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 (R-OH) were calculated for the first 30 min of the reaction. The R-OH for 1,4-naphthoquinone was initially very high and declined logarithmically over the 30-min interval, while anthracene had a low initial R-OH 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.

Automated summary

The formation of humic-like substances (HULIS) from the photolysis of polycyclic aromatic hydrocarbons (PAH) is influenced by the production of hydroxyl radicals (OH). Although OH is an important factor for HULIS formation, pathways that do not involve OH can also contribute to the formation of HULIS from PAH.