SO₂ Initiates the Efficient Conversion of NO₂ to HONO on MgO Surface

Abstract

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 SO 2 on the heterogeneous reactions of NO 2 on MgO as a mineral dust surrogate was investigated. The reactivity of MgO to NO 2 is weak, while coexisting SO 2 can increase the uptake coefficients of NO 2 on MgO by 2–3 orders of magnitude. The uptake coefficients of NO 2 on SO 2 -aged MgO are independent of NO 2 concentrations in the range of 20–160 ppbv and relative humidity (0–70%RH). The reaction mechanism was demonstrated to be a redox reaction between NO 2 and surface sulfite. In the presence of SO 2, NO 2 was 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 SO 2 on the heterogeneous conversion of NO 2 to HONO may have a significant contribution to the unknown sources of HONO observed in polluted areas (for example, in China).