Optical properties, chemical functional group, and oxidative activity of different polarity levels of water-soluble organic matter in PM2.5 from biomass and coal combustion in rural areas in Northwest China

Biomass and coal combustion are major sources of water-soluble organic matter (WSOM) in PM₂.₅. This study investigated the carbon content, light absorption capacity, molecular structures, and oxidative activity of three types of WSOM (neutral humic-like substances (HULIS-n), acidic HULIS (HULIS-a), and high-polarity WSOM (HP-WSOM)) produced from biomass and coal combustion in Northwest China. The carbon emission factors of HULIS-n, HULIS-a, and HP-WSOM were 5.52 ± 4.29, 3.82 ± 2.83, and 3.24 ± 2.85 g/kg from biomass burning, and 1.77 ± 0.65, 4.41 ± 1.24, and 3.94 ± 1.78 g/kg from coal combustion, respectively. HULIS-n had the highest light absorption coefficient at 365 nm (bₐbₛ₋₃₆₅) and the mass absorption efficiency at 365 nm (MAE₃₆₅) both for biomass and coal burning, suggesting that HULIS-n contains more chromophores than the others. The Fourier transform infrared spectra results revealed that aromatic substances and polysaccharides were predominated in HULIS-n and HP-WSOM. Reactive oxygen species (ROS) in coal combustion followed a decreasing order of HULIS-n > HP-WSOM > HULIS-a. In terms of WSOM from biomass combustion, the highest ROS was in HULIS-n for corn straw and wheat straw in fire kangs but the highest ROS was in HP-WSOM for pepper rod and sesame rod in hanging stoves as well as wood block fire. HULIS-a produced by HS of biomass burning had higher ROS activity, and oxidative activity of HULIS-n produced by burning biomass by HB and SS did not differ greatly. High ROS and strong light absorption of HULIS-n in both biomass burning and coal burning may attribute to its molecular structures such as abundance of R–ONO₂ organic nitrate. Our results also highlighted that the carbon content of HULIS-n occupied only 17.5% to HULIS + HP-WSOM in coal combustion, whereas HULIS-n contributed up to 58.3% and 64.8% of the total light absorption and ROS. Overall, this study revealed the complex emission properties of WSOM, which would improve the assessment of their environmental and climate impacts at a regional scale.