Enhanced light absorption by ambient brown carbon aerosols in the eastern Himalayas

Abstract

This study investigates the light absorption properties of organic aerosols in PM₁₀ collected at a high-altitude location (2700 m a.s.l.) in the eastern Himalayas from March 2019 to February 2020. The analysis reveals an enhanced light-absorbing signature of methanol-soluble brown carbon (MeS-BrC) extracts compared to water-soluble brown carbon (WS-BrC) within the optical wavelength range of 300–700 nm. MeS-BrC exhibits approximately twice the absorption compared to that of WS-BrC at 365 nm. The highest light absorption coefficients at 365 nm (b_abs365) are observed during spring for both MeS-BrC (9 ± 4.6 Mm⁻¹) and WS-BrC (5.9 ± 4.2 Mm⁻¹). Notably, the contribution of absorption from the water-insoluble fraction is relatively higher during the summer monsoon (45.2 ± 19.5%) and autumn (44.1 ± 18.4%). A significant linear relationship between WSOC and WS-BrC as well as OC and MeS-BrC at 365 nm suggests similar sources for BrC and WSOC (OC). Furthermore, significant positive correlations of b_abs365 (WS-BrC and MeS-BrC) with the water-soluble fraction of total nitrogen (WSTN) and organic nitrogen (WSON) indicate the presence of nitrogenous organic chromophores playing a crucial role in BrC absorption during spring and autumn. The mass absorption efficiency at 365 nm (MAE₃₆₅) reveals that BrC in spring aerosols (WS-BrC: 1.5 ± 0.6 m² g⁻¹; MeS-BrC: 2.07 ± 0.8 m² g⁻¹) absorbs UV-visible light more efficiently compared to aerosols collected during other seasons. The enhanced MAE₃₆₅ during spring resulted the highest simple forcing efficiency of 8.7 ± 3.9 W g⁻¹ and 10.8 ± 5.2 W g⁻¹ for WS-BrC and MeS-BrC, respectively, for a specific solar geometry and surface properties. This may be attributed to intense biomass burning followed by atmospheric processing of organic aerosols in the aqueous phase. These findings confirm the significant role of anthropogenic sources in enhancing BrC light absorption and radiative effects in this highly sensitive region of the eastern Himalayas. Such insights are crucial for devising effective strategies for mitigating climate change impacts in the Himalayan ecosystem.