Light absorption and molecular composition of brown carbon in Nanjing, China: Large contribution of biomass burning and secondary formation.

Brown carbon (BrC) impacts global climate through solar radiation absorption, yet its optical effects (especially those of water-insoluble fractions) and molecular links between chromophores and sources remain poorly constrained, hindering climate assessments. To address these gaps, this study systematically investigated BrC's optical properties, molecular compositions (focusing on nitroaromatic compounds, NACs) and sources in Nanjing, East China, across four seasons (November 2014-July 2015), by comparing water-soluble (WS-BrC) and methanol-soluble BrC (MS-BrC, total BrC proxy). MS-BrC exhibited 2.1-fold higher average light absorption (11.9 ± 7.7 Mm^-1) than WS-BrC (5.8 ± 3.8 Mm^-1), with its direct radiative effect relative to black carbon (10.5 ± 3.9 %) being 2.5 times that of WS-BrC (4.8 ± 2.6 %), highlighting the long-overlooked climate significance of water-insoluble BrC. Nine NAC species (9.1 ± 7.9 ng m^-3) contributed 0.33 ± 0.14 % to BrC absorption-2.4-17 times their mass contribution, with distinct seasonal shifts: nitrocatechols (NCs) dominated winter/autumn, whereas nitrosalicylic acids (NSAs) prevailed spring/summer. A NAC-constrained Positive Matrix Factorization (PMF) model identified distinct seasonal BrC sources. For MS-BrC, dominant sources were multiphase chemical processes (27.0 %) and biomass burning (25.0 %); for WS-BrC, biomass burning (28.4 %), and photochemical processes (25.2 %) prevailed. Combined secondary formation contributed ∼47 % to BrC light absorption, higher than in northwest China. During aerosol pollution, BrC light absorption and radiative effects intensified, with biomass burning contributions rising from 21.6 % to 36.6 %. These findings advance understanding of water-insoluble BrC's climate role, refine BrC source apportionment via molecular tracers, and offer a robust basis for climate models and targeted air pollution control.