The Solar Ultraviolet Environment at the Ocean

• Published in: Photochemistry and photobiology Vol. 94; no. 3; pp. 611 - 617
• Main Authors: Mobley, Curtis D., Diffey, Brian L.
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01-05-2018
• Subjects: Atmospheric models; Backscattering; Computer simulation; Exposure; Irradiance; Measuring instruments; Ocean models; Ozone; Pollution monitoring; Radiance; Radiation; Radiative transfer; Sky; Sun; Surface roughness; Ultraviolet radiation; Water column; Wavelengths; Wind speed; Zenith
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1111/php.12885

Atmospheric and oceanic radiative transfer models were used to compute spectral radiances between 285 and 400 nm onto horizontal and vertical plane surfaces over water. The calculations kept track of the contributions by the sun's direct beam, by diffuse‐sky radiance, by radiance reflected from the sea surface and by water‐leaving radiance. Clear, hazy and cloudy sky conditions were simulated for a range of solar zenith angles, wind speeds and atmospheric ozone concentrations. The radiances were used to estimate erythemal exposures due to the sun and sky, as well as from radiation reflected by the sea surface and backscattered from the water column. Diffuse‐sky irradiance is usually greater than direct‐sun irradiance at wavelengths below 330 nm, and reflected and water‐leaving irradiance accounts for <20% of the UV exposure on a vertical surface. Total exposure depends strongly on solar zenith angle and azimuth angle relative to the sun. Sea surface roughness affects the UV exposures by only a few percent. For very clear waters and the sun high in the sky, the UV index within the water can be >10 at depths down to two meters and >6 down to 5 m.
When the sun is near the zenith, erythemal exposure to the skin equivalent to a UV index of at least seven occurs to depths of 2 m or more in waters attractive to swimmers. Above the sea surface, UV exposure is due more to diffuse sky light than to the sun's direct beam, and reflectance by the sea surface contributes less than a UV index of 0.7 to overall erythemal exposure.