Bimodal size distribution influences on the variation of Angstrom derivatives in spectral and optical depth space

The variation of the aerosol optical depth and its first and second spectra l derivatives (a and alpha') can be largely described in terms of the spect ral interaction between the individual optical components of a bimodal size distribution. Simple analytical expressions involving the separate optical components of each mode explain virtually all the features seen in spectra of the aerosol optical depth and its derivatives. Illustrations are given for a variety of measured optical depth spectra; these include comparative simulations of the diurnal behavior of alpha and alpha' spectra as well as the diurnal and general statistical behavior of alpha and alpha' as a funct ion of optical depth (optical depth space). Each mode acts as a fixed "basi s vector" from which much of the behavior in spectral and optical depth spa ce can be generated by varying the extensive (number density dependent) con tributions of fine and coarse mode optical depths. Departures from these ba sis vectors are caused by changes in aerosol type (average size and refract ive index) and thus are associated with differing synoptical air masses, so urce trajectories or humidity conditions. Spectral parameters are very sens itive to interband errors in measured optical depth data. Third-order polyn omial fits within the visible-NIR spectral region effectively filter such e rrors while representing the limit of useful extractable information.