Brown carbon (BrC) has the potential to play a significant role in the coupling of atmospheric chemistry and climate, but its impact on the Earth’s radiation budget remains uncertain due to its chemical complexity. Nighttime atmospheric chemistry is a critical contributor to the formation of secondary BrC, yet the associated process-level understanding is not well-constrained. In this work, we simulated BrC formation from the nighttime chemistry of heterocyclic volatile organic compounds (VOCs), a group of reactive VOC precursors from biomass burning and atmospheric secondary processing, in a 10 m<sup>3</sup> Teflon-filmed chamber. The chemical composition and optical properties of the generated BrC were characterized by a suite of complementary online and offline analyses. Our results indicated that light-absorbing anhydrides and their structural analogues can react with protic solvents like methanol, while aprotic solvents like acetonitrile (ACN) would be a more ideal solvent to prevent such solvent artifacts. Moreover, furan-derived secondary BrC formation under different environmental conditions, including nitrate radical levels and pre-existing particles, was studied by online measurements coupled with ACN-based offline analysis. Our results showed that the mass absorption coefficient (MAC) profiles within 290-700 nm can be upraised by the increased nitrate radical levels but depressed in the presence of pre-existing particles. Multi-instrumental analysis revealed that higher nitrate radical levels can promote the generation of light-absorbing products, while pre-existing particles may facilitate the partitioning of non-absorbing organics into the particle phase. Furthermore, a variety of carbonyl chromophores were characterized in BrC samples from all the heterocyclic systems. The contributions of carbonyl chromophores to SOA mass and total light absorption of ACN-extracted BrC samples were estimated, revealing divergent effects under different nitrate radical levels. In summary, secondary BrC from nighttime oxidation of heterocyclic VOCs exhibits complicated physicochemical properties due to a variety of chemical composition, especially chromophores, which can be affected by the change of environmental conditions. More research is needed to constrain the radiative effects of secondary BrC in the context of a changing climate.

Chemical composition and optical properties of secondary brown carbon from nighttime oxidation of heterocyclic volatile organic compounds

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