Enhanced NO2-driven multiphase formation of particulate nitrate and sulfate under high-humidity conditions

Abstract Fast atmospheric particulate nitrate and sulfate formation under high-humidity conditions has been extensively observed; however, the underlying chemical mechanisms and their relative contributions remain poorly understood. This study examined the characteristic high-humidity events (HHEs) in southern China during spring, providing field observation evidence for the crucial role of NO 2 -driven multiphase reactions in particulate nitrate and sulfate formation. Our findings revealed efficient nitrate formation during early HHEs, likely facilitated by enhanced NO 2 uptake via disproportionation reaction. As humidity increased and fog formed, S(IV) oxidation competitively consumed NO 2 and N(III), causing rapid sulfate formation. The resulting N(III), produced from the oxidation of S(IV) by NO 2 (aq), further oxidizes S(IV) effectively in droplets due to its slow liquid-gas mass transfer rate. A state-of-the-art multiphase box model demonstrated that NO 2 uptake and SO 2 oxidation by NO 2 /N(III) represent dominant formation pathways during HHEs, accounting for 45.4% and 63.6% of the total nitrate and sulfate production, respectively. These results highlight the critical importance of NO 2 -driven multiphase chemistry in particulate pollution under high-humidity environments.