The uptake of HNO₃ and HCI on water-ice films has been investigated using a coated wall flow reactor at temperatures typical of the upper troposphere. The uptake was followed by sensitive detection of gaseous species using a quadruple mass spectrometer. The uptake coefficients and surface coverage have been measured as a function of temperature (205 – 250) and reactant partial pressure (~10^-7 Torr). Using a Langmuir absorption isotherm and the surface coverage data, the enthalpy and entropy of absorption have been evaluated. The uptake coefficients showed a negative temperature dependence in both cases. It was concluded that HNO₃ has a higher propensity for absorption to ice than HC1 under these conditions. Competitive absorption experiments confirmed this conclusion. HNO₃ was found to promote HC1 desorption when both gases were exposed to an ice surface simultaneously. The surface coverage of HC1 on HNO₃-doped ice was found to be an order of magnitude lower and completely reversible compared to the uptake on bare ice. It was concluded that in the presence of HNO₃, reactions which require absorbed HC1 may be slower than anticipated on the basis of experiments which do not account for competitive absorption. One such reaction is C1ONO₂ + HC1 → C1₂ + HNO₃. The reactive uptake of C1ONO₂ has been studied at 218 and 228 K and at partial pressures ~ 10^-7 Torr. On bare ice, HOC1 was observed in the gas phase immediately upon exposure of C1ONO₂. The uptake coefficient of C1ONO₂ was found to have a weak negative partial pressure dependence. On HC1-doped ice, C1₂ was observed immediately upon C1ONO₂exposure. In a final set of experiments, HC1 was taken up onto HNO₃-doped ice. The surface coverage of HC1 was an order of magnitude lower than on bare ice.

Automotive three-way catalytic converters employ well-established technology for the control of NO<SUB>x</SUB>, CO, and hydrocarbon emissions, which are known to contribute to air pollution. This technology exploits the catalytic properties of platinum, palladium, and rhodium metals, of which rhodium is the crucial component in relation to the reduction of NO<SUB>x. </SUB> In order to minimise costs and meet increasingly stringent emission legislation, vehicle manufacturers have an interest in improving the performance of current catalyst systems, as well as developing catalyst compatible with lean-burn (oxygen rich) engines, which offer substantial improvements in fuel economy. The first part of this research, "Idealised system", has focused on studying the promotional effect of alkali metals in order to (i) enhance the catalytic properties of supported platinum-group metals, and (ii) promote the reduction of NO<SUB>x </SUB> in the presence of excess oxygen. Atmospheric microreactor measurements were used to demonstrate that, in a similar manner to platinum and palladium, the addition of sodium to rhodium catalysts enhances the activity and nitrogen selectivity for the reduction of NO in the absence of oxygen. This promotional effect was also apparent for sodium-promoted platinum and palladium catalysts operated under simulated exhaust conditions, with oxygen present in the feed stream. Significantly, however, under identical conditions sodium addition to rhodium has a detrimental effect as manifested by severe poisoning and decreased selectivity. A single mechanism has been established to account for the catalytic behaviour of the metals in the presence of sodium, based on the electronic influence of sodium on the coverages of the reactants over the metal surface. In the second part, "Practical systems", the relationship between ageing conditions and catalytic performance has been investigated for commercial three-way catalysts. Variations in oxygen storage and activity are observed depending on the ageing conditions experienced, and the original position of the catalyst in the exhaust system. Extensive catalyst characterisation was performed, including techniques used for the first time with these types of samples, in an attempt to explain the differences in performance.

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Air samples. from four aircraft campaigns and two surface experiments covering a large geographical area over both hemispheres have been analysed using the technique of gas chromatography mass spectroscopy (GCMS). Around 40 trace gases have been detected and where possible quantified, including most of the CFC's, HCFC's, and halon's, HFC-134a, HFC-152a, HFC-365mfc, CCI4, CH3CCb, CH3Br, CH3CI, and a number of short lived halogenated compounds including CH31, CHBr3, CH2BrCI, and n-C3H7Br. From the data obtained it has been possible to investigate the spatial and temporal variability of halogenated compounds, including variations with altitude, longitude, and interhemispheric ratios. Significant quantities of the short lived bromocarbons (CHBr3, CH2Br2, CH2BrCI, CHBr2CI, CHBrCI2, n-C3H7Br) and methyl iodide (CH31) have been observed in air samples collected within the upper troposphere lower stratosphere (UTLS) region of the atmosphere, implying that the contribution of these compounds to stratospheric ozone loss is potentially significant. Evidence has also been found which suggests that sub-tropical regions of the Pacific Ocean and' coastal waters around the Phillipines are particularly productive source regions for all of these compounds. In addition using data from a plant enclosure study the potential for terrestrial ecosystems to both emit and take up a number of halogenated compounds has been identified. Although the significance of these results remains unclear. The atmospheric mixing ratios of all four CFCs measured in this study are decreasing due to the reduction in global emissions of these compounds as a result of their regulation under the Montreal Protocol. A significant decrease in the mixing ratios of carbon tetrachloride (CCI4) and methyl chloroform (CH3CCb) has also been observed in the 2000 - 2006 CARIBIC data. In contrast there is evidence to suggest that halon's 1211 and 2402 are still slowly increasing in the atmosphere. For all the HCFC's and HFC's measured in this study (HCFC-21, HCFC22, HCFC-141b, HCFC-142b, HCFC-124, HCFC-123, HFC-134a, HFC-152a, and HFC365mfc), clear inter hemispheric gradients were observed with higher mixing ratios in the NH, consistent with a predominantly NH anthropogenic source for these compounds. However there is evidence to suggest that emissions from Asia are becoming increasingly important for a number of these compounds. In addition evidence was found which suggests a continuing source for CFC-11, CFC-12, CFC-113 and CFC-114.

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