Effect of volcanic aerosols on stratospheric radiance at wavelengths between 8 and 13 microm

A set of stratospheric aerosol optical models is employed in a radiative-transfer computation to study the effects of postvolcanic particle size distributions and compositions on the spectral radiance in the 8-13-microm window region. The models are based on direct measurements of post El Chichon aerosol size distributions and vertical profiles. They represent various maturity states of aerosols composed of sulfuric acid aqueous solutions, formed and evolved in the stratosphere following a massive volcanic eruption. Comparisons are made with concurrent radiance observations obtained on balloon flights 6 months after the eruption and 1 yr later. Most of the calculations are done using the LOWTRAN-6 code (either alone or combined with a multiple scattering calculation), revised so as to include the appropriate aerosol optical models. A better quantitative agreement is found between the calculated and observed spectral radiances when the aerosol loading is relatively high, indicating that additional minor radiance sources of a yet unresolved nature should be incorporated in the present version of the calculation.