Low-pressure dependency of the isotopic enrichment in ozone: stratospheric implications

Abstract

Stratospheric ozone is enriched in the heavy isotopes (¹⁷O and ¹⁸O) relative to the ambient oxygen from which it is formed. This enrichment varies with altitude, attaining very high values between 30 and 40 km. A recent theory of Y. Q. Gao and R. A. Marcus explores the reasons for isotopic enrichment in the process of ozone formation and is particularly useful to understand the laboratory results. The stratospheric variations are sought to be explained in terms of temperature dependence of isotopic enrichment, but the magnitude of variation does not match with predictions accurately. We demonstrate here that isotopic enrichment in ozone generated by oxygen photolysis depends on the pressure of the oxygen reservoir and can have very high values (at about 15 torr), comparable to the highest observed stratospheric values. Analysis of the data shows that secondary enrichment through ozone dissociation can add to the primary enrichment associated with ozone formation. The effect of dissociation is found to be more pronounced in the pressure range of 15 to 50 torr, resulting in high enrichment. It is shown that the relative kinetics, pressure and temperature conditions of ozone formation and dissociation play an important role in determining the ultimate value of the enrichment. The results are particularly useful to understand the stratospheric data and resolve the observed discrepancy.