Aerosol size distribution. Two columns are expected: The radius [micrometer] and the particle number. Together with aerosol_refrac_index or aerosol_refrac_file this forms the input to Mie calculations of the aerosol optical properties. Please note that only the single-scattering albedo, the scattering phase function, and the wavelength-dependence of the extinction coefficient are affected by the Mie calculation while the absolute value of the extinction coefficient is taken from other sources; generally, the extinction coefficient at the first internal wavelength is taken from whatever is available (either default shettle89 or user-defined); the extinction at all other wavelengths is scaled according to the Mie calculation. For this reason, the absolute numbers are not relevant - only the shape of the size distribution matters. For details see also the description of aerosol_refrac_file. It is recommended to avoid this option and rather to calculate the aerosol optical properties externally e.g. with mie and to pass them to uvspec with aerosol_files.