Valley wind systems and their influence on nocturnal ozone concentrations

In September 1992 the measuring campaign of the EUROTRAC subproject TRACT was performed to investigate the boundary layer structure and air pollutant transport over complex terrain. During the campaign a large number of measurements were carried out by more than 40 international working groups. In addition, data were collected from approx. 350 ground based stations operated by national weather services and environmental protection agencies. This paper relies mainly on the data received from ground based operational networks. It consists of two parts: In Part I four local wind systems within the TRACT area (Rench valley, Kinzig valley, Dreisam valley and Murg valley) with their diurnal characteristics are introduced. A distinction was made between two groups of valley-wind systems, differing in behaviour as regards the ratio of daytime wind velocity to night time wind velocity. Part II deals with the influence of the topographically induced flow systems on the ozone concentrations. Stations in side-valleys of the upper Rhine valley often showed less nocturnal ozone removal (compared to stations on flat terrain) or even a clear secondary ozone maximum in the night due to orographically channeled down-valley advection of ozone-rich and little polluted air from the mountains.