The overlooked formation of environmentally persistent free radicals on particulate matter collected from biomass burning under light irradiation

Abstract

BackgroundThe illumination process may be an important contributor to environmentally persistent free radicals (EPFRs) in atmospheric particles, but the ability of light to generate EPFRs in combustion products remains unclear. ObjectiveThis paper studies the characteristics and formation mechanism of EPFRs in combustion particles after photoexcitation. MethodThe secondary photochemical processes and the generation and decay capability of EPFRs in size-resolved (<10 µm) biomass combustion particles were analysed by electron paramagnetic resonance (EPR) spectroscopy. ResultOur results indicated that secondary EPFRs can be generated after illumination and the produced EPFRs have a lifetime of approximately 1 day. The content of secondary EPFRs after light exposure increased by 20 %–30 % compared to that of the original EPFRs. Through the analysis of components of different polarities, it was found that non-extractable substances were the main contributors to secondary EPFRs (75 %), followed by extractable organics. This study showed that metal species and quinones are important precursors for the formation of secondary EPFRs from non-extractable and extractable PM components, respectively. We found that O2 molecules are an important factor for the formation of secondary EPFRs from organic substances without oxygen functional groups. ConclusionsThis study presents information about the effects of light and O2 on the generation of EPFRs, and the unstable nature of secondary EPFRs has important implications for assessing the health risks of atmospheric particles.