COMBINED MEASUREMENTS OF UV-A ACTINIC FLUX, UV-A IRRADIANCE AND GLOBAL RADIATION IN RELATION TO PHOTODISSOCIATION RATES

At present, photodissociation rates at ground level are mainly estimated from empirical expressions that relate the irradiance (UV-A or global) to photodissociation rates. In this paper, a comparison is made between the UV-A actinic flux, which is the appropriate radiometric quantity to estimate photodissociation rates, and the two other commonly used radiometric quantities: UV-A irradiance and global radiation. The comparison is based on measurements of the 3 quantities at various locations: the Azores (37 degrees N), Antarctica (74 degrees S) and the Netherlands (52 degrees N). The observed variation in UV-A actinic flux, simultaneously measured UV-A irradiance and global radiation is a measure of the accuracy that can be obtained with empirical relations that relate UV-A irradiance or global radiation to photodissociation rates. It is shown that solar zenith angle, surface albedo and the ratio of direct downward to total downward irradiance are the main factors that determine the relation between UV-A actinic flux and UV-A irradiance at ground level. However, the angular distribution of the radiation field is also important. Model calculations using a doubling-adding code show large variations in the actinic flux for a given irradiance because of the anisotropy of the radiation field. When the atmosphere is cloud-free and unpolluted and also when the sky is completely overcast, UV-A actinic fluxes for a given UV-A irradiance are found to vary by about 10% at ground level. Therefore we also estimate that empirical relations between photodissociation rates and UV-A irradiance are accurate within 10% under these well-defined circumstances. Horizontal inhomogeneity of the atmosphere enlarges the uncertainty in the relation between actinic flux and irradiance. Variations of up to 25% in UV-A actinic flux for a given UV-A irradiance were observed on a day with partial cloud cover. Variations in UV-A actinic fluxes for a given global radiation measurement were always less than 20%, i.e., for clear sky, completely overcast conditions and partial cloud cover conditions.