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Superoxide (O2• –) is produced photochemically in natural waters by chromophoric dissolved organic matter (CDOM) via the reaction of molecular oxygen with photoproduced one-electron reductants (OERs) within CDOM. In the absence of other sinks (metals or organic radicals), O2• – is believed to undergo primarily dismutation to produce hydrogen peroxide (H2O2). However, past studies have implicated the presence of an additional light-dependent sink of O2• – that does not lead to H2O2 production. Here, we provide direct evidence of this sink through O2• – injection experiments. During irradiations, spikes of O2• – are consumed to a greater extent (∼85–30% loss) and are lost much faster (up to ∼0.09 s–1) than spikes introduced post-irradiation (∼50–0% loss and ∼0.03 s–1 rate constant). The magnitude of the loss during irradiation and the rate constant are wavelength-dependent. Analysis of the H2O2 concentration post-spike indicates that this light-dependent sink does not produce H2O2 at low spike concentrations. This work further demonstrates that simply assuming that the O2• – production is twice the H2O2 production is not accurate, as previously believed. 

One-electron reductants (OER) photoproduced by chromophoric dissolved organic matter (CDOM) have been shown to be likely precursors for the formation of superoxide and subsequently hydrogen peroxide. An improved method that employs a nitroxide radical probe (3AP) has been developed and utilized to determine the photoproduction rates of OER from a diverse set of CDOM samples. 3AP reacts with OER to produce the hydroxyl- amine, which is then derivatized with fluorescamine and quantified spectrofluorometrically. Although less sensitive than traditional methods for measuring RO2•−, measuring RH provides a simpler and faster method of estimating RO2•− and is amenable to continuous measurement via flow injection analysis. Production rates of OER (RH), superoxide (RO2•−), and hydrogen peroxide (RH2O2) have a similar wavelength dependence, indicating a common origin. If all the OER react with molecular oxygen to produce superoxide, then the simplest mechanism predicts that RH /RH2O2 and RO2•−/RH2O2 should be equal to 2. However, our measurements reveal RH /RH2O2 values as high as 16 (5.7−16), consistent with prior results, and RO2•−/RH2O2 values as high as 8 (5.4−8.2). These results indicate that a substantial fraction of superoxide (65−88%) is not undergoing dismutation. A reasonable oxidative sink for superoxide is reaction with photoproduced phenoxy radicals within CDOM.