•OH Photochemistry Aging Reduces the Light Absorption of Imidazoles in Solid Fuel Combustion Emissions and Enhances Their Health Risks

Solid fuel combustion is an important source of nitrogen-containing brown carbon (BrC) and secondary aerosol precursors. However, knowledge of the emission of imidazoles (IMs) in nitrogen-containing BrC and their transformation remains limited. This study investigated the emissions of IMs from 11 solid fuel combustions and subsequent photochemical evolution. The reaction pathways and light absorption of IMs were estimated through quantum chemical calculations. The potential toxicity of IMs for aging products was predicted with machine learning. The results showed that emission factors (EFs) of IMs were higher from biomass burning (BB) than from coal combustion (CC). Maize stalks exhibited the highest ∑IMs EFs, while anthracite coal showed the lowest ones. Alkyl IMs (Alk-IMs) and benzene-ring IMs (Bn-IMs) dominated the ∑IMs in BB (85%) and CC (52%). During aging, Alk-IMs underwent •OH substitution reactions, forming poorly oxygenated hydroxyimidazole derivatives (O/N = 0.5). Conversely, based on density functional theory calculations, •OH oxidation of Bn-IMs and carboxyl IMs occurred more easily, forming highly oxygenated imidazole derivatives (O/N = 1–2) with carboxyl groups. The light absorption of these highly oxygenated derivatives decreased by 17–28%, while their health risk probability increased by 1.12–2.27 times compared to those of their precursors. These findings improved our understanding of the residence time and interaction with other pollutants of IMs.