Measurement report: Formation and brownness of aqueous secondary organic aerosol from the aged biomass-burning emissions in the Sichuan Basin, China

Abstract. Secondary organic aerosol (SOA), formed via complex chemical mechanisms, is a major contributor to atmospheric aerosol pollution and climate forcing worldwide. Aqueous-phase oxidation serves as an important pathway for SOA formation, with aqueous SOA (aqSOA) exhibiting light absorption across the ultraviolet–visible range. This study investigates the formation and absorption properties of aqSOA in the Sichuan Basin, China. Results indicate that aqSOA mainly originated from aged biomass-burning emissions through aqueous-phase processing rather than from gas-phase photochemical oxidation, particularly under high aerosol liquid water content conditions during pollution periods. Substantial enhancement of brown carbon absorption by SOA was observed between 370 and 660 nm (27.5 %–43.2 %). These findings highlight the significant contribution of aqSOA formation from aged biomass-burning emissions to the brown carbon budget and absorption, especially at night. The mean aerosol absorption Ångström exponents between 370 and 880 nm (AAE370–880) were 1.95, which is higher than the values reported for fresh and photochemically aged biomass-burning emissions. This study elucidates the formation and light-absorbing characteristics of aqSOA derived from aged biomass-burning emissions and highlights the important role of aqueous-phase reactions in aerosol pollution and radiative absorption.