Comparison of Residential Coal Combustion Brown Carbon Generated during the Initial and Stable Stages: Insights into Their Molecular Composition, Optical Properties, and Oxidative Potential

The molecular composition, optical properties, and oxidative potential of methanol-soluble organic carbon (MSOC, i.e., brown carbon) generated during different stages (initial vs stable) of residential coal combustion were systematically investigated. The findings demonstrated notable disparities in MSOC, characterized by relatively higher abundance of unsaturated aromatic substances produced in the initial combustion stage, whereas more highly oxygenated polar and S-containing compounds were produced in the stable combustion stage for bituminous coals. Additionally, results indicated that MSOC generated in the initial combustion stage exhibited stronger light-absorbing capacity than that generated in the stable combustion stage, which was positively correlated with aromaticity. Spearman's rank correlations were utilized to screen 507 potential light-absorbing molecules, predominantly comprising CHO and CHON compounds with aromatic and condensed aromatic structures. The oxidative potential of MSOC generated in the stable combustion stage was higher than that generated in the initial combustion stage, which may be attributed to its abundance of S-containing compounds. In summary, the initial stage of residential coal combustion releases a substantial quantity of light-absorbing pollutants, contributing to light absorption by atmospheric aerosols, while the high-temperature stable combustion stage releases large quantities of S-containing compounds, posing health risks.