| Airborne particles have attracted intense scientific and public attention in recent years due to their adverse impact on visibility, human health and global climate. The water-soluble species have great impact on the chemical and optical properties of aerosols by affecting their hygroscopicity. We have been experiencing severe and persistent haze pollution in recent years, and it was found that the severe haze episodes were driven, to a large extent, by the formation of secondary aerosols. As an important component of secondary organic aerosols(SOA), WSON could increase the hygroscopicity of aerosols and promote the formation of haze. Previous studies focused mainly on water-soluble inorganic nitrogen, and very little is known about the origin of water-soluble organic nitrogen(WSON) in ambient aerosols. The objective of this study is to investigate the contribution of WSON to ambient aerosols, and to get a better understanding of the sources of SOA.PM2.5 samples were collected at Baoshan of Shanghai from May 2013 to April2014 using a high-volume sampler. The concentrations of carbonaceous aerosols,inorganic ions and the elements of carbon and nitrogen in PM2.5 were measured with an optical carbon analyzer, an ion chromatograph and an element analyzer. Also, we determined the concentrations of water-soluble total nitrgogen(WSTN) and WSON with wet chemical oxidation and spectrophotometer analysis. By these methods, we investigated the seasonal variation, formation mechanism, sources of the WSON in PM2.5 in Shanghai, and also the potential impact of WSON on haze formation. The following conclusions were reached:The seasonal concentrations of WSON in PM2.5 in Shanghai during the sampling periods were 1.45 μg N/m3 in spring, 0.90 μg N/m3 in summer, 0.92 μg N/m3 in autumn and 1.76 μg N/m3 in winter. The concentrations of WSON were higher in winter and spring, while lower in summer and autumn. The contribution of WSON to WSTN was the highest in summer(26%), followed by autumn(20%) and spring(15%), while the lowest in winter(13%). Under low temperature in winter, ammonium nitrate inclinedto exist in particle phase instead of gas phase, which would lower the WSON/WSTN ratio. Moreover, high temperature and strong solar radiation were in favor of the formation of WSON, which would increase the WSON/WSTN ratio in summer.WSON was strongly correlated with nss-SO42-、NH4+、NO3- and nss-K+, but not correlated with Na+ and nss-Ca2+, indicating that WSON was mainly formed from anthropogenic pollutants and marine sources or soil had insignificant contributions.Principal component analysis(PCA) also showed that WSON was correlated with secondary species such as SO42- and NO3-, and should thus be from the secondary transformation of anthropogenic pollutants.Significant positive correlation was found between particle acidity, expressed as([SO42-] + [NO3-]- [NH4+])/([SO42-] + [NO3-]) equivalent ratio, and WSON/WSTN,suggesting that acidic condition could enhance the formation of WSON.Potential source contribution function(PSCF) analysis suggested that WSON in PM2.5 in Shanghai was mainly from local sources and emission sources in Zhejiang and Anhui in summer and winter, while in spring and autumn, continental pollutants from northern China could be transported to Shanghai through the Yellow Sea and contributed significantly to the WSON in PM2.5 in Shanghai. |
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