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.

Marine Photochemistry of Organic Matter

“…In addition, evaluation of errors associated with AQY determinations and with the assumption that AQYs are constant with photon exposure (i.e., irradiance integrated over time of exposure) are discussed in detail elsewhere (Neale and Kieber, 2000;Reader and Miller, 2012). Understanding and evaluating these errors and assumptions are key to accurately modeling photochemical processes in seawater on a global scale (Fichot and Miller, 2010;Powers and Miller, 2014; see Section V in this chapter).…”

“…Global models of CO photoproduction have used average CDOM absorbance spectra to estimate the proportion of light absorbed by CDOM Zafiriou et al, 2003). However, variations in estimates of CDOM light absorption can have a significant influence on the proportion of light absorbed by CDOM and resultant calculations of photochemical rates (Powers and Miller, 2014;Reader and Miller, 2011). An improvement in this regard utilizes remote sensed CDOM levels to provide an estimated global ocean CO photoproduction rate (~41 × 10 12 g-C year −1 ; Fichot and Miller, 2010).…”

“…Additionally, quantum yields for pure compounds undergoing a direct photolysis are generally independent of wavelength (Calvert and Pitts, 1966). However, CDOM is a complex mixture of organic compounds dissolved in an aqueous matrix together with a suite of inorganic solutes, and empirical evidence reveals that AQYs involving CDOM generally decrease exponentially with increasing wavelength (e.g., Blough, 1997;Gao and Zepp, 1998;Miller et al, 2002;Powers and Miller, 2014;Valentine and Zepp, 1993;Weiss et al, 1995;Zepp and Andreae, 1994). Consequently, spectrally and temperature-dependent AQYs need to be determined when studying CDOM-driven photoreactions.…”

“…In order to achieve this, researchers model solar irradiance reaching the surface of the ocean, the penetration of sunlight into the water column, and the proportion and spectral dependence of the water column light field absorbed by CDOM. The reader is referred to Fichot and Miller (2010), Miller et al (2002), Moran and Zepp (1997), Powers and Miller, 2014;Smyth (2011), and Zafiriou et al (2003 for various models and assumptions used in determining the photon flux absorbed by CDOM in the ocean. In the open ocean, CDOM dominates light absorbance at photochemically relevant wavelengths between ~280 and 400 nm , whereas in turbid estuarine waters particles compete for available light (Stubbins et al, 2011).…”

Ammonia Oxidation in the Ocean Can Be Inhibited by Nanomolar Concentrations of Hydrogen Peroxide

“…Of these ROS species, H 2 O 2 has the longest half-life (Kieber et al, 2003, and references therein; Powers and Miller, 2014). H 2 O 2 is primarily introduced by photochemical reactions with colored dissolved organic matter (CDOM; Figure 1B; Cooper et al, 1994;Blough and Zepp, 1995); rates of this production vary with solar irradiance, temperature, and [CDOM], but generally range from 9 to 134 nM h −1 in marine systems (reviewed in Kieber et al, 2003).…”