Sources and light absorption of brown carbon in urban areas of the Sichuan Basin, China: Contribution from biomass burning and secondary formation

Abstract

The optical properties of brown carbon (BrC) and their correlation with chemical characteristics remained inadequately understood in different regions worldwide. This study investigated the correlations and estimated the subsequent radiative effects using real-time measurements during wintertime in the Sichuan Basin, China. The average light absorption of BrC (Abs_BrC) at 370 nm constituted 35.5 ± 8.2 % of total absorption, significantly higher than those at 470 nm (20.9 ± 4.3 %), 590 nm (14.0 ± 2.5 %), and 660 nm (7.7 ± 2.1 %) (p < 0.001). The contributions of various organic aerosol (OA) sources to Abs_BrC varied by wavelength, with biomass-burning OA (BBOA) and semi-volatile oxygenated OA (SVOOA) exhibiting the higher Abs (14.4 Mm⁻¹ and 13.5 Mm⁻¹), absorption Ångström exponents (AAE) (4.81 and 4.35), and contributions to Abs_BrC (24.4 % and 22.8 %). Additionally, secondary BrC likely formed from BBOA through aqueous-phase reactions during winter. The transport of BBOA and SVOOA from northern regions (i.e., Guang'an in Sichuan and Hechuan in Chongqing) significantly contributed to elevated Abs_370,BrC levels. The mean simple forcing efficiency for BrC (SFE_BrC) was 60.5 W g⁻¹, accounting for 14 % of SFE_BC in the 370–880 nm range during winter. Overall, this study enhanced the understanding of Abs_BrC and its evolution with sources, providing a more accurate assessment of its radiative effects, and emphasized the importance of biomass burning emissions.