Characteristics and sources of polycyclic aromatic hydrocarbons and fatty acids in PM2.5 aerosols in dust season in China

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Atmospheric Environment 40 (2006) 3251–3262 www.elsevier.com/locate/atmosenv

Characteristics and sources of polycyclic aromatic hydrocarbons and fatty acids in PM2.5 aerosols in dust season in China Ximei Houa, Guoshun Zhuanga,b,c,, Yele Suna, Zhisheng Anc a

Center for Atmospheric Environmental Study, Department of Chemistry, Beijing Normal University, Beijing 100875, China Center for Atmospheric Chemistry Study, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China c State key Laboratory of Loess & Quaternary Geology, Institute of Earth Environment, CAS, Xian, 710075, China

b

Received 28 October 2005; accepted 1 February 2006

Abstract The concentrations and distributions of 10 polycyclic aromatic hydrocarbons (PAHs) and seven fatty acids (FAs) in PM2.5 collected in dust season in six sites located in the source regions and on the pathway of the two dust storm episodes over China in 2004 were observed. The average concentrations of the total PAHs in Beijing were 10 times higher than those measured in 1980s, indicating that the PAH pollution has been much aggravated due to the rapid development and the motorization in the past 20 years. In non-dust storm days, the highest PAH was found at Yulin (YL) in the dust source region, as there are three large coal mines located around YL. The characteristic ratios of the individual PAH at the six sites indicated that coal combustion and traffic emission were the major contributors to PAHs in urban areas. The high abundances of oleic acid and linoleic acid in the aerosols suggested that cooking emission was the main source of FAs in urban areas. In dust storm episodes, the total concentrations of PAHs and FAs decreased sharply after the dust storm, while the ratios of PAH/PM2.5 and FAs/PM2.5 increased, demonstrating that the organic pollutants were cleared out through deposition or transport of the dust particles after dust episodes passed. The ratio of PAH(4)/PAH(5,6) was developed to be a tracer to deduce the sources of PAHs in the aerosols based on the different environmental behavior of PAH(4) and PAH(5,6) during the long-range transport. The higher ratios of PAH(4)/PAH(5,6) in BNU-DE2 suggested that the PAHs in the dust aerosols in the urban area in the dust storm episode were from the long-range transport, whereas the lower ratios in YL-DE2 and DL-DE2, which were similar to the ratios in non-dust days, implied that the local emission contributed greatly to the organic pollutants in the aerosols in the dust source regions. The ratio of FAs, C18:1/C18:0, could also be used to trace the sources of the organic pollutants in the dust storm episode. r 2006 Elsevier Ltd. All rights reserved. Keywords: Aerosol; PM2.5; PAH; Fatty acids; Dust storm

1. Introduction

Corresponding author. Department of Environmental Science & Engineering, Fudan University, Center for Atmospheric Chemistry Study, Shanghai 200433, China. Tel.: +86 21 55664579; fax: +86 21 65642080. E-mail address: [email protected] (G. Zhuang).

1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.02.003

Dust storms occur mainly in spring in Northern China, as it is called dust season. About 65% dust episodes occurred in Northeast China between March and May from 1951–2001 (Wang et al., 2004). Dust storms not only influence inland China but also transport to Japan, Korea (Chun et al.,

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2001), Pacific, and even contribute to the formation of haze layers in the North American Arctic and the west America (Rahn et al., 1977; Husar et al., 2001; Perry et al., 1999). Most of those studies on the dust episodes concentrated on addressing the characteristics and the long-range transport of those inorganic components in the dust aerosols and how they affect the global biogeochemical cycle and earth’s climatic system (Zhang et al., 1992, 1993; Zhuang, et al, 2001; Kobayashi et al., 2002; Ito et al., 2002; Sun et al., 2004a, 2005; Guo et al., 2004a; Wang et al., 2005a, b). However, the information of organic characteristics in dust episodes is scarce (Fang et al., 1999; Guo et al., 2004b), especially, there is no report on those trace organic matters, such as polycyclic aromatic hydrocarbons (PAHs) and their long-range transport in the dust episodes. Organic aerosols are the large contributor to the fine aerosols in the atmosphere in many highly industrialized urban areas (Sun et al, 2004a; Xu et al., 2005). The concentrations of carbonaceous species contributed 18.4% and 37.2% of PM2.5 in summer and winter, respectively, in Beijing, of which organic carbon (OC) accounted for 70% or more of the total carbon (TC) (Dan et al., 2004, Zheng et al., 2005). Moreover, PAHs and fatty acids (FAs) contain some of the useful biomarkers, which have been successfully used for source appointment (Simoneit and Mazurek, 1982; Simoneit, 1986; Simoneit et al., 1991; Schauer et al., 1996; Zheng et al., 2000, 2005). The study on PAHs and FAs can provide not only the characteristics of organic aerosols, but also the information of the sources of those organic aerosols. Thus, the role of organic aerosols, especially those trace organic matters, such as PAHs, in those long-range transported dust aerosols should be paid much attention to.

In this study, we report 10 PAHs and 7 FAs in PM2.5 aerosols, which were collected in the dust season over northern China from the dust source areas, the pathway of the dust, and the urban cities and coastal cities that dust passed through. We probe into the origin and the characteristics of organic pollutants in dust episodes and try to address the question of how these trace organic matters are influenced by dust storm through the comparison of the composition and the distribution patterns of PAHs and FAs in non-dust storm days and in the dust episodes in six sampling sites. 2. Experimental 2.1. Sampling The aerosol samples were simultaneously collected at six sites: Beijing Normal university (BNU), Miyun (MY), Yulin (YL), Shanghai (SH), Qingdao (QD), and Duolun (DL) in March and April 2004; the detail information of sampling is shown in Table 1. Among these sampling sites, YL and DL are located in dust source regions, BNU, QD, and SH are in urban areas, and MY is located in suburban area of Beijing. YL is located in the border area between Maowusu desert and the Loess plateau in northwest China, DL is located in Hunshandake Desert of Inner Mongolia, which are the major source areas of the dust storms in China (Wang et al., 2004). Beijing is a large city in Northern China, where dust storms often occur. QD and SH are the coastal cities in China. QD is often on the transport pathway of those dust storms from the source areas to the Pacific (Guo et al., 2004b), whereas SH is also often affected by those Asian dust storms.

Table 1 Sampling locations Station

Sampling period

Sum of samples

Height (m)

Ambient mean Relative temp. (1C) humidity (%)

Site description

BNU

9 March–9 April

19

40

28

28

MY YL

9 March–7 April 11 March–7 April

19 14

4 12

7

26

SH

11 March–19 April

10

10

12

67

QD

14 March–9 April

10

10

8

63

DL

11 March–7 April

10

12

1

29

Mixed urban site, commercial, residential and heavy traffic regions Suburban site, near to the Miyun reservoir Residential and heavy traffic regions, sand land Urban site: commercial, residential and heavy traffic regions Urban site: commercial, residential and heavy traffic regions Residential regions, sand land

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PM2.5 aerosol samples were simultaneously collected from 9 March to 15 April, 2004 using medium volume samplers (model (TSP/PM10/PM2.5)-2) made by Beijing Geological Instrument—Dickel Co., Ltd., at ow rates of 77.59 l min 1 on a quartz fiber filters (90 mm, Whatman Company, UK). The sampling time was 12 h for every 3 days in nonepisodic days, while it was throughout the dust lasting time in the dust episodes. The filters had been baked out at 500 1C for 2 h prior to sampling to minimize the background of the organic contaminants, and the sample filters were put in the polyethylene plastic bags right after sampling and stored at 20 1C in a laboratory refrigerator before analysis. All of those filters were weighed before and after sampling with an analytical balance (Sartorius 2004MP, reading precision 10 mg) after stabilizing under constant temperature (2075 1C) and humidity (4072%). A total of 82 aerosol samples, including 10 dust episodic samples, were collected and used in this study. All the procedures were strictly quality-controlled to avoid any possible contamination of the samples. Two or three sequential samples were combined to be one in measuring to lower the method detection limit (MDL), as the concentration of these trace organic compounds in each sample was too low to measure it. 2.2. Extraction The samples were ultrasonically extracted thrice with 20 ml of dichloromethane (HPLC grade, Fisher, USA) each for 15 min, and then the extracts were combined together to be filtered with glass fiber filter to remove those insoluble particles and fibers. The filtrates were concentrated to 3 ml by a rotary evaporator at 40 1C under gentle vacuum. These concentrated filtrates were divided into two fractions and spiked with deuterated tetracosane (nC24D50, Alltech, USA) and hexamethylbenzene (Aldrich Chem. Co., USA) for the internal standards and recovery tests of FAs and PAHs, respectively. Both fractions were reduced to near dryness by gentle evaporation with a stream of highpurity N2, and re-dissolved in 3 ml hexane. The fraction for measuring PAHs was cleaned up on silica SPE cartridges (500 mg, 6 ml) (Phenomenex, Torrance, CA, USA), and then eluted from the cartridge with 3 ml benzene/hexane (1:1) after the cartridge was washed with 3 ml hexane. The final concentrated filtrates were adjusted to 500 ml by adding hexane for GC/MS analysis. The fraction for

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measuring FAs was derivatized by 500 ml of 10% BF3-methanol (Supelco, USA) to obtain the corresponding methyl esters for 30 min at 80 1C. After extraction and dryness, the extracts in hexane were concentrated to 500 ml and analyzed by GC/MS. The uncertainty of derivatization of the seven FAs was less than 10%. 2.3. GC/MS analysis In all, 14 PAHs and seven FAs in PM2.5 aerosols were identified and quantified by using a Trace GC/ MS spectrometer (ThermoQuest, Finnigan San Jose, CA, USA) with a DB-5MS capillary column (J&W Scientific, Folsom, CA, USA, 30 m  0.25 mm I.D., 0.25 mm film thickness). High-purity helium (99.999%) was used as carrier gas at a flow rate of 1.0 ml min 1. The GC temperature program for PAH analysis was from 100 1C (2 min) to 200 1C at 10 1C min 1 and hold at 200 1C for 3 min, then ramp to 290 1C at 5 1C min 1 and hold at 290 1C for 7 min. The temperature program for FA analysis was first hold at 60 1C for 2 min, ramp to 160 1C at 20 1C min 1 and hold at 160 1C for 5 min, then ramp to 250 1C at 10 1C min 1 and hold at 250 1C for another 3 min. The target compounds were identified by comparing the retention times and the mass spectra in the NIST98 (National Institute of Standards and Technology) standard library of those authentic standards. Relative response factors calculated for the standard compounds of 14 PAHs (Chem. Service) were as below: naphthalene (99.2%, Nap), acenaphthene (91.4%, Ace), fluorene (99%, Flu), anthracene (98.1%, Anth), fluoranthene (98.0%, Flua), pyrene (98%, Pyr), benzo[a]anthracene (98%, B[a]A), chrysene (98%, Chr), benzo[b]fluoranthene (99%, B[b]F), benzo[k]fluoranthene (97.5%, B[k]F), benzo[a]pyrene (99.2%, B[a]P), dibenz[a,h]anthracene (98.3%, Dib[a,h]A), benzo [ghi]perylene (99.2%, B[ghi]P), and indeno[1,2,3cd]pyrene (99.5%, IndP), and the seven methylated FAs (Sigma) were lauric acid, myristic acid, hexadecanoic acid, heptadecanoic acid, linoleic acid, oleic acid, and octadecanoic acid. The detection limits were 0.6–10 pg for PAHs and 1.3–61 pg for FAs. The background of five blank filters and field blank samples were below the detection limit, and thus no blank correction was applied to the sample filter measurement. The average relative standard deviations of seven duplicate measurements for PAHs and FAs were 6.1% (0.4–10.1%)

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and 3.2% (0.2–6.4%), respectively. Recovery tests were carried out through the same experimental procedure as that used for the samples, i.e. by spiking the blank filters with a standard mixture containing the 14 PAHs and four FAs (hexadecanoic acid, linoleic acid, oleic acid and octadecanoic acid, Sigma). The recovery was mostly in the range of 75–96%, except Nap, Ace and Flu, which were in the range of 50–70%. 3. Results and discussion 3.1. General descriptions of the dust episodes in 2004 During the whole sampling period, two dust episodes occurred: Dust Episode 1 (DE1) on 9–10 March 2004 and Dust Episode 2 (DE2) on 28–29 March 2004. According to the report of Monitoring and Warning system of dust storm of China Meteorological Administration (CMA), DE1 influenced a wide region, including most areas in Northern China, and it even reached the delta of Yangtze River, whereas DE2 was believed to be the strongest dust episode in 2004, which caused serious economic losses in Inner Mongolia. In Beijing area, the dust weather occurred in the nighttime of 28 March and the yellow dust covered the whole city at 2:00 PM on 29 March, then the dust moved eastward (http://www.cma.gov.cn). High Air Pollution Index (API) on 10 March (446), 28 March (338), and 29 March (239) were reported by Beijing Environmental Protection Bureau, which confirmed the occurrence of the dust episode (http:// www.bjepb.gov.cn). Dust samples from the two dust episodes were collected in BNU and MY sites, and DE2 samples were also collected in YL and DL sites. 3.2. The total concentrations of PAHs and FAs As the PAH compounds with 2- and 3-ring (namely Nap, Ace, Flu and Anth) of those priority pollutants publicized by EPA are of low molecular mass and high volatility, only those PAHs with 4- to 6-ring were presented and discussed in this study. The semivolatile PAHs with 4-ring are Flua, Pyr, B[a]A, Chr and the practically non-volatile PAHs with 5- and 6-ring are B[b]F, B[k]F, B[a]P, Dib[a,h]A, B[ghi]P, and IndP. Earlier studies indicated that the particle-bound fraction of PAHs with 3-and 4-ring could be from a few percent up to 75% of their total in the atmosphere, whereas of

those PAHs with 5- and 6-ring could be 90–100%, mainly depending on temperature and aerosol concentration (Schauer et al., 2003). The results measured of those PAHs with 4-ring could represent the lower limit to the actual abundance in the sampled air masses because only the fraction bound to aerosol particles or absorbed on the filter could be detected (Schauer et al., 2003). The average concentrations of the 10 PAHs in PM2.5 aerosols and the total mass concentrations of the aerosols collected from the six sites were presented in Table 2. Those data of PAHs from the dust season in QD and Hong Kong, and from non-dust days in Beijing were also listed for comparison. The total concentrations of the 10 PAHs in QD in this study (8.36 ng m 3) was similar to that of the earlier study (10.20 ng m 3) during the non-dust days (Guo et al., 2004b). However, the average concentrations of the total PAHs in both BNU (urban site of Beijing) and MY (suburban site of Beijing) were 26.48 and 4.45 ng m 3, 10 times higher than those from the early study (PK-1, urban, 2.56 ng m 3, 1986; PK-2, suburban, 0.35 ng m 3, 1987) (Simoneit et al., 1991). The results indicated that the PAH pollution in Beijing has been much aggravated due to the rapid development and the motorization in the past 20 years. The concentration of the seven individual FAs (C12–C18) and their total concentrations were listed in Table 3. The total concentrations of the seven FAs in the six sites ranged from 11.81 to 307.83 ng m 3. In the non-dust days, the average concentration of the total FAs in the urban site, BNU, was 54.01 ng m 3, five times higher than that in the suburban site, MY, (11.90 ng m 3), indicating that the pollution level of FAs in urban areas is much serious than in suburban areas. This pattern was the same as the early study in Hong Kong (Zheng et al., 2000). 3.3. Distribution and sources of PAHs and FAs in non-dust days 3.3.1. PAH The average concentrations and their standard deviations of the total PAHs at the six sites in nondust days were shown in Fig. 1. The highest of the total PAHs was 78.77 ng m 3 in YL site, ranging from 34.23 to 135.02 ng m 3, the second in BNU (26.48 ng m 3). As PAHs are almost exclusively produced from combustion and the sources of

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Table 2 Concentrations of PAH in PM2.5 at six sites over China (ng m 3) Station

Sampling during

Flua

Pyr

B[a]A

Chr

B[b]F

B[k]F

B[a]P

IndP

Dib[a,h]A

B[ghi]P

Sum

MY-DE1 MY-DE2 MY-N BNU-DE1 BNU-DE2 BNU-N YL-DE2 YL-N SH-N QD-N DL-DE2 DL-N PK-1 urbana PK-3 suburbana QD-Nb

9–10 March 2004 28–29 March 2004 11 March–7 April 2004 9–10 March 2004 28–29 March 2004 11 March–9 April 2004 29 March 2004 11 March–15 April 2004 11 March–19 April 2004 14 March–9 April 2004 29 March 2004 11 March–7 April 2004 June 1986

2.70 3.13 1.58 11.90 4.14 6.47 26.64 15.00 0.92 1.26 2.81 0.84 0.233

1.37 1.61 0.81 6.68 2.10 3.85 23.65 8.49 0.51 0.69 1.50 0.43 0.582

0.24 — 0.15 3.96 — 1.29 12.37 6.57 0.25 0.26 0.83 0.25 —

0.72 0.92 0.55 10.28 1.00 2.39 12.94 8.58 0.76 0.80 1.16 0.39 0.998

0.33 0.48 0.37 6.89 0.50 1.84 13.28 10.93 0.87 0.93 1.36 0.51 n.d.

0.10 0.17 0.11 1.87 0.15 0.86 9.44 3.84 0.35 0.33 0.45 0.20 n.d.

0.10 0.16 0.19 3.33 0.27 0.83 19.19 7.26 0.81 1.79 0.76 0.52 0.083

0.11 — 0.24 3.08 — 3.15 6.87 8.72 0.76 0.91 0.75 0.16 0.316

— — 0.13 1.05 — 2.47 1.45 1.87 0.24 — — — 0.015

0.13 — 0.33 6.40 0.34 3.33 11.83 7.51 0.87 1.39 0.78 0.19 0.333

5.81 6.48 4.45 55.43 8.49 26.48 137.65 78.77 6.34 8.36 10.41 3.47 2.557

April 1987, 25 April 2002

0.136

0.002

0.182

n.d.

n.d.

0.005

0.022

0.001

0.004

0.352

6 May 2002, 20 March 2002 7–8 April 2002 1–2 April 1996 9–10 May 1996

1.27

1.17

0.55

1.47

2.61

0.67

1.11

—

1.36

10.20

5.76

5.02

2.27

6.12

10.56

2.53

4.07

—

5.12

41.45 15.2, 21.4 25.6, 22.6

QD-DEb HK-Nc HK-DEc

—

—, not detected; n.d., not determined. Flua: fluoranthene; Pyr: pyrene; B[a]A: benzo[a]anthracene; Chr: chrysene; B[b]F: benzo[b]fluoranthene; B[k]F: benzo[k]fluoranthene; B[a]P: benzo[a]pyrene; Dib[a,h]A: dibenz[a,h]anthracene; IndP: indeno[1,2,3-cd]pyrene; B[ghi]P: benzo[ghi]perylene. a Simoneit et al. (1991). b Guo et al. (2004a, b). c Fang et al. (1999).

PAHs are mostly from the anthropogenic-like fossil fuel (Zheng and Fang, 2000). YL is located on the one of the dust source regions, the north-west of China. However, there are three large coal mines in Yu Lin city (www.sxyl.gov.cn). The high concentration of PAHs in the aerosols in YL indicated that the aerosols in YL was much influenced by the local coal mine although YL is located in a dust source region. Contrary, in another sampling site, DL (Doulun in Inner Mongolia of northern China) is also located in one of the dust source regions, the average concentration of PAHs was only 3.47 ng m 3, the lowest among these six sampling sites. This result indicated clearly that around DL, there is lack of those local industry emission sources. PAH concentration in BNU, the urban site, was much higher than that in MY, the suburban site, as BNU is located in the center of the downtown of Beijing, the heavy traffic areas, where it was influenced by the emission from the serious traffic and heating, whereas, MY is located in the northeastern suburb of Beijing and the average concentration of the total PAHs was 4.45 ng m 3, a little higher than the background concentration of PAHs

(o2.5 ng m 3) in Hong Kong. This result also suggested that the city plumes from downtown of Beijing may affect on the air in MY, the suburb areas, due to the prevailing southwesterly wind in spring (Sun et al., 2004b; Dan et al., 2004). The average concentrations of the total PAHs at SH (6.34 ng m 3) and QD (8.36 ng m 3) were lower than that in YL and BNU. These results showed that PAH pollution in Shanghai is less serious, possibly due to that there is no heating activity and more precipitation in the sampling period in SH. In QD in the sampling period, there was enhanced precipitation that could wash out more PAHs from the air, thus, the concentration of PAHs was much lower than that in BNU, the urban area of Beijing, although in QD there is heating activity in winter and in the early spring (Schauer et al., 2003). Fig. 2 illustrates the average relative contributions of the individual PAH to the total concentrations of PAHs in the six sites in the non-dust storm days. It was found that PAH(4) contributed most to the total concentrations of PAHs, 70% and 58% in MY and BNU, respectively, whereas it contributed less in SH (39%) and QD (42%). The partitioning of semivolatile PAH(4) (PAH with 4-ring) between

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Table 3 Concentrations (ng m 3) of fatty acids in PM2.5 and the ratios of C18:1/C18:0 in six sites over China Station

Sampling during

C12

C14

C16

C17

C18:2

C18:1

C18:0

Sum

MY-DE1 MY-DE2 MY-N BNU-DE1 BNU-DE2 BNU-N YL-DE2 YL-N SH-N QD-N DL-DE2 DL-N

9–10 March 2004 28–29 March 2004 11 March–9 April 2004 9–10 March 2004 28–29 March 2004 11 March–7 April 2004 29 March 2004 11 March–15 April 2004 29 March 2004 11 March–7 April 2004 11 March–19 April 2004 14 March–9 April 2004

0.79 1.39 0.60 4.52 2.94 3.32 7.01 2.96 1.32 1.55 9.34 2.57

1.25 3.85 1.40 8.96 5.58 5.84 17.73 7.03 2.66 2.84 22.06 6.96

10.86 14.82 5.52 43.15 23.40 23.12 122.67 74.08 26.53 28.49 141.84 72.67

0.21 0.50 0.19 1.32 0.58 0.83 37.24 4.51 0.40 0.34 0.00 0.00

0.44 0.75 0.28 2.81 1.13 1.57 22.13 7.59 1.49 0.49 11.91 12.48

2.94 2.91 1.13 13.00 3.02 7.13 40.13 39.87 5.45 5.15 46.27 28.48

5.86 9.34 2.78 27.49 10.69 12.21 60.92 33.88 9.00 12.17 70.23 34.32

22.35 33.56 11.90 101.24 47.33 54.01 307.83 169.92 46.85 51.02 301.66 157.48

C12: lauric acid; C14: myristic acid; C16: hexadecanoic acid; C17: heptadecanoic acid; C18:2: linoleic acid; C18:1: oleic acid; C18:0: octadecanoic acid.

100%

140 Mass Fraction [%]

PAH [ng m-3]

120 100 80 60

40

B[ghi]P Dib[a, h]A IndP B[a]P B[k]F B[b]F Chry B[a]A Pyr Flua PAH (4)

80% 60% 40% 20%

20 0

0%

MY-N

BNU-N

YL-N

SH-N

QD-N

DL-N

Fig. 1. Average concentrations and standard deviation of the total PAHs at six sites in non-dust storm days.

particles and the gas phase in the aerosols can be significantly influenced by the relative humidity (RH) (Schauer et al., 2003). Schauer et al. (2003) found that the PAH(4) concentration in particle was negatively correlated with RH, whereas PAH(5,6) (PAH with 5- and 6-ring) exhibited no correlation with RH. As there was a higher RH in SH (63%) and QD (66%) in the sampling periods, PAH(4) bound on the particle phase was reduced. However, the average RH in Beijing was 28%, which was much lower than that in SH and QD, thus PAH(4) contributed to the total PAHs in MY and BNU which was much higher than that in SH and QD. As B[ghi]P was used to be a tracer of traffic sources of PAHs (Simcik et al., 1999; Zheng et al., 2000) because of the close association of B[ghi]P with vehicular exhaust (Baek et al., 1991), the higher contributions of B[ghi]P at BNU (13%), QD (16%), and SH (14%) sites were likely due to the exhaust emission from the heavy traffic in these megacities

MY-N BNU-N YL-N

SH-N

QD-N

DL-N

Fig. 2. Concentration fraction of the individual PAH at six sites in non-dust storm days. Flua: fluoranthene; Pyr: pyrene; B[a]A: benzo[a]anthracene; Chr: chrysene; B[b]F: benzo[b]fluoranthene; B[k]F: benzo[k]fluoranthene; B[a]P: benzo[a]pyrene; Dib[a,h]A: dibenz[a,h]anthracene; IndP: indeno[1,2,3-cd]pyrene; B[ghi]P: benzo[ghi]perylene; PAH(4) include Flua, Pyr, B[a]A and Chr.

(Simoneit, et al., 1991; Zheng et al., 1997; Zheng and Fang, 2000). It is generally known that most of the PAHs are of anthropogenic origin, and there are two main sources of PAHs: mobile (vehicular) and stationary (residential heating and power plants) (Zheng et al., 2000). As mentioned above, the concentration ratios of the individual PAH can be used to trace the emission sources (Schauer et al., 2003). The characteristic ratios of some individual PAH from the earlier studies and the average ratios of these individual PAHs in the aerosols collected in the non-dust days in this study were listed in Table 4. The ratios of Pyr/B[a]P and B[a]A/Chy in MY, BNU, and YL were in the range of those characteristic ratios (Pyr/B[a]P:1–6; B[a]A/Chy: gasoline, 0.28–1.2; diesel, 0.17–0.36) of traffic source

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Table 4 Individual PAH ratios in six sampling sites BaP/B[ghi] P

Pyr/B[a]P

B[a]A/Chy

Traffic

0.3–0.44a

1–6a

0.28–1.2 (gasoline)b

Coal combustion MY BNU YL SH QD DL

0.9–6.6a 0.57 0.25 0.97 0.92 1.29 2.75

o1a 4.31 4.62 1.17 0.63 0.38 0.83

1.0–1.2b 0.28 0.54 0.77 0.33 0.32 0.65

a

IndP/B[ghi]P 0.17–0.36 (diesel)b

Traffic 0.9b 0.75 0.95 1.16 0.87 0.65 0.85

From Tang (1993). From Simcik et al. (1999).

b

3.3.2. Fatty acids (FAs) Fig. 3 presented the average concentrations and the relative standard deviations of FAs in these six sampling sites in the non-dust storm days. The concentrations of FAs in the aerosols at YL and DL were 169.92 and 157.48 ng m 3, respectively, 3 times higher than that at BNU, QD, and SH sites (54.01, 51.02, and 46.85 ng m 3, respectively). Besides microbial activities, cooking has been found to be an important contributor of those oC20 FAs in urban areas (Rogge et al., 1991). The relative abundances of linoleic acid (C18:2) and oleic acid (C18:1) were much higher in YL and DL than that in other sampling sites, as shown in Fig. 4. It has been reported that linoleic acid was mostly the products released from Chinese cooking (He et al., 2004), and n-hexadecanoic acid (C16), n-octadecanoic acid

250

Fatty acids [ng m-3]

(Tang, 1993; Simcik et al., 1999), and the ratios of B[a]P/B[ghi]P or IndP/B[ghi]P were close to that of those in coal combustion emissions (B[a]P/(B[ghi]P: 0.9–0.66; IndP/B[ghi]P: 0.9). In SH, QD, and DL, the average ratios of B[a]A/Chy were in the range of those characteristic ratios of traffic source, and the average ratios of Pyr/B[a]P were all lower than 1, just in the range of the characteristic ratio (o1) of coal combustion. These results confirmed that both coal combustion and vehicle emissions were the major sources of the PAHs in the six sites in the non-dust storm days. It is noticed that the source of the PAHs in SH was mostly related to the coal combustion, which was likely from the large power plant and the huge steel industry in SH, although there is basically no heating activity in the sampling period in this city.

200 150 100 50 0

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Fig. 3. Average concentrations and standard deviation of fatty acids at six sites in non-dust days.

(C18), oleic acid (C18:1) and their oxidation products were the predominant compounds emitted from the meat cooking (Rogge et al., 1991; Schauer et al., 1999, 2002). Thus, the high abundances of linoleic acid and oleic acid in FAs at YL and DL could imply that cooking may be the major contributors to FAs in the aerosols in YL and DL. 3.4. The characteristics of PAHs and FAs in dust storm episodes During dust storm episodic periods, the concentrations of PAHs and FAs in the aerosols could be affected by those fine particles because of their emission, deposition, transport, and dilution (Schauer et al., 2003). In Figs. 5 and 6, the ratios of PAH/PM2.5 and FAs/PM2.5 were compared with the total concentrations of the PAHs and FAs at the four sampling sites (BNU, MY, YL, and DL), where the dust storm occurred. Throughout the sampling period, the following important trends of

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Dao on 20–21 March and 7–8 April 2002 (Fang et al., 1999; Guo et al., 2004b). These results suggested that in these dust storm episodes the PAHs and FAs were accumulated in the aerosols because the pollutants from the longrange transport from the source areas or from the pathways of the dust storm could overlap/ mix with those locally emitted pollutants, which would increase the total concentrations of these pollutants in the aerosols collected in the dust storm days (Guo et al., 2004a). (3) The total concentrations of the PAHs and FAs in most dust episodes decreased sharply after the dust storm passed, while the relative abundance of PAHs and FAs to fine particles, PAH/PM2.5 and FAs/PM2.5 increased, as illustrated in Figs. 5 and 6. For example, the total concentrations of PAHs and FAs in BNU-DE1 (at BNU in DE1 on 9–10 March) decreased from 55.42 and 101.24 ng m 3 on 9 March to 29.15 and 67.42 ng m 3 on 11 March, respectively, whereas the ratios of PAH/PM2.5 and FAs/PM2.5 increased from 90.08 and 164.54 mg g 1 to 221.06 and 511.28 mg g 1. This variation indicated that the pollutants, PAHs and FAs, were all or partially cleared out by deposition or transport with those dust particles when the dust storm passed away plus the input of fine aerosols with high pollution from local emissions increased, which resulted in the increase of the ratios of PAH/PM2.5 and FAs/PM2.5. However, the total concentration of PAHs in MY, the suburban site, increased after DE1 ended. This exception might be due to the input to the suburban area of those PAHs from the surrounding urban area polluted with higher concentration of PAHs after the dust storm passed away.

the variation of PAHs and FAs were found in the two dust storm episodes (DE1 and DE2). (1) In BNU-DE2 (at BNU in the Dust storm Episode 2 on 28–29 March), the total concentrations of PAHs and FAs reached the highest peak of 110.67 and 47.33 ng m 3 before the dust storm occurred and reduced to 65.91 and 8.49 ng m 3, respectively, in the dust storm episode. This result could be because the accumulated pollutants in the aerosols were cleared out before the pure dust appeared. This variation accorded with the mechanism of the concentration change of those pollution components in the super dust storm in 2002 in Beijing described in the earlier study (Guo et al., 2004a). (2) Except in BNU-DE2, the total concentrations of PAHs and FAs in the dust storm episode were much higher than that in the non-dust days. This variation was similar to the changes of the PAHs and FAs in the dust storms days detected in Hong Kong on 9–10 May 1996 and in Qing

C18:0 C18:1 C18:2 C17 C16 C14 C12

80% 60% 40% 20% 0%

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Fig. 4. Mass fraction of individual fatty acids at six sites in nondust storm days. C12: lauric acid; C14: myristic acid; C16: hexadecanoic acid; C17: heptadecanoic acid; C18:2: linoleic acid; C18:1: oleic acid; C18:0: octadecanoic acid.

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0.1

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1

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Fig. 5. Concentration (ng m 3) of the total PAHs in PM2.5 and the ratio of PAH/PM2.5 (mg g 1) in the dust storm episodes.

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1

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10

Fig. 6. Concentration (ng m 3) of the total FAs in PM2.5 and the ratio of FAs/PM2.5 (mg g 1) in the dust storm episodes.

3.5. The sources of PAHs and FAs in the aerosols of dust storm It was found that even in dust storm episodes, the concentration of organic pollutants emitted from those anthropogenic sources was very high in some dust source regions. For example, the total concentration of PAHs in YL-DE2 (at Yulin in the DE2 on 28–29 March) was as high as 137.64 ng m 3. Early studies reported that the persistent organic pollutants (POPs), such as PAHs, were subject to long-range atmospheric transport, which could result in the contamination of remote areas, such as the Arctic (Kawamura, 1998; Hung et al., 2005). It would be no doubt that those organic pollutants, PAHs and FAs with such a high concentration could be transported to other far away downwind areas in dust storm episodes. However, we do need to address such a question: whether these organic pollutants in the urban areas observed in the dust storm episodes derived from the local emission or from the long-range transport? The concentration of various PAHs could be affected by degradation, deposition, and those meteorological parameters in the long-range transport (Christensen, 1997; Gong et al., 1997a, b; Halsall et al., 2001). However, the environmental behaviors of PAH(4) and PAH(5,6) in the atmosphere were definitely different, as PAH(4) are semivolatile organics and exist in both gas and particle phases, while PAH(5,6) exist only in particle phase. Early studies reported that the particlebound PAH fractions were more efficiently removed from the atmosphere than those low particle-bound components by the photochemical breakdown and deposition (Halsall et al., 1997, 2001). It was found

that the observed profiles at the three Arctic sampling stations dominated by those lighter compounds of PAH(3,4), such as Flu, Phe, Flua and Pyr, while most of the heavier compounds, such as PAH(5,6), would have been removed from the atmosphere before reaching the Arctic (Halsall et al., 2001; Hung et al., 2005). Fig. 7 illustrated the ratios of PAH(4)/PAH(5,6) in the entire sampling period through the comparison of the abundances of PAH(4) (Flua, Pyr, B[a]A, and Chr) with PAH(5,6) (B[b]F, B[k]F, B[a]P, Dib[a,h]A, B[ghi]P, and IndP). In YL, DL, QD, and SH, the average ratios of PAH(4)/ PAH(5,6) were in the range 0.71–1.32. Among these four sampling sites, YL and DL were in the dust source regions, and the ratios of PAH(4)/PAH(5,6) in YL and DL were in the range 0.78–1.23 and 0.98–1.53, respectively. In the urban site, BNU, excluding BNU-DE2, the average ratio of PAH(4)/ PAH(5,6) was 1.44. However, it must be noted that in MY, the suburban site, the ratios of PAH(4)/ PAH(5,6) were much higher, ranging from 2.71 to 7.00, compared with the ratios in all of other sites. This result demonstrated that PAH(4) dominated in the suburban area, MY, which would indicate that PAH in MY, mostly in the form of PAH(4), were likely from the atmospheric transport of the city plumes. Simoneit et al. (1991) found that one of PAH(4), Flua, was predominant, and the heavier PAH (B[a]P, IndP and B[ghi]P) was of the minor fraction in the aerosols collected in a suburban sampling site, and they hypothesized that the semivolatile PAH(4) were the predominant components in the distribution of PAH from the longdistance transport. The results mentioned above in this study were consistent with the early report of

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0

Fig. 7. The ratios of PAH(4)/PAH(5,6) at six sampling sites.

Simoneit et al. (1991) and further demonstrated their hypothesis. Consequently, the ratio of PAH(4)/PAH(5,6) we derived could be used in evaluating the origin of PAH, i.e., the higher ratio of PAH(4)/PAH(5,6) implied longer distance of transport, while the lower ratio would suggest that those PAH might be mostly from the emission of the local source. For example, in the two dust storm episodes in BNU, the ratio of PAH(4)/PAH(5,6) in BNU-DE1 was 1.45, close to the ratios of that in the aerosols collected in the non-dust days, whereas the ratio in BNU-DE2 was much higher (5.9). This result indicated clearly that in DE1 most of the PAH were likely from the local emission, such as the resuspened dust, while in DE2 most of the PAH could be from the long-range transport. For FAs in the dust storm episodes, compared with the saturated FAs (octadecanoic acid, C18:0), the unsaturated FA (oleic acid, C18:1) is very unstable and can be rapidly oxidized and degraded in the environment (Simoneit and Mazurek, 1982; Kawamura and Gagosian, 1987). Thus FAs from the long-range transport would have a low ratio of C18:1/C18:0 (Simoneit et al., 1991; Fang et al., 1999). In the second dust storm episode (DE2), the ratios in BNU-DE2 and MY-DE2 were 0.28 and 0.31, respectively, lower than that in the aerosols collected in the non-dust storm days (0.61 in BNU and 0.42 in MY). These results provided supportive evidence that FAs in DE2 in Beijing were derived from long-distance transport (Fang et al., 1999; Guo et al., 2003). Contrarily, the higher ratios in YL-DE2 (0.66) and DL-DE2 (0.66) would imply that the FAs in YL and DL, the dust source regions, were likely from the local emission. In the first dust episode (DE1), the high level of the ratio, C18:1/ C18:0, in BNU-DE1 (0.47) also suggested that the FAs in the fine aerosols in BNU were derived from the local emission. It could be seen that the conclusion on the sources of FAs in the dust storm episode was consistent with the conclusion on the

sources of PAH in the dust storm episodes mentioned above. 4. Conclusion The concentrations and distributions of 10 polycyclic aromatic hydrocarbons (PAHs) and seven fatty acids (FAs) in PM2.5 fine particles collected in dust season in six sites located in the source regions and on the pathway of the two dust storm episodes over China in 2004 were observed. The average concentrations of the total PAHs in Beijing were ~10 times higher than those measured in 1980s, indicating that the PAH pollution has been much aggravated due to the rapid development and the motorization in the past 20 years. In non-dust storm days, the highest PAH was found at Yulin (YL) in the dust source region, as there are three large coal mines located around YL. The characteristic ratios of the individual PAH at the six sites indicated that coal combustion and traffic emission were the major contributor to PAHs in urban areas. The high abundances of oleic acid and linoleic acid in the aerosols suggested that cooking emission was the main source of FAs. In dust storm episodes, the total concentrations of PAHs and FAs decreased sharply after the dust storm, while the ratios of PAH/PM2.5 and FAs/PM2.5 increased, demonstrating that the organic pollutants were cleared out through deposition or transport of the dust particles after dust storm passed. The ratio of PAH(4)/ PAH(5,6) was developed to be a tracer to deduce the sources of PAHs in the aerosols based on the different environmental behavior of PAH(4) and PAH(5,6) during the long-range transport. The higher ratios of PAH(4)/PAH(5,6) in BNU-DE2 suggested that the PAHs in the dust aerosols in the urban area in the dust storm episode were from the long-range transport, whereas the lower ratios in YL-DE2 and DL-DE2, which were similar to the ratios in non-dust storm days, implied that the local

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emission contributed greatly to the organic pollutants in the aerosols in the dust source regions. The ratio of FAs, C18:1/C18:0, could also be used to trace the sources of the organic pollutants in the dust storm episode. The lower ratio of C18:1/C18:0 in BNU-DE2 than that in the non-dust storm days also provided supportive evidence that FAs in DE2 in Beijing were derived from long-range transport. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 30230310, 20477004, and 40575062), Beijing Natural Science Foundation (Grant no. 8991002 and 8041003), and also in part supported by SKLLQG, the Institute of Earth Environment, CAS and LAPC, the Institute of Atmospheric Physics, CAS, and the Swedish International Development Cooperation Agency (SIDA) through the Asian Regional Research Program on Environmental Technology (ARRPET) at the Asian Institute of Technology. References Baek, S.O., Field, R.A., Goldstone, M.E., Kirk, P.W., Lester, J.N., Perry, R., 1991. A review of atmospheric polycyclic aromatic hydrocarbons: sources, fate and behavior. Water, Air, and Soil Pollution 60, 279–300. Christensen, J.H., 1997. The Danish Eulerian hemispheric model—a three-dimensional air pollution model used for the Arctic. Atmospheric Environment 31, 4169–4191. Chun, Y., Kim, J., Choi, J.C., Boo, K.O., Oh, S.N., Lee, M., 2001. Characteristic number size distribution of aerosol during Asian dust period in Korea. Atmospheric Environment 35, 2715–2721. Dan, M., Zhuang, G., Li, X., Tao, H., Zhuang, Y., 2004. The characteristics of carbonaceous species and their sources in PM2.5 in Beijing. Atmospheric Environment 38, 3443–3452. Fang, M., Zheng, M., Wang, F., Chim, K.S., Kot, K.S., 1999. The long-range transport of aerosols from northern China to Hong Kong—a multi-technique study. Atmospheric Environment 33, 1803–1817. Gong, S.L., Barrie, L.A., Blanchet, J.P., 1997a. Modeling seasalt aerosols in the atmosphere 1. Model development. Journal of Geophysical Research 102D, 3805–3818. Gong, S.L., Barrie, L.A., Prospero, J.M., Savoie, D.L., Ayres, G.P., Blanchet, J.P., Spacek, L., 1997b. Modeling sea-salt aerosols in the atmosphere 2. Atmospheric concentrations and fluxes. Journal of Geophysical Research 102D, 3819–3830. Guo, Z.G., Sheng, L.F., Feng, J.L., Fang, M., 2003. Seasonal variation of solvent extractable organic compounds in the aerosols in Qingdao, China. Atmospheric Environment 37, 1825–1834.

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