During the POLLUMET field campaign, high resolution aircraft measurements at approximately 4 km altitude have been taken over the Alps. These measurements show gravity wave activity and wave breaking. The analysis of the data suggests that wave breaking is a frequent event and the associated turbulence provides a mechanism for effective vertical mixing. This has important implications for the transport of pollutants. The analysis of the data has been compared to simulations using a non-linear gravity-wave and turbulence model.