Aerosol Water Content Drives Changes in Hydroxymethanesulfonate in Beijing Winter from 2015 to 2021.

Hydroxymethanesulfonate (HMS) is an abundant secondary organosulfur aerosol during winter in the North China Plain, while the variation and affecting factors of HMS with implementing clean air actions remain unclear. Here, we conducted long-term field measurements in Beijing during the winter, including Phase I (2015-2017) and Phase II (2018-2021). Our results showed that HMS concentrations decreased significantly in Phase I, consistent with changes in PM2.5, organic matter (OM), and sulfate, and showed an increasing trend in Phase II in contrast to the decreasing trend of PM2.5, OM, and sulfate. In addition, HMS contributions to PM2.5, OM, and particulate sulfur increased significantly during Phase II. HMS contributed up to 6% of PM2.5 and 24% of OM during heavy pollution periods in 2019. Despite the continuous reduction in gaseous precursors, the changes in aerosol water content driven by relative humidity and total chemical composition concentrations were the main factors influencing the interannual changes in HMS concentrations from 2015 to 2021 winter. Our findings highlight the importance of HMS during clean air actions and indicate that adverse meteorological conditions may somewhat offset the impact of precursor emission reductions on secondary aerosol formation.