This paper explores the role of the secondary inorganic aerosol(SIA) species ammonium, NH+4, nitrate, NO-3, and sulfate,SO2-4, during haze and fog events using hourly mass concentrations of PM2.5measured at a suburban site in Hangzhou,China. A total of 546 samples were collected between 1 April and 8 May 2012. The samples were analyzed and classified as clear, haze or fog depending on visibility and relative humidity(RH). The contribution of SIA species to PM2.5mass increased to ～50% during haze and fog. The mass contribution of nitrate to PM2.5increased from 11% during clear to 20%during haze episodes. Nitrate mass exceeded sulfate mass during haze, while near equal concentrations were observed during fog episodes. The role of RH on the correlation between concentrations of SIA and visibility was examined, with optimal correlation at 60%–70% RH. The total acidity during clear, haze and fog periods was 42.38, 48.38 and 45.51 nmol m-3,respectively, indicating that sulfate, nitrate and chloride were not neutralized by ammonium during any period. The nitrate to sulfate molar ratio, as a function of the ammonium to sulfate molar ratio, indicated that nitrate formation during fog started at a higher ammonium to sulfate molar ratio compared to clear and haze periods. During haze and fog, the nitrate oxidation ratio increased by a factor of 1.6–1.7, while the sulfur oxidation ratio increased by a factor of 1.2–1.5, indicating that both gaseous NO2 and SO2were involved in the reduced visibility.
