Jens-Uwe Grooss

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

Numerical schemes for the calculation of photolysis rates are usually employed in simulations of stratospheric chemistry. Here, we present an improvement of the treatment of the diffuse actinic flux in a widely used stratospheric photolysis scheme (Lary and Pyle, 1991). We discuss both the consequences of this improvement and the correction of an error present in earlier applications of this scheme on the calculation of stratospheric photolysis frequencies. The strongest impact of both changes to the scheme is for small solar zenith angles. The effect of the improved treatment of the diffuse flux is most pronounced in the lower stratosphere and in the troposphere. Overall, the change in the calculated photolysis frequencies in the region of interest in the stratosphere is below about 20%, although larger deviations are found for H2O, O2, NO, N2O, and HCl.

The relevance of reactions of the methyl peroxy radical (CH3O2) and methylhypochlorite (CH3OCl) for Antarctic chlorine activation and ozone loss

Abstract The maintenance of large concentrations of active chlorine in Antarctic spring allows strong chemical ozone destruction to occur. In the lower stratosphere (approximately 16–18 km, 85–55 hPa, 390–430 K) in the core of the polar vortex, high levels of active chlorine are maintained, although rapid gas-phase production of HCl occurs. The maintenance is achieved through HCl null cycles in which the HCl production is balanced by immediate reactivation. The chemistry of the methyl peroxy radical (CH3O2) is essential for these HCl null cycles and thus for Antarctic chlorine and ozone loss chemistry in the lower stratosphere in the core of the polar vortex. The key reaction here is the reaction ; this reaction should not be neglected in simulations of polar ozone loss. Here we investigate the full chemistry of CH3O2 in box-model simulations representative for the conditions in the core of the polar vortex in the lower stratosphere. These simulations include the reaction CH3O2 + Cl, the product methylhypochlorite (CH3OCl) of the reaction CH3O2 + ClO, and the subsequent chemical decomposition of CH3OCl. We find that when the formation of CH3OCl is taken into account, it is important that also the main loss channels for CH3OCl, namely photolysis and reaction with Cl are considered. Provided that this is the case, there is only a moderate impact of the formation of CH3OCl in the reaction CH3O2 + ClO on polar chlorine chemistry in our simulations. Simulated peak mixing ratios of CH3OCl ( ppb) occur at the time of the lowest ozone mixing ratios. Further, our model simulations indicate that the reaction CH3O2 + Cl does not have a strong impact on polar chlorine chemistry. During the period of the lowest ozone concentrations in late September, enhanced values of CH3O2 are simulated and, as a consequence, also enhanced values of formaldehyde (about 100 ppt) and methanol (about 5 ppt).