Characteristics of the Size Distribution of Water-soluble Ions During a Heavy Pollution Episode in the Winter in Tianjin

Abstract

To characterize the size distribution of water-soluble inorganic ions (WSⅡ) during a heavy pollution episode, particle samples were collected by an Andersen cascade sampler in Tianjin in January 2014, and the concentrations of eight WSⅡ (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, and SO42-) during a typical haze episode were analyzed by ion chromatography. The sources and formation mechanisms of WSⅡ were analyzed based on their size distributions. The results showed that the daily average concentrations of PM2.5 and PM10 were (138±100) μg·m-3 and (227±142) μg·m-3, respectively, and the average concentration of total WSⅡ concentrations (TWSⅡ) in the coarse and fine particles were (34.07+6.16) μg·m-3 and (104.16+51.76) μg·m-3, respectively. The concentrations of SO42-, NO3-, and NH4+ in the fine particles were much higher than concentrations of the other ions, and there were strong correlations between these three ions. The TWSⅡ on clear days, light pollution days, and heavy pollution days were (41.55±12.41) μg·m-3, (94.46±31.19) μg·m-3, and (147.55±27.76) μg·m-3, respectively. On clear days, SO42- showed a unimodal distribution, peaking at 0.43-0.65 μm; and NO3- showed a trimodal distribution, peaking at 0.43-0.65 μm, 2.1-3.3 μm, and 5.8-9.0 μm. NH4+ had a bimodal distribution, peaking at 0.43-0.65 μm and 4.7-5.8 μm. On heavy pollution days, however, the size distributions of these three secondary inorganic ions switched to a unimodal size distribution, peaking at 0.65-1.1 μm. Unimodal NH4+ mainly coexisted with SO42- and NO3-, and the excess NH4+ was found to be combined with Cl- in the fine particles. In the coarse particles, NH4+ completely coexisted with SO42- and NO3-.