Acidity of Aerosols during Winter Heavy Haze Events in Beijing and Gucheng, China

We investigated the acidity and concentrations of water-soluble ions in PM 2.5 aerosol samples collected from an urban site in Beijing and a rural site in Gucheng, Hebei Province from November 2016 to January 2017 to gain an insight into the formation of secondary inorganic species. The average SO 4 2– , NO 3 – , and NH 4 + concentrations were 8.3, 12.5, and 14.1 μg m –3 , respectively, at the urban site and 14.0, 14.2, and 24.2 μg m –3 , respectively, at the rural site. The nitrogen and sulfur oxidation ratios in urban Beijing were correlated with relative humidity (with correlation coefficient r = 0.79 and 0.67, respectively) and the aerosol loadings. Based on a parameterization model, we found that the rate constant of the heterogeneous reactions for SO 2 on polluted days was about 10 times higher than that on clear days, suggesting that the heterogeneous reactions in the aerosol water played an essential role in haze events. The ISORROPIA II model was used to predict the aerosol pH, which had a mean (range) of 5.0 (4.9–5.2) and 5.3 (4.6–6.3) at the urban and rural site, respectively. Under the conditions with this predicted pH value, oxidation by dissolved NO 2 and the hydrolysis of N 2 O 5 may be the major heterogeneous reactions forming SO 4 2– and NO 3 – in haze. We also analyzed the sensitivity of the aerosol pH to changes in the concentrations of SO 4 2– , NO 3 – , and NH 4 + under haze conditions. The aerosol pH was more sensitive to the SO 4 2– and NH 4 + concentrations with opposing trends, than to the NO 3 – concentrations. The sensitivity of the pH was relatively weak overall, which was attributed to the buffering effect of NH 3 partitioning.