Stratospheric photolysis frequencies: Impact of an improved numerical solution of the radiative transfer equation

Numerical schemes for the calculation of photolysis rates are usually emplo yed in simulations of stratospheric chemistry. Here, we present an improvem ent of the treatment of the diffuse actinic flux in a widely used stratosph eric photolysis scheme (Lary and Pyle, 1991). We discuss both the consequen ces of this improvement and the correction of an error present in earlier a pplications of this scheme on the calculation of stratospheric photolysis f requencies. The strongest impact of both changes to the scheme is for small solar zenith angles. The effect of the improved treatment of the diffuse f lux is most pronounced in the lower stratosphere and in the troposphere. Ov erall, the change in the calculated photolysis frequencies in the region of interest in the stratosphere is below about 20%, although larger deviation s are found for H2O, O-2, NO, N2O, and HCl.