Revisiting Multiphase Reactions in Deliquesced Aerosol Particles in the Presence of NO ₂ and SO ₂

Nitrogen dioxide (NO 2 ) is a crucial precursor to nitrate via heterogeneous hydrolysis. It is a key oxidant in multiphase sulfur dioxide (SO 2 ) oxidation, which is recognized as the primary source of sulfate production during Beijing haze episodes. However, the multiphase reaction of NO 2 and SO 2 in deliquesced aerosol particles remains elusive due to competing pathways. Here, we examine the kinetics of multiphase reactions in deliquesced aerosol particles in the presence of NO 2 and SO 2 . Results show pH-dependent competition between nitrate and sulfate production. We find enhanced sulfate production in air compared with N 2, indicating an unaccounted-for reaction pathway involving the NO 2 + SO 2 reaction to form sulfite radical anions, which then react with molecular oxygen, rather than with additional NO 2, to initiate the radical chain reaction for sulfate production. As a result, the present pathway yields sulfate production ∼1 order of magnitude higher than the conventional NO 2 pathway. Kinetic calculations predict that the updated NO 2 pathway shows the highest sulfate production rate among the prevailing oxidation pathways involving ozone, transition-metal ions, and hydrogen peroxide at pH > 4.5 under the haze conditions.