Measurement Report: Size-resolved secondary organic aerosol formation modulated by aerosol water uptake in wintertime haze

Abstract. This study investigated the potential effects of inorganics changes on aerosol water uptake and thus secondary organic aerosol (SOA) formation in wintertime haze, based on the size-resolved measurements of non-refractory fine particulate matter (NR-PM_2.5) in Xi’an, Northwest China. The composition of inorganic aerosol showed significant changes in winter 2018–2019 compared to winter 2013–2014, shifting from a sulfate-rich to a nitrate-rich profile. In particular, the fraction of sulfate and chloride decreased but nitrate increased in the entire size range, while ammonium mainly increased at larger particle sizes. These changes thus resulted in size-dependent evolution in water uptake. Increased water uptake was observed in most cases mainly associated with enhanced contributions of both nitrate and ammonium, with the highest increase ratio reaching 5–35 % at larger particle sizes and higher relative-humidity (RH). The non-negligible influence of chloride on aerosol water uptake was also emphasized. The random forest analysis coupled with a Shapley additive explanation algorithm (SHAP) further showed enhanced relative importance of aerosol water in impacting SOA formation. Aerosol water contributed to the SOA formation in most cases in winter 2018–2019, and the SHAP value increased as aerosol water increased, especially at larger particle sizes. This implies the majority of enhanced aerosol water uptake at larger particle sizes and high RH might facilitate the efficient aqueous-phase SOA formation. This study highlights the key role of aerosol water as a medium to link inorganics and organics in their multiphase processes. As challenges to further improve China's air quality remain and SOA plays an increasing role in haze pollution, these results provide an insight into the size-resolved evolution characteristics and offer a guidance for future control.