Due to thermally induced wind systems during the summer months Alpine valleys contribute effectively to vertical mixing between the atmospheric boundary layer and the lower free troposphere (LFT). In former studies it was shown that up to five times the valley volume is exported to the LFT in one up-wind phase in the Mesolcina valley, southern Switzerland. During high-pressure episodes with weak synoptic forcing, emissions from the valley floor are carried upwards in thermal plumes and start to accumulate in the so-called injection layer above the mountain tops. Due to dilution of the air pollutants the ozone production efficiency (molecules ozone produced per molecule NOx consumed) might increase while the air masses are lifted from the valley floor to the mountain tops.

Within the CHAPOP (Characterization of High Alpine Pollution Plume) project partially sponsored by CAATER (Co-ordinated Access to Aircraft for Transnational Environmental Research) field measurements were carried out in the Leventina valley in southern Switzerland during the summer of 2001. The Leventina is part of the most important transalpine road trafficroute in Switzerland, The results will enable us to quantify the vertical transport of air pollut-ants in the transalpine traffic regions and to understand the chemical processes in the transported air mass.

The investigated valley is characterised by very steep slopes, high rising crests up to 3 km asl, a valley floor increas-ing from 0.5 km asl in the south-east to 1.2 km asl in the north-west. On three flight days three aircrafts could be used to simultaneously acquired meteorological, chemical and aerosol parameters within the valley and the injection layer. One aircraft carried a nadir-pointing aerosol LIDAR that supplied important information about boundary layer development and aerosol distribution by flying at 8 km asl.

The PSI mobile laboratory MOSQUITA measured gases and aerosols along the valley floor and at the slopes. Con-tinuous measurements of trace gases, and meteorological parameters were conducted during one month at three sites, including formaldehyde and detailed VOC analysis at one site within the valley. Another station, on top of the crest 2.2 km asl, measured within the injection layer and the LFT. Wind profiles up to 1km agl were also taken at this site.

The recorded data set will be analysed with respect to the air mass balance in the valley. First results indicate high vertical transports out of the valley with a multi-layer structure between 3 and 4 km asl. Furthermore dynamics are discussed by analysis of the spatial distribution of more inert trace gases like carbon monoxide.