Comparison of water-soluble and 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, however, the chemical composition and association with the light absorption ability of BrC remain poorly understood. In this study, focusing on one major source of BrC, water-soluble and water-insoluble organic carbon (WSOC; WISOC) from residential solid fuel combustions were characterized at the molecular level, and evaluated for their quantitative relationship with mass absorption efficiency (MAE). The MAE values at λ=365 nm from biomass burning were significantly higher than the coal combustion smokes. Thousands of peaks were identified in the m/z range of 150–800, with the most intense ion peaks of 200–500 m/z for WSOC and 600–800 m/z for WISOC, respectively. CHO group was the most abundant component in the WSOC extracts from biomass burning emissions compared to coals; while sulfur-containing compounds (CHOS+CHONS, SOCs) were more intense in the WISOC extracts, especially in coal emissions. Emissions of the CHON group were positively correlated with the fuel N content (r=0.936, p<0.05), which explained higher CHON emissions in coal emissions compared to biomass burning emissions. The SOCs emissions were more predominant in flaming phases, as seen from a positive correlation between SOCs and 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 WISOC extracts, coal combustion emissions had significantly high fractions of condensed aromatics (32–59 %) which was only 4.3–9.7 % in biomass burning emissions. The CHOS group in biomass burning emissions was characterized by larger condensed aromatic compound fractions compared to coal combustion. The CHOS aromatic compounds fractions were positively correlated with MAE values, in both WSOC (r=0.714, p<0.05) and WISOC extracts (r=0.929, p<0.001), suggesting the abundance of CHOS aromatic compounds explained MAE variabilities across the different fuels.