Abstract
To characterize the size distribution of water soluble inorganic ions (WSⅡ) in haze days, particle samples were collected by an Andersen cascade impactor in Shijiazhuang from October 15 to November 14 in 2013, and the concentrations of eight kinds of WSⅡ (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-and SO42-) during a typical haze episode were analyzed by ion chromatography. Sources and formation mechanism of WSⅡ were analyzed based on their size distributions. The results showed that Shijiazhuang suffers serious air pollution during the autumn season. The daily average concentrations of PM10and PM2.5were (361.2±138.7) μg·m-3 and (175.6±87.2) μg·m-3 and the daily average concentration of PM2.5was 2.3 times as high as the national secondary standard. The total water soluble inorganic ion concentrations (TWSⅡ) in clean days, light haze days and heavy haze days were(64.4±4.6)μg·m-3, (109.9±22.0)μg·m-3 and (212.9±50.1) μg·m-3 respectively. In addition, the ratio of secondary inorganic ions (SNA:SO42-, NO3- and NH4+) in TWSⅡ increased from 44.9% to 77.6% as changed from clean days to the heavy haze days, suggesting the evolution of haze episodes mainly originated from the formation and accumulation of SNA. The size distributions of SO42-, NO3- and NH4+ were bimodal in clean days, peaking at 0.43 -0.65 μm and 4.7-5.8 μm, respectively, which changed to unimodal distribution in both the light and heavy haze days, peaking at 0.65-1.1 μm. Owing to high humidity during the heavy haze days, the aqueous phase reactions of SO42- and NO3- were likely promoted, which led to the transformation of condensation mode in clean days to the droplet mode in haze days. The size distributions of Na+, Mg2+ and Ca2+ were different with that of SNA, which showed a coarse mode peaking at 4.7-5.8 μm both in clean and haze days, whereas K+and Cl- showed a bimodal distribution both in clean and haze days, although the modal size was different in clean and haze days.
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