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 PMinline-formula_2.5 in winter. Nitrate and sulfate are the main constituents of PMinline-formula_2.5, but their formations via inline-formulaNO₂ and inline-formulaSO₂ 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 PMinline-formula_2.5 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 PMinline-formula_2.5 under the relatively high RH condition. The significant correlation between NOR, NOR inline-formula= inline-formula $M8inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}/(chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}+chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}})$ 93pt16ptsvg-formulamathimgfbdf95935737ce6770e25fa825e596bd acp-20-4153-2020-ie00001.svg93pt16ptacp-20-4153-2020-ie00001.png , and inline-formula[NO₂]² inline-formula× inline-formula[O₃] during the nighttime under the inline-formulaRH≥60 % condition indicated that the heterogeneous hydrolysis of inline-formulaN₂O₅ 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] inline-formula× [DR] (direct radiation) inline-formula× [inline-formulaNO₂] compared to its occurrence with [Dust] inline-formula× [inline-formulaNO₂] during the daytime under the 30 % inline-formula< RH inline-formula< 60 % condition provided convincing evidence that the gas-phase reaction of inline-formulaNO₂ with OH played a pivotal role in the diurnal formation of nitrate at moderate RH levels. The extremely high mean values of SOR, SOR inline-formula= inline-formula $M23inlinescrollmathmlchem{\mathrm{normal\; SO}}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}/(chem{\mathrm{normal\; SO}}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}+chem{\mathrm{normal\; SO}}_{\mathrm{normal\; 2}})$ 100pt17ptsvg-formulamathimg3d2a8cef5a5eb55d09cff2d5c0bdb742 acp-20-4153-2020-ie00002.svg100pt17ptacp-20-4153-2020-ie00002.png , during the whole day under the inline-formulaRH≥60 % condition could be ascribed to the evident contribution of inline-formulaSO₂ 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 inline-formulaH₂O₂ rather than inline-formulaNO₂ was found to contribute greatly to the aqueous-phase formation of sulfate.