On the use of Isaksen‐Luther method of computing photodissociation rates in photochemical models

Isaksen's algorithm of the photolysis rate calculation in Luther's modification (I‐L method) is analyzed. The interpretations suggested by Thompson [1984] and Madronich [1982] are also considered. The accuracy of the I‐L method is tested by (1) evaluation of the transmittance and reflectance coefficients for a single homogeneous Rayleigh scattering layer against delta Eddington and discrete ordinate results and (2) comparison of I‐L and delta Eddington photolysis rates. These tests show that if the I‐L method is applied for calculation of twilight UV photolysis rates, its inaccuracy can exceed 50% (compare 10% according to the earlier estimations). The possible effect of such inaccuracy on model predictions is illustrated by comparing the results obtained by the same one‐dimensional PCM successively employing I‐L and delta Eddington radiation codes.