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Tropospheric Ozone Isotope Measurements and the Ozone Formation Process


Since it was first observed on a stratospheric balloon flight in 1980, numerous laboratory experiments and stratospheric studies have examined the enrichment in heavy ozone isotopes. From laboratory investigations, the dependence of the enrichment on pressure and temperature have been found, while stratospheric measurements have shown that the enrichment may be highly variable, and often changes with altitude. Until this thesis, however, the enrichment in tropospheric ozone has not been measured. To this end, an ozone collector has been developed and used to determine the isotopic composition of tropospheric ozone. After numerous measurements, the enrichment in tropospheric ozone has been found to be constant, at 8.8+/-1.0% for mass 50 and 6.3+/-1.6% for mass 49, though it changes slightly in winter and summer due to the known temperature dependence of the enrichment. Furthermore, the enrichments in mass 49 and 50 are in agreement with predictions from laboratory studies under the same conditions of pressure and temperature. This provides the first confirmation that laboratory studies are adequately simulating conditions in part of the atmosphere, as measurements of the enrichment in stratospheric ozone have not always agreed with results from the laboratory.

To date, the heavy isotope enrichment in ozone has not been adequately explained by any theory, although a number of models have been developed. The most promising assumes a symmetry-dependent transition between a metastable state and the ground state of ozone, which leads to the isotope fractionation. As metastables tend to be extremely reactive, it is possible that the presence of such metastable ozone molecules in the atmosphere would have implications for atmospheric modeling. Therefore, what began with the observation of an enrichment in the heavy isotopes of ozone may lead to a new understanding of ozone in the atmosphere.

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