The lognormal size distributions of spherical particles depend on the standard deviation and on the modal radius rm ,
Only the mode of small particles is considered: we assume a standard deviation 
= 0.403, a set of ten modal radii rm, from 0.05 to 0.15 µm and a refractive index m=1.47-0.01i.
Reference : Vermeulen A., Devaux C., and Herman M.
Retrieval of the scattering and microphysical properties of the aerosols from ground-based optical measurements including polarization.
I, Method, Appl. Opt., 39(33), 6,207-6,220, 2000.