Interface-dominated hydroxymethanesulfonate and its isomer formation provides key mechanisms for reconciling the atmospheric sulfur budget gap in polluted and cold environments

Abstract. Hydroxymethanesulfonate (HMS) is a critical source of particulate sulfur, formed by formaldehyde (HCHO) and sulfur dioxide (SO₂) in droplets. Current models relying on bulk aqueous-phase HMS formation only explain ~one-third of unexplained sulfate concentrations, leaving gaps in atmospheric sulfur budget, especially in polluted and cold environments. Using Born–Oppenheimer molecular dynamics simulations, we explored HMS and its isomer hydroxymethyl sulfite (HMSi) formation mechanisms across aqueous phase and air–water/ice interfaces. Air–water interfaces enable nearly barrierless HMS formation (0.6 kcal mol^-1) via unique stepwise water-mediated proton transfer, preferring HMS over HMSi (0.6 vs. 6.1 kcal mol^-1), which contrasts sharply with the competitive pathways observed in the bulk aqueous phase (7.7 vs. 7.6 kcal mol⁻¹). In contrast, protonation of formaldehyde under strongly acidic conditions reverses reaction selectivity, favoring HMSi formation over HMS. Importantly, these reaction mechanisms remain viable at air–ice interfaces in cold environments including polar areas and the upper troposphere, revealing ice surfaces as previously overlooked yet significant sites for atmospheric organosulfate formation. Our findings establish that interfacial mechanisms dominate HMS and HMSi formation in both polluted and cold environments, helping to reconcile model-observation discrepancies in the atmospheric sulfur budget.