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Hydrogen peroxide (H 2 O 2 ) is an important reactive oxygen species (ROS) in natural waters, affecting water quality via participation in metal redox reactions and causing oxidative stress for marine ecosystems. While attempts have been made to better understand H 2 O 2 dynamics in the global ocean, the relative importance of various H 2 O 2 sources and losses remains uncertain. Our model improves previous estimates of photochemical H 2 O 2 production rates by using remotely sensed ocean color to characterize the ultraviolet (UV) radiation field in surface water along with quantitative chemical data for the photochemical efficiency of H 2 O 2 formation. Wavelength- and temperature-dependent efficiency (i.e., apparent quantum yield, AQY) spectra previously reported for a variety of seawater sources, including coastal and oligotrophic stations in Antarctica, the Pacific Ocean at Station ALOHA, the Gulf of Mexico, and several sites along the eastern coast of the United States were compiled to obtain a “marine-average” AQY spectrum. To evaluate our predictions of H 2 O 2 photoproduction in surface waters using this single AQY spectrum, we compared modeled rates to new measured rates from Gulf Stream, coastal, and nearshore river-outflow stations in the South Atlantic Bight, GA, United States; obtaining comparative differences of 33% or less. In our global model, the “marine-average” AQY spectrum was used with modeled solar irradiance, together with satellite-derived surface seawater temperature and UV optical properties, including diffuse attenuation coefficients and dissolved organic matter absorption coefficients estimated with remote sensing-based algorithms. The final product of the model, a monthly climatology of depth-resolved H 2 O 2 photoproduction rates in the surface mixed layer, is reported for the first time and provides an integrated global estimate of ∼21.1 Tmol yr −1 for photochemical H 2 O 2 production. This work has important implications for photo-redox reactions in seawater and improves our understanding of the role of solar irradiation on ROS cycling and the overall oxidation state in the oceans. 

Abstract Superoxide () is a reactive oxygen species (ROS) that is primarily produced by the one‐electron transfer of photooxidized chromophoric dissolved organic matter (CDOM) to O₂in sunlit natural waters. Here we examine the environmental and chemical parameters (pH, ionic strength, buffer, and halides) that may influence photochemical production rates and decay pathways in natural water. Using the enzyme superoxide dismutase and H₂O₂measurements, we present results from an irradiated freshwater CDOM source indicating that reductive decay pathways (P/P_SOD) dominate with increased pH and NaCl additions and maximal photoproduction rates () increase with carbonate compared to borate buffer. Over 2 h of irradiation, a significant decline in was seen for all samples along with a minor increase in oxidative pathways. These results imply shifts in decay pathways and production rates that seem to vary across natural waters and as a function of irradiation history.