Characteristics and Sources of Water-Soluble Ions in PM2.5 in the Sichuan Basin, China

Abstract

To track the particulate pollution in Sichuan Basin, sample filters were collected in three urban sites. Characteristics of water-soluble inorganic ions (WSIIs) were explored and their sources were analyzed by principal component analysis (PCA). During 2012–2013, the PM2.5 concentrations were 86.7 ± 49.7 μg m−3 in Chengdu (CD), 78.6 ± 36.8 μg m−3 in Neijiang (NJ), and 71.7 ± 36.9 μg m−3 in Chongqing (CQ), respectively. WSIIs contributed about 50% to PM2.5, and 90% of them were secondary inorganic ions. NH4+ and NO3− roughly followed the seasonal pattern of PM2.5 variations, whereas the highest levels of SO42− appeared in summer and autumn. PM2.5 samples were most acidic in autumn and winter, but were alkaline in spring. The aerosol acidity increased with the increasing level of anion equivalents. SO42− primarily existed in the form of (NH4)2SO4. Full neutralization of NH4+ to NO3− was only observed in low levels of SO42− + NO3−, and NO3− existed in various forms. SO42− and NO3− were formed mainly through homogeneous reactions, and there was the existence of heterogeneous reactions under high relative humidity. The main identified sources of WSIIs included coal combustion, biomass burning, and construction dust.