Prelimunary Study In Seconday Organic Aerosol In China: Spatial Distribution And Source Apportionment

Secondary organic aerosol(SOA) formed through atmospheric oxidation of reactive organic gases condensed onto preexisting aerosol particles and aging of atmospheric organic aerosols. SOA takes the dominant part of fine particle and affects the earth’s radiation balance and global climate. There are still many uncerntaintyies in the source and formation machnism about SOA and great gap between model and field observation results. China now faces severe pollutions caused by fine particles and SOA is the difficult part in controlling the pollution. Understanding of the distribution, composition and origin of SOA will help to grasp its climate effects. Nationwide observation of SOA through tracer-based method, we analyze the distribution and composition of SOA. Relavant results showed below:Filter-based particle samples were simultaneously collected at 14 sites across 6 regions of China during summer 2012. These filters were analyzed for secondary organic aerosol(SOA) tracers from biogenic precursors(isoprene, monoterpenes and β-caryophyllene) and anthropogenic aromatics. The sum of all SOA tracers ranged from 29.9 to 371 ng m-3 with the dominance from isoprene(123 ± 78.8 ng m-3), followed by monoterpenes(10.5 ± 6.64 ng m-3), β-caryophyllene(5.07 ± 3.99 ng m-3) and aromatics(2.90 ± 1.52 ng m-3). The highest levels of biogenic SOA tracers were observed in East China; while the highest concentrations of aromatics SOA tracer, 2,3-dihydroxy-4-oxopentanoic acid(DHOPA) occurred in North China. All the biogenic SOA tracers exhibited positive correlations with temperature, implying that the emission of biogenic precursors was the major factor determining the spatial distribution of biogenic SOA on the national scale during summer. Among the isoprene SOA tracers, the low-NOx products, 2-methyltetrols were the dominant compounds. However, at some urban sites, the contribution of high-NOx product, 2-methylglyceric acid significantly increased, implying more influence of NOx on isoprene SOA formation in urban areas. For monoterpene SOA tracers, the ratio of first-generation products(cis-pinonic acid plus pinic acid) to high-generation product(3-methyl-1,2,3-butanetricarboxylic acid) exhibited a negative correlation with the amount of high-generation products, indicating that this ratio could be served as an indicator to trace the aging of monoterpene SOA. The ratio ranged from 0.89 to 21.0 with an average of 7.00 ± 6.02 among these sites, suggesting monoterpene SOA was generally fresh over China during summer. As a typical anthropogenic SOA tracer, DHOPA exhibited higher levels at urban sites than remote sites. The SOA tracers were further applied to attribute SOA origins by SOA-tracer method. The total concentrations of secondary organic carbon(SOC) and SOA were estimated in the range of 0.37-2.47 μg C m-3 and 0.81-5.44 μg m-3, respectively with the high levels in the eastern regions of China. Isoprene(46 ± 14%) and aromatics(27 ± 8%) were two major contributors to SOC in each region. In North China, aromatics even played the dominant role.High-elevation areas are sensitive to global climate change. However, at present, SOA origins and seasonal variations are understudied in remote high-elevation areas. In this study, particulate samples were collected from July 2012 to July 2013 at the remote Nam Co(NC) site, Central Tibetan Plateau and analyzed for SOA tracers from biogenic(isoprene, monoterpenes and β-caryophyllene) and anthropogenic(aromatics) precursors. Among these compounds, isoprene SOA(SOAI) tracers represented the majority(26.6-44.2 ng m-3), followed by monoterpene SOA(SOAM) tracers(0.97-0.57 ng m-3), aromatic SOA(SOAA) tracer(2,3-dihydroxy-4-oxopentanoic acid, DHOPA, 0.25-0.18 ng m-3) and β-caryophyllene SOA tracer(β-caryophyllenic acid, 0.090-0.10 ng m-3). SOAI tracers exhibited high concentrations in the summer and low levels in the winter. The similar temperature dependence of SOAI tracers and isoprene emission suggested that the seasonal variation of SOAI at the NC site was mainly influenced by isoprene emission. The ratio of high-NOx to low-NOx products of isoprene(2-methylglyceric acid to 2-methyltetrols) was the highest in the winter and the lowest in the summer, due to the influence of temperature and relative humidity. The seasonal variation of SOAM tracers was impacted by monoterpenes emission and tracers partitioning. The similar temperature dependence of SOAM tracers and monoterpenes emission was only observed during winter to spring. SOAM tracer levels did not elevate with increased temperature in the summer, probably resulting from the counteraction of temperature effects on gas/particle partitioning and monoterpenes emission. The concentrations of DHOPA were 1-2 orders of magnitude lower than those reported in the urban regions of the world. Due to the transport of air pollutants from the adjacent Bangladesh and the eastern India, DHOPA presented relatively higher levels in the summer. In the winter when air masses mainly came from the northwestern India, massfractions of DHOPA in total tracers increased, although its concentrations declined. The SOA-tracer method was applied to estimated secondary organic carbon(SOC) from these four precursors. The annual average of SOC was 0.22-0.29 μg C m-3, with the biogenic SOC(sum of isoprene, monoterpenes and β-caryophyllene) accounting for 75%. In the summer, isoprene was the major precursor with its SOC contributions of 81%. In the winter when the emission of biogenic precursors largely dropped, the contributions of aromatic SOC increased. Our study implies that anthropogenic pollutants emitted in the Indian subcontinent could transport to the TP and have impact on SOC over the remote NC.