Insight into the molecular photooxidation evolution mechanism of brown carbon released from residential solid fuel combustion

Abstract

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% to 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.