Estimation of surface actinic flux from satellite (TOMS) ozone and cloud reflectivity measurements

Recently, a method has been proposed to estimate ultraviolet (UV) spectral irradiances at the Earth's surface, from satellite-based measurements of oz one and cloud reflectivity. Here, we extend this method to the determinatio n of spectral actinic fluxes needed for the calculation of photodissociatio n rate coefficients. It is shown that the modification of actinic flux by c louds is closely approximated by the cloud reduction factor applicable to t he irradiance, if considering averages over area or time instead of instant aneous point data. The conversion of irradiance estimates into actinic flux estimates introduces additional errors, but for most practical situations these fall below 10 to 20% in the UV spectral region, where the relevant ph otolytic reactions have their maximum sensitivity. Based on these results, large satellite-derived data sets (e.g., ozone column and cloud reflectivit y from the Total Ozone Mapping Spectrometer, TOMS) can be utilized to estim ate near-surface photolysis rate coefficients on a global scale.