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 was the major contributor to atmospheric aerosol pollution and climate forcing worldwide. The aqueous-phase oxidation was an important pathway for SOA formation and the aqueous SOA (aqSOA) exhibited absorption properties across ultraviolet to visible range. Here, we reported the direct ambient observation of SOA formation and absorption properties in the aqueous phase from the Sichuan Basin, China. Considerable aqSOA was originated from the aged biomass-burning emissions via aqueous-phase reactions instead of photo-chemical reactions under high aerosol liquid water content (ALWC) conditions, especially during the polluted period. The substantial impact on brown carbon (BrC) absorption from SOA was observed from 370 nm to 660 nm (27.5 %−43.2 %). This study highlighted the significant contribution of aqSOA formation from aged biomass-burning emissions to the BrC budget and absorption, especially at night. The mean aerosol absorption Ångström exponents from 370 nm to 880 nm (AAE_370−880) was 1.95, higher than that observed in fresh and photo-chemically aged biomass-burning emissions. This study revealed the aqSOA formation and brownness from aged biomass-burning emissions and highlighted the importance of aqueous-phase reactions on air quality and climate.