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. A vast area in China is currently going through severe haze episodes withdrastically elevated concentrations of PM 2.5 in winter. Nitrate andsulfate are the main constituents of PM 2.5 , 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 indifferent polluted cases, hourly samples of PM 2.5 were collectedcontinuously in Beijing during the wintertime of 2016. Three seriouspollution cases were identified reasonably during the sampling period, andthe secondary formations of nitrate and sulfate were found to make adominant contribution to atmospheric PM 2.5 under the relatively high RHcondition. The significant correlation between NOR, NOR  = NO 3 - / ( NO 3 - + NO 2 ) , and [NO2]2 × [O3] during the nighttime under the RH≥60  % condition indicatedthat the heterogeneous hydrolysis of N2O5 involving aerosolliquid water was responsible for the nocturnal formation of nitrate at theextremely high RH levels. The more often coincident trend of NOR and [HONO]  ×  [DR] (direct radiation)  ×  [ NO2 ] compared to its occurrence with [Dust]  ×  [ NO2 ] during the daytime under the 30 %   RH   60 % condition provided convincing evidence that the gas-phasereaction of NO2 with OH played a pivotal role in the diurnal formationof nitrate at moderate RH levels. The extremely high mean values of SOR, SOR  = SO 4 2 - / ( SO 4 2 - + SO 2 ) , during the whole dayunder the RH≥60  % condition could be ascribed to the evidentcontribution of SO2 aqueous-phase oxidation to the formation of sulfateduring the severe pollution episodes. Based on the parameters measured inthis study and the known sulfate production rate calculation method, theoxidation pathway of H2O2 rather than NO2 was found tocontribute greatly to the aqueous-phase formation of sulfate.

atmospheric chemistry and physics

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