Characteristics and sources of trace elements in PM2.5 in two megacities in Sichuan Basin of southwest China☆
Graphical abstract
Introduction
Particulate matters have attracted considerable attention due to their adverse effects on human health and environment (Englert, 2004; Lepeule et al., 2012; Nieuwenhuijsen et al., 2013; Tao et al., 2017b). Previous studies found that the finer particles, e.g., PM1.0 or PM2.5, caused more serious damage to human health than larger ones (Cifuentes et al., 2000; Tsiouri et al., 2015), partly due to their higher contents of toxic chemical components such as trace metals and polycyclic aromatic hydrocarbons (Costa and Dreher, 1997; Rogula-Kozlowska et al., 2013b). Seven trace elements including As, Cd, Cr, Ni, Pb, Co, and V are classified as carcinogenic or probably carcinogenic agents to humans by the International Agency for Research on Cancer (http://monographs.iarc.fr/ENG/Classification/index.php). Besides the effects on human health, trace metals could also damage ecological environments through dry and wet deposition to terrestrial and aquatic ecosystems (Pan and Wang, 2015; Wright et al., 2018).
Trace elements are typically identified as markers for source apportionment analysis of particles because of their unique source features (Hang and Oanh, 2014; Mantas et al., 2014; Salcedo et al., 2016). In contrast, major water-soluble ions such as SO42−, NO3− and NH4+ and carbonaceous components are at much higher levels and are generally from common sources, thus are not ideal to be used as tracers. The water-soluble portion (K+) typically account for more than 90% of the total element K in mass concentration (Rogula-Kozlowska et al., 2013a). Either K or K+ has been used as biomass burning tracers (Pan et al., 2013; Phillips-Smith et al., 2017; Praznikar et al., 2014; Rogula-Kozlowska et al., 2013a; Tao et al., 2016). Abundant Ca is probably from local construction activities besides crustal dust (Duan et al., 2012; Sahin et al., 2016). Tian et al. (2015) evaluated that coal combustion primarily contributed to the heavy metal emission, representing more than 50% of the total emission, e.g., As and Pb are commonly linked to coal combustion (Manoli et al., 2002; Pacyna et al., 2007; Tian et al., 2010, 2012; Zhang et al., 2009). Cr is mainly contributed by fuel combustion and metallurgical industry like chrome plating and steel production (Dall'Osto et al., 2008; Song and Gao, 2011). Mn might be released from ferrous metal smelting (Duan and Tan, 2013; Szefer and Szefer, 1986; Tian et al., 2015). Ni and V are typical tracers of heavy oil combustion, especially derived from ship emissions in coastal areas (Bressi et al., 2014; Mazzei et al., 2008; Mueller et al., 2011; Pandolfi et al., 2011; Tao et al., 2017a). Cu and Zn are well known to be associated with traffic related sources (Harrison et al., 2012; Pio et al., 2013; Sanders et al., 2003; Sternbeck et al., 2002; Thorpe and Harrison, 2008). Although traffic emissions appear to be key contributors to Zn particles, industry emissions like ferrous and nonferrous smelters should not be ignored (Dall'Osto et al., 2013; Kfoury et al., 2016). Apparently, trace elements in aerosols originate from diversified sources.
Several methods including enrichment factor (EF), correlation analysis, principal component analysis, and conditional probability function (CPF) have been widely applied in literature to explore the possible sources of trace elements (Cheng et al., 2013; Das et al., 2015; Lin et al., 2015; Zhai et al., 2014; Zhou et al., 2014). Among these, CPF method is typically used to analyze local source impacts from different wind directions and then ascertain the most possible source locations of each identified sources (Bressi et al., 2014; Hsu et al., 2017; Squizzato and Masiol, 2015; Wang et al., 2018a).
The two megacities, Chengdu and Chongqing, in southwest China have been suffering from serious air pollution due to the rapidly increasing coal combustion, adverse meteorological conditions, and special topography (Qiao et al., 2015). Earlier studies have focused on characterizing chemical compositions and formation mechanisms of PM2.5 under heavy polluted conditions (Chen et al., 2017a; Liao et al., 2017; Tian et al., 2017; Wang et al., 2018b), but little effort has been attributed to characterizing the contents and sources of trace elements (Chen et al., 2017c; Tao et al., 2014). Therefore, the present study aims to fill this knowledge gap by analyzing contents of trace elements in PM2.5 using the same daily samples collected in our previous study (Wang et al., 2018b). In addition, the sources of these trace elements are identified for the first time in this region, proving the much needed information for assessing their detrimental effects on humans and ecological environments as well as the scientific basis for making future emission control policies.
Section snippets
Sites description
Daily PM2.5 samples were collected at two urban sites, 260 km apart, in Sichuan Basin of southwest China, one in Chengdu (CD) and another in Chongqing (CQ). The sampling site at CD was located on the roof of a sixth-floor building in the Sichuan Academy of Environmental Science, and at CQ was on the roof top of Chongqing Monitoring Center (Wang et al., 2018b). The locations of the two sampling sites and the major industrial plants are shown in Fig. 1. These two cities are among the top three
Seasonal variations and site differences of trace elements in PM2.5
Table 1 shows the annual mean concentrations of 13 trace elements in PM2.5 at CD and CQ during the sampling periods. The amounts of V and As were below the detection limits in more than 50% of the samples at CQ, thus they were not discussed here. On annual basis, the total mass concentrations of 11 trace elements in PM2.5 were 2.1 and 2.8 μg m−3, accounting for 3.1% and 4.0% of PM2.5 mass at CD and CQ, respectively. Among these elements, K, Fe, Al, and Ca were the dominant ones at CD,
Conclusion
Annual mean concentrations of the analyzed trace elements in total were 2.1 and 2.8 μg m−3 at CD and CQ, respectively. The crustal elements (Al, Ca, Fe, and Ti) contributed 48.5% at CD and 66.6% at CQ, and the rest was from trace metals (K, Cr, Zn, Cu, Mn, Pb, Ni, and V) and As. Zn was the only trace metal having high concentration, e.g., annual mean of 238 and 113 ng m−3 at CD and CQ, respectively, comparable to some of the crust elements. The remaining heavy metals and As had annual mean
Acknowledgements
This work was partially supported by the National Natural Science Foundation of China (No. 41405027, 41375123), the National Key R&D Program of China (No. 2016YFC0200400), the Strategic Priority Research Program of Chinese Academy of Sciences (No. KJZD-EW-TZ-G06), the Fundamental Research Funds for the central University, and Chongqing Science and Technology Commission (No. cstkjcxljrc13).
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This paper has been recommended for acceptance by Dr. Yong Sik Ok.