New insights into the molecular characteristics-dependent light absorption variation of water-soluble organic matter in biomass burning smoke

Forest fire (mainly wood burning) and crop residue burning (mainly herb burning) are two ways to produce biomass burning smoke water-soluble organic matters (BBS-WSOMs), largely altering atmospheric light absorption. However, their molecular characteristics-dependent light absorption remains unknown. Hence, this study combined Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis with UV–Vis spectrum to deeply investigate the different molecules-dependent light absorption of wood- and herb-derived BBS-WSOMs from different burning temperatures. The results showed that at the tested burning temperatures (300 and 600 °C), biomass types showed a stronger influence on the light absorption at 200–550 nm than burning temperatures. Herb-derived BBS-WSOMs had a stronger light absorption than wood-derived BBS-WSOMs. This was because in herb-derived BBS-WSOMs, more conjugated diene structures or more CHO compounds of low molecular mass and high aromaticity were responsible for their light absorption at 200–300 nm, and more CHON compounds (mainly nitroaromatics compounds) were responsible for their light absorption at 365–550 nm. The CHO/CHOS compounds in wood-derived BBS-WSOMs and CHON compounds in herb-derived BBS-WSOMs were respectively responsible for their light absorption at 365–550 nm. Interestingly, O-containing groups played an increasingly important role in enhancing light absorption with the increasing wavelength at 220–275 nm, while highly aliphatic structure with low O content played an increasingly important role in enhancing light absorption with the increasing wavelength at 365–550 nm. This study is beneficial for deeply understanding the different molecules-dependent light absorption of BBS-WSOMs, having significant implications in atmospheric environment management and quality control.