Occurrence of Polybrominated Diphenyl Ethers in Air and Precipitation

Nov 12, 2009 - Academy of Sciences, Beijing 100049, China, and Air Quality. Research ... samples were collected in the Pearl River Delta (PRD) in sout...
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Environ. Sci. Technol. 2009 43, 9142–9147

Occurrence of Polybrominated Diphenyl Ethers in Air and Precipitation of the Pearl River Delta, South China: Annual Washout Ratios and Depositional Rates B A O - Z H O N G Z H A N G , †,‡ Y U - F E N G G U A N , †,‡ S H A O - M E N G L I , § A N D E D D Y Y . Z E N G * ,† State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, Graduate School, Chinese Academy of Sciences, Beijing 100049, China, and Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario M3H 5T4, Canada

Received July 2, 2009. Revised manuscript received October 28, 2009. Accepted October 30, 2009.

On the basis of a one-year (from October 2006 to September 2007) sampling campaign, 34 air samples and 23 bulk precipitation samples were collected in the Pearl River Delta (PRD) in southern China and analyzed for polybrominated diphenyl ethers (PBDEs). Fifteen tri- to deca-BDE congeners (sum of which is defined as Σ15PBDE) were detected in more than 70% of the samples. In three urban-rural regions, Σ15PBDE concentrations ranged from 77 to 372 pg/m3 in air (particulate + vapor) and 1.98 to 15.5 ng/L in rain (particle + dissolved) from Dongguan, from 195 to 1450 pg/m3 in air and 4.71 to 17.2 ng/L in rain from Shunde, and from 23.7 to 148 ng/L in rain from Guangzhou. Among the BDE congeners, BDE-209 was the predominant component. Linear correlations between the gas-particle partition coefficients (Kp) and the subcooled vapor pressures (PoL) of individual BDE congeners were observed for both the wet and dry seasons, but the slopes (-0.572 to -0.525) of the fitted equations all substantially deviated from equilibrium condition (slope ) -1). The total washout ratio by bulk rainfalls was determined to be 2 × 103 for tri-BDEs and 6 × 104 for BDE209. The estimated annual dry and wet depositional rates were 6720 and 2460 kg/yr, respectively, for BDE-209, and 7270 and 2940 kg/yr for Σ15PBDE in the PRD, indicating a dominant pathway for PBDEs input into the PRD soil and aquatic environments.

Introduction The atmosphere is an important transitional zone for the transport of semivolatile organic compounds (SOCs) (1), such as polybrominated diphenyl ethers (PBDEs). The effectiveness for atmospheric transport of SOCs is dictated by meteorological conditions and residence times of SOCs, with the latter depending upon the distribution of the target compounds between the gaseous and particulate phases in * Corresponding author phone: 86-20-85291421; fax: 86-2085290706; e-mail: [email protected]. † Guangzhou Institute of Geochemistry. ‡ Graduate School, Chinese Academy of Sciences. § Environment Canada. 9142

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the atmosphere (1). At the same time, SOCs can also be removed from the atmosphere through various depositional and photochemical processes, such as wet and dry depositions, gaseous adsorption, photolytic degradation, and reaction with OH radicals (2). Repeated volatilization and deposition not only result in the ubiquity of SOCs in environments surrounding the points of emissions, but also lead to the occurrence of the SOCs in areas far away from emission sources (3). Brominated flame retardants (BFRs) are a diverse group of different chemical mixtures, one of which is PBDEs present in three major commercial mixtures, i.e., Penta-, Octa-, and Deca-BDE formula, and are widely used in electronic and electrical appliances, automotive equipment, household appliances, plastic products, foam, and textiles to improve fireproof properties (4). As a class of SOCs, PBDEs have the potential for long-range transport in the atmosphere (5) and can be accumulated in both the aquatic and terrestrial systems (6). PBDEs may cause neural development deficits, thyroid hormone disruption, and potential carcinogenesis (7). In the Pearl River Delta (PRD), an area of 53,580 km2 (8) within Guangdong Province in southern China (Figure S1 of the Supporting Information; “S” designates tables, figures and other contents in the Supporting Information hereafter), the annual mean air temperature is 19-23 °C and the annual mean precipitation is 1500-2200 mm (9). Large quantities of PBDEs have been used in manufacture of electrical and electronic devices, plastic products, vehicles, garments, and textiles in the PRD (10). As a result, PBDEs, especially BDE209 which is the main component of Deca-BDE technical mixture, have been detected in a wide range of environmental media from the PRD, such as riverine runoff (11), river and coastal sediments (12), watershed soils (13), air (14, 15), edible fish (16), and maternal milk and fetal blood (17). Additionally, improper handling of electronic waste containing PBDE residues also adds to the widespread occurrence of PBDEs in the PRD’s environment (18). Despite numerous efforts to uncover the extent of environmental contamination by PBDEs in the PRD, atmospheric processes that play a critical role in distributing PBDEs and other SOCs have not been adequately examined. The present study was thus conducted to fill the knowledge gap by measuring seasonal variability, compositional profiles, gas-particle partition coefficients, washout ratios, and atmospheric depositional rates of PBDEs.

Materials and Methods Field Sampling. Air and precipitation were sampled at two rural sites, located in Shunde (806 km2 with an urban area of 82 km2) and Dongguan (2465 km2 with an urban area of 650 km2) (19), respectively. An urban site in Guangzhou (7434 km2 with an urban area of 3843 km2) (10) was also chosen for sampling precipitation (Figure S1). Particle and gas samples were collected using a high-volume air sampler that housed a glass fiber filter (GFF; 20.3 × 25.4 cm2, 0.6 µm nominal pore size, Whatman, Maidstone, England) for particles and a polyurethane foam plug (PUF; 6.5 cm diameter and 8.0 cm thick with a density of 0.030 g/cm3) for gases. To prevent breakthrough of PUF plugs, different sampling flow rates were set in different seasons and sampling sites. Six to 12 pairs of 24-h particle and gas samples were collected in each season from October 2006 to September 2007, yielding a total of 34 pairs with sampled air volumes ranging from 170 to 890 m3. Prior to sampling, GFFs were baked at 450 °C for 6 h and PUF plugs were Soxhlet extracted for 48 h with 10.1021/es901961x CCC: $40.75

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methanol (MeOH) and for another 48 h with dichloromethane (DCM). Upon sampling, loaded GFFs were wrapped with prebaked aluminum foils and sealed with double layers of polyethylene bags, whereas PUF plugs were stored in acetonerinsed glass jars with aluminum foil-lined lids. They were then transported to the laboratory and stored at -20 °C until extraction. Meteorological data and air sample records are presented in Table S1. Twenty-three bulk rainfall samples with volumes of 2090 L were collected with round stainless steel containers (80 cm diameter and 30 cm high) and transferred into 10-L brown glass bottles which were scrubbed with cotton wetted with acetone and rinsed with purified water prior to use at the three sampling sites from October 2006 to September 2007; these samples were transported on ice to the laboratory where they were refrigerated at ∼4 °C until further processing. Meteorological data and bulk rainfall sample records are detailed in Table S2. Sample Preparation and Extraction. All rainfall samples were filtered immediately upon delivery to the laboratory, and suspended particulate matter (SPM) was collected with GFFs (142 mm diameter and 0.7 µm nominal pore size; Millipore, Billerica, MA) precombusted at 450 °C for at least 6 h. The SPM-loaded GFFs were freeze-dried. Total suspended particulate (TSP) contents in rainwater were determined by weighing the GFFs before filtration and after freeze-drying. Each filtrate sample was passed through a glass column (15 mm i.d. and 400 mm length) containing a mixture of XAD-2 and XAD-4 resin (1:1 in weight) precleaned with MeOH and DCM. The loaded resin column was eluted 3 times with 50 mL of MeOH each, and the resin was further extracted 3 times with 50 mL of MeOH:DCM (1:1 in volume) each in an ultrasonic bath. All effluents were combined and spiked with a known amount of two surrogate standards (PCB-209 and 13 C-PCB-141), and ∼500 mL of distilled water and an appropriate amount of NaCl were added to the extract to remove MeOH. The extract was liquid-liquid extracted 5 times with 60 mL of DCM each and the DCM extracts were combined. Half of the combined DCM extract used for measuring PBDEs was concentrated to ∼1 mL with a Zymark TurboVap 500 (Hopkinton, MA), solvent-exchanged to hexane, and further concentrated to ∼2 mL. The concentrated extract was purified on a glass column (10 mm i.d.) packed with, from bottom to top, neutral alumina (6 cm, 3% deactivated), neutral silica gel (2 cm, 3% deactivated), 25% sodium hydroxide silica (5 cm), neutral silica gel (2 cm, 3% deactivated), 44% sulfuric acid silica (8 cm), and anhydrous sodium sulfate (2 cm). The fraction containing PBDEs was eluted on this glass column first with 30 mL of hexane and then 60 mL of hexane/DCM (1:1 in volume), and the eluant was concentrated to ∼0.5 mL and further reduced to100 µL under a gentle nitrogen stream. An internal standard, 13CPCB-208, was added to the final extract prior to instrumental analysis. Before the extraction of each particulate sample (air or precipitation), approximately one-twentieth of each GFF was cut out for determination of total organic carbon (TOC) content. The remaining GFF and each PUF plug were spiked with the surrogate standards prior to Soxhlet extraction with a 1:1 (v:v) acetone/hexane mixture for 48 h. Half of the extract was cleaned up with the same protocol as used for the filtrate samples. Detailed information about the chemical standards used is presented in the Supporting Information. Instrumental Analysis. The concentrations of the target analytes were determined with a Shimadzu model 2010 gas chromatograph-negative chemical ionization-mass spectrometer in the selected ion monitoring mode. Sample injection was conducted with an AOC-20i auto injector (Shimadzu, Japan). A DB-5MS (30 m × 0.25 mm i.d. with 0.25

µm film thickness) capillary column was used to separate 39 BDE congeners which were then quantified with an internal calibration procedure. In addition, a DB-5MS (15 m × 0.25 mm i.d. with 0.10 µm film thickness) capillary column was employed to separate six heavily brominated BDEs (BDE196, -197, -206, -207, -208, and -209) that were then quantified with an external calibration method. Other parameters have been detailed by Guan et al. and Mai et al. (11, 12). Quality Assurance and Control. Field, procedural, and spiked blanks and matrix spiked samples (target analyte standards spiked into pre-extracted field samples) were processed with each batch of 9 field samples. The recoveries of the surrogate standards in all samples were 81 ( 13% for 13 C-PCB-141 and 79 ( 19% for PCB-209. The recoveries of all BDE congeners ranged from 58 to 112% with a standard deviation (SD) of BDE-100 and -153 in the particle phase and BDE-47 > BDE-99 > BDE-100, -153, and -183 in the vapor phase. Furthermore, the relative abundance of BDE-47 was higher in the vapor phase than in the particulate phase (p < 0.05) (Figure 2), similar to the results from other studies (14, 20). Additionally, the relative abundances of BDE47 and -99 were higher in air from Dongguan than from Shunde (p < 0.05). Similar BDE congener profiles were observed in the rainwater samples. Briefly, BDE-209 remained the major constituent in rain (particle and dissolved), ranging from 87% (Dongguan) to 94% (Guangzhou) in the particle phase and from 65% (Dongguan) to 71% (Shunde) in the dissolved phase. The order of relative abundances of other important BDE congeners was as follows: BDE-183 > BDE-99 and -47 > BDE-153 > BDE-100 in the particulate phase and BDE-47 > BDE-99 and -183> BDE-100 and -153 in the dissolved phase. In addition, higher relative abundance of BDE-47 was also found in rainwater from Dongguan compared to Shunde (p < 0.05) (Figure 3), but the cause for the difference remains to be clarified. Phase Partitioning of Polybrominated Diphenyl Ethers. Gas-particle partitioning for SOCs is the key factor controlling the transport and depositional processes in the atmosphere (1), and is characterized by Kp (eq S1). When both adsorptive and absorptive mechanisms exist, log Kp can be related to log P Lo by log Kp ) mrlog PLo + br

(1)

where P Lo (Pa) is the compound’s subcooled liquid vapor pressure and mr and br are fitting constants. Under certain conditions, the value of mr may be indicative of whether adsorption or absorption is the dominant mechanism dictating the partitioning of SOCs between the gas and particle phases and it should be close to -1 for either adsorption or absorption at equilibrium under some assumptions (29). For the present study, the correlations between log Kp and log P Lo for BDE-17, -28, -47, -49, -66, -99, -100, -153, -154, -183, and -209 are significantly linear with r2 in the range of 0.90-0.93 in both the dry and wet weather seasons (p < 0.0001; Figure S2). In addition, all mr values range from -0.525 to -0.572, slightly higher than those (from -0.632 to -0.762) obtained by Chen et al. (14) in Guangzhou (Figure S1), but lower than those (-0.20 for suburban and -0.33 for urban sites) in Izmir, Turkey (30). It should be noted that the deviation of mr values from -1 does not consequentially suggest a disequilibrium partitioning. It may simply indicate that atmospheric particles have different sorbing properties for SOCs (29), or that continuous emissions from land to air or some slow partitioning congeners resulted in nonequilibrium partitioning (30). Washout Ratios. Wet deposition is an important pathway to transport SOCs from the atmosphere to land or water. The

TABLE 1. Comparison of Average Concentrations of PBDEs in Air (Particulate + Vapor, pg/m3) and Volume Weighted Mean Concentrations in Precipitation (Particle + Dissolved, pg/L) from the Pearl River Delta of South China and Other Regions or Countries location

type

sampling date

BDE-47

BDE-209

Σ PBDE

references

Guangzhou (South China) Guangzhou (South China) Guiyu (South China) Izmir (Turkey) Ottawa (Canada) Chicago (U.S.) Chicago (U.S.) Shunde (South China) Dongguan (South China)

atmosphere (particulate + vapor) urban June 2004 580 urban 2003-2004 299 e-waste site September 2005 2750 suburban September 2004 1.4 sanitary landfill 2004-2005 2.0 urban 1997-1999 33 urban 2002-2003 17 rural 2006-2007 5.9 rural 2006-2007 6.8

1420 6900 1950 19 12.7 0.3 60.1 583 139

2450 7740 8860 24 20 52 100 692 209

14 34 15 30 24 23 22 present study present study

Gotska Sando¨n (Baltic Proper) Malmo¨ (Sweden) Lund (Sweden) Chicago (U.S.) Cleveland (U.S.) Sleeping Bear Dunes (U.S.) Shunde (South China) Dongguan (South China) Guangzhou (South China)

precipitation (particle + dissolved) island 2001 200 MSW planta 2001-2002 2400b utban 2000 NAc urban 2005v2006 53000 urban 2005-2006 600 rural 2005-2006 160 rural 2006v2007 30 rural 2006-2007 65 urban 2006-2007 78

1700 14400b NAc 1300 3200 320 10200 7660 58200

2400 20600b 209 94000 4400 730 11500 8940 62100

5 28 27 20 20 20 present study present study present study

a

Municipal solid waste plant.

b

Median concentration. c No available data.

FIGURE 2. Relative abundances (mean with standard deviation) of individual BDE congeners normalized to Σ15PBDE (sum of BDE-17, -28, -47, -49, -66, -99, -100, -153, -154, -183, -196, -206, -207, -208, and -209) in air samples (particulate + vapor). effectiveness of precipitation in removing SOCs from air can be measured with the total washout ratio (WT) expressed as: WT ) Crain /Cair ) Wv(1 - φ) + Wpφ

(2)

where Crain (pg/L) and Cair (pg/L) are the annual mean concentrations of a chemical in precipitation (particle + dissolved) and air (particulate + vapor), Wv and Wp are the vapor and particle scavenging ratios, and φ is the particleassociated fraction in air (28). Because on average more than 97% of the total PBDEs exist in the particle phase of rain, similar to the previous studies (5, 28), the total washout ratio (WT) presumably only depends on the particle scavenging ratio (Wp). Additionally, the vapor washout ratios (Wv) are discussed in Supporting Information and the calculated results were not consistent with the Henry’s law equilibrium.

FIGURE 3. Relative abundances (mean with standard deviation) of individual BDE congeners normalized to Σ15PBDE (sum of BDE-17, -28, -47, -49, -66, -99, -100, -153, -154, -183, -196, -206, -207, -208, and -209) in bulk precipitation samples (particle + dissolved). The annual mean WT value for Σ15PBDE was 41,000 and ranged from 4500 for tri-BDE to 59,000 for deca-BDE in Donguan, and ranged from 1700 for tri-BDE to 18,000 for deca-BDE with 17,000 for Σ15PBDE in Shunde (Figure 4), suggesting that deca-BDE was the easiest to be removed from the atmosphere by rain. The WT values estimated in the present study were 1 order of magnitude lower than those (ranging from 80,000 for tri-BDE to 1280,000 for deca-BDE with 610,000 for total PBDEs) in the Baltic Proper (5). Another previous study obtained a median WT value of 540,000 for total PBDEs (List S1) in both urban and rural sampling sites from southern Sweden (28). Bidleman (1) reported that the values of WT for SOCs in a region depend on the physiochemical properties of SOCs, precipitation forms (fog, rain, or snow), meteorological conditions, TSP contents, temperVOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Annual washout ratios (WT) by bulk rainfalls for trito deca-BDE homologues and Σ15PBDE in Dongguan and Shunde from October 2006 to September 2007. ature, and other factors. Koester and Hites (31) found that the scavenging effectiveness of particle-bound SOCs was higher at the beginning of a rain event and then dropped with increasing rain volume. Therefore, the relatively lower values of WT for PBDEs in the PRD from the present study were possibly associated with the high volume of each rain sample, nonconcurrent sampling of air and rain, meteorological conditions (e.g., continuous and intense rain events) and low TSP contents in the wet season during which most precipitation samples were collected (Table S2). Atmospheric Depositional Rates of PBDEs. Rates of dry particle and wet deposition integrated over the study areas, labeled as Fdry and Fwet, respectively, can be estimated by (20) Fdry ) CpvdA ) (Cp,uAu + Cp,rAr)vd

(3)

Fwet ) (VWM)pA ) (VWMuAu + VWMrAr)p

(4)

where Cp is the atmospheric particle-phase concentration (kg/m3) of a specific BDE congener or total PBDEs, vd is the particle dry deposition velocity in air (m/yr), A is the total area (m2) and p is the annual precipitation rate (m/yr). In addition, A is expressed as the sum of the urban (Au) and rural (Ar) areas, and consequently the total depositional rate can be estimated by the sum of those in the urban and rural areas (20). At the same time, the PBDE concentration in air or rain from Guangzhou is used to represent urban concentration (Cp,u or VWMu) whereas the mean PBDE concentration from Dongguan and Shunde is used to represent the rural concentration (Cp,r or VWMr). Because no air sample from Guangzhou was collected in the present study, the results of Chen et al. (14) were adopted, i.e., the average concentrations in the particulate phase of air at 1420 and 1480 pg/m3, respectively, for BDE-209 and total PBDEs (List S4). It should be noted that the use of Chen et al.’s results to represent the average PBDE levels may underestimate the actual values by 35-50% because Chen at al.’s data were acquired in the wet weather season only (June 2004); whereas the dry season levels of Σ15PBDE in air are 1.7-2.0 times higher than in the wet weather season from the present study. The different BDE congeners used by Chen et al. (14) and the present study should not cause more than 15% in error. Dry deposition velocity of particles is dependent on particle size, boundary layer turbulence, and surface characteristics, and often shows a wide range (0.2-49 cm/s) at different locations (20). Because no measured dry deposition velocities of PBDEs in South China are available so far, a velocity of 0.5 cm/s was chosen for vd in the present study, which is often applied to 9146

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SOCs such as PCBs (32). A sensitivity analysis (Table S7) shows that the value assigned to Vd strongly influences Fdry (detailed in the Supporting Information) and becomes the largest source of error in the estimated Fdry (20, 32). The annual dry deposition rates of BDE-209 and Σ15PBDE were estimated at 1340 and 1450 kg/yr within the combined area of Guangzhou, Dongguan, and Shunde. Given the average precipitation rate of 1.7 m/year within the combined area in 2007 (8), the annual wet deposition rates of BDE-209 and Σ15PBDE were estimated at 490 and 590 kg/yr. Because the combined study area (10,700 km2) accounts for 20% of the entire PRD area (53,580 km2), the deposition rates of PBDEs to the entire PRD region can be estimated if the present urban and rural data are representative of the PRD. The dry deposition rates estimated this way for the PRD are 6700 and 7300 kg/yr for BDE-209 and Σ15PBDE, respectively, while the corresponding wet deposition rates are 2500 and 2900 kg/yr, respectively. It should be noted that the wet deposition rates of PBDEs are possibly underestimated due to the limited number of bulk rain samples, most of which were collected during high rainfall events resulting in lower VWM concentrations (31). Obviously, wet deposition is also an important mode to remove PBDEs from the atmosphere of the PRD. A detailed comparison of atmospheric deposition rates of PBDEs in different regions remains a challenge because the BDE congeners, atmospheric depositional processes, and parameters used to estimate atmospheric depositional rates of PBDEs are often different (5, 20, 33). Therefore, the following comparison is qualitative at best. Venier and Hites (20) recently estimated that dry depositional rates of BDE209 and Σ7PBDE (List S3) to the Great Lakes (with an area of 169,800 km2) were 80 and 129 kg/yr while wet depositional rates were 69 and 344 kg/yr. Ter Schure et al. (5) estimated atmospheric depositional rates (dry + wet) of BDE-209 and Σ8PBDE (List S8) to the Baltic Sea (with the sea surface area of 257,300 km2) at 166 and 236 kg/yr. The annual input of PBDEs through atmospheric deposition to the PRD is more than an order of magnitude higher than that to the Great Lakes even though the area of the Great Lakes is almost three times as large as that of the PRD. It is interesting to note that the annual inputs of BDE-209 and Σ17PBDE (List S5) from all riverine runoff outlets of the PRD to the coastal ocean were estimated at 1960 and 2140 kg/yr (11), lower than the amounts of BDE-209 (2500 kg/yr) and Σ15PBDE (2900 kg/yr) transported by wet deposition in the PRD. Another study by Zou et al. (13) reported the mass inventories of BDE-209 and Σ10PBDE (List S6) in the watershed soils of the PRD were 44,400 and 48,380 kg, respectively. For comparison, the atmospheric depositional rates (dry and wet depositions combined) of BDE-209 and Σ15PBDE to the PRD are estimated at 9200 and 10,200 kg/yr, respectively, from the present study. If a degradation half-life of 5 months in water or soil, derived by Wania and Dugani (35), is applicable for the target PBDEs, approximately 80% of the atmospheric deposition of BDE-209 and Σ15PBDE will be decomposed in a year. This means that the remaining 20%, i.e., 1840 kg/yr of BDE-209 and 2040 kg/yr of Σ15PBDE through atmospheric deposition, will be accumulated every year in the PRD. To further estimate the atmospheric depositional rates of PBDEs in the PRD from a different perspective, the reported Vd values of 7 PBDE congeners (List S7) in Izmir by Cetin et al. (33) were also employed, resulting in dry deposition rates of BDE-209 and total PBDEs (List S7) in the PRD at 43,500 and 44,100 kg/yr, respectively (Table S7). Apparently, the atmospheric depositional rates estimated using 0.5 cm/s for Vd were approximately seven times lower than those obtained using the Vd values from Cetin et al. (33). If urban concentrations (Cp,u) of 6900 and 7740 pg/m3 for BDE-209 and total PBDEs (List S6) in Guangzhou (34), respectively, were used as the upper limits, the dry depositional rates of BDE-209

and Σ15PBDE in the PRD were estimated at 26,500 and 29,870 kg/yr, respectively, which are approximately four times higher than those estimated using the data of Chen et al. (14) and 0.5 cm/s for Vd (Table S7). These comparisons suggest that the atmospheric depositional rates estimated from the present study are conservative.

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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (40532013 and 40821003) and the Earmarked Fund of the State Key Laboratory of Organic Geochemistry (SKLOG2008A05). This is contribution No. IS1130 from GIGCAS.

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Supporting Information Available Additional tables and figures containing detailed information about the sampling locations, meteorological parameters, and detailed concentration data. This material is available free of charge via the Internet at http://pubs.acs.org.

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