Hydroxyl radical (OH) formation during the photooxidation of anthracene and its oxidized derivatives

Polycyclic aromatic hydrocarbons (PAH) are a ubiquitous atmospheric pollutant, comprising between 2% and 27% of PM2.5 by mass. They undergo rapid photooxidation forming oxidized products (oxPAH), and humic-like substances (HULIS). HULIS has known climate-forcing potential, and oxPAH have been reported to be more toxic than their parent PAH. The hydroxyl radical (OH) is associated with the photolysis of PAH. Anthracene and a selection of oxPAH (1-naphthol, 2-naphthol, 1,4-naphthoquinone, 1,4-anthraquinone, and 9,10-anthraquinone) were exposed to simulated sunlight for 16 h and OH concentration was measured. Formation rates (ROH) were calculated for the first 30 min of the reaction. The ROH for 1,4-naphthoquinone was initially very high and declined logarithmically over the 30-min interval, while anthracene had a low initial ROH which then increased linearly over the 30-min interval. It was determined that the production of OH is not a sole predictor of HULIS formation: photolysis of both 1-naphthol and 2-naphthol result in HULIS formation, but 2-naphthol does not result in a substantial OH concentration. Conversely, neither 1,4-anthraquinone or 9,10-anthraquinone result in HULIS, but OH was seen with 9,10- anthraquinone photolysis. This suggests that, while OH is important, pathways resulting in HULIS from PAH which do not include OH are also significant.