Ionic Strength Inhibits the Multiphase Reaction Rate between Dissolved SO₂ and HCHO in Aqueous Aerosols: Implications for Sources of Atmospheric Hydroxymethanesulfonate

Hydroxymethanesulfonate (HMS) is an important reservoir of particulate sulfur, especially during winter haze episodes. However, the kinetics and mechanisms of HMS formation in aqueous aerosols remain poorly understood. In this study, we investigated the kinetics of the multiphase reaction between sulfur dioxide (SO₂) and formaldehyde (HCHO) in aqueous ammonium nitrate aerosol particles buffered by gas-phase NH₃ at pH of 4.6 to 5.1 via well-controlled flow tube experiments. We found that high ionic strength inhibits the HMS formation rate in aqueous aerosol particles by up to a factor of 10 compared to that in dilute bulk solutions. Using the new kinetic parameters predicts a negligible formation rate of HMS in aerosol water relative to that in cloud/fog water for typical winter haze conditions in the North China Plain. Our results suggest that atmospheric models should consider the effects of ionic strength on the HMS formation rate in aqueous aerosol particles to better describe the HMS chemistry and evaluate its impact on air quality.