S. K. Bhattacharya

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

New Evidence for Symmetry Dependent Isotope Effects: O+CO Reaction

The isotopic fractionation associated with the O + CO reaction has been studied using oxygen atoms produced by room temperature O2 photolysis at two different wavelengths, 185 and 130 nm. A large mass-independent isotopic fractionation is observed in the product CO2, extending the range of this type of reaction beyond O + O2 and SF5 + SF5. Kinetic evaluation of the data restricts the source of the mass-independent fractionation mechanism to the O + CO recombination step rather than O2 photolysis, secondary ozone formation, or O2 photodissociation. At least one, and most likely two other fractionation processes appear to occur in the experiments, and interpretation of the isotopic results is tentative at present. Based on the relevant reaction rates and the value for the reduced partition function for isotopic exchange between O and CO, it is suggested that this process may occur prior to the δ17O≅δ18O recombination process. Secondary CO2 photolysis may superimpose an additional fractionation. The experimental data are also examined in the context of a model based upon energy randomization rates versus the lifetime of the activated complex.

Anomalous oxygen isotope enrichment in CO2 produced from O+CO: Estimates based on experimental results and model predictions

The oxygen isotope fractionation associated with O+CO-->CO(2) reaction was investigated experimentally where the oxygen atom was derived from ozone or oxygen photolysis. The isotopic composition of the product CO(2) was analyzed by mass spectrometry. A kinetic model was used to calculate the expected CO(2) composition based on available reaction rates and their modifications for isotopic variants of the participating molecules. A comparison of the two (experimental data and model predictions) shows that the product CO(2) is endowed with an anomalous enrichment of heavy oxygen isotopes. The enrichment is similar to that observed earlier in case of O(3) produced by O+O(2) reaction and varies from 70 0/00 to 136 0/00 for (18)O and 41 0/00 to 83 0/00 for (17)O. Cross plot of delta (17)O and delta (18)O of CO(2) shows a linear relation with slope of approximately 0.90 for different experimental configurations. The enrichment observed in CO(2) does not depend on the isotopic composition of the O atom or the sources from which it is produced. A plot of Delta(delta (17)O) versus Delta(delta (18)O) (two enrichments) shows linear correlation with the best fit line having a slope of approximately 0.8. As in case of ozone, this anomalous enrichment can be explained by invoking the concept of differential randomization/stabilization time scale for two types of intermediate transition complex which forms symmetric ((16)O(12)C(16)O) molecule in one case and asymmetric ((16)O(12)C(18)O and (16)O(12)C(17)O) molecules in the other. The delta (13)C value of CO(2) is also found to be different from that of the initial CO due to the mass dependent fractionation processes that occur in the O+CO-->CO(2) reaction. Negative values of Delta(delta (13)C) ( approximately 12.1 0/00) occur due to the preference of (12)C in CO(2)* formation and stabilization. By contrast, at lower pressures (approximately 100 torr) surface induced deactivation makes Delta(delta (13)C) zero or slightly positive.

Anomalous enrichment of O17 and C13 in photodissociation products of CO2: Possible role of nuclear spin

Oxygen and carbon isotope fractionation associated with products (CO and O(2)) of gas phase photodissociation of CO(2) have been studied using photons from Hg lamp (184.9 nm) and Kr lamp (123.6 and 116.5 nm). In dissociation by Hg lamp photons both CO and O(2) are enriched in (17)O by about 81 per thousand compared to the estimate based on a kinetic model. Additionally, CO is enriched in (13)C by about 37 per thousand relative to the model composition. In contrast, in dissociation by higher energy Kr lamp photons no such anomaly was found in O(2). The observed isotopic enrichments in case of Hg lamp dissociation are proposed to be due to a hyperfine interaction between nuclear spin and electron spins or orbital motion causing enhanced dissociation of isotopologues of CO(2) containing (17)O and (13)C. The (17)O enrichment is higher than that of (13)C by a factor of 2.2+/-0.2 which can be explained by the known magnetic moment ratio of (17)O and (13)C due to differing nuclear spins and g-factors. These results have potential implications in studies of the planetary atmospheres.

Oxygen isotope exchange between O2 and CO2 over hot platinum: an innovative technique for measuring Δ17O in CO2.

The isotopic composition of carbon dioxide provides a powerful tool and has been widely used for constraining the sources and sinks of atmospheric CO2. In this work, we demonstrate a simple, rapid, and clean way for measuring the triple oxygen isotope ratio of carbon dioxide with high precision. The method depends on isotope exchange between O2 and CO2 in the presence of platinum at high temperature and allows rapid measurement of Δ(17)O of CO2. The method has been established and confirmed through several tests by using artificially made CO2 with known Δ(17)O values. The analytical precision obtained for determining Δ(17)O in CO2 is 0.045‰ (1 - σ standard deviation).

17O excess transfer during the NO2 + O3 → NO3 + O2 reaction

The ozone molecule possesses a unique and distinctive (17)O excess (Δ(17)O), which can be transferred to some of the atmospheric molecules via oxidation. This isotopic signal can be used to trace oxidation reactions in the atmosphere. However, such an approach depends on a robust and quantitative understanding of the oxygen transfer mechanism, which is currently lacking for the gas-phase NO(2) + O(3) reaction, an important step in the nocturnal production of atmospheric nitrate. In the present study, the transfer of Δ(17)O from ozone to nitrate radical (NO(3)) during the gas-phase NO(2) + O(3) → NO(3) + O(2) reaction was investigated in a series of laboratory experiments. The isotopic composition (δ(17)O, δ(18)O) of the bulk ozone and the oxygen gas produced in the reaction was determined via isotope ratio mass spectrometry. The Δ(17)O transfer function for the NO(2) + O(3) reaction was determined to be: Δ(17)O(O(3)∗) = (1.23 ± 0.19) × Δ(17)O(O(3))(bulk) + (9.02 ± 0.99). The intramolecular oxygen isotope distribution of ozone was evaluated and results suggest that the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The results obtained in this study will be useful in the interpretation of high Δ(17)O values measured for atmospheric nitrate, thus leading to a better understanding of the natural cycling of atmospheric reactive nitrogen.

Effect of Isotopic Exchange upon Symmetry Dependent Fractionation in the O + CO → CO2 Reaction

Abstract In a recent study, it was demonstrated that the mechanism associated with the O + CO reaction produces a large, mass independent isotopic fractionation in the product CO2. A kinetic treatment of the data demonstrated that isotopic exchange between the O atom, produced by O2 photolysis and CO, occurred prior to the O + CO recombination. It was concluded that the likely source of the mass independent fractionation was the O + CO recombination. The present paper includes a kinetic evaluation of the added role of O + O2, along with O + CO, isotopic exchange. The new determinations provide a better fit of the experimental data.