Reaction between a NO2 Dimer and Dissolved SO2: A New Mechanism for ONSO3- Formation and its Fate in Aerosol.

Abstract

Experimental observations indicate that sulfate formation in aerosol is sensitive to the concentrations of nitric oxide (NO2). While it also widely exists as a dimer in the gas phase, previous studies focus on the monomer of NO2. In this study, we employ quantum chemical calculations and ab initio molecular dynamics simulations to investigate the reaction between the NO2 dimer (ONONO2) and sulfite (HSO3-/SO32-) in the gas phase and in an aerosol. Gas-phase reactions turn out to be barrierless. In an aerosol, the reaction between adsorbed ONONO2 and HSO3- to form ONSO3- follows a stepwise mechanism with proton and electron transfer processes. The reaction between ONONO2 and SO32- is more straightforward. Nevertheless, both reactions occur at a picosecond time scale. Decomposition of ONSO3- can form an NO molecule and SO3-, which gives a complementary pathway for sulfate formation in an aerosol. Hydrolysis of ONSO3- to form HNO and HSO4- is highly impossible in an aerosol, which calls for a revisit of the atmospheric N2O formation mechanism. The results presented in this study deepen our understanding of the interaction between NO2 and SO2 pollutants in the atmosphere.