Hydroxymethanesulfonate (HMS) Formation under Urban and Marine Atmosphere: role of aerosol ionic strength

Abstract. Hydroxymethanesulfonate (HMS) has emerged as a critical organosulfur species in ambient aerosols under polluted atmosphere, with limited observations conducted in pristine and marine environment. The impact of aerosol properties, particularly ionic strength (IS), on the formation of HMS remains uncertain, especially considering their significant variations that may occur under different atmospheric conditions. Here, HMS levels in a continental city (Nanjing, China) and, for the first time, in marine atmosphere (Yellow Sea and Bohai Sea) were quantified at 0.30±0.10 and 0.05±0.01 μg m^–3, respectively. Elevated HMS concentration was recorded during a Nanjing haze event resulted from enhanced HMS formation rates, which can be largely attributed to reduced IS levels on hazy days as the IS-dependent enhancement on HMS formation increased with decreasing IS within the continental IS range (6–20 mol kg^–1). This arises from the fact that elevated IS can boost HMS formation rate constants but also hinder the solubility of HMS precursor (SO₂) and its further dissociation. Consequently, the IS-dependent enhancement initially rose with increasing IS, peaking at 4 mol kg^–1 before declining. This also elucidated the negative correlation between HMS level and IS found in urban Nanjing (R=-0.62). Additionally, marine aerosols with lower IS levels (2.0–6.0 mol kg^–1) likely exhibited more pronounced enhancements on marine HMS formation. Furthermore, the study highlights the significant impact of ambient humidity on aerosol IS (R=-0.89), suggesting the integration of ionic strength into chemical models to enhance the representation of particulate sulfur chemistry, particularly in humid environments.