Characterization and source analysis of water-soluble inorganic ionic species in PM2.5 in Taiyuan city, China
Introduction
Particle matters < 2.5 μm in aerodynamic diameter (PM2.5) have been paid more attention in recent decades due to its distinct impacts on human health, air quality and global climate change. Being characterized by a higher surface/mass ratio, small particles were more likely to absorb toxic constituents, and cause many kinds of healthy effects (WHO, 1999, EPA, 2015). PM2.5 could directly scatter and absorb the sunlight contributing to light extinction, and affect the atmosphere visibility (Yuan et al., 2006).
Many studies have reported the PM2.5 levels were higher in Chinese cities than other countries because of excessive coal usage (Tan et al., 2014, Tan et al., 2016, Qiu et al., 2013, Zhang et al., 2011). Compared with those reported in foreign countries (e.g. Khan et al., 2010, Aldabe et al., 2011), the high PM2.5 pollutions were found in some Chinese cities and its levels exceeded the limit value of PM2.5 issued by EPA (e.g. Tan et al., 2016, Tao et al., 2012, Zhang et al., 2011). Water-soluble inorganic ions (WSI), including NO3−, SO42 −, NH4+, Mg2 +, and Ca2 + species, accounted for about 20–45% or even > 70% of the mass of PM2.5 (Tan et al., 2016, Zhang et al., 2011, Li et al., 2010, Kulshrestha et al., 2009, Khan et al., 2010, Tao et al., 2012, Dai et al., 2013, Tao et al., 2013), and played a decisive role in the process of aerosol hygroscopicity which could exacerbate visibility impairment (Shen et al., 2009). SO42 −, NO3− and NH4+ were the major species of WSI and contributed 32% and 43% to PM2.5 mass in Shanghai and Fuzhou, respectively (Wang et al., 2006, Xu et al., 2012). Compared with vehicle emission, the high level of SO42 − and NO3− in Chinese cities were mainly related to coal combustion, especially in winter in some northern cities (Xiao and Liu, 2004, Hu et al., 2002, Wang et al., 2005, Xu et al., 2012, Tan et al., 2016). In addition, WSI in particles were also affected by the variation of sources, such as industrial source, agricultural source and et al., which could lead to the variation of F−, Cl−, NH4+, K+, Mg2 + and Ca2 + in particles (Nair et al., 2006, Rengarajan et al., 2011, Li et al., 2010). For example, fertilizer usage in agricultural activity may result in more formation of NH4+ in atmospheric particles (Zhang et al., 2011).
WSI in particles existed in some specific chemical forms, which can influence the characteristics of PM2.5. (NH4)2SO4, a typical chemical form of two important ions of SO42 − and NH4+, not only have hygroscopic effect on particulate matters, but also enhance the hygroscopic growth of mixtures of organic salts with ammonium sulfate (Wu et al., 2011). Compared with abundant NH4NO3 of Beijing in 2001–2003 and Shanghai in 2001, the major ions were in the form of (NH4)2SO4, Ca(NO3)2, CaCl2, and CaSO4 in aerosol particles in Shanghai in 2004 (Wang et al., 2005, Wang et al., 2006, Yao et al., 2002). The fine mode NH4NO3 increased greatly in recent year in East Asia due to the increasing emissions of pollutants (both including NH3 and NOX emission from anthropogenic sources) and the decreasing concentration of coarse particles (Kim et al., 2006).
The ratio of NO3−/SO42 − could be reasonably used to evaluate the contribution of mobile and stationary sources to sulfur and nitrogen in the atmosphere. The ratio was higher in cities with heavy traffic density, such as Guangzhou (0.79, Tan et al., 2009), Shanghai (0.64, Wang et al., 2006), Beijing (0.58, Yao et al., 2002), Xiamen (0.52, Zhang et al., 2012a), and lower in coal-dominated cities, such as Guiyang (0.14; Xiao and Liu, 2004) and Qingdao (0.35; Hu et al., 2002). Previous studies have indicated that the major source of F− in atmosphere in urban area was coal burning, while the main sources of Cl− were coal burning, biomass burning and soil dust (Kulshrestha et al., 2009, Watson et al., 2001). Besides dust and coal combustion which were dominate contributor of Mg2 + and Ca2 + in atmosphere, industrial emission was another important source of Mg2 + in some magnesium production areas (Osada et al., 2002, Zhang et al., 2002). In general, NH4+ and K+ were suggested as the main tracers of biomass burning (Nair et al., 2006), but Zhang et al. (2012a) reported that coal combustion was also an important source of NH4+, especially in urban area.
As the largest coal base of China, atmospheric SO2, NOX and particles in Taiyuan was one of the highest in China (MEP, 2015). SO42 −, NO3−, Ca2 + and NH4+ were the main components in wet precipitations in recent years, and high deposition flux of S and N should be concerned in Taiyuan (Guo et al., 2015). In spite of high atmospheric SO2 and NOX level, there were still few studies about the SO42 − and NO3− level and corresponding contribution in the related particle phase. The purpose of this paper was to discuss the PM2.5 and WSI levels, and fully illustrate the key influence factors on its seasonal variation and chemical forms in Taiyuan.
Section snippets
Site description
Taiyuan, with a population of 4.2 million and a land area of about 1500 km2, is the capital of Shanxi Province. The terrain resembles a dustpan surrounded by mountains in its west, north and east directions, with valley plain in the south, which was conducive to the air pollutants confluent and accumulation in the city. Located in the continental interior, Taiyuan belongs to the warm temperate zone continental monsoon climate. The annual average temperature is 9.5 °C, with the highest in summer
PM2.5 levels
Fig. 2 showed the daily concentrations of PM2.5 and water-soluble ions in Taiyuan, from summer, 2009 to spring, 2010. The daily PM2.5 levels in the field samples varied from 49.90 to 477.93 μg/m3 with the mean of 209.54 μg/m3 (Table 2), which all largely exceeded the PM2.5 24-h limitation value of 35 μg/m3 in EPA (EPA, 2006). Except for 2nd Aug, 2009, the daily PM2.5 concentrations in all samples were higher than the daily limitation value (75 μg/m3) issued by MEP (MEP, 2012). High PM2.5 levels in
Conclusion
PM2.5 samples were collected from August 2009 to April 2010 in Taiyuan to discuss the characterization of PM2.5 and WSI in PM2.5. The daily PM2.5 levels in the field samples varied from 49.90 to 477.93 μg/m3 with the mean of 209.54 μg/m3, which all largely exceeded the PM2.5 24-h limitation value of 35 μg/m3 in EPA and 75 μg/m3 in MEP. The WSI average concentration was 68.86 μg/m3 and occupied about 32.86% of PM2.5. SO42 − and NH4+ were the most abundant anion and cation, accounting for 43.53 and
Acknowledgements
This study was supported by the funding of the National Natural Science Foundation of China (No. 41501543, 41472316); The key project of the national Ministry of Education of China (No. 211026); We gratefully acknowledge the partial supports from the One-hundred talent program supported by The Organization Department of Shanxi Province Committee.
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