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