen and Wexler, 1995; Seinfeld and Pandis, 1998); and the coarse mode SO2− 4 can be attributed to a combination of sea salt sulfate and heterogeneous reactions of SO2 on sea salt and soil particles (Pakkanen et al., 1996; Zhuang et al., 1999). The size distribution of sulfate measured in this study indicates that most of sulfate should be produced from in-cloud aqueous oxidation of SO2, which is consistent with the conclusion of a previous study in Shenzhen by Huang et al. (2006b). The mean size distribution of nitrate had comparable droplet and coarse modes and a small condensation mode, as shown in Fig. 2b. Fine mode NO− 3 is governed by the thermodynamic equilibrium of HNO3(g) + NH3(g) = NH4NO3 (s, aq) (John et al., 1990) while coarse mode NO− 3 is formed by the heterogeneous reaction of gaseous HNO3 with sea salt or soil particles (Yoshizumi and Hoshi, 1985; Zhuang et al., 1999). The chemistry favors the production of semi-volatile NH4NO3 in the fine mode at low temperature (Allen et al., 1989; Yao et al., 2003a). As expected, the fraction of fine mode nitrate in total nitrate in this study showed a high correlation with ambient temperature (R2 = 0.82). This feature for nitrate was also previously observed in this region by Zhang et al. (2008). The bigger amount of NO− 3 in the droplet mode rather than in the condensation mode may reflect that most NO− 3 was in aged aerosols that had experienced in-cloud processing and got internally mixed with sulfate. Chloride shown in Fig. 2c showed a similar size distribution pattern to that of nitrate. The coarse mode chloride was clearly an indicator of sea salt aerosols, while the fine mode chloride could be mostly attributed to semivolatile NH4Cl. The size distribution of NH+ 4 was concentrated in the droplet mode and had a small condensation mode and a small coarse mode, as shown in Fig. 2d, which was a sum of contributions from secondary (NH4)2SO4, NH4NO3, and NH4Cl. K+ in fine particles can serve as an effective tracer for biomass burning aerosols (Andreae, 1983; Yamasoe et al., 2000). The mean K+ size distribution shown in Fig. 2e was dominated with a droplet mode and had small condensation and coarse modes. Such mode characteristics for K+ were also previously observed in Shenzhen by Huang et al. (2006b) and the droplet mode K+ can be attributed to incloud processing of primary biomass burning particles in the condensation mode, which are effective cloud condensation nuclei (CCN) due to large water-soluble organic contents (Novakov and Corrigan, 1996; Kaufman and Fraser, 1997).

The coarse mode K+ could originate from both soil and sea salt particles. 3.4. Size distribution modes of EC and OC Knowledge of size distribution characteristics of EC and OC is essential in many aspects, such as studying the formation and growth of aerosol particles, wet and dry deposition of aerosols, aerosol light extinction, regional and global climate, as well as the effects of aerosols on human health. This is especially the case in PRD, since carbonaceous aerosol can contribute as much as 30–40% of the total PM2.5 mass in PRD (Cao et al., 2004; Hagler et al., 2006). The information about size distributions of EC and OC, however, are very limited compared to that of the inorganic ions in the literature. In this study, we have systematically measured and discussed the size distributions of aerosol EC and OC. The average EC size distribution is shown in Fig. 2f. Unlike other species, EC showed a bigger condensation mode than the droplet mode, and a small coarse mode. The mean concentrations of EC in the condensation, droplet, and coarse modes were 2.20, 1.18, 0.64 μg m−3, respectively. The size distribution of EC particles from primary vehicular emissions in this region was previously measured in a roadway tunnel and the results showed a dominant condensation mode peaking at ~ 0.4 μm (Huang et al., 2006a). Previous studies on the temporal and spatial distributions of PM2.5 in PRD indicated that local vehicular exhaust was the dominant source of EC in the urban atmosphere in PRD (Cao et al., 2004; Hagler et al., 2006). Therefore, the dominant condensation mode of EC found in this study implies that the ambient EC particles in Shenzhen were dominated by fresher EC particles from vehicular emissions. On the other hand, the condensation mode EC also showed some correlation with the condensation mode K+ (R2 = 0.55, in Table 2), indicating that fresh biomass burning emissions might also contribute to the condensation mode EC. Since even fresh EC particles emitted from vehicles in this region were suggested to be effective CCN, therefore the droplet mode EC can be attributed to aged and cloud-processed particles (Huang et al., 2006a, 2008). Both the condensation mode EC and the droplet mode EC have significant correlation with NOx (R2 = 0.61, in Table 2), consistent with their common combustion sources. The coarse mode EC showed some weak correlation with fresh and aged sea salt species like the coarse mode Cl−, NO− 3 , and SO2− 4 , implying a potential marine source, most likely to be vessel emissions. A recent study by Schembari et al. (2010) has found a bimodal EC size distribution for vessel emissions, with a large coarse mode EC. Other possible sources of the coarse mode EC include the re-suspension of EC-containing soil/dust particles and tire abrasion (Glaser et al., 2005). The size distribution characteristics of EC found in this study indicate that relatively fresher and thus locally emitted EC particles contributed mostly to aerosol EC in the urban atmosphere of Shenzhen. OC showed comparable droplet and coarse modes and a smaller condensation mode, as shown in Fig. 2g. The mean concentrations of OC in the condensation, droplet, and coarse modes were 2.29, 3.34, and 3.51 μg m−3 respectively. OC in the condensation mode showed high correlation with K+, NOx, and EC (R2 = 0.86, 0.73, and 0.71, respectively, in

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Table 2 The list of correlation coefficients among species in different modes and SO2 and NOx. R2 (n = 24) Condensation mode

Droplet mode droplet

Coarse mode coarse

EC OC Oxalate EC OC Oxalate EC OC Oxalate

SO2− 4

NO− 3

Cl−

NH+ 4

K+

EC

OC

SO2

NOx

0.26 0.23 0.46 0.27 0.31 0.79 0.27 0.38 0.20

0.19 0.58 0.33 0.04 0.25 0.02 0.37 0.23 0.49

0.00 0.07 0.00 0.00 0.05 0.00 0.28 0.11 0.21

0.51 0.51 0.81 0.16 0.20 0.40 0.01 0.01 0.01

0.55 0.86 0.74 0.00 0.00 0.02 0.19 0.03 0.22

1.00 0.71 0.71 1.00 0.50 0.45 1.00 0.38 0.46

0.71 1.00 0.77 0.50 1.00 0.30 0.38 1.00 0.31

0.14 0.33 0.19 0.30 0.28 0.02 0.01 0.29 0.02

0.61 0.73 0.58 0.61 0.29 0.13 0.02 0.42 0.22

Table 2), indicating that it was dominantly from primary emissions (e.g., biomass burning and vehicular emissions). The OC/EC ratio in the condensation mode was about 1, further supporting the primary origin of OC in the condensation mode. The condensation mode OC also showed some + correlation with NO− 3 and NH4 , indicating some contribution from secondary formation similar to NH4NO3. The droplet mode OC showed weak correlation with EC and sulfate, suggesting that it was a complicated result of both primary and secondary origins. A high OC/EC ratio of the droplet mode (2.8), however, indicates that secondary formation should be the leading source of the droplet mode OC. For the coarse mode OC, it only showed weak correlation with EC and sulfate, indicating its sources were very complex and could include comparable primary and secondary sources. In addition, the significant amount of the coarse mode OC could also come from biogenic aerosols (e.g., soil microorganisms, plant debris, and pollen). The size distribution characteristics of OC in Shenzhen indicate that the OC sources were more primary in the condensation mode while more secondary in the droplet mode.

Table 2), indicating that both biomass burning and photochemical production contributed significantly to oxalate in this mode. The droplet mode oxalate and sulfate were found to be well correlated (R2 = 0.79, in Table 2), suggesting their similar secondary formation pathways. Prevalent good correlations between sulfate and oxalate in ambient fine particles were found and discussed by Yu et al. (2005) and attributed to common in-cloud secondary formation of both sulfate and oxalate, which is further supported by our moderesolved aerosol dataset. The coarse mode oxalate showed some correlation with the coarse mode nitrate, implying that gaseous oxalic acid could be adsorbed onto alkaline coarse particles like gaseous HNO3 does (Neusüss et al., 2000; Mochida et al., 2003). Alternatively, it was ever suggested that the gaseous precursors could also react on coarse particles to produce oxalate (Kerminen et al., 1999; Mochida et al., 2003). Another potential source was metabolic processes of fungi in soil particles (Dutton and Evans, 1996; Yao et al., 2003b). The size distribution characteristics of aerosol oxalate found in this study strongly confirm the previous conclusion that aerosol oxalate in this region is mostly produced from in-cloud formation (Yu et al., 2005).

3.5. Size distribution modes of oxalate 3.6. Size distribution modes of PAHs Dicarboxylic acids are a significant fraction of atmospheric organic aerosols and were detected in aerosols across the world (e.g., Kawamura and Ikushima, 1993; Saxena and Hildemann, 1996; Huang et al., 2005). Despite their relatively low mass concentrations compared with aerosol inorganic ions, they have been shown to have potential to alter the hygroscopic property of atmospheric aerosols and hence to change global radiation balance (Facchini et al., 1999). Oxalic acid is typically the most abundant dicarboxylic acid in atmospheric aerosols and its sources have been suggested to include both primary and secondary origins. The size distribution of oxalate found in this study was dominated by a droplet mode and had a small condensation mode and a coarse mode, as shown in Fig. 2h. The condensation, droplet, and coarse mode concentrations of oxalate were 0.05, 0.14, and 0.05 μg m−3, respectively, which accounted for 2.2%, 4.2%, and 1.2% of the corresponding mode OC on a carbon mass basis. Biomass burning has been suggested to be a significant primary source of aerosol oxalate (Narukawa et al., 1999), while vehicle exhaust has been excluded from being a significant primary source of aerosol oxalate (Huang and Yu, 2007). The condensation mode oxalate showed good correlation with the condensa+ 2 tion mode NH+ 4 and K , (R = 0.81 and 0.74, respectively, in

PAHs, which are formed during incomplete combustion, have been proved to be mutagenic and carcinogenic to human bodies long before. In this study, the sixteen sets of MOUDI samples collected in 2009 were measured for PAHs in the accumulation mode size range (0.1–3.2 μm). The observed mean size distributions of sixteen PAHs are shown in Fig. 4. Among all the measured PAHs, dibenzo[ah]anthracene (DBahA) had the lowest mean concentration of 0.08 ng/m3, while benzo[b + k]fluoranthene (B[b + k]F) had the highest mean concentration of 1.88 ng/m3 in the accumulation mode. In Fig. 4, it is seen that all the PAHs (except fluoranthene) showed a size distribution pattern peaking at 0.32–0.56 μm, while the size distributions of the 5, 6-ring PAHs seemed to have more intensive mass distribution in the 0.32–1 μm size range than the 3, 4-ring PAHs. PAHs with 3 or 4 rings have been demonstrated to be semi-volatile and partition comparably in both the gas and particle phases, while PAHs with more rings are roughly non-volatile and mostly reside in the particle phase (Bidleman et al., 1986; Seinfeld and Pandis, 1998; Zheng et al., 2000). Therefore, low-ring PAHs in the particle phase might evaporate into the air and subsequently re-condense onto other particles, resulting in more uniform size distributions (Offenberg and Baker, 1999), as shown in

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Fig. 4. The average size distributions of the 16 measured PAHs in the accumulation mode (a–p) and total PAH amounts according to aromatic rings (q–t) (PHE: Phenanthrene; ANT: Anthracene; FLU: Fluoranthene; PYR: Pyrene; BaA: Benz[a]Anthracene; CHR/Triphenylene: Chrysene/Triphenylene; BghiF: Benzo[ghi] Fluoranthene; B(b + k)F: Benzo[b + k]Fluoranthene; BaF: Benzo[a]Fluoranthene; BeP: Benzo[e]Pyrene; BaP: Benzo[a]Pyrene; PER: Perylene; DBahA: Dibenzo[ah] Anthracene; IcdP: Indeno[cd]Pyrene; BghiP: Benzo[ghi]Perylene).

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Fig. 4q and r. This evaporation/re-condensation process would obscure the representativeness of the condensation and droplet modes for fresh and aged aerosols, respectively, in terms of the size distributions of low-ring PAHs in this study. The roughly non-volatile 5, 6-ring PAHs showed a quite similar size distribution pattern to that of EC, intensively peaking at 0.32–0.56 μm as shown in Fig. 4s and t, implying a larger condensation mode than a droplet mode. This is quite reasonable when considering both PAHs and EC are from primary combustion sources. The similar size distributions of EC and 5, 6-ring PAHs indicate that PAHs in the urban atmosphere of Shenzhen were also predominantly from local emissions. 4. Conclusions Size distributions of EC, OC, oxalate, PAHs, and inorganic ions in aerosols in the urban atmosphere of Shenzhen were systematically measured in this study. Totally 24 sets of samples were collected using a ten-stage MOUDI sampler during October 2009 to February 2010. In fine particles, carbonaceous aerosol (OM + EC) averagely contributed 44.7% of the total estimated mass, indicating a key role of carbonaceous aerosol. PMF model was used to resolve different aerosol modes contained in the size distributions of species. Three lognormal modes were resolved to correspond to the condensation, droplet, and coarse modes, respectively, and their peak diameters were 0.34, 0.84, and 5.4 μm, respectively. The mean concentrations of EC in the condensation, droplet, and coarse modes were 2.20, 1.18, and 0.64 μg m−3, respectively, and the modal characteristics of EC indicate that fresher local combustion emissions contributed mostly to aerosol EC in the urban atmosphere of Shenzhen. The mean concentrations of OC in the condensation, droplet, and coarse modes were 2.29, 3.34, and 3.51 μg m−3, respectively, and the modal characteristics of OC indicate that its sources were more primary in the condensation mode while more secondary in the droplet mode. The size distributions of aerosol oxalate showed a predominant droplet mode, which could be mainly attributed to in-cloud secondary formation. The modal characteristics of PAHs suggest that they were mostly from local combustion emissions in the urban atmosphere of Shenzhen. Acknowledgements This work was supported by the National Natural Science Foundation of China (20777001). References Allen, A.G., Harrison, R.M., Erisman, J.W., 1989. Field measurements of the dissociation of ammonium nitrate and ammonium chloride aerosols. Atmospheric Environment 26, 1591–1599. Andreae, M.O., 1983. Soot carbon and excess fine potassium—long range transport of combustion-derived aerosols. Science 220, 1148–1151. Bidleman, T.F., Billings, W.N., Foreman, W.T., 1986. Vapor particle partitioning of semivolatile organic compounds–estimates from field collections. Environmental Science & Technology 20, 1038–1043. Cao, J.J., Lee, S.C., Ho, K.F., Zou, S.C., Fung, K., Li, Y., Watson, J.G., Chow, J.C., 2004. Spatial and seasonal variations of atmospheric organic carbon and elemental carbon in Pearl River Delta Region, China. Atmospheric Environment 38, 4447–4456.

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