Hydroxyl Radical Photoproduction in the Sea and Its Potential Impact on Marine Processes

Photochemical production rates and steady-state concentrations of hydroxyl radicals (⋅OH) were measured in sunlight-irradiated seawater. Values ranged from 110 nanomolar per hour and 12 × 10 -18 molar in coastal surface water to 10 nanomolar per hour and 1.1 × 10 -18 molar in open ocean surface water. The wavelengths responsible for this production are in the ultraviolet B region (280 to 320 nanometers) of the solar spectrum. Dissolved organic matter (DOM) appears to be the main source for ⋅OH over most of the oceans, but in upwelling areas nitrite and nitrate photolysis may also be important. DOM in the deep sea is degraded more readily by ⋅OH (and its daughter radicals), by a factor of 6 to 15, than is DOM in open-ocean surface water. This finding may in part bear on major discrepancies among current methods for measuring dissolved organic carbon in seawater.