Salting-in of HCHO Sustains Fast Hydroxymethanesulfonate Formation in Deliquesced Sulfate Aerosols

The multiphase formation of hydroxymethanesulfonate (HMS) in aerosol water plays a crucial role in sulfur chemistry during haze events, yet the impact of aerosol composition on the kinetics remains poorly constrained. This study directly measured the HMS formation rate from the reaction between sulfur dioxide (SO2) and formaldehyde (HCHO) in aqueous ammonium sulfate and mixed ammonium nitrate and ammonium sulfate aerosols at ionic strengths of ∼4 to 20 mol kg–1. We demonstrate that sulfate aerosols enhance the HMS formation rate due to a “salting-in” effect of HCHO, offsetting the suppression of the reaction rate constant by high ionic strength. A kinetic model is developed to describe the HMS formation in mixed ammonium nitrate and ammonium sulfate aerosols and applied to winter haze conditions in Beijing and Fairbanks. The results reveal that shifts in the nitrate-to-sulfate molar ratio from 4:1 to 1:4 can increase the HMS formation rate by a factor of 4. This study highlights the necessity of incorporating aerosol composition-dependent kinetics into atmospheric models to accurately represent the HMS chemistry across diverse environments.