Letter Cite This: Environ. Sci. Technol. Lett. 2018, 5, 56−61
pubs.acs.org/journal/estlcu
Contribution of Biomass Burning to Ambient Particulate Polycyclic Aromatic Hydrocarbons at a Regional Background Site in East China Shuduan Mao,†,‡ Jun Li,† Zhineng Cheng,† Guangcai Zhong,† Kechang Li,† Xiang Liu,† and Gan Zhang*,† †
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Biomass burning has a significant impact on regional air quality, public health, and climate change. It is an important source of particulate polycyclic aromatic hydrocarbons (PAHs), which make up a major class of toxic air pollutants. To estimate the contribution of biomass burning to ambient particulate PAH concentrations, 15 PAHs and three anhydrosugars (levoglucosan, galactosan, and mannosan) were analyzed in particulate samples collected at a background site in east China from August 2012 to August 2015. Higher concentrations of all species were observed in fall and winter. Indoor biofuel combustion in north China was considered to be the major contributor to the high concentrations of anhydrosugars in fall and winter, because there were few fires detected on a fire count map for this period. A tracer-based approach, using the ratio of PAHs to levoglucosan (PAHs/lev) in fresh biomass burning aerosols, was proposed and used to estimate the contribution of biomass burning to PAHs. The results showed that biomass burning contributed nearly 11% of the total particulate PAHs. The estimation of the contribution from biomass burning using PAHs/lev agreed well with the results obtained from an independent positive matrix factorization analysis.
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INTRODUCTION Biomass burning in both open field fires and domestic biofuel combustion can emit large amounts of greenhouse gases, trace gases, and particulate matter, which can affect air quality, public health, and climate change.1,2 Studies of ambient volatile organic compounds (VOCs) at a receptor site in the Pearl River Delta during October and November 2008 have revealed that biomass burning accounts for 9−37% of the mixing ratios of selected VOC species, with the figure being >30% for aromatics, formaldehyde, and acetaldehyde.3 Model- and observation-based estimations have indicated that open biomass burning accounted for 37% of PM2.5 in the Yangtze River Delta during the harvest period in summer 2011.4 Radiocarbon (14C)-based source apportionment studies on the Hainan Island from May 2005 to August 2006 and on the Jeju Island, from April 2013 to April 2014 have shown that biomass burning could account for >30% of the organic carbon (OC) concentrations in China.5,6 Among the trace substances emitted from biomass burning, there are also polycyclic aromatic hydrocarbons (PAHs), an important class of persistent organic pollutants (POPs).7,8 PAHs are generated by the incomplete combustion of biomass and fossil fuels.9 They are ubiquitous in the atmosphere and are considered to be a major class of toxic air pollutant.10,11 They can cause serious health effects because of their carcinogenicity and mutagenicity. In recent decades, PAHs have attracted © 2018 American Chemical Society
considerable public and scientific attention, especially in China where high PAH concentrations are frequently reported.11,12 Efforts have been made to estimate PAH emissions. An emission inventory in 2004 indicated that China was one of the largest PAH emitters in the world, and that biomass combustion and coke ovens were the most important sources.13 Recently, an updated global PAH emission inventory for the period from 1960 to 2008 suggested that biomass fuels were the major global sources of PAHs.12 Because of economic considerations and the promotion of coal consumption reduction policies, biomass fuel has become a popular alternative energy source to fossil fuels in households and in manufacturing and service industries in rural regions.14,15 Therefore, assessment of the contribution of biomass fuels to ambient PAH levels is of great significance. Molecular markers, especially levoglucosan (lev), can be used to quantify the contribution of biomass burning to atmospheric pollutants, such as particulate matter and OC aerosol.16,17 A lev-based approach may therefore also have the potential to quantify the contribution of biomass burning to PAH concentrations. Received: Revised: Accepted: Published: 56
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3, 2018 29, 2018 30, 2018 30, 2018 DOI: 10.1021/acs.estlett.8b00001 Environ. Sci. Technol. Lett. 2018, 5, 56−61
Letter
Environmental Science & Technology Letters
Quality Control. Laboratory and field blank filters were analyzed. The method detection limit (MDL) was defined as the average of all blanks plus 3 times the standard deviation. The MDLs of each target compound are listed in Table S1. The average recoveries were 75 ± 15, 91 ± 12, 101 ± 14, 100 ± 19, and 95 ± 14% for acenaphthene-d10, phenanthrene-d10, chrysene-d12, perylene-d12, and 13C-labeled lev, respectively. All reported values were corrected for recovery in this study. Air Mass Back Trajectories. Five-day back trajectories were calculated at 6 h intervals for the campaign period using the HYSPLIT model (http://ready.arl.noaa.gov/HYSPLIT. php). All individual trajectories were clustered for each season, as shown in Figure S1. The air mass mostly originated from the west Pacific and the South China Sea in summer and passed over the continental regions of Mongolia and north China during other seasons.
A biomass burning plume not only has a local impact but also has effects at the regional scale through long-range transport.18 As one of the most economically developed regions in China, east China experiences serious air pollution. Although it is not a high-emission intensity area for PAHs from biomass burning, the outflow flux of PAHs from north China has resulted in elevated concentrations of atmospheric PAHs in this region.19 Additionally, it has been reported that most PAHs were transported out of China through the eastern boundary.19 Therefore, the Ningbo Atmospheric Environment Observatory (NAEO, 29°40.8′N, 121°37′E, 550 m ASL) in east China, which is a key site on the outlet transport route for aerosols from the Chinese continent to the Pacific Ocean, was selected as a regional background site for monitoring PAHs in this study (Figure S1). The sampling site has been described in a previous report.20 The aim of this study was to apportion the sources of PAHs and estimate the contribution of biomass burning to particulate PAHs in east China.
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RESULTS AND DISCUSSION Temporal Variations and Correlations between Levoglucosan and Particulate PAHs. The concentrations of 15 particulate PAHs, three anhydrosugars, OC, and EC are listed in Table S2. The composition of PAHs in this study was dominated by high-molecular weight PAHs, with low vapor pressures. Anhydrosugars were composed primarily of lev (91 ± 4.4%), followed by mannosan (6.1 ± 3.3%) and galactosan (3.3 ± 1.7%). A time series of particulate PAHs, three anhydrosugars, OC, and EC from August 2012 to August 2015 is shown in Figure S2. The average concentrations of each species in different seasons are listed in Table S3. Seasonal characteristics were observed for each species, with the highest average concentrations being in winter (December to February) and fall (September to November) and the lowest in summer (June to August). Five-day backward trajectories showed that air masses passed over north China in winter, where biofuel and fossil fuel are typically used for household heating. During summer, the air masses originated from the west Pacific or the South China Sea. The low abundance of components at this time was mostly attributed to clean air masses originating from marine areas. According to fire count maps for each season (Figure S3), produced by the satellite-based Moderate Resolution Imaging Spectroradiometer (MODIS), few wild fires occurred in China during winter and fall when the highest concentration of lev was observed. Moreover, air masses passed over north China during winter and fall. These results suggested that high concentrations of three anhydrosugars were mostly attributed to indoor biofuel combustion in north China (such as domestic heating by biomass and combustion of agricultural crops and wood in domestic stoves), which could not be detected by satellites. Correlations between lev (a well-known biomass burning tracer) and individual selected particulate PAH compounds were estimated, and the Pearson correlation coefficients (r) are listed in Table 1. The correlation coefficients (r) were above 0.75 for 71 samples with each particulate compound. The strong correlations between lev and particulate PAHs, as shown in Table 1, would suggest a strong association of the PAHs with the particulate, from the source or origin to atmospheric processes. Tracer-Based Estimation of the Contribution of Biomass Burning to Particulate PAHs. Levoglucosan concentrations and the ratios of OC to lev (OC/lev) or EC to lev (EC/lev) have frequently been used to estimate the
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MATERIALS AND METHODS Sampling. Since the year 2012, successive 24 h total suspended particle samples were collected once a week at NAEO by high-volume air samplers fitted with quartz microfiber filters (QFFs) (grade GF/A, 20.3 cm × 12.7 cm; Ahlstrom Munktell, Falun, Sweden) operating at a rate of 300 L/min. The QFFs were baked at 450 °C for 6 h before sampling and then sealed and stored at −20 °C after collection. A total of 73 QFF samples, i.e., one from each fortnight from August 2012 to August 2015, were used in this study. Sample Pretreatment and Analysis. Details of the sample treatment and analytical method can be found in Text S1 of the Supporting Information. Briefly, a section of each filter sample was spiked with perdeuterated PAHs as surrogates and extracted in a Soxhlet apparatus for 24 h with dichloromethane (DCM). The extracts were further purified on a multilayer neutral silica gel column. Fifteen PAHs were then quantified by gas chromatography−mass spectrometry (GC− MS), with a DB-5MS capillary column (30 m × 0.25 mm × 0.25 μm, model 7890/5975, Agilent, Santa Clara, CA): acenaphthene (Ace), acenaphthylene (Acy), fluorene (Flu), phenanthrene (Phe), anthracene (Ant), fluoranthene (Fla), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), dibenzo[a,h]anthracene (DahA), benzo[g,h,i]perylene (BghiP), and indeno[1,2,3-c,d]pyrene (IcdP). A section of each sample was spiked with 13C-labeled lev as a recovery standard and Soxhlet extracted for 24 h with DCM and methanol (40:3 by volume) to analyze anhydrosugars. The extracts were anhydrated with an anhydrous sodium sulfate column, spiked with methyl β-L-xylanopyranoside (m-XP) as an internal standard, and then dried completely in a gentle nitrogen stream. Finally, a derivatization reagent [a mixture of N,O-bis(trimethylsilyl) trifluoroacetamide (1% trimethylsilyl chloride) and pyridine] was added for the sequent reaction (70 °C, 1 h). The resulting solution was then immediately analyzed by GC−MS. A punch of each filter was analyzed to determine the organic carbon (OC) and elemental carbon (EC) concentrations by a DRI model 2015 Thermal/Optical Carbon Analyzer (Atmoslytic Inc., Calabasas, CA). Details of the analytical method can be found in Text S1. 57
DOI: 10.1021/acs.estlett.8b00001 Environ. Sci. Technol. Lett. 2018, 5, 56−61
Letter
Environmental Science & Technology Letters
ratios of particulate PAH compounds to lev vary by type of biomass, which have been confirmed by the ratios of the three anhydrosugars. In this study, the ratios of lev to mannosan (lev/Man) and mannosan to galactosan (Man/Gal) were 14.5 ± 5.8 and 1.8 ± 0.7, respectively, which were within the typical ranges of hardwood fires (Table S4). Therefore, (PAHs/lev)bb ratios for hardwood were used to estimate the contribution of biomass burning to ambient particulate PAH concentrations. Despite the (PAHs/lev)bb ratios in fresh source aerosols that were obtained from previous studies, the different atmospheric residence times and reactivities between lev and individual PAH compounds will inevitably result in changes of these ratios during atmospheric transport from the source to receptors.24,25 The varied ratios will potentially lead to biased results. To minimize the bias, decays of particulate PAHs and lev by heterogeneous photochemical reactions were taken into account. Because reaction with OH radicals was the dominant pathway for heterogeneous degradation of PAHs compared with reactions with NO2 and O3,26 average percent decays of individual PAHs after exposure to OH radicals were cited from the study by Jariyasopit et al. [Table S5; the particulate matterbound PAHs exposed to an average tropospheric OH radical concentration (1.0 × 106 molecules cm−3) for ∼6−7 days].27 Levoglucosan has also been reported with degradation by OH radicals under ambient conditions and in laboratory studies.28−30 Kessiler et al.29 studied the heterogeneous oxidation of pure lev particles and plotted the remaining mass fraction of lev to OH exposure. Accordingly, an approximate decay rate of 20% for lev was deduced when the OH exposure was 0.6 × 1012 molecules cm−3. Therefore, corrected ratios of individual PAHs to lev [(PAHs/lev)*bb] during atmospheric transport from the source to the sampling site can be calculated (Table S6) on the basis of decay rates of PAHs and lev that were obtained from previous studies. Thus, the PAHs from biomass burning (PAHsbb) can be estimated as follows:
Table 1. Correlations between Levoglucosan and Individual Particulate PAH Compounds during the Whole Sampling Campaign lev vs particulate PAH lev lev lev lev lev lev lev lev lev lev lev lev lev
vs vs vs vs vs vs vs vs vs vs vs vs vs
Phe Ant Fla Pry BaA Chr BbF BkF BaP IcdP DahA BghiP Σ12PAHs
a N, number of points. probability.
b
Na
rb
Pc
71 71 71 71 71 71 71 71 71 71 71 71 71
0.75 0.80 0.78 0.79 0.80 0.80 0.82 0.80 0.83 0.82 0.80 0.83 0.83
