Molecular probe measurements of photochemical production of one-electron reducing intermediates from chromophore dissolved organic matter: relation to O2- and H2O2 production

The nitroxide, 3-amino-2,2,5,5,-tetramethy-1-pyrrolydinyloxy (3ap), was employed to determine the initial rates of photoproduction of one-electron reductants (R_H) from SRFA and SRHA. We provide evidence that O₂ and 3ap compete for the same pools of photoproduced reductants and thus that R_H can provide reliable estimates of the initial rates of O₂^- production (R_O2-). Under identical irradiation conditions, the initial rates of H₂O₂ production (R_H2O2) were far lower than R_H, indicating that the ratio of R_O2-/ R_H2O2 is larger than most previous reports, approximately 6-10. These results thus imply that a substantial fraction of the O₂^- is lost through oxidative pathways rather than simply through dismutation to H₂O₂. Our results further suggest that 3ap may provide more reliable measurements of photochemically produced one-electron reductants of O₂ in aquatic environments.