General information

The Lidar technique (LIght Detection And Ranging) uses a pulsed laser beam to probe the atmosphere. Time-resolved detection of the backscattered light allows precise determination of the altitude z of the scattering process:

z = c / 2 * t 

where c is the velocity of light and t the time.
The main scattering processes are molecular scattering (Rayleigh scattering) and aerosol scattering. Rayleigh scattering follows the well-known (wavelength)-4 law and can be computed quantitatively, provided the air density is known (by meteorological soundings). Aerosol scattering (often called Mie scattering, yet this term only applies to spherical scatterers) is an additional scattering process, which enhances the intensity of the backscattered light. The enhancement relative to the pure Rayleigh backscatter signal is called the backscatter ratio R and is a measure for the aerosol load in the air. In addition to the detection of the elastic backscatter of aerosols and molecules, inelastic Raman signals due to scattering on nitrogen and water molecules are detected as well. This allows the calculation of both extinction and backscatter coefficients (Ansmann, A.; Wandinger, U.; Riebesell, M. Weitkamp, C: Independent measurements of extinction and backscatter profiles in cirrus clouds by using a combined Raman elastic-backscatter Lidar; Applied Optics, 1992, Vol. 31, 7113-7131).
Depolarisation measurements enable conclusions of shape and therefore the state of aggregation of aerosols. 

At the moment, there are three different Lidar systems based in the observatory of the “Network for the Detection of Atmospheric Composition Changes” (NDACC):

The Stratospheric System is in operation since 1988 and used for the detection of Polar Stratospheric Clouds (PSCs) and the determination of ozone profiles.  

With the Koldewey-Aerosol-Raman-Lidar (KARL), which has been designed between 1998 and 2001, the determination of aerosols and water vapour in the arctic troposphere is performed.  

A Micro Pulse Lidar (MPL), owned by the National Institute of Polar Research (NIPR), Japan, is also based at the NDACC observatory. This system records clouds and aerosol concentrations in automatic and permanent operation (see data).