A vast area in China is currently going through severe haze episodes with drastically elevated concentrations of PMinline-formula2.5 in winter. Nitrate and sulfate are the main constituents of PMinline-formula2.5, 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-formula2.5 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-formula2.5 under the relatively high RH condition. The significant correlation between NOR, NOR inline-formula= inline-formula
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, 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
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, 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.