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Physical chemistry of acid systems relevant to stratospheric ozone depletion

Thermodynamic properties of various binary H 2S0 4/H 20, HNO3/H 20 and HCl/H20 as well as ternary H 2S0 4/HNO3/H 20 solutions were studied using differential scanning calorimetry. Stable as well as metastable hydrates were identified. The supercooling trend of the solutions was also studied for both bulk and small samples. In general, H2S0 4/H 20 solutions supercooled more readily than the other binary solutions, suggesting that H 2S0 4/H 20 solutions are in liquid form in the stratosphere. A time study was also conducted to investigate the likelihood that various ternary solutions (H2S0 4/HNO3/H 20) would freeze. Binary H2 S0 4 /H 2 0 and HNO3/H 20 solutions were also studied to compare freezing probabilities of these binary solutions to the ternary solutions under the same equilibrium stratospheric conditions. A PSC formation mechanism in which NAT crystallizes out first is introduced based on results from this time study. Finally, an experiment has been designed to study heterogeneous reactions on a liquid surface and measure vapor pressures of various solutions. The apparatus consists of a wetted-wall flow tube in conjunction with a molecular beam mass spectrometer. Preliminary data is presented to ensure the feasibility of the experiment. Thesis Supervisor: Mario J. Molina Title: Martin Professor of Atmospheric Chemistry