| Water-soluble organic carbon (WSOC) has become a frontier and hot topic inatmospheric sciences, since it is strongly associated with secondary organic aerosol(SOA), and its absorption spectrum shows similar characteristics with brown carbon.Based on a continuous and long-term field measurement, this study investigated theprimary versus secondary nature of WSOC under a condition of high biomass burningemission. Combining with analysis results of near source samples, the main factors thatinfluenced the mass absorption efficiency (MAE) of WSOC have been identified.Besides, light absorption properties of water-insoluble organics were investigated basedon methanol-extraction method, and whether absorption of WSOC could be applied astracer for total brown carbon was studied.Compared with the United States and other European countries,concentration of levoglucosan in PM2.5has been found to be at a high level inBeijing. Similar temporal variation of levoglucosan and WSOC indicated asignificant contribution of biomass burning to WSOC in Beijing. Severalevidences suggested that WSOC were mainly secondary products even under acondition of high biomass burning emission: the substantially higher WSOC/OCin summer,strong correlations between WSOC with secondary organic andinorganic species, co-variation between WSOC/EC and relative humidity andthe results of source apportionment performed with PMF model.Analysis of light absorption spectrum of WSOC showed that the averagelight absorption intensity of WSOC in Beijing was6.9±6.9Mm-1, much higherthan that observed in developed countries. The ngstr m Absorption Exponent (AAE)of WSOC in Beijing was similar in different seasons (7.6±0.5), and close to that ofWSOC in Los Angeles and the southeastern United States. MAE of WSOCshowed distinct seasonal variations, which was higher in fall and winter(0.88~1.26m2/g）, and lower in spring and summer (0.51~0.92m2/g). MAE ofWSOC in Beijing was much higher than the southeastern United States butsimilar with that of Los Angeles, where anthropogenic emission was thedominant source of air pollution, while biogenic source dominates in the southeastern United States. The spatial and temporal variation of MAE, as wellas evidences from smog chamber experiments suggested that precursor type wasa main factor controlling the light absorbing properties of WSOC. In this case,the MAE of WSOC would be higher when contribution of anthropogenic sourcesis more significant. Additionally, results from both sources apportionment andanalysis of near source samples indicated that primary WSOC could be stronglylight absorbing (with a MAE as high as3m2/g), although its contribution toWSOC mass concentration is less than10%. From the optical perspective,primary sources contribute significantly to water-soluble brown carbon in theatmosphere.Methanol-extraction method could be applied to approximately estimate theabsorption of total brown carbon, since its extraction efficiency of organics in PM2.5was87%and77%in winter and summer, respectively. AAE of organics extracted bymethanol was lower that WSOC, and was7.1±0.2and6.3±0.5in winter andsummer, respectively. MAE of organics extracted by methanol was1.69±0.46and0.96±0.48m2/g in winter and summer, respectively, which were higher thanthat of WSOC. The light absorption intensity of organics extracted by methanolwas higher than WSOC. However, the temporal variations of light absorption oforganic extracted by methanol and WSOC were similar in both winter andsummer, and the conversion factor from absorption of WSOC to absorption oftotal brown carbon were estimated to be2.5±0.5and1.8±0.5in winter andsummer, respectively. |
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