MODEL STUDIES OF THE INFLUENCE OF O-2 PHOTODISSOCIATION PARAMETERIZATIONS IN THE SCHUMANN-RUNGE BANDS ON OZONE RELATED PHOTOLYSIS IN THE UPPER-ATMOSPHERE

A new parameterization for atmospheric transmission and O-2 photodisso ciation in the Schumann-Runge band region has been developed and teste d with a 1D radiative-photochemical model. The parameterization is bas ed on the O-2-column along the line of sight to the Sun and the local temperature. Line-by-line calculations have served as a benchmark for testing this method and several other, commonly used, parameterization s. The comparisons suggest that differences between the line-by-line c alculations and currently accepted parameterizations can be reduced si gnificantly by using the new method, particularly at large solar zenit h angles. The production rate of O-atoms computed with this method sho ws less than 6% deviation compared to the line-by-line calculations at any altitude, all solar zenith angles and in all seasons. The largest errors are found toward the shorter wavelengths in the Schumann-Runge region at low altitudes. Transmittance is approximated to better than 4% at any altitude and/or solar zenith angle. The total O-production rate above 20 km is approximated to better than 2%. The new parameteri zation is easily implemented in existing photochemical models and in m any cases it may simply replace the existing algorithm. The computatio nal effort exceeds that of other parameterizations but in view of the total computation time needed for the actual calculation of the parame terized Schumann-Runge bands this should not lead to significant perfo rmance degeneration. The first 14 coefficients of the parameterization are included in this study. Both the complete sets of coefficients an d a simple algorithm can be obtained by contacting the authors. A phot ochemical model study shows the largest effect of the parameterization method is on odd hydrogen concentrations. Subsequent interaction with an odd oxygen family causes differences in the ozone concentrations b etween the different parameterizations of more than 10% at selected al titudes. Although it is already established that deficiencies in the t reatment of Schumann-Runge band absorption are unlikely to explain the current underestimation of ozone concentration at the stratopause in a variety of photochemical models, this study does show that the choic e of parameterization has a large impact on the accuracy of the result s at large solar zenith angles and in different seasons.