Nitrate-Enhanced Oxidation of SO2 on Mineral Dust: A Vital Role of a Proton.

Heterogeneous oxidation of SO2 on mineral dust is a significant source of sulfate in the atmosphere. Given that a large fraction of nitrate is deposited on the mineral aerosols, the determination of the effect of nitrate on the SO2 oxidation on mineral dust and its in-depth mechanism are much desired. In this work, we report nitrate-enhanced SO2 oxidation on authentic mineral dust. By comparing the SO2 uptake behaviors on Arizona test dust (ATD, a typical proxy of mineral dust) with or without nitrate, we found that although nitrate hinders the initial SO2 uptake, it substantially accelerates SO2 uptake and oxidation after a pronounced induction period. In other words, a hindering-then-accelerating feature in the SO2 uptake profile was observed on nitrate-containing ATD (N-ATD) particles. In addition, HONO was released in the accelerating period as the reduction product of nitrate. The accumulation of protons (H+) from SO2 oxidation during the induction period plays a key role in the acceleration of SO2 oxidation. Our work suggests that the nitrate-participating SO2 oxidation on mineral dust can be one of the important contributions of the sulfate source in the atmosphere.