Insight Into the Molecular Photooxidation Evolution Mechanism of Brown Carbon Released From Residential Solid Fuel Combustion

Solid fuels were one of the important emission sources of brown carbon (BrC). This study investigates molecular composition of BrC derived from combustion of five types of residential solid fuels (i.e., firewood, corn cob, bituminous coal, anthracite, and biomass briquette) and their photooxidation evolution mechanisms using an oxidation flow reactor. Results show that nitrogen‐containing and sulfur‐containing organic compounds are the main components of molecular composition of methanol‐soluble organic compounds (MSOC) emitted from these fuels, with their intensity being 33%–43% and 23%–56%, respectively, depending on fuel type. The photooxidation processes mediated by NO3⋅ and ⋅ OH significantly altered the molecular composition and distribution of aged MSOC. The formation of typical nitrogen‐containing BrC chromophores (e.g., nitro‐aromatic compounds) also proves the role of NO3 ⋅ in the photooxidation reaction. However, the photo‐enhancement effect of NO3 ⋅ mediated photooxidation reaction could not offset the destruction of the molecular conjugation degree and aromaticity of BrC by ⋅ OH oxidation, resulting in a significant reduction in the light absorption capacity of BrC. The mass absorption coefficient at 365 nm of BrC aged for 12 days derived from the remaining fuels decreased by 47.0%–55.8% compared to that of fresh BrC, except for firewood, which only decreased by 5.3%. These findings on the molecular evolution and oxidation mechanism of BrC generated from solid fuels are useful in reducing uncertainties in climate change studies involving BrC aerosol. Plain Language Summary The atmospheric oxidation process and mechanism of brown carbon (BrC) derived from solid fuel combustion (e.g., biomass burning, coal combustion) are less known. To address this knowledge gap, fresh and aged BrC aerosols emitted from solid fuel were collected based on a potential aerosol mass‐oxidation flow reactor (PAM‐OFR) in this study with the aim of determine the molecular evolution and photooxidation mechanisms of BrC. The results show that the molecular composition of BrC was changed by ∙OH and NO3∙ mediated photooxidation. Although NO3∙ promotes the formation of nitroaromatic compounds, which has the effect of enhancing light absorption, the photooxidation reaction of ∙OH destroys the structure of BrC chromophore, resulting in the decrease of light absorption capacity of aging BrC. The results provide insights into the photooxidation evolution of BrC derived from sol