Three-dimensional survey of pollution during winter in French Alps valleys

We developed and installed a scientific payload on-board an ultra-light aircraft dedicated to ozone, nitrogen oxides and aerosol-scattering profilers to study the spatial distribution of pollution in the Chamonix and Maurienne valleys (French Alps). Meteorological parameters (temperature, pressure and relative humidity) and altimetry (GPS measurements) were recorded simultaneously. A synergy of measurements has been ensured with a ground mobile station equipped with a new lidar emitting at 532 nm. The experiment undertaken in January 2003 during the daytime allowed us to document, for the first time, the pollution level on horizontal and vertical scales during typical winter situations in the Alps valleys. Our results show a strong inversion of temperature up to about 150±50 m above the valley bottom, in the Chamonix valley, which traps the pollutants close to the snow-covered surface. Their concentrations are thus closely linked to the proximity of their sources and seem to have a local influence. In the Maurienne valley, with a bottom surface mostly free of snow, more efficient mixing due to convection is observed and a multi-layer structure between the planetary boundary layer and the free troposphere is detected on lidar vertical profiles. These lidar measurements allow a characterization of valley aerosols, with a backscatter to extinction ratio of 0.028 sr−1 very close to the value retrieved during the INDian Ocean EXperiment for a mixing of aerosols from biomass and fossil fuel burning.