Global mapping of underwater UV irradiances and DNA-weighted exposures using total ozone mapping spectrometer and sea-viewing wide field-of-view sensor data products

The global stratospheric ozone layer depletion results in an increase in bi ologically harmful ultraviolet (UV) radiation reaching the surface and pene trating to ecologically significant depths in natural waters. Such an incre ase can be estimated on a global scale by combining satellite estimates of UV irradiance at the ocean surface from the Total Ozone Mapping Spectromete r (TOMS) satellite instrument with the Sea-viewing Wide Field-of-view Senso r (SeaWiFS) satellite ocean color measurements in the visible spectral regi on. In this paper we propose a model of seawater optical properties in the UV spectral region based on the case I water model in the visible range. Th e inputs to the model are standard monthly SeaWiFS products: chlorophyll co ncentration and the diffuse attenuation coefficient at 490 nm. Penetration of solar UV radiation to different depths in open ocean waters is calculate d using the radiative transfer quasi-single scattering approximation (QSSA) . The accuracy of the QSSA approximation in the water is tested using more accurate codes. Sensitivity studies of underwater UV irradiance to atmosphe ric and oceanic optical properties have shown that the main environmental p arameters controlling absolute levels of UVB (280-320 nm) and DNA-weighted irradiance underwater are solar zenith angle, cloud transmittance, water op tical properties, and total ozone. Monthly maps of underwater UV irradiance and DNA-weighted exposure are calculated using monthly mean SeaWiFS chloro phyll and diffuse attenuation coefficient, daily SeaWiFS cloud fraction dat a, and the TOMS-derived surface UV irradiance daily maps. The results inclu de global maps of monthly average UVB irradiance and DNA-weighted daily exp osures at 3 and 10 m and depths where the UVB irradiance and DNA-weighted d ose rate at local noon are equal to 10% of their surface values.