Actinic flux and O1D photolysis frequencies retrieved from spectral measurements of irradiance at Thessaloniki, Greece

The results of two methods retrieving actinic flux and ozone photolysis frequencies (JO(1)D), from measurements of irradiance with a Brewer MKIII spectroradiometer are investigated in this paper. The first method uses actinic flux retrieved from irradiance measurements by the use of known formulas while the second is an empirical method converting irradiance to JO(1)D through polynomials extracted from a study of synchronous actinic flux and irradiance measurements. When examining the actinic fluxes derived from the first method to those measured by an actinic flux spectrometer data agree within +/-10% for solar zenith angles lower than 75degrees for the UV-B and the UV-A wavelength band. Also, the actinic to global irradiance ratio derived, deviates within +/-6% for solar zenith angles lower than 70degrees compared with cloudless sky calculations of the TUV model. For both cases the deviations are in the order of the magnitude of the measurement or model uncertainties. Values of JO(1)D calculated by the second method show a mean ratio of 0.99+/-0.10 (1sigma) and 0.98+/-0.06 for all data and for cloudless skies respectively when compared with values of JO(1)D derived by a Bentham actinic flux spectroradiometer. Finally, the agreement of the two methods is within +/-5% comparing two years' data of JO(1)D retrieved from irradiance measurements at Thessaloniki, Greece. The use of such methods on extensive data sets of global irradiance can provide JO(1)D values with acceptable uncertainty, a parameter of particular importance for chemical process studies.