The Role of Dissolved Organic Matter Composition in Determining Photochemical Reactivity at the Molecular Level

Dissolved organic matter (DOM) composition influences its ability to form photochemically produced reactive intermediates (PPRI). While relationships have been established between bulk DOM properties and triplet DOM (³DOM) and singlet oxygen (¹O₂) quantum yields, contradictory evidence exists for hydroxyl radical (^•OH) and hydroxylating species. Furthermore, little is known about these relationships at the molecular level. We evaluated DOM composition and photochemical reactivity of water samples from a wastewater treatment plant and the St. Louis River in Minnesota and Wisconsin, U.S.A. Bulk characterization using ultraviolet-visible spectroscopy demonstrates that color and apparent size of DOM decrease downstream, while molecular composition analysis using Fourier-transform ion cyclotron resonance mass spectrometry reveals that saturation and chemodiversity is highest near Lake Superior. ³DOM quantum yield coefficients and ¹O₂ quantum yields increase downstream and correlate strongly with saturated formulas. Similar results are observed for carbon-normalized photodegradation rate constants of atorvastatin, carbamazepine, and venlafaxine, which react primarily with ³DOM and ¹O₂. In contrast, ^•OH quantum yields are lowest downstream and correlate with less saturated, more oxygenated DOM, suggesting that ³DOM is not its major precursor. Mixed relationships are observed for DEET, which reacts with multiple PPRI. Molecular-level compositional data reveal insights into the differing formation pathways of individual PPRI, but information about specific contaminants is needed to predict their photochemical fate.