Abstract
Abstract. To investigate the characteristics of PM2.5 and its major chemical components, formation mechanisms, and geographical origins in the two megacities, Chengdu (CD) and Chongqing (CQ), in Sichuan Basin of southwest China, daily PM2.5 samples were collected simultaneously at one urban site in each city for four consecutive seasons from autumn 2014 to summer 2015. Annual mean concentrations of PM2.5 were 67.0 ± 43.4 and 70.9 ± 41.4 µg m−3 at CD and CQ, respectively. Secondary inorganic aerosol (SNA) and organic matter (OM) accounted for 41.1 and 26.1 % of PM2.5 mass at CD, and 37.4 and 29.6 % at CQ, respectively. Seasonal variations of PM2.5 and major chemical components were significant, usually with the highest mass concentration in winter and the lowest in summer. Daily PM2.5 concentration exceeded the national air quality standard on 30 % of the sampling days at both sites, and most of the pollution events were at the regional scale within the basin formed under stagnant meteorological conditions. The concentrations of carbonaceous components were higher at CQ than CD, likely partially caused by emissions from the large number of motorcycles and the spraying processes used during automobile production in CQ. Heterogeneous reactions probably played an important role in the formation of SO42−, while both homogeneous and heterogeneous reactions contributed to the formation of NO3−. Geographical origins of emissions sources contributing to high PM2.5 masses at both sites were identified to be mainly distributed within the basin based on potential source contribution function (PSCF) analysis.
Highlights
IntroductionFine particles (PM2.5, particulate matter with an aerodynamic diameter smaller than 2.5 μm) have adverse effects on human health (Anderson et al, 2012; Lepeule et al, 2012; Taus et al, 2008), deteriorate air quality (Zhang et al, 2008; Paraskevopoulou et al, 2015), reduce atmospheric visibility (Fu et al, 2016; Cao et al, 2012; Baumer et al, 2008), impact climate (Ramanathan and Feng, 2009; Hitzenberger et al, 1999; Mahowald, 2011), and affect the ecosystem (Larssen et al, 2006)
Organic matter (OM), estimated from Organic carbon (OC) using a conversion factor of 1.6 to account for other elements presented in organic compounds (Turpin and Lim, 2001), was the most abundant species in PM2.5, accounting for 26.1 and 29.6 % of PM2.5 mass at CD and CQ, respectively
Such a finding was supported by back trajectory analysis, which showed that air masses reaching both sites originated within the basin and only traveled for short distances on heavy polluted days
Summary
Fine particles (PM2.5, particulate matter with an aerodynamic diameter smaller than 2.5 μm) have adverse effects on human health (Anderson et al, 2012; Lepeule et al, 2012; Taus et al, 2008), deteriorate air quality (Zhang et al, 2008; Paraskevopoulou et al, 2015), reduce atmospheric visibility (Fu et al, 2016; Cao et al, 2012; Baumer et al, 2008), impact climate (Ramanathan and Feng, 2009; Hitzenberger et al, 1999; Mahowald, 2011), and affect the ecosystem (Larssen et al, 2006). Ambient Air Quality Standards (NAAQS) for PM2.5 was promulgated by the Chinese government in 2012, and strict strategies have been implemented nationwide, e.g., controlling SO2 emissions by installing desulphurization system in coal-fired power plants and conversion of fuel to natural gas (Lu et al, 2011), mitigating NOx emissions through traffic restrictions, and reducing biomass burning through straw shredding. Despite these efforts, there are still many cities that have not yet met the current NAAQS (Tao et al, 2017). According to the “2013–2015 Reports on the State of Environment of China”, annual mean concentration of PM2.5 in 74 major cities across China was 72, 64, and 50 μg m−3 in 2013, 2014 and 2015, respectively, and only 4.1, 12.2, and 22.5 % of the monitored cities met the NAAQS (35 μg m−3)
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