Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 53

gas ( λ min ) experiences nearly not absorption.

The two frequencies are chosen so close to each other that no difference in the
aerosol backscatter is detectable and the influence of the aerosol can be elimi-
nated by subtracting the two signals from each other. The detected intensity of the
backscattered light at frequency

λ max ) while the other frequency (

λ max is inversely proportional to the concentration
of the absorbing trace gas. Principally, a DIAL is a backscatter LIDAR. The ratio of
the backscattered optical intensities at the two frequencies is (Colls 2002 )

λ max ) / λ min ) = − / σ

I ( I ( exp( 2 r N ), (3.20)

where r is the distance of the backscattering air volume, N is the number con-
centration of the absorbing trace gas, and σ is the absorption cross section of the

absorbing gas.
The following types of laser are available for a DIAL: (a) dye laser with fre-
quency multiplier (the wave lengths may be chosen arbitrarily but these instruments
require much attendance and adjustment, so that they are not suitable for moni-
toring purposes), b) Nd:YAG laser (1064 nm) with frequency multiplier (266 nm)
and subsequent Raman shift in deuterium (289 nm) and/or hydrogen (299 nm), (c)
krypton fluoride excimer laser (248 nm) and subsequent Raman shift in deuterium
(268, 292, 319 nm) and/or hydrogen (277, 313 nm). Wulfmeyer ( 1998 ) developed
an enhanced DIAL based on an alexandrite laser with reduced noise and reduced
systematic errors that is able to measure water vapour fluctuations for moisture flux
determinations.
A DIAL yields a vertical resolution of about 75 m for ozone measurements, a
temporal resolution of about 60 s for a height range between several hundreds of
metres and many kilometres.

3.5.3 Raman-LIDAR

A further possibility for the detection of trace gas profiles in the atmosphere is
the operation of a Raman LIDAR, which records radiation from Raman scatter-
ing at trace gas molecules (Cooney 1970 , 1972 ; Ansmann et al. 1990 ; Turner et al.
2002 ; Weitkamp 2005 ). With normal elastic scattering a molecule absorbs a pho-
ton, enters an excited state, and immediately emits (backscatters) the photon again
while returning to the ground state. With inelastic Raman scattering, the molecule
does not return to the ground state but to a neighbouring energy level so that a pho-
ton with a slightly changed (usually reduced) frequency is emitted. The observed
frequency shift is characteristic for a given trace gas species (Fig. 3.10 ). The pro-
cedure is named after the Indian physicist, Chandrasekhara Venkata Raman, who
first reported on the experimental discovery of this frequency shift in 1928. The dis-
advantage of the method is that the inelastically backscattered signal intensities are
two to three orders of magnitude lower than with the elastic Rayleigh scattering.
Therefore, measurements with a Raman LIDAR are possible only at nighttime or
with special provisions to exclude other scattered light.