ULTRAVIOLET OPTICAL-MODEL OF VOLCANIC CLOUDS FOR REMOTE-SENSING OF ASH AND SULFUR-DIOXIDE

The total ozone mapping spectrometer (TOMS) instruments have detected every significant volcanic eruption from November 1978 to December 199 4 on the Nimbus 7 and Meteor 3 satellites and since July 1996 on the n ew satellites, TOMS-Earth Probe and ADEOS. We apply a radiative transf er model to simulate the albedos of these fresh eruption clouds to stu dy the limitations of the present algorithm which assumes an absorbing cloud above a scattering atmosphere. The conditions are found to be a pproximated when the total absorption optical depth is less than 2 (i. e., 100 Dobson units (DU) SO2 at 312 nm or 300 DU SO2 at 317 nm). The spectral dependence of the albedo of a nonabsorbing Rayleigh atmospher e can be specified by only two parameters which are uniquely different when ash or sulfate aerosols are present in the stratosphere. However , the interaction between ash scattering and SO, absorption within a v olcanic cloud produces a nonlinear effect at strongly absorbing wavele ngths that accounts for overestimation of sulfur dioxide in ash-laden volcanic clouds by the Krueger ct al. [1995] algorithm. Correction of this error requires knowledge of the ash properties. A method for dete rmining two of the ash parameters from the longer TOMS wavelengths is described. Given the altitude of the cloud, surface reflectivity, and an estimate of effective variance of the ash size distribution, the op tical thickness and either the effective radius or the index of refrac tion can be deduced. The ash retrievals are also needed to evaluate th e tephra/gas ratio of eruptions and to compare the ash properties of d ifferent volcanoes.