Backscatter lidar measurement of aerosol stratification in the atmosphere

The atmospheric aerosol, its stratification and the principal dynamics controlling the air exchange at the top and the base of the aerosol layers are of key importance for understanding critical atmospheric phenomena such as the transport and impact of air pollution, the destruction of the ozone layer and the evolution of the greenhouse effect. In particular, it is the detection of stratification within the atmospheric boundary layer, the lower Troposphere and the regions around the Tropopause that are of importance to comprehending these phenomena. Elastic backscatter lidar is a novel tool promising to complement and partially to replace established atmospheric measuring techniques for quantifying these effects. The present thesis is investigating the theoretical and practical relevance of lidar in fulfilling this promise. The thesis performs an evaluation of two lidar methods to identify the structure of the boundary layer as well as their validation with respect to traditional methods such as radiosondes employed in atmospheric measurements. The validation shows that the lidar derived structure of the boundary layer is congruent with the structure derived from radiosonde temperature measurements. This result substantiates the use of elastic backscatter lidar for considerably advanced boundary layer monitoring compared to presently accepted methods. These measurements are then used to determine the frequency of the boundary layer top height variation. It is demonstrated that this method permits the identification of gravity waves through the boundary layer, consequently allowing the use of lidar for the investigation of the vertical exchange of pollution constituents. The monitoring of the height of the boundary layer has subsequently been carried out over a one-year period of measurements above the urban site of Basel, Switzerland. This campaign demonstrates the key role of lidar in conducting measurements over such long intervals, which is otherwise not possible with established instruments. The thesis provides complementary results collected by two airborne lidars in the upper Troposphere and the lower Stratosphere. The detection of specific clouds in this atmospheric region has critical importance for two aspects in the stratospheric ozone balance: polar stratospheric clouds are responsible for initiating ozone destruction, and the ultra-thin optical cirrus are linked to the water and radiation budget as well as to large-scale transport. The significance of the present thesis lies in conclusively demonstrating that elastic backscatter lidar contributes with both unique and complementary information to the present inventory of atmospheric instruments. This is substantiated by showing that the obtained results allow straightforward interpretation of the probed atmosphere, where they are particularly advantageous in terms of simplicity, reliability and information content, with respect to traditional, established methods.

Thèse de doctorat : Université de Neuchâtel, 2006 ; 1913