Oxygen isotopic fractionation during UV and visible light photodissociation of ozone

Abstract

Stratospheric ozone is essentially in a steady state due to the simultaneous formation and dissociation and found to be enriched (mass-independently) in heavy oxygen isotopes. Though there have been a number of experimental and theoretical studies on the mechanism(s) associated with the formation of isotopically heavy ozone, the decomposition processes were not studied in necessary detail. Here we report a novel feature in the isotopic fractionation of ozone during photodissociation in the UV and visible wavelengths. Photodissociation of ozone produces isotopically light oxygen, enriching the leftover ozone pool. Interestingly, the isotopic fractionation patterns are not similar in the two wavelength regions. Dissociation at visible wavelengths displays a mass-dependent slope (Δδ¹⁷O/Δδ¹⁸O = 0.54) whereas UV dissociation shows a mass-independent character (Δδ¹⁷O/Δδ¹⁸O = 0.63). O₃ photodissociation in UV wavelengths is normally associated with another effective channel of dissociation, i.e., O₃+O(¹D). It is demonstrated for the first time that pure UV photodissociation of ozone [i.e., without the O(¹D) channel] gives a slope of unity intriguingly similar to that obtained in the ozone formation process. A combination of the two processes i.e., mass dependent O(¹D)+O₃ reaction and pure UV photodissociation is responsible for the observed slope of 0.63.