The photochemistry of quinones and combustion-derived particles in forming hydroxyl radicals and singlet oxygen in the atmosphere

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are byproducts of combustion processes and can undergo photooxidation in cloud water upon being emitted into the atmosphere. Quinones are among the products formed from the photochemical aging of PAHs and can undergo subsequent reactions to form environmentally persistent free radicals (EPFRs) and reactive oxygen species (ROS). This study showed that 9,10-anthraquinone (9,10-ANTQ), 1,4-anthraquinone (1,4-ANTQ), and 1,4-naphthoquinone (1,4-NAPQ) undergo different pathways when forming ROS. Singlet oxygen (¹O₂), which was monitored during the photoaging of these quinones, formed logarithmically over time in low yields (0.5 ± 0.1 μM M⁻¹ to 1.6 ± 0.1 μM M⁻¹) with 9,10-ANTQ and 1,4-ANTQ. However, with 1,4-NAPQ, it formed rapidly within the first 5 min, reaching high yields of 21 ± 7 μM M⁻¹, before undergoing decay. This work suggests that 9,10-ANTQ can behave as a photocatalyst to continuously generate hydroxyl radicals (^•OH) in the atmosphere; however, it will more likely exist in its triplet state (³9,10-ANTQ∗) as an EPFR. Contrarily, 1,4-ANTQ and 1,4-NAPQ will photodegrade into other products that generate ROS. Among these products is juglone, which was shown to generate ^•OH exponentially upon photoaging as well as degrade into other products. The varying photochemical behaviors between these different quinones help explain the overall photochemistry of combustion-derived particles, which catalytically generated ¹O₂ and ^•OH in this study. This work provides insight into the environmental fates of combustion-derived material and their oxidized derivatives and shows the potential of these compounds to generate excess atmospheric ROS.