Characterization of the size-segregated water-soluble inorganic ions in the Jing-Jin-Ji urban agglomeration: Spatial/temporal variability, size distribution and sources

To investigate the characteristics of aerosols in north China, the samples of water-soluble ions, including anions (F-, Cl-, NO2-, NO3-, SO42-) and cations (NH4+ K+, Na+, Ca2+, Mg2+) in 8 size-segregated particle fractions, are collected using a sampler from Sep. 2009 to Aug. 2010 at four sites in urban areas (Beijing, Tianjin and Tangshan) and a background region (Xinglong) in the Jing-Jin-Ji urban agglomeration. High spatial variability is observed between the urban areas and the background region. The results of chemical composition analysis showed that secondary water soluble ions (SO42- + NO3- + NH4+ (SWSI) composed more than half the total ions, and are mainly found in fine particles (aerodynamic diameters less than 2.1 mu m), while Mg2+ and Ca2+ contributed to a large fraction of the total water-soluble ions in coarse particles (aerodynamic diameters greater than 2.1 mu m and less than 9.0 mu m). The concentrations of SO42-, NO3- and NH4+ are higher in summer and winter and lower in spring and autumn. Mg2+ and Ca2+ are obviously abundant in winter in Beijing, Tianjin and Tangshan. In contrast, Mg2+ and Ca2+ are abundant in autumn in Xinglong. The SWSI showed a bimodal size distribution with the fine mode at 0.43-1.1 mu m and the coarse mode at 4.7-5.8 mu m, and had different seasonal variations and bimodal shapes. NH4+ played an important role in the size distributions and the formations of SO42- and NO3-. Heterogeneous reaction is the main formation mechanism of SO42- and NO3-, which tended to be enriched in the coarse mode of aerosol. The sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) indicated high photochemical oxidation property over the whole Jing-Jin-Ji urban agglomeration. (C) 2013 Elsevier Ltd. All rights reserved.