Effect of the reaction HO2+ O3→ OH + 2O2on stratospheric ozone

THE recent measurements1–3of large values (8.1 ± 1.5 and (8.2 ± 2.7 × 10−12cm3s−1) of the rate coefficient for the reaction NO + HO2→ NO2+ OH (1) have led to some important revisions in our understanding of stratospheric photochemistry4. In particular, the predicted ozone perturbations due to the injection of nitrogen oxides by supersonic transport aircraft have become net ozone gains instead of losses5, while the predicted ozone depletion due to ozone-active chlorine from chlorofluoromethanes has almost doubled4. An unfavourable result of the adoption of the larger value of the rate coefficient for reaction (1) has been the prediction of excessive amounts of stratospheric ozone (10–20% more than the measured concentrations) both in terms of the total column abundance and the concentration profile at altitudes below 40 km. Attempts to remedy this discrepancy between theory and observation by altering the simulated lower stratosphere transport and by increasing the assumed amount of ozone-active chlorine were not successful. It is the purpose of this letter, however, to show that another recent measurement by Zahniser and Howard6of the rate coefficient, k2, for the reaction HO2+ O3→ OH + 2O2(2) largely resolves these problems of excessive stratospheric ozone in models. Zahniser and Howard have reported the first direct measurement of the reaction between HO2and ozone (reaction 2). Their temperature-dependent rate coefficient is k2= (1.4 ± 0.4) × 10−14exp [−(580 ± 100)/T] cm3s−1which at stratospheric temperatures is 4 to 5 times larger than the value recommended in the report of the NASA Chlorofluoromethane (CFM) Workshop4, that is, 7.3 × 10−14exp (−1,275/T) cm3s−1.