Photochemical production of hydrogen peroxide and methylhydroperoxide in coastal waters

Hydrogen peroxide (H2O2) has been observed in significant concentrations in many natural waters. Because hydrogen peroxide can act as an oxidant and reductant, it participates in an extensive suite of reactions in surface waters. Hydrogen peroxide is produced as a secondary photochemical product of chromophoric dissolved organic matter (CDOM) photolysis. Apparent quantum yields for the photochemical production of hydrogen peroxide were determined in laboratory irradiations of filtered surface waters from several locations in the Chesapeake Bay and in Arctic coastal waters with varying levels of CDOM. The apparent quantum yield for H2O2 decreases by about an order of magnitude from 280 nm to 500 nm, and the majority of H2O2 production occurs at wavelengths less than 340 nm. The apparent quantum yield for H2O2 production at 290 nm ranged from 4.2 × 10− 4 to 2.1 × 10− 6 mol H2O2 (mol photons)− 1 from freshwater to marine waters. A linear relationship was found between the production of H2O2 and change in CDOM absorbance characterized as photobleaching (loss of absorbance). No significant relationship was observed between DOC concentration and peroxide production. Methylhydroperoxide (CH3O2H) was the only short chain peroxide produced during the irradiations, and its production is at least an order magnitude less than that of hydrogen peroxide. Peroxide production was greatest in waters containing significant amounts of terrigenous C in the form of humic substances. Surface waters whose synchronous fluorescence spectra indicated the presence of polyaromatic and/or extensive conjugated compounds exhibited the greatest peroxide production. CDOM photobleaching is not significantly linked to apparent quantum yields for peroxide production.