Photodissociation in the atmosphere of Mars: Impact of high resolution, temperature‐dependent CO₂ cross‐section measurements

We have investigated the impact of high resolution, temperature‐dependent CO 2 cross‐section measurements (Lewis and Carver, 1983) on calculations of photo dissociation rate coefficients in the Martian atmosphere. Since the cross‐section measurements exhibit large variations on a 10–20 Å scale, the calculations are sensitive to the wavelength resolution. We find that the adoption of 50 Å intervals for the purpose of computational efficiency result in errors in the calculated values for photodissociation of CO 2 , H 2 O and O 2 which are generally ≤10% but as large as 20% in some instances. These are acceptably small errors, especially considering the uncertainties introduced by the large temperature dependence of the CO 2 cross‐section. The inclusion of temperature‐dependent CO 2 cross‐sections is shown to lead to a decrease in the diurnally averaged rate of CO 2 photodissociation as large as 33% at some altitudes, and increases of as much as 950% and 80% in the photodissociation rate coefficients of H 2 O and O 2 , respectively. The actual magnitude of the changes depends on the assumptions used to model the CO 2 absorption spectrum at temperatures lower than the available measurements, and at wavelengths longward of 1970 Å. Given our current understanding, these results minimize or even reverse the sense of the CO 2 chemical stability problem.