Semi-volatile Organic Compounds (SOCs) in Fine Particulate Matter


Apr 9, 2018 - Few efforts have been made to elucidate the influence of weather conditions on the fate of semi-volatile organic compounds (SOCs). Here ...
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Environmental Processes

Semi-volatile Organic Compounds (SOCs) in Fine Particulate Matter (PM2.5) during Clear, Fog, and Haze Episodes in Beijing Winter Ting Wang, Mi Tian, Nan Ding, Xiao Yan, She-Jun Chen, Yangzhi Mo, Weiqiang Yang, Xinhui Bi, Xinming Wang, and Bi-Xian Xian Mai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06650 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Semi-volatile Organic Compounds (SOCs) in Fine Particulate Matter

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(PM2.5) during Clear, Fog, and Haze Episodes in Beijing Winter

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Ting Wang,†,‡ Mi Tian,§ Nan Ding,†,‡ Xiao Yan,ǁ She-Jun Chen,*,† Yang-Zhi Mo,†,‡

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Wei-Qiang Yang,†,‡ Xin-Hui Bi,† Xin-Ming Wang,† and Bi-Xian Mai†

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Environmental Protection and Resources Utilization, Guangzhou Institute of

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Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China

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University of Chinese Academy of Sciences, Beijing, 100049, China

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Key Laboratory of Reservoir Aquatic Environment of CAS, Chongqing Institute of

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Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714,

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China

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ǁ

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Sciences, Ministry of Environmental Protection, Guangzhou, 510530, China

State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of

Center for Environmental Health Research, South China Institute of Environmental

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Abstract Art

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ABSTRACT Few efforts have been made to elucidate the influence of weather conditions on the fate

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of semi-volatile organic compounds (SOCs). Here, daily fine particulate matter (PM2.5)

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during clear, haze, and fog episodes collected in winter in Beijing, China was analyzed

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for polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants (BFRs), and

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organophosphate flame retardants (OPFRs). The total concentrations of PAHs, OPFRs,

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and BFRs had medians of 45.1 ng/m3, 1347 pg/m3, and 46.7 pg/m3, respectively. The

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temporal pattern for PAH concentrations was largely dependent on coal combustion for

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residential heating. OPFR compositions that change during colder period were related to

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enhanced indoor emissions due to heating. The mean concentrations of SOCs during haze

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and fog days were 2−10 times higher than those during clear days. We found that BFRs

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with lower octanol/air partition coefficients tended to increase during haze and fog

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episodes and/or be removed from PM2.5 during clear episodes. For PAHs and OPFRs,

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pollutants that are more recalcitrant to degradation were prone to accumulate during haze

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and fog days. The potential source contribution function (PSCF) model indicated that

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southern and eastern cities were major source regions of SOCs at this site.

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1. INTRODUCTION

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Frequent haze-fog pollution is one of the most prominent environmental issues in

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recent decades in China due to the substantial adverse impacts on human health,

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ecosystems, and the climate.1,2 Haze-fog pollution is characterized by elevated

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concentrations of particulate matter (especially PM2.5, particulate matter with an

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aerodynamic diameter less than 2.5 µm) and decreased visibility.3 Haze and fog pollution

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can be derived from increased primary emissions of PM (e.g., from various combustion

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and industrial sources) or secondary aerosol formations (including new particle formation

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and particle growth) under certain meteorological conditions. Previous studies have

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shown that local vehicle emissions play an important role in haze and fog formation in

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urban regions.4,5 However, extensive haze and fog events in northern China during the

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cold season are, to a large extent, attributed to coal combustion for heating.6,7 Huang et al.

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investigated the 2013 haze pollution events in several urban locations in China and found

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that secondary aerosols contributed 30–77% of PM2.5.8

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Semi-volatile organic compounds (SOCs) are an important constituent of PM and

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largely originate from primary sources, such as combustion and industrial emissions.

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Although SOCs may comprise a small fraction of the organic matter in aerosols, many

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SOCs are persistent, bio-accumulative, and toxic. Subsequently, these chemicals have the

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ability to transport over long distances to remote regions and pose adverse effects on

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human health, which has been a great concern for decades.9-11 For instance, polycyclic

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aromatic hydrocarbons (PAHs) are formed mainly as a result of the incomplete 4

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combustion and pyrolysis of fossil fuels, biomass and plastic, and are ubiquitous in the

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environment. Brominated flame retardants (BFRs) are widely used in a variety of

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commercial products (e.g., electronic equipment, furniture, car interiors, and construction

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materials) to reduce fire risks.12 Most FRs are not chemically bonded to the original

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material; thus, they tend to release into the environment by volatilization or abrasion.13

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Several BFRs, such as polybrominated diphenyl ether (PBDE) commercial mixtures and

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hexabromocyclododecane, were listed under the Stockholm Convention on persistent

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organic pollutants due to their widespread occurrence and toxicity.14,15 With the reduction

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of restricted BFRs, there has been increasing evidence on the environmental occurrence

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of novel alternatives in recent years.16,17 Lee et al. (2016) monitored global-scale air

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concentrations of numerous new flame retardants, and the distributions indicated distinct

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use or emission patterns.18 Long-term temporal variations in flame retardant levels in the

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Great Lakes atmosphere showed decreasing trends for PBDEs and increasing trends for

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replacement flame retardants over the period 2005–2013.19 Recent research also indicated

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the global occurrence of organophosphate flame retardants (OPFRs), which have been

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proposed as alternatives for BFRs.20-23

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During haze and fog pollution episodes, the physical and chemical characteristics of

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aerosols likely undergo substantial alterations compared to those during clear days not

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only because of increased primary emissions but also the secondary formation of aerosols

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and varying meteorological factors. Previous studies have suggested enhanced

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conversions from NOx and SO2 to secondary inorganic aerosols, especially under higher 5

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humidity conditions, and significant secondary organic aerosol (SOA) formation during

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haze-fog episodes.24-26 Thus, changes in the size distribution of particles (i.e., the shift

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from fine to larger particles due to heterogeneous reactions or hygroscopic growth)

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during the transition from clean to polluted periods have been frequently observed.26,27

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Severe fog may have the capacity to scavenge fine particles during haze episodes.28 In

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addition, SOCs may act as important SOA precursors or might be subject to atmospheric

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transformations, which has been proposed in recent research.29,30 Therefore, it is

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speculated that under different atmospheric pollution conditions, SOCs are likely subject

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to different processes; however, to our knowledge, few efforts have been made to

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elucidate this hypothesis. In particular, there remain challenges in understanding the

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sources and environmental fates of novel FRs.

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In this study, daily PM2.5 samples in wintertime were collected in Beijing, China,

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where frequent regional haze-fog pollution events with considerably high PM2.5

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concentrations during the cold season have been reported.24 A number of SOCs

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(including PAHs, BFRs, and OPFRs) in PM2.5 were determined. The primary objectives

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are to investigate the contamination status of these pollutants in this region, provide

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insights into their sources and environmental behaviors under different weather

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conditions (i.e., clear, haze, and fog), and understand the principal factors influencing

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SOC concentrations in fine PM.

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MATERIALS AND METHODS

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A detailed description of the materials and methods is given in the Supporting 6

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Information (SI)

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Sample collection. Sampling was conducted on the rooftop of a four-story building on

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campus at the University of Chinese Academy of Sciences, which is located in a

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suburban region (Huairou) of northern Beijing (Figure S1 in the SI). Beijing is the capital

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of China and the center of the Beijing-Tianjin-Hebei city cluster. Beijing has a high

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population density and is surrounded by several industrial cities. This region has

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experienced the frequent occurrence of severe fog-haze events during the past two

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decades due to various anthropogenic activities. A total of 65 samples were obtained

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consecutively in winter (over the period of October 2014 to January 2015), except for

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several-day breaks due to technical problems. PM2.5 samples were collected on Whatman

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quartz fiber filters for 24 h using an active large-volume air sampler (TE-6001, Tisch

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Environment Inc., US) at a flow rate of 1.13 m3/min. The loaded filter was wrapped in

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aluminum foil, sealed in a small polyethylene zip bag and stored at -20 ºC until

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extraction.

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Sample preparation and analysis. In this study, 18 PAHs, numerous BFRs (including

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13 PBDE congeners, decabromodiphenyl ethane (DBDPE), pentabromotoluene (PBT),

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and hexabromobenzene (HBBz)), and 12 OPFRs in PM2.5 were analyzed (Table 1).

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Briefly, the samples were Soxhlet extracted with a mixture of hexane and acetone (v:v =

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1:1) for 48 h. Prior to the extraction, surrogates for PAHs (Nap-d8, Acy-d10, Phe-d10,

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Chr-d12, and Per-d12), BFRs (BDE77, BDE181, and BDE205), and OPFRs (TnPP-d21,

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TnBP-d27, TCPP-d18, and TPhP-d15 ) (see the footnote in Table 1) were added to monitor 7

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their recoveries. The extracts were concentrated to a volume of 1 mL using a rotary

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evaporator then purified and fractioned with a solid-phase extraction cartridge

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(Supelclean ENVI-Florisil, 3 mL, 500 mg). The column was eluted with 5 mL of hexane

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and, subsequently, 5 mL of 1:1 hexane:dichloromethane (v/v) for fractions containing

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PAHs and BFRs. The third fraction containing OPFRs was obtained by elution with 8 mL

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of ethyl acetate. The effluent fractions were concentrated to near dryness under a gentle

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nitrogen stream then dissolved in 300 µL of isooctane. The quantitation internal standards

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(BDE118 and BDE128 for BFRs and TCEP-d12 and TDCPP-d15 for OPFRs) were added

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prior to the instrumental analysis.

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PAHs and OPFRs were analyzed using an Agilent 7890 gas chromatograph coupled

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with an Agilent 5975 mass spectrometer, which operated in a selected ion monitoring

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mode and used electron impact ionization (GC-EI-MS). Measurement of the PAHs and

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OPFRs was achieved using a DB-5MS capillary column (30 m × 0.25 mm i.d., 0.25 µm

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film thickness) (J&W Scientific). BFRs were analyzed with a Shimadzu 2010 GC-MS,

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which operated in electron capture negative ionization mode (ECNI).

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Quality control. A procedural blank was run with each batch of samples (n = 11).

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Only a few PAHs and OPFRs (TCEP, TCPP, and TEHP) were detected in the procedural

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and field blanks, and the concentrations in the sample extracts were blank-corrected.

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Recoveries of the surrogated standards (mean ± standard deviation) were 33.0 ± 18.3%

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for Nap-d8, 70.0 ± 20.5% for Acy-d10, 85.9 ± 14.6% for Phe-d10, 88.3 ± 10.9% for Chr-d12,

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99.5 ± 16.6% for Per-d12, 104.9 ± 13.9% for BDE77, 95.6 ± 8.4% for BDE181, 94.7 ± 8

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11.9% for BDE205, 91.4 ± 15.4% for TnPP-d12, 92.2 ± 14.4% for TnBP-d27, 87.9 ± 15.5%

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for TCPP-d18, and 91.8 ± 15.8% for TPhP-d15. An indoor dust standard reference material

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(SRM 2585) was analyzed to evaluate the method accuracy. The method detection limits

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were defined by the mean blank mass plus three standard deviations, or a signal of 10

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times the noise level for the non-detectable compounds in the blank, which were 0.05−10

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pg/m3 for PAHs, 0.01−3 pg/m3 for BFRs, and 0.25−10 pg/m3 for OPFRs.

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Definition of weather conditions. In this study, weather conditions were categorized

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into clear, haze, and fog days according to the Chinese Meteorological Administration.

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Specifically, haze was defined by weather conditions with visibility under 10 km and

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relative humidity (RH) less than 80%; fog had the same reduced visibility as haze but an

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RH over 90%, and clear episodes had visibilities greater than 10 km. Precipitation was

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excluded from the three weather conditions. Fog and haze situations that lasted longer

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than 3 h a day were utilized.31 For several days with RH levels between 80-90% and

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visibility < 10 km, if RH levels over 80% lasted longer than 6 h or if mist was reported,

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these days were considered fog days. The meteorological parameters and categories for

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these weather conditions are given in Table S1.

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Air mass back trajectory and potential source contribution function (PSCF). The

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24-h back trajectories, starting at a height 100 m above ground level at the sampling site,

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were calculated using the NOAA HYSPLIT model. The PSCF method, based on the

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results of the HYSPLIT model, was used to evaluate likely source regions of transported

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aerosols. Details are given in the SI. 9

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Data analysis. The student’s t-test or the Mann-Whitney rank sum test for comparing

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differences between data groups, as well as the Pearson (2-tailed) or Spearman

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correlation analyses, were performed with Sigmaplot 12.5. The principal component

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analysis (PCA) was conducted for the SOC concentration data using the SPSS software

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package 19.0, where only compounds with detection frequencies > 60% were included,

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and the values were log-normalized. A confidence level of 95% was used for the

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statistics.

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3. Results and discussion

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Overall SOC concentrations and variations. Table 1 summarizes the statistics for the

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concentrations of SOCs in the PM2.5 samples, and Figure S2 shows their daily variations

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during the sampling period. The total PAH concentrations varied greatly from 1.30 to 167

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ng/m3 (median = 45.1 ng/m3), with a relative standard deviation (RSD) of 79%. Despite

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these large variations, we observed an increasing trend in the concentrations of PAHs

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from the beginning of the sampling campaign (28 October) to the end of November; after

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that, PAH concentrations generally remained at high levels. This temporal pattern was

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consistent with the pattern of sulfur dioxide (SO2) in the air (which is an indicator of coal

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combustion emissions) and opposite the temperature pattern during the same period

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(Figure S2). This result clearly revealed that increased coal combustion, which is China’s

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primary energy source for residential heating in Beijing and the surrounding regions, was

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responsible for the enhanced emissions of PAHs into the atmosphere in winter.

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The concentrations of lower brominated PBDEs, which were mostly from technical 10

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penta-BDE products,32 also showed a large variation ranging from 0.02-19.2 pg/m3 (RSD

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= 112%), with a median of 2.68 pg/m3. These concentrations did not depend on

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residential heating. For lower brominated PBDEs, which have been restricted in many

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countries, emissions from old indoor furniture and equipment, historically contaminated

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matrices (e.g., soil), and waste incineration and recycling could be the main sources, as

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suggested in previous research.33-35 In contrast, the concentrations of highly brominated

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PBDEs, which are the main components of the currently used technical deca-BDE

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products, were relatively stable (RSD = 43%, excluding two outliers), ranging from 12.7

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to 292 pg/m3 (median = 36.1 pg/m3). A similar result was also observed for DBDPE,

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which is a novel replacement for technical deca-BDE products, with concentrations of

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15.4−514 pg/m3 and a median = 35.0 pg/m3. Both deca-BDEs and DBPDE are currently

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used in flame retardants in large quantities (mostly in the electronic equipment

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manufacturing industry). China is a large producer and consumer of these two classes of

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BFRs.36 Thus, in addition to the potential sources of waste incineration and recycling,

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local or regional industrial activities are significant sources of these chemicals, which

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was also observed in southern China in our previous study.37

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OPFRs were detected in all samples, with concentrations ranging from 257 to 8358

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pg/m3 (RSD = 86%) and a median value of 1347 pg/m3, which were much higher than

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those in BFRs. The concentrations of aryl-OPFRs (TPhP, EHDPP, and TCrP) (median =

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541 pg/m3) were comparable to those of chlorinated OPFRs (TCEP, and TCPP, and

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TDCPP) (median = 430 pg/m3), and they both were significantly higher than those of 11

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alkyl-OPFRs (TEP, TnBP, TnPP, TBEP, and TEHP) (243 pg/m3). The variations in TCPP,

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TDCPP, TCrP, TPhP, TnPP, and TEHP concentrations were similar, and TnBP, TCEP and

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TEP essentially showed temporal patterns similar to theirs. However, EHDPP and TBEP

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had similar variations, which were distinct from other OPFRs (Figure S2). The different

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temporal patterns of the OPFRs reflected the similarities and differences in their sources

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and atmospheric processes. OPFRs are used in a range of polymers, depending on the

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side chain of the phosphate ester.38 For instance, both TCPP and TDCPP are used in

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building/construction materials, and TPhP and EHDPP are mostly used in

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electrical/electronic products and floor coverings, respectively.39

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The PAH concentrations in the present study were lower than those reported recently

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in winter PM2.5 in the Beijing downtown area (averages = 64.4-94.3 ng/m3) and other

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cities in northern China.40-43 The total PBDE levels in this study (mean = 50.7 pg/m3)

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were comparable to the average levels in PM2.5 in ten Chinese cities (35 pg/m3) and PM10

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in Chinese northern cities (77.1 pg/m3), which has been investigated recently.44,45 The

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BDE209 concentrations in Beijing were much higher than those in the urban air of the

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Great Lakes (with rough geometric means of 10-15 pg/m3) and the Czech Republic (