PM2.5 acidity during haze episodes in Shanghai, China

Environmental context Aerosol acidity, or aerosol aqueous phase pH, can affect various environmental processes. Based on high frequency measurements of particulate compositions, along with thermodynamic calculations, this work studies particle acidity in the course of severe episodes of haze in Shanghai and considers the effect of this on the production of nitrate. The results will provide new perspectives on our interpretation of PM2.5 acidity during haze episodes in megacities. Abstract Aerosol acidity is one of the most important parameters that can influence climate change and human health, which has been inadequately analysed in China. Here, hourly measurements of particulate compositions and the E-AIM II model (assuming thermodynamic equilibrium) were used to study particle acidity during severe episodes of haze in Shanghai. The total concentration of sulfate, nitrate and ammonium was 138.9 ± 50.6 μg m−3, maximum 241.3 μg m−3; and the PM2.5 to PM10 (PM2.5/PM10) ratio was 0.60. The fine particles detected were somewhat acidic, with a pH range of 0.04–4.50, average 2.34, which is higher than in some areas of the US and China. The relatively low particle acidity is attributed to particle water content levels. Furthermore, the growth rate of sulfate, nitrate and ammonium during a haze episode (Case 2) was faster than that during a clean episode (CE), owing to exacerbated effects of PM2.5 acidity in the event of high relative humidity (RH) on hazy days. Finally, the detected significant correlations of [NO3−]/[SO42−] with [NH4+]/[SO42−] in conditions of abundant NH4+ indicate that NO3− in Shanghai is primarily formed through homogeneous reaction between ambient NH3 and HNO3. These findings provide new perspectives on our interpretation of PM2.5 acidity during haze episodes in megacities.