Field evidence of brown carbon absorption enhancement linked to organic nitrogen formation in Indo-Gangetic Plain

Atmospheric brown carbon (BrC), a short-lived climate forcer, absorbs solar radiation and is a substantial contributor to warming of Earth’s atmosphere. BrC composition, absorption properties, and their evolution are poorly represented in climate models, especially for aqueous media (e.g., fog, clouds). Aqueous phenomena such as clouds and fog are quite prevalent during wintertime in Indo-Gangetic Plain (IGP). In this study, we examined the evolution of water-soluble BrC (WS-BrC) mass absorption efficiency at 365 nm (MAEWS-BrC-365) and chemical characteristics, viz., low-volatile fraction of organic carbon and water-soluble organic nitrogen (WSON) to water-soluble organic carbon (WSOC) ratio (org-N/C) on diurnal scale and under varying aerosol liquid water content (ALWC) and relative humidity (RH) conditions during wintertime in central IGP. Diurnally, highest MAEWS-BrC-365 was observed during late night and morning time and was coincided with enhanced WSON formation and relative abundance of low-volatile organics. The lower MAEWS-BrC-365 during the afternoon time was due to photochemical aging; whereas fresh WS-BrC emissions due to increased biomass burning and vehicular emissions activities caused lower MAEWS-BrC-365 values during evening and early night time. Further, we observed that MAEWS-BrC-365 and org-N/C ratio increased with decrease in ALWC and RH, signifying that drying of bulk aerosol particle and aqueous droplets would be potentially accelerating the formation of nitrogen containing organic chromophores resulting in enhancement in WS-BrC absorptivity. We also obtained that direct radiative forcing of WS-BrC relative to that of EC was ~19% during wintertime at Kanpur and ~40% of this contribution was in UV-region. These findings highlight the importance of incorporating this atmospheric phenomenon in BrC framework for fog and clouds in global climate models.