Study on pollution behavior and sulfate formation during the typical haze event in Nanjing with water soluble inorganic ions and sulfur isotopes

The typical haze event in Nanjing was selected to study pollution behavior and sulfate formation by field measurement. Based on the concentrations of water soluble inorganic ions in PM2.5, pollution characteristics of the haze were investigated with phase clustering analysis. Besides, delta S-34 values of SO2 and sulfate in PM2.5 were determined in order to explore sulfur sources and sulfate formation. The result showed that PM2.5 pollution during the haze event was significantly serious, which was mainly from coal combustion, vehicle exhaust emission and biomass burning. Sulfate formation was attributed to aqueous phase sulfur oxidation reactions promoted by high relative humidity and NO2 concentration under the alkaline condition. The color of sky on 22 Dec. was ascribed to the combination of sunset glow and fine particles in high-moisture atmosphere. delta S-34 values of SO2 are found to be lower than those of sulfate in PM2.5 indicating there was presence of sulfur isotopic fractionation during SO2 oxidation. The average contribution of SO2 homogenous oxidation to sulfate was about 51.2% during the haze events. The ratio of SO2 heterogeneous and homogeneous oxidation to sulfate was mainly attributed to the concentrations of gaseous pollutants (NO2, SO2 and O-3) and relative humidity of the atmosphere.