Aerosol size spectra from spectral extinction data: the use of a linear inversion method

A modification of the Twomey-Phillips linear constrained inversion method is used to retrieve aerosol size spectra from simulated spectral optical depths in the wavelength range from 0.37 to 2.2 microm. It is found that the transition between nonphysical solutions to well-behaved ones, which is driven by the smoothing parameter gamma is gradual and that negative components, when present in the solution, always belong to the smallest particle range. When values of gamma greater than 0.05 are required to obtain well-behaved solutions, a strong bias on the retrieved solution is posed by the form of the first-guess solution. Moreover, even if the retrieved size distribution computed for low values of gamma is not well-behaved, much information on the shape of the true solution in the medium and large particle range is contained in the retrieved solution. The effect of realistic random errors added to the simulated optical depths is also discussed.