Optical techniques permit a rapid and non-invasive technique to measure the properties of aerosols. Since optical properties are influenced by the properties of the particles, a determination of the size distribution of the particles by inversion of the optical data is desirable. Unfortunately, the inversion problem is ill-posed, the solutions are ambiguous and many techniques have been developed to select realistic solution functions. An information parameter kappa close to 1 is necessary for a meaningful inversion. For extinction measurements in the visible, this applies to the particle size range between 0.3 and 2 mum in diameter. Inversions of simulated extinction data have shown that a determination of the mean diameter of monomodal size distributions is possible for diameters between 0.35 and 1 mum, a determination of the volume (mass) of the suspended particles is possible within a factor of 1.6, but that the determination of the width of the size distribution is difficult. For bimodal distributions having an accumulation mode and a coarse mode, only the accumulation mode can be retrieved, with a tendency to wider size distributions. Inversions of angular scattering data give similar results. Since the scattering pattern of particles larger than 1 mum is quite distinct, the range of retrievable sizes is extended to larger particles compared to extinction measurements.
