A ceilometer has been modified so as to measure the amount of light backscattered from aerosols. By studying the signal generated by aerosols trapped within thermal stratifications occurring in the first four kilometers of the atmosphere, one can determine the height of the Mixing Layer.

A Vaïsala CT 25 K ceilometer's software has been adapted by the constructor to access the backscatter signal. The data acquired is an average over a variable duration ranging from 15 to 135 seconds of the instrument's internal data. Visualization of the data has been done with a Vaïsala-supplied task-specific software as well as with other softwares.

This system is currently being evaluated over a very long measurement campaign at Météo-France's Trappes instrument park. It has also been used during the ESCOMPTE campaign (June-July 2001), where it could be compared with radiosoundings, with the aim of monitoring thermal stratifications and aerosol accumulation, in ozone plumes maxima areas.

Preliminary results indicate a good fit between zones of strong vertical backscatter gradient on the one hand and radiosounding-observed thermal stratifications on the other. Furthermore we have noticed a striking correlation between some backscatter visualization images and humidity profiles.

In the poster the most characteristic results seen so far will be presented.