资源环境科技发展态势分析平台

In this study, we investigated how the ion balance causes variations in size segregated aerosol acidity and atmospheric processing on clean versus hazy days using a 9-stage sampler. We calculated the ratios (in charge equivalents, R-c/A) between measured cations (Na+, NH4+, K+, Mg-2 (+), and Ca-2(+)) and anions (SO42-, NO3- and C1-) for different aerosol size fractions. The ratios were typically close to unity in the accumulation mode (0.65-2.1 gm), and increased significantly when the particle size increased or decreased. In the coarse size range (aerodynamic diameter > 2.1 mu m), high R-c/A values were most likely caused by the undetermined CO32- and HCO3- content of the mineral dust. In contrast, the high R-C/A values for submicron aerosols ( < 1.1 gm) were likely caused by the presence of water-soluble organic anions. The Rett, values for all size fractions were lower on hazy days than clean days, indicating that aerosol acidity was enhanced on polluted days. Simiar temporal trend between R-C/A and in-situ pH indicated that R-C/A was a good indicator of aerosol acidity in fine mode aerosol. The SO42- and NO3- contents in fine particles were completely neutralized as the Rcm values for PM2.1 approached unity, and mean values of R-C/A were 1.34 and 1.16 during the transition and polluted periods, respectively. The lowest Rc/A values were observed in the size fraction with the highest concentrations of SO42-, NO3- and NH4+ (SNA) and concentrations of SNA increased with the increasing aerosol acidity. Significant correlations between [NO3-]/[SO42-] and [NH4+ V[SO42-] during NH4+-rich conditions in fine size fractions indicated fine mode NO3- in Beijing was mainly formed by gas-phase homogeneous reaction between the ambient NH3 and HNO3.