[Ozone in the layer of the atmosphere closest to the ground].

High ozone concentrations are regularly observed and of wide-spread appearance in the planetary boundary layer during summerly high pressure conditions. The ozone is a product of photochemical reactions involving as precursors nitrogen oxides and volatile organic compounds. The emission of both precursors in Switzerland is discussed. During summer-smog episodes, the time series of ozone concentration shows a typical diurnal pattern, with low values in the early morning and high values in the mid-afternoon. At mountain stations this pattern is much less pronounced. Historical data show that the natural ozone background is considerably lower than today's measured values at mountain top stations. In Switzerland, pronounced spatial differences in the ozone levels, caused by the complex topography, can occur. Reduction of the emission of precursors is the only way to reduce high ozone levels. Several reduction scenarios are discussed. In Switzerland the combined reduction of both precursors is the most successful strategy.