Overlapping Chromophores Overlook Coal Combustion Contribution to Brown Carbon by Emphasizing Biomass Burning and Secondary Formation

Brown carbon (BrC) stems from complex emission sources and atmospheric physicochemical processes, thus modulating its light-absorbing properties. This study conducted an in-depth investigation of BrC chromophores to reveal the impacts of coal combustion on its optical properties in northern coastal China. During the coal-fired heating period, the light-absorption coefficient (6.36 ± 2.37 Mm–1) and mass absorption efficiency (1.22 ± 0.52 m2g–1) of methanol-extracted BrC at 365 nm were about 1.5 times those during the nonheating period. Nontarget analysis identified 1339–1649 organic molecules, enabling BrC chromophore characterization at molecular levels. The detected relative intensity of nitrogen-containing compounds increased by 2 to 3 folds during the coal-fired heating period, contributing to more than one-third of the relative intensity of BrC molecules and bulk absorption. However, molecular signatures of potential BrC chromophores typically associated with biomass burning and secondary formation processes were significantly elevated during coal-fired heating, potentially leading to an underestimation of contributions from coal combustion emissions and an overestimation from secondary formation and biomass burning. Distinctively, 12 sulfur-containing compounds with considerable absorption were determined, of which ten unique compounds emerged during the heating period, contributing considerably to the bulk absorption (mean: 13.2%; maximum: 24.0%). Our results highlight the necessity of distinguishing coal combustion-derived BrC chromophores, particularly sulfur-containing tracers, from those of biomass burning to improve the source apportionment accuracy. This study underscores the need for refined molecular tracers to better assess the contribution of different emission sources to BrC.