Optical and microphysical parameters of the Mt. Pinatubo aerosol as determined from MIPAS-B mid-IR limb emission spectra

High-resolution mid-IR limb emission spectra were recorded during a flight of the Michelson interferometer for passive atmospheric sounding, balloon-borne version (MIPAS-B) from Kiruna, northern Sweden (68 degrees N) on March 14/15, 1992. These spectra are affected by the Mt. Pinatubo stratospheric aerosol, which caused an enhanced continuum emission, especially in spectra of low tangent altitudes. Aerosol extinction coefficients were retrieved from MIPAS-B spectra at approximately 60 spectral positions in the 750-980 cm(-1) and 1180-1380 cm(-1) spectral ranges. Retrieved aerosol extinction coefficients range from 6x10(-4) km(-1) to 3x10(-3) km(-1) in tangent altitudes 11.3 km and 14.5 km and from 5x10(-5) km(-1) to 1x10(-3) km(-1) in 16.1 km. Their distinct spectral shape indicates the presence of H2SO4-H2O droplets. Compositions and size distribution parameters were retrieved by least squares fitting of Mie-generated spectral extinction coefficients to the ones derived from the spectra. Estimated spectral, single-scattering albedos between 0.08 and 0.3 indicate the significance of thermal multiple scattering. Multiple-scattering corrections led to an increase of spectral extinction coefficients by 5-50% with highest changes at lowest tangent altitudes. Accordingly, estimated volume densities have increased by 4-20% to values of 3.66, 2.85, and 0.93 mu m(3) cm(-3) for tangent altitudes 11.3, 14.5, and 16.1 km, respectively. Retrieved H2SO4 weights of 66-70% are in good agreement with values derived from stratospheric temperatures and water vapor partial pressures. Estimated surface densities are systematically low in comparison with in situ size distribution measurements. This finding is explained by the underestimation of small particles by the use of a monomodal size distribution in the analysis. Retrieved effective radii of up to 0.8 mu m were found to be consistent with the temporal evolution of the Mt. Pinatubo aerosol.