Remote Sensing of Stratospheric Ozone with the Airborne OLEX Lidar

The OLEX (Ozone Lidar Experiment) aboard the DLR research aircraft Falcon 20 uses two different lidar techniques: The backscatter lidar technique performed at different wavelengths gives the optical properties of aerosols and PSCs (Polar Stratospheric Clouds) while the differential absorption lidar technique (DIAL) applied in the ultraviolet spectral range allows to retrieve the ozone number density in the stratosphere with high spatial resolution. The DIAL technique does not need any calibration of the received signals and, being an active optical method, is an independent measurement technique particularly well suited for the study of ozone depletion mechanisms (see figure), as well as ozone filaments at the edge of the polar vortex. A further application with increasing importance is the validation of passive satellite sensors by under-flying the satellite's orbit.

The OLEX system on board of the research aircraft Falcon is a vertically upward pointing 4-wave-length lidar. It consists of two lidar transmitters. One is a Nd:YAG laser with second and third harmonic generation (i.e. 1064nm, 532nm, and 355nm). The other one is a Xe:Cl excimer laser emitting at 308nm. The two UV wavelengths (308nm and 355nm) are used to retrieve stratospheric ozone profiles from about 2 km above the flight level to 26 km altitude by means of the differential absorption lidar (DIAL) technique. From the ratio of the backscatter signals at these two wavelengths - one of them is being absorbed by ozone while the second is not - profiles of ozone number density can be calculated. Furthermore, the returns at the three Nd:YAG wavelengths contain information about the size distribution of the scattering particles. The capability to measure the cross polarised return at 532 nm allows for phase discrimination, i.e. ice/water discrimination and PSC classification.