Comparison of water-soluble and water-insoluble organic compositions attributing to different light absorption efficiency between residential coal and biomass burning emissions

Abstract. There are growing concerns about the climate impacts of absorbing organic carbon (also known as brown carbon, BrC) in the environment, yet its chemical composition and association with the light absorption capabilities remain poorly understood. This study characterized water-soluble and water-insoluble organic carbon (WSOC and WIOC) from residential solid fuel combustion at the molecular level and evaluated their quantitative relationship with mass absorption efficiency (MAE). The MAE values at λ = 365 nm from biomass burning were significantly higher than those from coal combustion (p < 0.05). Thousands of peaks were identified in the m/z range of 150–800, with the most intense ion peaks occurring between m/z 200–500 for WSOC and m/z 600–800 for WIOC, respectively. The CHO group predominated in the WSOC extract from biomass burning emissions, while sulfur-containing compounds (SOCs) including CHOS and CHONS were more intense in the WIOC extract, particularly from coal emissions. Emissions of the CHON group were positively correlated with the fuel nitrogen content (r = 0.936; p < 0.05), explaining their higher abundance in coal emissions compared to biomass. The SOC emissions were more predominant during flaming phases, as indicated by a positive correlation with modified combustion efficiency (MCE) (r = 0.750; p < 0.05). The unique formulas of coal combustion aerosols were in the lower H/C and O/C regions, with higher unsaturated compounds in the van Krevelen (VK) diagram. In the WIOC extract, coal combustion emissions contained significantly higher fractions of condensed aromatics (32 %–59 %) compared to only 4.3 %–9.7 % in biomass burning emissions. In contrast, the CHOS group in biomass burning emissions was characterized by larger condensed aromatic compound fractions than those in coal combustion. Moreover, the CHOS aromatic compound fractions were positively correlated with MAE values in both WSOC (r = 0.714; p < 0.05) and WIOC extracts (r = 0.929; p < 0.001), suggesting that these compounds significantly contributed to MAE variabilities across different fuels.