The chemical composition of inorganic and carbonaceous materials in PM2.5 in Nanjing, China
Abstract
PM2.5 samples were collected at an urban and a suburban site in Nanjing, China in 2001. They were analyzed for inorganic ions, elemental carbon, organic carbon (OC), water-soluble organic carbon (WSOC), and individual WSOC and nonpolar organic species. Sulfate and organic matter were the two most abundant constituents in these samples. Sulfate accounted for an average of 23% (urban site) and 30% (suburban site) of the identified aerosol mass. Organic matter accounted for an average of 37% (urban) and 28% (suburban) of the identified aerosol mass. WSOC was a significant portion of OC, accounting for about one-third of OC at the urban site and 45% of OC at the suburban site. The suburban-urban gradient in the WSOC/OC ratio also reflected that the aerosol OC was more aged at the suburban location. The correlations of WSOC with sulfate and nitrate suggest that the WSOC fraction was dominated by secondary organics. More than 30 individual WSOC species in the compound classes of organic anions, amino acids, aliphatic amines, and carbohydrates were quantified, accounting for approximately 8% of the WSOC on a carbon mass basis. In addition, 46 individual nonpolar organic compounds in the compound classes of n-alkanes, hopanes, and polycyclic aromatic hydrocarbons were quantified using an in-injection port thermal desorption technique. These nonpolar organic species accounted for less than 7% of the OC on a carbon mass basis. The quantification of individual compounds allowed the identification of major aerosol sources through principal component analysis. Coal combustion, vehicular emissions, secondary inorganic and organic aerosols, and road/sea salt were the major contributing sources to the identified PM2.5 aerosol mass.
Keywords: Aerosol chemical composition; Water soluble organic compounds; Chinese aerosols; coal combustion; Principal component analysis; Aerosol source identification.
1. Introduction
Ambient aerosols play important roles in global radiative forcing by scattering or absorbing light directly or indirectly through modulating cloud condensation nuclei (CCN) and they consequently have an impact on climate change (IPCC, 2001). Aerosols aerodynamically smaller than 2.5 micro;m (i.e., PM2.5) are of particular concern because they are the major culprit in visibility degradation (Watson, 2001) and because of their established harmful effects on human health (NRC, 1998).
As the most populated country and one of the fastest growing economies in the world, China is important in assessing the impact of anthropogenic pollution on the global environment. There have been an increasing number of studies on Chinese aerosols in the recent decade; however, our knowledge of the physical and chemical properties of ambient aerosols in China remains rather limited. Many of the published studies on Chinese aerosols have focused
mainly on the elemental constituents and major inorganic ions (e.g., Zhang and Friedlander, 2000 and references therein). In comparison, studies on the composition of the carbonaceous fraction of aerosols in China have been scant, with the number increasing only in recent years. Organic carbon (OC) and elemental carbon (EC) data have been collected mostly in large cities and in economically developed areas, such as Beijing (Bergin et al., 2001; He et al., 2001; Dan et al., 2004;), the Yangtze Delta region (Shanghai, Linan, Sheshan, Changshu) (Xu et al., 2002; Ye et al., 2003), and the Pearl River Delta region (Hong Kong, Guangzhou, Shenzhen, and Zhuhai) (Ho et al., 2003; Cao et al., 2003, 2004; Yu et al., 2004). Studies on the chemical composition of the OC fraction on a molecular level are especially sparse. Measurements of a limited number of organic compound classes have been documented only for a few large cities including Beijing (e.g., Simoneit et al., 1991; Yao et al., 2003), Qindao (Guo et al., 2003), Nanjing (Wang et al., 2002b), Shanghai (Yao et al., 2002), Guangzhou (e.g., Bi et al., 2002), and Hong Kong (e.g., Zheng et al., 2000).
12 In this study, we collected PM2.5 aerosol samples in Nanjing, a metropolitan area in the Yangtze Delta, in February and September of 2001. We report here on the concentrations of OC, EC, and water-soluble organic carbon (WSOC) in addition to the concentrations of major inorganic cations and anions in these aerosol samples. We also determined the concentrations of nonpolar organic species including n-alkanes, hopanes and PAHs as well as the concentrations of low-molecular weight carboxylic acids, amino acids, aliphatic amines in the WSOC fraction. As far as we are aware, this is one of the most comprehensive measurements of the chemical composition in PM2.5 aerosols in China. Our work adds to the increasing chemical database on Chinese aerosols.
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