Tropospheric ozone in the pre-alpine and alpine regions

Surface ozone measurements taken at different altitudes in the south-western and eastern alpine regions from 1987 to 1991 are compared in terms of seasonal and daily fluctuations. The annual mean levels increase 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 processes and the exchanges with the free troposphere are the main processes influencing its distribution in these regions. In the warm season, sites at intermediate altitudes up to about 1800 m may suffer from higher ozone exposure than sites at high altitudes. For instance, in July 1991, 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 the advection of an ozone front during the day and the persistent high levels during the night. The highest hourly levels (up to 185 ppbv) observed in the years 1989–1991 were associated to subsidences of upper dry layers on a regional scale.