The spectral radiometer provides a direct measurement of solar actinic UV flux and determination of photolysis frequencies. The actinic flux is measured between 280 - 650 nm (<1 nm resolution) using a spectral radiometer (Ocean Optics QE5000) attached to a quartz receiver optic (Metcon). Photolysis rates are derived from the product of absorption cross-section of the precursor molecule, the quantum yield of the photo-product and the actinic flux density (cm-2s-1nm-1).