IMPACT OF AVERAGED PHOTOLYSIS RATES CHEMICAL-MODELS

An evaluation is made of the effect of two approximations to the diurn al variation of photolysis rates on the simulation of stratospheric ch emistry. The daylight average approximation uses photolysis rates that are given a value representative of the daylight average over that po rtion of the day that the Sun is shining, and zero otherwise. Although this distorts the diurnal cycle, the zonal average concentrations usi ng this approximation are within about 5% of those from the fully reso lved diurnal cycle for most of the important species in the stratosphe re. Larger discrepancies occur in the partitioning of inorganic chlori ne, although even this is in error by only about 10% for the dominant species. The 24-hour average photolysis approximation gives perpetual daylight except in the polar night. The error associated with this is quite large, especially for those species such as NO3 and N2O5 that ar e produced during night. Over an integration time of a month, it can l ead to differences in the simulated concentrations of major species su ch as ozone of the order of 20% or more.