Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.459677
Title: Studies of reactions relevant to atmospheric photochemistry
Author: Horner, Martin Grenville
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1976
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Abstract:
Three main catalytic chain processes are believed to contribute to the loss of ozone in the stratospheric region of the earth's atmosphere. These are the HOx , the ClOx and the NOx catalytic processes. This thesis describes experiments conducted to elucidate mechanisms involved in the first two of these; the HOx and the ClOx chain processes. In addition the efficiency of energy transfer from O(1D) to molecular oxygen is measured. A fast-flow/photolysis system with mass spectrometric detection has been developed to study the reactions of O(1D) generated by the photolysis of ozone at 2537Å. The method of relative quantum yields, wherein the quantum yield for loss of ozone in a series of reactions is quantitified by reference to the loss of ozone in the photolysis of O3/N2 and O3/He mixtures, is used to monitor ozone loss. Under suitable concentration-time conditions the relative quantum yield for ozone loss in the photolysis of O3/N2 and O3/He mixtures is 1.0 and 2.0, respectively. From measurement of the relative quantum yield for ozone loss in oxygen carrier gas the efficiency of energy transfer to produce O2(1∑g+) in the reaction O(1D) + O per O(1D) atom deactivated is found to be 0.63 ± 0.18. Observations of the relative quantum yield for ozone loss in the photolysis of O3/H2 mixtures in helium carrier gas failed to show any evidence for the occurence of a catalytic chain process. On this basis it is believed that the reaction of vibrationally excited OH* , produced by the reaction H + O3, with ozone does not generate H atoms as products. The observations are consistent with the reaction OH* + O3 producing HO with unit efficiency. This result implies that the Norrish mechanism is the one most likely to subsist in the HOx catalytic loss of ozone in the gas phase. The rate of quenching of O(1D) by sulphurhexafluoride relative to the rate of reaction with ozone is found to be (2.1 ± 1.0) x 10-4 from observations of the relative quantum yield for the photolysis of ozone in SF6 carrier gas. Qualitative observations show that the relative rate of quenching of O(1D) by carbon tetrafluoride is unlikely to be significantly faster than this. The relative quantum yields for the photolysis of ozone in the helium carrier gas and in the presence of CF3CL, CF2Cl2, CFCl3 and CCl4 are found to be ca 1.5 , 2.0, 1-2 (dependant on [O3] and t , and 2-4 , respectively. The quantum yields for the photolysis of CF3Cl/O3, CF2Cl2/O3 and CFCl3/O3 mixtures show an increase on addition of nitric oxide. This is interpreted in terms of a catalytic chain involving formation of ClO by abstraction of chlorine fronythe freon by O(1D), the reaction ClO + NO ⟶ Cl + NO2 and regeneration of ClO in the reaction Cl + O3. Calculation shows that the efficiency for ClO production in the reaction O(1D) + CF2Cl2 is approximately 0.3. The other major reaction pathway is believed not to be physical quenching of O(1D). Observation of a product peak at m/e ≃ 66 is interpreted in terms of the production of chlorine dioxide in the second reactive pathway.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.459677  DOI: Not available
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