Blending remote sensing data products to estimate photochemical production of hydrogen peroxide and superoxide in the surface ocean

Abstract

Hydrogen peroxide (H₂O₂) and its precursor, superoxide (O₂⁻), are well-studied photochemical products that are pivotal in regulating redox transformations of trace metals and organic matter in the surface ocean. In attempts to understand the magnitude of both H₂O₂ and O₂⁻ photoproduction on a global scale, we implemented a model to calculate photochemical fluxes of these products from remotely sensed ocean color and modeled solar irradiances. We generated monthly climatologies for open ocean H₂O₂ photoproduction rates using an average apparent quantum yield (AQY) spectrum determined from laboratory irradiations of oligotrophic water collected in the Gulf of Alaska. Because the formation of H₂O₂ depends on secondary thermal reactions involving O₂⁻, we also implemented a temperature correction for the H₂O₂ AQY using remotely sensed sea surface temperature and an Arrhenius relationship for H₂O₂ photoproduction. Daily photoproduction rates of H₂O₂ ranged from <1 to over 100 nM per day, amounting to ∼30 μM per year in highly productive regions. When production rates were calculated without the temperature correction, maximum daily rates were underestimated by 15–25%, highlighting the importance of including the temperature modification for H₂O₂ in these models. By making assumptions about the relationship between H₂O₂ and O₂⁻ photoproduction rates and O₂⁻ decay kinetics, we present a method for calculating midday O₂⁻ steady-state concentrations ([O₂⁻]_ss) in the open ocean. Estimated [O₂⁻]_ss ranged from 0.1–5 nM assuming biomolecular dismutation was the only sink for O₂⁻, but were reduced to 0.1–290 pM when catalytic pathways were included. While the approach presented here provides the first global scale estimates of marine [O₂⁻]_ss from remote sensing, the potential of this model to quantify O₂⁻ photoproduction rates and [O₂⁻]_ss will not be fully realized until the mechanisms controlling O₂⁻ photoproduction and decay are better understood.