Lidar Measurements: Atmospheric Constituents ...
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useful that senses oxygen, another gas with a height-independent mixing ratio. From the
deviation of the ratio of the signals measured in the O2 and N2 channels from the expected
ratio the attenuation behavior of the aerosol can be deduced and corrected for.
Raman lidar offers yet another interesting aspect as to the determination of aerosol properties.
Whereas the elastic backscatter signal is affected by both the backscattering and the extinction
coefficient of an aerosol, the Raman signal undergoes extinction by the aerosols, but remains
unaffected from its backscattering behavior. With an elastic and one (N2 or O2) Raman channel
it is thus possible to measure the aerosol contributions to atmospheric backscattering and
extinction, (3 and 0, separately and independently of assumptions about relations between the
two.
The mathematical treatment of the procedure is developed in Ansmann et al. (1990), a detailed
error analysis can be found in Ansmann et al. (1992).
10.2.4 Fluorescence lidar
Fluorescence lidar is based on the direct absorption of radiation into a molecular or atomic
transition. Differently from the Raman effect the irradiating photons have to satisfy a resonance condition. Accordingly, fluorescence is orders of magnitude more intense than Raman
scattering. Although fluorescence always relies on some resonance effect, the return of the atom
or molecule to its ground state with emission of the primary wavelength is often but inconsistently called resonance fluorescence, whereas this term is not used when deexcitation occurs
to another level and the return radiation is shifted in wavelength.
In the troposphere and lower stratosphere applications of fluorescence Ii dar have so far remained
unimportant. Naturally occuring atmospheric gases do not fluoresce, and fluorescence of pollutants such as aromatic hydrocarbons is too weak for quantitative measurements. The technique
has, however, been used for the pursuit of plumes or puff-like emissions to which a fluorescent
tracer like fire orange was purposely added (Kyle et al., 1982); in one case the material could
be traced up to several hundred kilometers downwind from the emission, although 50 kg of dye
had to be used for the experiment (Uthe et aI., 1985).
Fluorescence lidar has been very successfully used for stratospheric studies of atomic and ionic
species present in heights between 75 and 110 km. Sodium, potassium, lithium and other
elements have been observed (Sandford and Gibson 1970; Hake et al., 1972; Felix et al., 1973;
Megie et al., 1978; Pelon et al., 1980; Beattyet al., 1988; Tilgner and von Zahn, 1988). In the
maximum of the distribution around 90 km the concentration of Na atoms is only 5 particles
per mm 3 , and thus almost 16 orders of magnitude less than the density of air molecules at
standard conditions. Concentrations of K and Li are even lower. Measurements have been
made down to 10- 3 of the maximum concentration. This proves the sensitivity of the method
which lends itself to the study of the movements of the upper part of our atmosphere known
as gravity waves (Hauchecorne and Chanin, 1980).
10.2.5 DIALEX and ground reflection
If the measurement system is mounted on an airplane or satellite and the ground is used as a
(topographic) reflector, the differential absorption technique with two wavelengths can be used
even with a non-depth-resolving system (and, thus, cw lasers) to determine the total amount or
average concentration of a gas in the air column. Such a system called DIALEX was proposed
by Wiesemann et aI. (1978) and Boscher et al. (1980). Although not a lidar, a device of this
kind could be useful for rapid mapping of average loads of pollutants over extended areas.
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useful that senses oxygen, another gas with a height-independent mixing ratio. From the
deviation of the ratio of the signals measured in the O2 and N2 channels from the expected
ratio the attenuation behavior of the aerosol can be deduced and corrected for.
Raman lidar offers yet another interesting aspect as to the determination of aerosol properties.
Whereas the elastic backscatter signal is affected by both the backscattering and the extinction
coefficient of an aerosol, the Raman signal undergoes extinction by the aerosols, but remains
unaffected from its backscattering behavior. With an elastic and one (N2 or O2) Raman channel
it is thus possible to measure the aerosol contributions to atmospheric backscattering and
extinction, (3 and 0, separately and independently of assumptions about relations between the
two.
The mathematical treatment of the procedure is developed in Ansmann et al. (1990), a detailed
error analysis can be found in Ansmann et al. (1992).
10.2.4 Fluorescence lidar
Fluorescence lidar is based on the direct absorption of radiation into a molecular or atomic
transition. Differently from the Raman effect the irradiating photons have to satisfy a resonance condition. Accordingly, fluorescence is orders of magnitude more intense than Raman
scattering. Although fluorescence always relies on some resonance effect, the return of the atom
or molecule to its ground state with emission of the primary wavelength is often but inconsistently called resonance fluorescence, whereas this term is not used when deexcitation occurs
to another level and the return radiation is shifted in wavelength.
In the troposphere and lower stratosphere applications of fluorescence Ii dar have so far remained
unimportant. Naturally occuring atmospheric gases do not fluoresce, and fluorescence of pollutants such as aromatic hydrocarbons is too weak for quantitative measurements. The technique
has, however, been used for the pursuit of plumes or puff-like emissions to which a fluorescent
tracer like fire orange was purposely added (Kyle et al., 1982); in one case the material could
be traced up to several hundred kilometers downwind from the emission, although 50 kg of dye
had to be used for the experiment (Uthe et aI., 1985).
Fluorescence lidar has been very successfully used for stratospheric studies of atomic and ionic
species present in heights between 75 and 110 km. Sodium, potassium, lithium and other
elements have been observed (Sandford and Gibson 1970; Hake et al., 1972; Felix et al., 1973;
Megie et al., 1978; Pelon et al., 1980; Beattyet al., 1988; Tilgner and von Zahn, 1988). In the
maximum of the distribution around 90 km the concentration of Na atoms is only 5 particles
per mm 3 , and thus almost 16 orders of magnitude less than the density of air molecules at
standard conditions. Concentrations of K and Li are even lower. Measurements have been
made down to 10- 3 of the maximum concentration. This proves the sensitivity of the method
which lends itself to the study of the movements of the upper part of our atmosphere known
as gravity waves (Hauchecorne and Chanin, 1980).
10.2.5 DIALEX and ground reflection
If the measurement system is mounted on an airplane or satellite and the ground is used as a
(topographic) reflector, the differential absorption technique with two wavelengths can be used
even with a non-depth-resolving system (and, thus, cw lasers) to determine the total amount or
average concentration of a gas in the air column. Such a system called DIALEX was proposed
by Wiesemann et aI. (1978) and Boscher et al. (1980). Although not a lidar, a device of this
kind could be useful for rapid mapping of average loads of pollutants over extended areas.