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

We investigated the acidity and concentrations of water-soluble ions in PM2.5 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 SO42-, NO3-, and NH4+ concentrations were 8.3, 12.5, and 14.1 mu g m(-3), respectively, at the urban site and 14.0, 14.2, and 24.2 mu g 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 SO2 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 NO2 and the hydrolysis of N2O5 may be the major heterogeneous reactions forming SO42- and NO3- in haze. We also analyzed the sensitivity of the aerosol pH to changes in the concentrations of SO42-, NO3-, and NH4+ under haze conditions. The aerosol pH was more sensitive to the SO42- and NH4+ concentrations with opposing trends, than to the NO3- concentrations. The sensitivity of the pH was relatively weak overall, which was attributed to the buffering effect of NH3 partitioning.