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. Although the anthropogenic emissions of SO2 have decreasedsignificantly in China, the decrease in SO42- in PM2.5 ismuch smaller than that of SO2. This implies an enhanced formation rateof SO42- in the ambient air, and the mechanism is still underdebate. This work investigated the formation mechanism of particulatesulfate based on statistical analysis of long-term observations inShijiazhuang and Beijing supported with flow tube experiments. Our mainfinding was that the sulfur oxidation ratio (SOR) was exponentiallycorrelated with ambient RH in Shijiazhuang (SOR = 0.15+0.0032×exp⁡(RH/16.2)) and Beijing (SOR = -0.045+0.12×exp⁡(RH/37.8)). InShijiazhuang, the SOR is linearly correlated with the ratio of aerosol watercontent (AWC) in PM2.5 (SOR = 0.15+0.40×AWC/PM2.5).Our results suggest that uptake of SO2 instead of oxidation of S(IV) inthe particle phase is the rate-determining step for sulfate formation.NH4NO3 plays an important role in the AWC and the change ofparticle state, which is a crucial factor determining the uptake kinetics ofSO2 and the enhanced SOR during haze days. Our results show thatNH3 significantly promoted the uptake of SO2 and subsequently theSOR, while NO2 had little influence on SO2 uptake and SOR in thepresence of NH3.

Ammonium nitrate promotes sulfate formation through uptake kinetic regime