Subrata Chakraborty

Oxygen isotopic fractionation during UV and visible light photodissociation of ozone

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).u2002O3 photodissociation in UV wavelengths is normally associated with another effective channel of dissociation, i.e., O3+O(1D). It is dem...

Low‐pressure dependency of the isotopic enrichment in ozone: Stratospheric implications

[1]xa0Stratospheric 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.

Oxygen isotopic anomaly in surface induced ozone dissociation

Abstract The products of ozone dissociation occurring on glass surface are enriched in heavy oxygen isotopes ( 17 O and 18 O ) in a mass independent ( Δ δ 17 O / Δ δ 18 O =1.0 ) fashion. Such behavior is in contrast to the case of thermal dissociation where fractionation is mass dependent ( Δ δ 17 O / Δ δ 18 O =0.5 ). Even photo-dissociation by visible light is a mass dependent process. The mass independent fractionation in surface dissociation can probably be explained by assuming that the dissociation takes place via a short-lived complex involving the ozone molecule and an active surface site. The anomalous isotopic fractionation in surface dissociation can be useful to decipher the mechanism of surface reaction in some cases.