Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 54

Fig. 3.10 ( a ) Schematic of the arrangement of Raman-shifted backscattered wavelengths, and ( b )
evaluation methods for a Raman-LIDAR
In a relatively cool atmosphere, the scattering air molecules are nearly always in
their energetic ground state. Therefore, light from Raman scattering has usually less
energy than the radiation emitted from the LIDAR (Stokes scattering, small lines to
the right of the main lines in Fig. 3.10a ). With rising temperature an increasingly
number of air molecules can be in excited states, so that Raman scattering also
from jumps to energy levels below the initial level becomes possible (Anti-Stokes
scattering, small lines to the left of the main lines in Fig. 3.10a ). Then, scattered
light with a higher frequency than the emitted one can be detected as well. But
this more energetic radiation from anti-Stokes scattering is weaker than the less
energetic radiation from Stokes scattering.
A Raman LIDAR permits with different evaluation methods the detection of trace
gas profiles from characteristic frequency shifts, the determination of type and size
of aerosols from extinction measurements, the classification of aerosols from depo-
larization measurements, and the determination of air temperature from analysing
the temperature-dependent ratio of different Raman frequencies (see Fig. 3.10b ).
While stratospheric temperature profiles, which can be derived with the integration
method (Keckhut et al. 1990 ), are irrelevant for the boundary layer, temperature and
particle profiles in the troposphere are deducible with the rotational Raman method
(Cooney 1972 ; Behrendt and Reichardt 2000 ). Humidity and particle profiles are
obtainable with the vibrational Raman method (Cooney 1970 ; Behrendt et al. 2002 ).