Abstract
The relevant measure of radiation for atmospheric chemistry is actinic flux, but the great majority of UV measurements, not common in themselves, are UV irradiance (radiation on a flat surface). Ideally the radiation measurements should also give full spectral detail, however, only a subset of monitoring sites can provide this information. This study describes a method to derive the photodissociation rate of O-3 and NO2 from irradiance measurements with a moderate bandwidth filter radiometer that has five channels in the UV region with center wavelengths at 305, 313, 320, 340 and 380 run with bandpass function of 10 nm. The global irradiance measurements from GUV in Manchester, U.K. (18 m altitude, 53.28degrees N, 2.14degrees W) were compared with independently measured photolysis rates determined from the measured actinic flux on a single clear sky day. On the reference day a linear regression of GUV global irradiance to simultaneous photolysis rate measurements was performed to generate scale factors for each optical channel. This enables the derivation of an empirical equation by which the measured global irradiance is converted to the photodissociation rate using selected channels of GUV for all other days of the measurement period. The agreement between the measured and calculated photolysis rate from the empirical equation at different solar zenith angles agreed within 20% under all conditions tested. This compares wall with other methods of measuring photolysis rate, and uses a low maintenance UV instrument that can be used for long-term monitoring. The results were also compared to the derived photolysis rates from a radiative transfer model using the discrete ordinate method (DISORT) by using the derived O-3 column from GUV.