Deriving actinic flux and photolysis rates from spectral and multiband measurements of UV irradiance

Abstract

Actinic flux (the radiation incident on the surface of a small sphere) is the measure of radiation required for atmospheric chemistry. The photolysis rate for a given species is the product of its absorption cross-section, quantum yield and the actinic flux, integrated over all relevant wavelengths. Despite their importance, actinic flux and photolysis rate data are not routinely available since radiation monitoring sites usually report irradiances (radiation on a flat, horizontal surface). Using data from Reading (UK) we show that spectral UV actinic fluxes can be derived from spectral irradiance measurements with a scanning spectroradiometer (Bentham DTM300) to within an uncertainty of 20% full range, or 15% if sky conditions are known. Any photolysis rate dependent on these wavelengths can then be calculated from the actinic flux data. The disadvantage of spectroradiometers is their scan time of several minutes. The GUV-541 multi-band radiometer measures across 5 narrow UV channels simultaneously and samples at a rate of 2-3Hz. Using a semi-empirical formula, this multi-band irradiance data can be converted into photolysis rates for J(O 3 ) and J(NO 2 ). The GUV derived photolysis rates were within 20% of those calculated from spectral actinic fluxes under all conditions tested. This compares well with other methods of measuring photolysis rates.