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

Abstract

A novel combination of remote sensing products is used to estimate photochemical production rates of hydrogen peroxide and superoxide in the global surface ocean.

Abstract

Hydrogen peroxide (H ₂O ₂) and its precursor, superoxide (O ₂ ⁻), 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 H ₂O ₂ and O ₂ ⁻ 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 H ₂O ₂ 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 H ₂O ₂ depends on secondary thermal reactions involving O ₂ ⁻, we also implemented a temperature correction for the H ₂O ₂ AQY using remotely sensed sea surface temperature and an Arrhenius relationship for H ₂O ₂ photoproduction. Daily photoproduction rates of H ₂O ₂ 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 H ₂O ₂ in these models. By making assumptions about the relationship between H ₂O ₂ and O ₂ ⁻ photoproduction rates and O ₂ ⁻ decay kinetics, we present a method for calculating midday O ₂ ⁻ steady-state concentrations ([O ₂ ⁻] _ss) in the open ocean. Estimated [O ₂ ⁻] _ss ranged from 0.1–5 nM assuming biomolecular dismutation was the only sink for O ₂ ⁻, 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 [O ₂ ⁻] _ss from remote sensing, the potential of this model to quantify O ₂ ⁻ photoproduction rates and [O ₂ ⁻] _ss will not be fully realized until the mechanisms controlling O ₂ ⁻ photoproduction and decay are better understood.