Effect of particle non-sphericity on satellite monitoring of drifting volcanic ash clouds

The NASA total ozone mapping spectrometer (TOMS) instruments aboard the Nimbus-7, Meteor3, ADEOS, and Earth Probe satellites have produced a unique data set of global S0(2) volcanic emissions since 1978. Besides S0(2), a new technique has been developed which uses the measured spectral contrast of the backscattered radiances in the 0.34-0.38 mu m spectral interval (where gaseous absorption is negligible) in conjunction with radiative transfer models to retrieve properties of ash clouds including the optical depth and effective particle radius, R-eff. Using the T-matrix method for computing the scattering properties of randomly oriented spheroids, we have tested the sensitivity of the TOMS volcanic ash retrievals to particle shape. For the case of the August 19, 1992 Mt. Spurr ash cloud and the TOMS observational geometry, modeling the ash as spherical particles causes the TOMS-retrieved R-eff to be underestimated for R-eff > 0.1 mu m by as much as 30%. On the other hand, the optical depth will be overestimated by as much as 25%. In terms of the total mass of the cloud, the compensating errors in R-eff and optical depth tend to produce a 5-20% underestimation. (C) 1999 Elsevier Science Ltd. All rights reserved.