Direct Evidence of a Light-Dependent Sink of Superoxide within Chromophoric Dissolved Organic Matter.

Superoxide (O 2 •   - ) 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), O 2 •   - is believed to undergo primarily dismutation to produce hydrogen peroxide (H 2 O 2 ). However, past studies have implicated the presence of an additional light-dependent sink of O 2 •   - that does not lead to H 2 O 2 production. Here, we provide direct evidence of this sink through O 2 •   - injection experiments. During irradiations, spikes of O 2 •   - 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 H 2 O 2 concentration post-spike indicates that this light-dependent sink does not produce H 2 O 2 at low spike concentrations. This work further demonstrates that simply assuming that the O 2 •   - production is twice the H 2 O 2 production is not accurate, as previously believed.