TROPOSPHERIC OZONE IN THE PRE-ALPINE AND ALPINE REGIONS

Surface ozone measurements taken at different altitudes in the south-w estern and eastern alpine regions from 1987 to 1991 are compared in te rms of seasonal and daily fluctuations. The annual mean levels increas e with altitude, ranging from about 20 parts-per-billion by volume, i. e. 10(-9) v/v (ppbv), on the plain to 50 ppbv at 3500 m. The transport of ozone and/or precursors from the plain, the photochemical processe s and the exchanges with the free troposphere are the main processes i nfluencing its distribution in these regions. In the warm season, site s at intermediate altitudes up to about 1800 m may suffer from higher ozone exposure than sites at high altitudes. For instance, in July 199 1, the monthly mean levels were 65 ppbv at 490 m (Brione), 70 ppbv at 920 m (Mottarone) and 67 ppbv at 1650 m (Cimetta), all higher than 50 ppbv at 3580 m (Jungfraujoch). The irregular distribution is due to th e advection of an ozone front during the day and the persistent high l evels during the night. The highest hourly levels (up to 185 ppbv) obs erved in the years 1989-91 were associated to subsidences of upper dry layers on a regional scale.