Effect of the reaction HO₂ + O₃ → OH + 2O₂ on stratospheric ozone

Abstract

THE recent measurements^1–3 of large values (8.1 ± 1.5 and (8.2 ± 2.7 × 10⁻¹² cm³ s⁻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 Howard⁶ of 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 HO₂ and ozone (reaction 2). Their temperature-dependent rate coefficient is k₂ = (1.4 ± 0.4) × 10⁻¹⁴ exp [−(580 ± 100)/T] cm³ s⁻¹ which 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 × 10⁻¹⁴ exp (−1,275/T) cm³ s⁻¹.