SO₂Initiates the Efficient Conversion of NO₂to HONO on MgO Surface

Nitrous acid (HONO) is an important source of hydroxyl radical (OH) that determines the fate of many chemically active and climate relevant trace gases. However, the sources and the formation mechanisms of HONO remain poorly understood. In this study, the effect of SO2on the heterogeneous reactions of NO2on MgO as a mineral dust surrogate was investigated. The reactivity of MgO to NO2is weak, while coexisting SO2can increase the uptake coefficients of NO2on MgO by 2-3 orders of magnitude. The uptake coefficients of NO2on SO2-aged MgO are independent of NO2concentrations in the range of 20-160 ppbv and relative humidity (0-70%RH). The reaction mechanism was demonstrated to be a redox reaction between NO2and surface sulfite. In the presence of SO2, NO2was reduced to nitrite under dry conditions, which could be further converted to gas-phase HONO in humid conditions. These results suggest that the reductive effect of SO2on the heterogeneous conversion of NO2to HONO may have a significant contribution to the unknown sources of HONO observed in polluted areas (for example, in China).