Abstract
The isotopic fractionation associated with photodissociation of N2O, OCS and
CO2, at different altitudes in Earth’s atmosphere, is investigated theoretically using constructed quantum mechanical models of the dissociation processes (i.e. potential energy surfaces and relevant coupling elements for the important electronic states). The quality of these models are assessed by calculating a range of observable for the processes (e.g. UV-spectra and product state distributions) and comparing the results to availble experimental data. The main findings are that: (i) Photodissociation of N2O in Earth’s stratosphere is nearly mass dependent, and only a small fraction of the observed anomalous oxygen-17 excess can be attributed to N2O photolysis. In contrast, stratospheric photolysis produces a significant inverse clumped isotope effect.(ii) Stratospheric OCS photolysis significantly enrich the remaining OCS in
heavy carbon. The sulfur fractionation is weak and photolysis of OCS in the
stratosphere produces only a small and mass dependent enrichment of heavy sulfur
isotopes in the remaining OCS. Sulfur fractionation from the two remaining chemical sinks (oxidation by O(3P) and OH, respectively) is weak or moderate, and overall sulfur fractionation in the stratosphere is very weak which does not exclude OCS from being an acceptable background the Stratospheric Sulfate Aerosol layer. (iii) CO2 photolysis in the upper stratosphere and lower mesosphere is highly fractionating in both isotopes, enriching the remaining CO2 and depleting the produced CO in heavy isotopes. But photolysis of CO2 is too slow to produce significant fractionation in the remaining CO2. In contrast, the produced CO is highly fractionated and possesses a significant non mass dependent perturbation and a clumped isotope signal. Given that CO2 photolysis is a significant source of CO in the upper stratosphere and mesosphere these signals should be clearly traceable. These signals become small for photolysis at higher altitudes.
CO2, at different altitudes in Earth’s atmosphere, is investigated theoretically using constructed quantum mechanical models of the dissociation processes (i.e. potential energy surfaces and relevant coupling elements for the important electronic states). The quality of these models are assessed by calculating a range of observable for the processes (e.g. UV-spectra and product state distributions) and comparing the results to availble experimental data. The main findings are that: (i) Photodissociation of N2O in Earth’s stratosphere is nearly mass dependent, and only a small fraction of the observed anomalous oxygen-17 excess can be attributed to N2O photolysis. In contrast, stratospheric photolysis produces a significant inverse clumped isotope effect.(ii) Stratospheric OCS photolysis significantly enrich the remaining OCS in
heavy carbon. The sulfur fractionation is weak and photolysis of OCS in the
stratosphere produces only a small and mass dependent enrichment of heavy sulfur
isotopes in the remaining OCS. Sulfur fractionation from the two remaining chemical sinks (oxidation by O(3P) and OH, respectively) is weak or moderate, and overall sulfur fractionation in the stratosphere is very weak which does not exclude OCS from being an acceptable background the Stratospheric Sulfate Aerosol layer. (iii) CO2 photolysis in the upper stratosphere and lower mesosphere is highly fractionating in both isotopes, enriching the remaining CO2 and depleting the produced CO in heavy isotopes. But photolysis of CO2 is too slow to produce significant fractionation in the remaining CO2. In contrast, the produced CO is highly fractionated and possesses a significant non mass dependent perturbation and a clumped isotope signal. Given that CO2 photolysis is a significant source of CO in the upper stratosphere and mesosphere these signals should be clearly traceable. These signals become small for photolysis at higher altitudes.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Number of pages | 184 |
| Publication status | Published - 2013 |