Formation mechanisms of atmospheric nitrate and sulfate during the winter haze pollution periods in Beijing: gas-phase, heterogeneous and aqueous-phase chemistry

A vast area in China is currently going through severe haze episodes with drastically elevated concentrations of PM2.5 in winter. Nitrate and sulfate are the main constituents of PM2.5, but their formations via NO2 and SO2 oxidation are still not comprehensively understood, especially under different pollution or atmospheric relative humidity (RH) conditions. To elucidate formation pathways of nitrate and sulfate in different polluted cases, hourly samples of PM2.5 were collected continuously in Beijing during the wintertime of 2016. Three serious pollution cases were identified reasonably during the sampling period, and the secondary formations of nitrate and sulfate were found to make a dominant contribution to atmospheric PM2.5 under the relatively high RH condition. The significant correlation between NOR, NOR = NO3-/ (NO3- + NO2), and [NO2](2) x [O-3] during the nighttime under the RH >= 60 % condition indicated that the heterogeneous hydrolysis of N2O5 involving aerosol liquid water was responsible for the nocturnal formation of nitrate at the extremely high RH levels. The more often coincident trend of NOR and [HONO] x [DR] (direct radiation) x [NO2] compared to its occurrence with [Dust] x [NO2] during the daytime under the 30 % < RH < 60 % condition provided convincing evidence that the gas-phase reaction of NO2 with OH played a pivotal role in the diurnal formation of nitrate at moderate RH levels. The extremely high mean values of SOR, SOR = SO42-/ (SO42- +SO2), during the whole day under the RH >= 60 % condition could be ascribed to the evident contribution of SO2 aqueous-phase oxidation to the formation of sulfate during the severe pollution episodes. Based on the parameters measured in this study and the known sulfate production rate calculation method, the oxidation pathway of H2O2 rather than NO2 was found to contribute greatly to the aqueous-phase formation of sulfate.