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

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 ((DOM)-D-3*), singlet oxygen (102), and the hydroxyl radical ((OH)-O-center dot). 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, Phi(RI)) parameters. Multiple linear regression models consistently led to better predictions of R-RI and Phi(RI) for our data set but poor prediction of Phi(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 approximate to 120) in which E2:E3, S, and Phi(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)-O-center dot formation is, in general, decoupled from (DOM)-D-3* and O-1(2) formation, providing supporting evidence that (DOM)-D-3* is not a (OH)-O-center dot precursor. Finally, Phi(RI) for O-1(2) and (DOM)-D-3* 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 (3) DOM*.