Predicting Reactive Intermediate Quantum Yields from Dissolved Organic Matter Photolysis Using Optical Properties and...

The antioxidant capacity and formation of photochemically produced reactive intermediates (RI) was studied for water samples collected from the Florida Everglades with different spatial (marsh versus estuarine) and temporal (wet versus dry season) characteristics. Measured RI included triplet excited states of dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and the hydroxyl radical (^•OH). Single and multiple linear regression modeling were performed using a broad range of extrinsic (to predict RI formation rates, R_RI) and intrinsic (to predict RI quantum yields, Φ_RI) parameters. Multiple linear regression models consistently led to better predictions of R_RI and Φ_RI for our data set but poor prediction of Φ_RI for a previously published data set,1 probably because the predictors are intercorrelated (Pearson's r > 0.5). Single linear regression models were built with data compiled from previously published studies (n ≈ 120) in which E2:E3, S, and Φ_RI values were measured, which revealed a high degree of similarity between RI-optical property relationships across DOM samples of diverse sources. This study reveals that ^•OH formation is, in general, decoupled from ³DOM* and ¹O₂ formation, providing supporting evidence that ³DOM* is not a ^•OH precursor. Finally, Φ_RI for ¹O₂ and ³DOM* correlated negatively with antioxidant activity (a surrogate for electron donating capacity) for the collected samples, which is consistent with intramolecular oxidation of DOM moieties by ³DOM*.