Photoinduced formation of persistent free radicals, hydrogen radicals, and hydroxyl radicals from catechol on atmospheric particulate matter

Catechol is speculated to be a potential precursor of environmentally persistent free radicals (EPFRs) in the atmosphere. EPFRs absorbed on PM2.5 have attracted public attention because their toxicity is similar to cigarette smoke. In this study, we found that catechol could produce EPFRs, which were oxygen-centered phenoxy and semiquinone radicals. These free radical species had half-lives of up to 382 days. CaO, CuO, and Fe2O3 markedly promoted EPFR formation from catechol. The valence states of Cu and Fe changed during the photochemical reactions of catechol but no valence state changed for Ca. Alkaline nature of CaO is possibly the key for promoting the free radical formations through acid-base reactions with catechol. In addition to hydroxyl free radicals, hydrogen free radicals and superoxide anions formed from the photochemical reactions of catechol were first discovered. This is of concern because of the adverse effects of these free radicals on human health.