Size Distributions of Water-Soluble Inorganic Ions in Atmospheric Aerosols during the Meiyu Period on the North Shore of Taihu Lake, China

In order to research the impact on air pollution from Meiyu (a special period of seasonal heavy rainfall between late May and June in the middle-lower Yangtze area in China (Qian et al., 2009)), the pollutant gas concentrations, size distributions, and water-soluble inorganic ions of aerosols were measured on the north shore of Taihu Lake from June 10 to June 26, 2014. Results show that the PM_(1.1) (aerodynamic diameter ≤ 1.1 μm) grew in particle size, becoming PM_(1.1-2.1) (1.1 μm ≤ aerodynamic diameter ≤ 2.1 μm) and then CPM (coarse particle matter; aerodynamic diameter ＞ 2.1 μg) due to coagulation growth and chemical reactions, prior to Meiyu. The ions, anions, and cations all exhibited bimodal distributions before Meiyu, with peak positions of 0.43-1.1 and 9-10 μm, and trimodal distributions during Meiyu, with unfixed peak positions. The spectral size distributions of the SO_4 ^(2-), NO_3 ^-, and NH_4^+ were all bimodal or trimodal before Meiyu, and SNA (SO_4^(2-), NO_3^-, and NH_4^+) was the main component of the PM_(1.1) and PM_(1.1-2.1). Vehicle emissions were more important and the secondary formation of SO_4^(2-) from SO_2 occurred more readily during Meiyu than before Meiyu. The nitrogen oxidation ratio (NOR) was lower than the sulfur oxidation ratio (SOR), and the secondary formation of NO_3^– from NO_x infrequently occurred in Wuxi compared to that of SO_4^(2-) from SO_2. The samples collected during the pre-Meiyu season and Meiyu all fell below the line 1:1 (CE:AE) (cation equivalence:anion equivalence), suggesting that the volatilization of ammonium and nitrate as well as unmeasured hydrogen ions may have caused the loss of cations, which resulted in insufficient cations for neutralizing the anions and subsequently, acidic properties for the aerosol. [NH_4^+ + Ca^(2+)] are evidently sufficient for neutralizing [SO_4^(2-) + NO_3^- + NO_2^- + Cl^-].