Photolysis rates based on Schumann-Runge band approximations and irradiance measurements

Abstract

The photolysis rate (J₃) for CF₂Cℓ₂, (F-12) in the stratosphere has been computed for three altitudes using measured solar fluxes from a balloon borne spectrometer. The rate is compared with that obtained by computing the attenuated flux using effective cross sections appropriate for the Schumann-Runge bands (J₁) [Allen and Frederick, 1982]. The result shows that the error in the F-12 photolysis rate caused by using the Allen and Frederick approximation is about 10% (J₁ < J₃) for altitudes between 32 and 38 km with solar zenith angles less than about 62 degrees. For species other than O₂, this implies that the discrepancy in photolysis rates associated with the Schumann-Runge band approximations are not likely to be important. Within the Schumann-Runge band region the computed errors for F-12 photolysis are about 10.5% at 38.13 km and increase with decreasing altitude. If this error applies to O₂ photolysis, there may be the possibility of an incorrect calculation of net odd oxygen production in photochemical models.