Abstract
Superoxide (O-2(-)) is a key intermediate in the cycling of organic matter and trace metals in natural waters but production rates are difficult to determine due to low steady-state concentrations, rapid decay rates, and unstable standards. On the other hand, superoxide's dismutation product, hydrogen peroxide (H2O2), is relatively stable in filtered water. Thus, if the stoichiometry between O-2(-) and H2O2 is known, one can derive superoxide data from H2O2 measurements. The relationship between O-2(-) and H2O2 remains uncertain in seawater but work by Petasne and Zika (1987) presented a method for examining the relationship between O-2(-) and H2O2 during irradiations of coastal seawater using superoxide dismutase (SOD), which forces a 2:1 stoichiometry between O-2(-) and H2O2. Here we report the first O-2(-) apparent quantum yield (AQY) spectra following their approach; performing irradiations of various fresh and seawater samples and measuring H2O2 accumulation with and without added SOD. For all but a single riverine sample, H2O2 AQY spectra fell in a narrow range, but O-2(-) AQY spectra varied such that O-2(-):H2O2 ratios were always >2 and were highest for the clear waters of the Gulf Stream (similar to 3.4 02 per H2O2 generated). Because this approach eliminates the need to measure O-2(-) production rates directly, it represents a simple way to refine the stoichiometric relationships that would potentially allow global estimates of O(2)(-)photoproduction rates, O-2(-) steady-state concentrations ([O-2(-)](ss)), and related surface ocean redox reactions based on more manageable H2O2 photochemical studies.