Laser
remote-sensing techniques (lidar systems), have gained high
acceptance as long-range non-invasive probes of the chemical
composition and physical properties of the atmosphere. Through
its high spatial and temporal resolution, the lidar technique
is a powerful tool in monitoring the evolution of the basic
meteorological and atmospheric parameters. There are several
laser-remote-sensing techniques, as several physical processes
of interaction of light with matter can be exploited. Lidar
systems fall under one of the following categories: Elastic
backscattering lidar
Differential absorption and scattering lidar (DIAL)
Fluorescence lidar
Raman Lidar
Doppler lidar The detection and analysis of the received lidar signals
permits the retrieval of the relative concentration of the
suspended aerosol particles, and of the absolute concentration
of several air pollutants (ie. O3, NOx, SO2 etc.). In case
of a 3-dimensional (3D) scanning lidar system, a 3D image
of air pollution concentration, over the scanned area, can
be acquired, over a typical range of 3-5 km.
II.
Backscattering Lidar
When
a laser beam is sent into the atmosphere, it is widely scattered
by the suspended aerosol particles, molecules and atoms
present in the air. This scattering is essentially caused
by the N2 and O2 molecules (Rayleigh and Raman scattering)
and by the suspended aerosol particles (Mie scattering)
present in the atmosphere as dust, water droplets, black
carbon, etc.
In a typical lidar arrangement, the backscattered light
is collected by a telescope, usually placed coaxially with
the laser emitter. The signal is then focused onto a photodetector
through a spectral filter, adapted to the laser wavelength.
Since a pulsed laser is used, the intensity of the backscattered
light can be recorded as a function of time, and thus provide
the required spatial resolution of the measurement. The
basic lidar equation is given by:
where,
P(z) is the detected backscattered radiation from range
z, Po is the laser output power, t is the laser pulse duration,
c is the speed of light, b(z) is the volume backscatter
coefficient, a(z) is the total atmospheric extinction coefficient,
Atel is the total telescope area and O(z) is the overlap
function which takes into account geometrical and optical
factors of the receiver arrangement. The extinction term
a(z) includes the contribution of the different absorbing
atmospheric molecules (O3, NOX, SO2, etc.) and aerosol particles.[Go
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III.
DIAL technique
When
a laser beam is sent into the atmosphere, it is scattered
in every direction by particles and molecules present in
the air. This scattering is essentially caused by Rayleigh
scattering on nitrogen and oxygen molecules, and by Mie
scattering on aerosols (dusts, water droplets etc.). The
intensity of the received signal reflects the aerosol and
molecular concentrations as a function of distance, similarly
to an 'optical radar'. Moreover, molecular absorption allows
specific detection of a particular gaseous pollutant, using
the DIAL (Differential Absorption Lidar) method. This technique
is based on the use of a pair of wavelengths close to each
other, with a large absorption coefficient difference (denoted
Won and Woff, for on-resonance and off-resonance wavelength,
respectively). Such a pair of wavelengths, chosen for the
detection of a specific pollutant, is sent into the atmosphere
and backscattered signals at both wavelengths are compared.
If the pollutant is present in the air at a certain location,
it will produce a decrease of signal on the Won -channel
but not on the Woff -one. From this difference and by using
Beer-Lambert's law, the specific concentration of the considered
pollutant is retrieved as a function of distance. By scanning
the measurement direction in azimuth or elevation, 2D or
3D mappings are obtained, like a molecule-specific radar.[Go
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IV.
Doppler Lidar
Pulsed
Doppler LIDAR measures the radial (along-beam) velocity
as a function of range using light-scattering particles
in the air as tracers. When the LIDAR beam is directed straight
upward and the backscattered return as a function of height
is recorded, vertical aerosol profiles may be determined.
Various pointing and scanning schemes permit measurement
of a variety of mean and turbulent quantities based on often-met
assumptions about the flow. The remote-sensing character
of LIDAR offers the ability to measure flow parameters simultaneously
at all the heights in a profile. Alternatively, the LIDAR
scanning at lower elevation angles can probe flow characteristics
over a large area or volume. The vertical profiles of momentum
flux and turbulent intensity have been the object of experimental
measurements to understand and model the dynamics of the
atmosphere.
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