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I.
Tropospheric aerosols
Tropospheric aerosols arise from natural sources, such as wind-borne
dust, sea spray and volcanoes, and also from anthropogenic sources,
such as combustion of fossil fuels and biomass burning activities.
With the increasing urbanization and industrialization the content
of aerosols particularly in the lower troposphere increases continuously.
Moreover, the biomass burning activities in Africa, Asia and S.
America are responsible for the dramatic increase of the aerosol
concentration around the planet.
Aerosols can be emitted directly as particles (primary aerosol)
or formed by gas-to-particle conversion processes (secondary aerosol)
and can travel over long distances from their source region, as
their residence time in the troposphere is ranging from a few
days to a few weeks . Their composition and atmospheric concentrations
are highly variable. Atmospheric particles and mainly the mineral
dust particles, influence the earth's radiation balance and climate
in two ways: (a) by scattering and absorbing, both incoming and
out coming radiation depending on their chemical composition,
the so-called "direct aerosol effect", and (b) by acting
as cloud condensation nuclei (CCN) and thereby determining the
concentration of the initial droplets, albedo, precipitation formation
and life time of warm clouds, the so-called "indirect aerosol
effect".
A backscattering LIDAR system is able to provide accurate data
on the spatial and temporal evolution of the suspended aerosols
in the troposphere.
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II.
Cloud Monitoring
Clouds absorb and scatter the incoming solar radiation and emit
thermal radiation according to their temperature. They determine
to a large extent the variability of the radiative regime of Earth's
climatic system. The radiation balance depends, in particular,
on the cloud fraction, the shape of the clouds, their height and
the morphology of the cloud boundaries. Therefore, the spatial
distribution of clouds and the diurnal variation of cloud properties
are important parameters that have to be investigated.
The lidar technique is an efficient tool for continuous monitoring
of clouds and aerosols. Lidars are needed to provide long-term
accurate data on cloud heights, optical depth and their spatial
distribution. Knowledge of the formation, maintenance and dissipation
of clouds in the lower part of the troposphere is essential in
many operational and research applications.
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III.
Desert Dust
Every year large quantities of dust particles are emitted in
the atmosphere in desert regions of high convective activity. Most
of these particles are coarse (diameter 1 ìm) and are thus
deposited close to their source, while a large fraction of the smaller
particles can be transported over very large distances (thousands
of km). One estimation of the emission flux of desert aerosols that
is subject to long-range transport desert aerosols is 1500 Tg/yr.
In cases where mineral dust aerosols are located around or above
5 km height, they tend to change role in the radiative forcing budget
in the troposphere, from a cooling to a warming one. |
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IV.
Monitoring of the Planetary Boundary Layer (PBL) height
The Planetary Boundary Layer is the lowest part of the troposphere
that is directly influenced by the earth's surface, and responds
to surface forcing with a time scale of an hour or less. This layer
is of primary importance for our entire ecosystem. Air pollutants
concentrations in the PBL are generally orders of magnitudes higher
than in the free troposphere.
The convective PBL can be divided into three different layers -
the Surface Layer, the Mixed Layer, and the Entrainment Zone, the
last of which represents the transition zone
between the PBL and
the
free atmosphere. Accurate determination of the mixed layer height
is crucial for photochemical and dispersion models to accurately
predict pollutant concentration. This layer is typically moister
and has a greater aerosol content than the free troposphere, causing
more scattering of laser light. Therefore, lidars can easily detect
the boundary between these two layers with high spatial and temporal
resolution. |
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