It is proposed to investigate the mechanisms of heterogeneous reactions taking place between the important tropospheric constituents, NO2, NO, H2O2 and NH3 and certain simple carboxylic acids on water-ice surfaces. The systems employed will be relevant to the characterization of "missing" atmospheric chemistry, which has been suggested to be driven by cirrus cloud particulates. The core methodologies involved are based upon the successful water-ice, thin-film FT-RAIR studies and matrix isolation techniqu es previously performed in Professor Sodeau's laboratory. The experiments will be augmented by the complementary expertise possessed by Dr Hellebust to characterise ice surfaces and their catalytic activity by a variety of non-spectroscopic methods. Appro priate theoretical and empirical thermodynamic models will be used to consolidate the experimental results. All output information obtained will be made available to atmospheric modellers in order to improve our understanding of proposed sinks for the car boxylic acids and the release of compounds such as HONO by heterogeneous processes currently assessed to be important but with little confirmatory experimental proof.