<title>Empirical approach to converting spectral UV measurements to actinic flux data</title>

Abstract

The vast majority of radiation measurements, including UV, refer to the radiation incident on a flat horizontal plate. However, this may not be the most appropriate way to specify radiation for bodies affected by UV, since they are rarely flat or horizontal. In particular the target molecules involved in atmospheric chemistry are approximately spherical and the actinic flux would be a better measure of the incident radiation. The ADMIRA project is addressing the issue of converting spectral UV irradiances to spectral actinic fluxes that can then be weighted with any required cross-section or action spectrum to give photolysis rates or biologically effective radiation incident on a sphere. The success with which this conversion can be made will depend on the prevailing atmospheric conditions and the knowledge of such at the time the irradiance measurements were made. Several different approaches to the conversion are being assessed, together with their associated uncertainties. These range from the simple empirical method to more complex radiative-transfer model based algorithms. Here we report on a coordinated campaign of simultaneous irradiance and actinic flux measurements supported by a wide range of ancillary measurements and their application to a simple empirical approach to converting irradiances to actinic fluxes.