Rayleigh/Raman aerosol retrieval technique and lidar design
The Rayleigh/Raman aerosol lidar technique uses the backscattering properties of the air molecules and particles. In the absence of particles (aerosols and clouds, typically above 30 km), the number of photons collected by the lidar telescope is directly proportional to the number of backscattering air molecules. In the presence of aerosols the lidar signals contain additional scattering due to larger scattering elements (commonly called ?Mie? scattering as opposed to Rayleigh scattering for small scattering elements). The so-called ?backscatter ratio? can be calculated by scaling the Rayleigh/Mie lidar signals to a ?pure? Rayleigh profile (e.g., a concurrent number density profile). By using a concurrent pressure profile, the so-called ?backscatter coefficient? and other aerosol-related properties can be deduced. When an additional channel collecting light Raman backscattered by Nitrogen is used, it is not necessary to use a concurrent density profile, and with the help of a few theoretical assumptions, some particles properties such as their shape, size, and composition, can be estimated. When so-called ?perpendicular? and ?parallel? light polarization channels are used, spherical and non-spherical particles can be differentiated.
The eighteen aerosol/cloud NDACC lidar systems are dedicated to the study of stratospheric volcanic aerosols (20-40 km), polar stratospheric clouds (15-25 km), and cirrus clouds (10-18 km). Fourteen systems are at fixed locations, four systems are mobile. The fourteen fixed systems are located at Observatoire de Haute-Provence (France), Hohenpeissenberg and Zugzpitze (Germany), Table Mountain Facility (California), Mauna Loa Observatory (Hawaii), London (Canada), Ny-Alesund (Spitzbergen), Andoya (Norway), Reunion Island (Indian Ocean), Dumont d?Urville and McMurdo (Antarctica), Sondrestromfjord and Thule (Greenland), and Boulder (Colorado). Two of the four mobile systems are home based at the NASA Goddard Space Flight Center (USA), the two others are home based in Germany.Use the following link to see the aerosol data availability chart at NDACC.