Aerosols
Aerosols play an important part in the radiative
balance of the atmosphere. This influence is exerted in two ways:
Some aerosols, in
particular sulphate particles, have a direct effect through
reflection of sunlight back into space
Aerosols have an
indirect effect through their role as cloud condensation nuclei
The
long term effects of aerosols on climate forcing are still not fully
understood (see figure below) and we have only a limited knowledge
of their spatial and temporal distribution and
variability.
A network of ground based photometers has been set
up to measure aerosol properties at selected sites, but this does
not provide anything like global coverage as illustrated in the map
below.
AERONET sites around the world
Further information on
Aeronet may be found here.
A comprehensive global aerosol climatology can be
developed only by using satellite data. To do this, we must exploit
some feature of top-of-atmosphere radiance fields that is sensitive
to aerosol type and concentration.
Aerosol interaction with electromagnetic radiation
is wavelength dependent, and the degree of scattering depends
strongly on the angle between the solar beam and the direction of
sampling. The methodolgy used in the CHRIS-PROBA project is
therefore to sample top-of-atmosphere radiation in appropriate parts
of the spectrum, and from different directions nearly
simultaneously.
Aerosol reflectance depends
on the extinction coefficient. This in turn depends on
the aerosol particle size distribution and on the wavelength of the
light. In figure 5 the wavelength
dependence of the extinction coefficient is shown for different
aerosol effective radii.
The row of figures below show three colour polar
plots of the light leaving the top of the atmosphere for visible
light. The variation between the figures is due to the differences
in aerosol type and loading. The white dot corresponds to a ray of
light scattered back towards the sun. (The radial coordinate in the
polar plots represents the zenith angle with nadir being at the
centre while the angular coordinate represents the azimuth angle
from the solar principal plane)
URBAN
MARITIME
RURAL
Inversion of a set of top of atmosphere radiances,
suitably sampled in wavelength and direction (as will be available
from the CHRIS instrument), will enable the accurate retrieval of
aerosol loading.
The aerosol properties will
be retrieved by inversion of radiative transfer models. In an
earlier study, North (IEEE Transactions Of Geoscience and
Remote Sensing, 37(1) pp 526-537) showed how with just two
views and four spectral channels accurate information on aerosol
loading can be obtained. Figure 6
shows some typical results, demonstrating the accuracy of the
method. The enhanced spatial, directional and spectral sampling of
the CHRIS-PROBA system promises further improvements.
Validation of the method will be made against data
from sites in the AERONET system of sun photometers. Two sites are
selected for each of the major aerosol types (desert, maritime, urban
and continental/rural), chosen to give a good geographical spread.
The experiment is designed to last for a whole year's observations,
in order to sample the full seasonal variability of aerosol
properties.
Page last modified 17 April 2002