Estimation of Ultraviolet-A Irradiance from Measurements of 368-nm Spectral Irradiance

• Published in: Journal of atmospheric and oceanic technology 2005-12, Vol.22 (12), p.1853-1863
• Main Authors: Grant, R H, Slusser, J R
• Format: Article
• Language: English
• Subjects: Atmospheric aerosols; Atmospheric models; Earth surface; Error analysis; Errors; Irradiance; Marine; Measurement; Productivity; Radiative transfer; Spectra; Terrestrial ecosystems; Ultraviolet radiation; Zenith
• ISSN: 0739-0572; 1520-0426
• DOI: 10.1175/JTECH1823.1

The estimation of ultraviolet-A (UV-A) radiation across the earth's surface is needed to model plant productivity and future impacts of ultraviolet-B radiation on plant productivity. We have developed two models to estimate the UV-A irradiance from measurements of the diffuse and global spectral irradiance at 368 nm. The models were developed from 30-min-interval measurements made throughout 2000 at three locations across the United States and evaluated from 30-min measurements made throughout 2000 at three additional locations and throughout 2001 and 2002 at seven locations. UV-A irradiance was best estimated from measured global 368-nm irradiance and empirical functions defining the UV-A and 368-nm irradiance values estimated from a theoretical pseudospherical two-stream discrete-ordinates radiative transfer model. The radiative transfer model provided baseline irradiance relationships between UV-A irradiance and 368-nm spectral irradiance. The semiempirical model estimated the UV-A irradiance at seven locations across the United States with a mean bias error of 0.5 W m-2 and a root-mean-square error of 2 W m-2, corresponding to approximately c4% of a clear-sky irradiance of 50 W m-2 for a solar zenith angle of 30'. This model error was comparable to the combined effect of previously estimated UV-A and 368-nm irradiance measurement errors.