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). 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, certain properties of the particles such as shape, size, and composition, can be estimated. When so-called "perpendicular" and "parallel" light polarization channels are used, spherical particles (e.g., liquid water clouds) and non-spherical particles (e.g., cirrus/ice clouds) can be differentiated.

Data from nineteen aerosol/cloud lidar systems have contributed to NDACC since 1990. They are dedicated to the study of stratospheric volcanic aerosols (20-40 km), polar stratospheric clouds (15-25 km), and cirrus clouds (10-18 km). As of 2020, there remains nine operational systems at fixed locations, and two mobile systems. The 9 fixed systems are located at Observatoire de Haute-Provence (France), Hohenpeissenberg and Zugzpitze (Germany), Table Mountain Facility (California), Mauna Loa Observatory (Hawaii, 2 systems), Ny-Alesund (Spitzbergen), Reunion Island (Indian Ocean), Dome C (Antarctica), and Thule (Greenland). The 2 mobile systems are home based at the NASA Goddard Space Flight Center in Greenbelt, Maryland (USA).

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