Size-segregated Characteristics and Formation Mechanisms of Water-soluble Inorganic Ions during Different Seasons in Heshan of Guangdong, China

To identify the characteristics, sources, and formation mechanisms of aerosol particles during pollution episodes in the Pearl River Delta, 24 sets of size-segregated samples were collected in Heshan during July 2014 and January 2015 using a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI), and nine ions, viz., Na^+, NH_4^+, K^+, Mg^(2+), Ca^(2+), Cl^-, NO_2^-, NO_3^-, and SO_4^(2-), were investigated. The Na^+, Mg^(2+), and Ca^(2+) were mainly distributed in the coarse particles, and were mainly from soil, dust, and sea salt. The fine-mode K^+ during winter was mostly generated by biomass burning. The coarse-mode Cl^- originated from sea salt, whereas the fine-mode Cl^- resulted from the conversion of NH_4Cl to the particle phase. Both the SO_4^(2-) and the NO_3^- exhibited unimodal distributions during winter but bimodal ones during summer. The coarse-mode SO_4^(2-) and NO_3^- arose from sea salt and heterogeneous reactions, respectively. An increase in the nitrogen oxidation ratio (NOR) and a decrease in the sulfur oxidation ratio (SOR) were observed on polluted days, with the highest values occurring in the 0.56-1 μm particle size fraction. The formation of NO_3^- was chiefly related toμ homogeneous gas-phase reactions during winter and nocturnal heterogeneous reactions involving N_2O_5 during summer, whereas the formation of SO_4^(2-) was driven by gas-phase oxidation in the 0.056-0.32 μm size range and aqueous oxidation in the 0.56-3.2 μm range. Additionally, the SOR and the NO_2 concentration displayed a positive correlation in the 0.056-1.8 μm particle size fraction, indicating that the potential formation of SO_4^(2-) via aqueous reactions was accelerated by NO_2.