The influence of solar ultraviolet radiation on the photochemical production of H2O2 in the equatorial Atlantic Ocean

Hydrogen peroxide (H₂O₂) was measured in marine surface waters of the eastern Atlantic Ocean between 25°N and 25°S. H₂O₂ concentrations decreased from 80 nM in the north to 20 nM in the south, in agreement with earlier observations. A diel cycle of H₂O₂ production as a function of sunlight in surface waters was followed twice whilst the ship steamed southward. Around 23°N a distinct diel cycle could be measured which correlated well with irradiance conditions.The wavelength dependency of H₂O₂ formation was studied near the equator. For 16 hours, water samples were incubated with wavelength bands of the solar spectrum, i.e. visible (VIS: 400–700 nm), VIS and ultraviolet A radiation (UVAR: 320–400 nm) and VIS, UVAR and ultraviolet B radiation (UVBR: 280–320 nm). A significant relationship was found between wavelength band and the production of H₂O₂. In addition, a clear positive relationship between intensity and production was found. UVAR was 6.5 times more efficient than VIS in producing 1 nM of H₂O₂, whereas UVBR was 228 times more efficient than VIS. When these data were weighted with respect to the energy of the solar spectrum at zenith hour, 28% of the H₂O₂ was formed by VIS, 23% was formed by UVAR and 48% was formed by UVBR. Considering the strong attenuation of UVBR in marine waters as compared with UVAR and VIS radiation, the role of UVAR deeper in the water column is recognised. Furthermore results of this research emphasise the importance of VIS radiation in the formation of H₂O₂.