remotely sensed data. The method is based on the relationship between the direct beam, monochromatic, 
irradiance at the Earth's surface 1(A) and the extra-terrestrial irradiance I 0 (A) 
1(A) = I 0 (A).exp 
[-T(A).m] 
( 1 ) 
where, x(A) is atmospheric optical depth and m is the airmass. The optical depth x(A) can be expressed as the 
s um of Rayleigh scattering xr(A), aerosol optical depth x a (A), absorption by water vapour x w (A), absorption by 
ozone x 0 (A), and absorption by the uniformly mixed gases (CO 2 and O 2 ) Xg(A). The aerosol optical depth x a (X) 
can be derived at each of the 252 ATLAS spectral wavelengths as the other terms can be either measured or 
computed. 
3.1 Aerosol Size Distribution. Given a set of aerosol optical depths at known wavelengths it is possible to 
derive an estimate of the aerosol size distribution by inversion of the equation 
where, n<;(r) is the unknown aerosol size distribution and Q e xt( r >A,m) is the Mie extinction efficiency factor for 
and since it cannot be written analytically as a function of the x a (A) values, a numerical approach as given in 
King et al (1978) can be used. Further algorithms will be implemented to take advantage of the ability to make 
measurements in the solar aureole to constrain the results of the aerosol size distribution inversions. 
3.1.3 Sky Radiance Distribution. Data from the full sky scans can be visualised most easily by fitting an 
interpolated 3D mesh over the sampled data points for each of the 252 spectral channels. Figure 4 illustrates a 
typical sky radiance distribution at 550nm for the sun at an azimuth angle of 264.4" and a zenith angle of 57.6", 
ie. when the sun was low in the western sky. The most notable feature is the cone of high radiance around the 
solar position as a result of strong forward scattering by atmospheric aerosols forming a solar aureole, and the 
radiance minimum at 90 degrees away in the solar principal plane as a result of the dip in the dipole Rayleigh 
scattering phase function. There is also a limb brightening of the sky radiance towards the horizon. 
Figure 4. Sky radiance anisotropy at 550nm (Solar Azimuth = 264.4* Zenith = 57.6") from ATLAS data. 
Any analytical model that is developed to model the sky radiance distribution needs to be able to parameterize 
the main functional features illustrated above with as few parameters as possible. Work by Hooper and Brunger 
(1980) has formulated an equation 
( 2 ) 
spherical particles of radius r and complex refractive index m. To derive nc(r) the transform of (2) is required 
(3)