EFFECT OF CLOUDS ON PHOTOLYSIS RATES AT THE SURFACE - A PRACTICAL METHOD OF ESTIMATION AND ITS APPLICATION TO A MODELING STUDY OF THE FORMATION OF PHOTOCHEMICAL OXIDANTS

Up to now, theoretical parametrization of the influence of clouds on photolytic reactions still meets with difficulties. Thus we developed a practical method relying on measurements of the u.v. flux compared to that expected for cloudless conditions, over a diurnal range, on successive days. The impact of the depletion in the photolytic constants, varying according to five different groups of reactions, has been estimated by a simulation of photochemical smog with various chemical mechanisms. Under current weather conditions, the ozone and PAN decreases varied from 11 to 20%; on the contrary NO2 and HNO2 were found to increase. Limitations of the method are discussed. © 1990.