Existence of hydroxymethanesulfonate (HMS) during spring haze and sandstorm events in Beijing: Implications for a heterogeneous formation pathway on mineral aerosols

Abstract

Hydroxymethylsulfonate (HMS) is an abundant secondary organic aerosol from aqueous or heterogeneous processes and may be misidentified as sulfate in conventional measurements. High concentrations of HMS have been observed in humid winter and autumn haze in northern China, while its prevalence in other seasons is unclear and the production medium is controversial. In this study, our field measurements in Beijing during the 2021 spring first showed the presence of HMS in PM_2.5 during both haze and sandstorm events despite the different atmospheric conditions. HMS accounted for 0.44% of PM_2.5 during haze periods, higher than the proportion (0.097%) during sandstorms. The sum of HMS and sulfate was also higher during the haze (6.5 μg m⁻³) than during the sandstorm (2.6 μg m⁻³), while the HMS/sulfate molar ratio during the haze (0.021) was similar to the value during the sandstorm (0.019). HMS concentration showed a good positive correlation with aerosol water content (AWC), indicating multiphase production. During haze periods, relatively high AWC favored the formation of HMS. In contrast, relative humidity and AWC decreased significantly during sandstorm events, while high pH favored HMS formation. In addition, higher concentrations and proportions of HMS were observed in PM_2.5−10 than in PM_2.5. The presence of HMS in dust particles indicated a heterogeneous formation mechanism of HMS on mineral aerosols. Our findings broaden the prevalence of HMS in aerosols and indicate a new HMS formation mechanism from the perspective of observation.