You are confising the ozone layer, with ground level ozone. They are two different problems. The ozone layer shields us from excess radiation, whereas ground level ozone is the main contributer to smog. The cat will reduce those pollutants that cause ground level ozone, or smog, which is the principal air pollution problem in this country. It so happens that mobile sources (cars, trucks) either significantly contribute to, or are the main cause of ozone in many metropolitan areas (Dallas, TX, for instance). Diesels will also soon be forced to stricter levels of emissions, with the new low sulphur fuels, and their own version of cats that trap particulates.