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Anomalous oxygen isotope enrichment in produced from : Estimates based on experimental results and model predictions
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Image of FIG. 1.
FIG. 1.

Oxygen isotopic compositions (in ‰ relative to VSMOW) of product (set 1 samples) defining a line with slope of . The average and values of samples calculated from the KINTECUS model are and . Note that is enriched in heavy oxygen isotopes in an anomalous fashion.

Image of FIG. 2.
FIG. 2.

Oxygen isotopic compositions of samples from set 2. The best-fit line relating with gives a slope of suggesting anomalous enrichments in heavy oxygen isotopes. Ozone compositions used in these experiments are also shown in the plot. The average isotopic composition of samples as predicted by the KINTECUS model is and . The maximum enrichment seen in this set is 106.6‰ and 72.0‰ in and , respectively, corresponding to ozone compositions of 89‰ and 69‰ in and , respectively.

Image of FIG. 3.
FIG. 3.

A three-isotope plot of the experimental data observed in sets 3 and 4 along with results of model simulation. In set 3, mixture was photolyzed using visible lamp whereas in set 4, mixture was photolysed in UV light. The average and enrichments (difference between observed data and model prediction) are (126.8, 74.8) in set 3 and (128.1, 76.9) in set 4.

Image of FIG. 4.
FIG. 4.

Difference between value of the product and the value of initial CO (CO) plotted against CO pressure. In sets 1 and 2, between 270 and the average value is , which is close to the expected value (discussed in the text). The pattern suggests a surface effect at lower pressures. In sets 3 and 4, experiments were carried out in a chamber where the surface effect is smaller due to lower surface/volume ratio.

Image of FIG. 5.
FIG. 5.

Variation of factor (which enhances production by increasing ) with pressure. increases with decrease in CO pressure suggesting increased importance of surface induced stabilization of . For high CO pressure or experiments done in chamber, ranges from 1 to 55 whereas for low CO amount, value of could be as high as .

Image of FIG. 6.
FIG. 6.

A three isotope plot showing the oxygen isotopic composition of the product in case of set 4 and BT experiments. The average enrichment observed in BT experiments is and whereas in set 4 (present experiment) the values are and . Lower enrichment observed in BT case could be due to a larger amount of the product as compared to that of set 4.

Image of FIG. 7.
FIG. 7.

Three isotope plot showing correlation between enrichments in and of the produced from reaction. Enrichments in sets 1 and 2 fall on a line with a slope 0.87 (similar to the value in case of ozone formation by reaction). The enrichment varies from 70‰ to 136‰ and 41‰ to 83‰ in and . For sets 3 and 4, has higher enrichments (probably due to mass dependent fractionation) compared to that in sets 1 and 2 but the slope is similar .


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Table I.

Oxygen isotopic composition of produced from reaction for set 1. Ozone was made by Tesla discharge of . Average isotopic composition and amount based on control experiments are and and , respectively.

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Table II.

Oxygen isotope ratios of produced by reaction (Sets 2–4).

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Table III.

Change in value of product relative to the initial CO. ( is defined as []). is calculated for observed and also using model predicted value of . Corresponding CO pressures are also shown. A depletion of 4.3% is to be added to model value to account for stabilization.

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Table IV.

List of various gas phase reactions along with the rate constants used in the kinetic model (KINTECUS) for simulation of products from photolysis by visible light.

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Table V.

Effect of factor on the predicted oxygen isotopic composition of the product . Model A is for . In model B, was enhanced by to reproduce the observed amount of . The average difference between the two predictions is .

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Table VI.

List of reactions included in the KINTECUS model for simulating the photolysis reaction of mixture using UV light (set 4 and Bhattacharya-Thiemens data). Fractionation factors 0.983 (for species) and 0.989 (for species) are taken from Chakraborty and Bhattacharya (Ref. 33).

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Table VII.

Results of model simulation of Bhattacharya-Thiemens data [ and CO mixture was photolysed with UV lamp (case 1 to 8 Hg lamp and 9, 10 Kr lamp)] ( values are in ‰ relative to VSMOW). Enrichments defined as the difference of observed value from the model prediction are also shown.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Anomalous oxygen isotope enrichment in CO2 produced from O+CO: Estimates based on experimental results and model predictions