Aerosol features retrieved from solar aureole data: a simulation study concerning a turbid atmosphere.

The characteristics of the solar aureole were evaluated for several cases of a turbid atmosphere in the 3° ≤θ≤30° interval of scattering angles; for each case, the features of the aerosol were retrieved from the simulated aureole data. Computations were carried out with a recently set up radiative transfer code that uses the approximated delta-M method, corrected further for the 1st and 2nd scattering orders. Results showed that the software tested can work out both the direct and the inverse aureole problems with great accuracy and efficiency in several different situations, so it can reliably be used for handling experimental data measured in the field with an aureolemeter. Furthermore, the input parameters of ground albedo, complex refractive index, aerosol radius interval, and measurement angles were varied within a set of values to examine the sensitivity of the retrieval to improperly assumed values of these parameters and to evaluate the most suitable way of determining their correct values. Only data concerning diffuse radiation were elaborated. Results showed that (1) the scanned scattering angles have to be extended up to 40°; (2) the most suitable radius interval for aerosols appears to be from 0.05 to 15 µm; (3) ground albedo A should be independently determined within 15%; and (4) as to the complex refractive index m˜, the real part should be given within 3.5%, and the imaginary part within from 10% to 50%, according to its value. Finally, a procedure through which it is possible to derive A and m˜ by extending the information content of the aureole data is discussed. Improved calibration procedures are also proposed.