Water-soluble brown carbon in PM_2.5 at two typical sites in Guanzhong Basin: Optical properties, sources, and implications

Brown carbon (BrC) is a light-absorbing aerosol component that has a significant impact on atmospheric photochemistry and climate effects. Many studies on light absorbing characteristics of BrC (such as a fraction of water-soluble and/or water-insoluble) have been carried out in cities over the Guanzhong Basin, including radiative forcing, optical properties and sources. However, research on the Qinling Mountains is still lacking. Therefore, PM_2.5 samples were collected at the northern piedmont of Qinling Mountains (QL) and Xi'an (XN) in the winter of 2020, and the optical properties and radiation effects of water extracts were analyzed and evaluated. The mass absorption efficiency (MAE) of water-soluble organic carbon (WSOC) at 365 nm (MAE₃₆₅) obtained in QL and XN were 0.18 ± 0.03 m² g⁻¹ and 0.78 ± 0.96 m² g⁻¹, respectively. In the ultraviolet range, the relative light absorption of WSOC relative to elemental carbon (EC) was 6.76% and 33.41% in QL and XN, respectively, and the simple forcing efficiency (SFE_280–400) were 0.71 ± 0.43 and 2.82 ± 1.71 W g⁻¹ in QL and XN. It may have important effects on the radiation balance of regional climate systems. The chromophores in WSOC of XN and QL are mainly composed of humic-like and protein-like substances, and humus-like substances play a dominant role in two sites (52.61% and 71.13%). Biomass combustion has a limited contribution to chromophore abundance in WSOC of QL, which is more affected by urban transmission. The fluorescence index revealed that the chromophores in WSOC had autogenous characteristics and that the organic matter was mostly newly generated. Furthermore, the molecular weight and aromatic degree in XN samples were higher than that in QL, indicating a greater capacity for light absorption. This work will be instrumental in assessing the interaction and influence between the city and the northern piedmont of the Qinling Mountains and improve the capability of air pollution prevention and control of Guanzhong Basin.