Hydroxymethanesulfonate (HMS) formation in urban and marine atmospheres: role of aerosol ionic strength

Abstract

Hydroxymethanesulfonate (HMS) has emerged as a critical organosulfur species in ambient aerosols, yet the impact of aerosol properties, particularly ionic strength (IS), on the formation of HMS remains uncertain. Here, HMS levels in wintertime in urban Nanjing, China, were quantified at 0.30 ± 0.10 µg m−3, where the contribution of in-cloud formation probably carries minor significance, due to the barrier resulting from stable stratification. Elevated HMS concentrations were recorded during a Nanjing haze event, resulting from enhanced HMS formation rates, which can be largely attributed to reduced ISs on hazy days, as IS-dependent enhancement of 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. Additionally, for the first time, particulate HMS levels in marine atmospheres (Yellow Sea and Bohai Sea) were quantified, at 0.05 ± 0.01 µg m−3. Lower ISs (2.0–6.0 mol kg−1) observed for marine aerosols exhibited more pronounced enhancements in HMS formation; this can render the aerosol-phase HMS formation a process comparable to that in cloud/fog droplets in marine environments. Furthermore, the study highlights the significant impact of ambient humidity on aerosol IS (R = −0.89), suggesting the integration of ionic strength in chemical models to better represent particulate sulfur chemistry, particularly in humid environments.