Abstract :
A technique is described for efficient retrieval of families of smooth model distributions, such as lognormals or modified gammas, from the lower moments of the particle size distribution from which aerosol optical properties can be accurately computed. The Multiple Isomomental Distribution Aerosol Surrogate (MIDAS) technique, along with the quadrature technique of McGraw et al. (1995 Geophys. Res. Lett. 22, 2929-2932), is evaluated by computing the extinction efficiency, asymmetry parameter, backscatter fraction, 180° backscattering cross section, upscatter fraction, mass scattering efficiency, and a direct shortwave forcing at 8 wavelengths for 28 test distributions derived from field observations of marine, continental, urban and stratospheric aerosols. For the 224 single wavelength evaluations with retrieved modified gammas the average magnitude of error for each of the computed optical properties was 2% or less, with the exception of the 180° backscattering cross section (4%), establishing the accuracy of the technique. It is concluded that this approach is useful for obtaining aerosol optical properties from the first 6 moments of the size distribution, permitting confident determination of these properties from models in which aerosol evolution is represented by evolution of the lower-order moments.
