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

The NASA total ozone mapping spectrometer (TOMS) instruments aboard the Nim bus-7, Meteor3, ADEOS, and Earth Probe satellites have produced a unique da ta 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 t he backscattered radiances in the 0.34-0.38 mu m spectral interval (where g aseous absorption is negligible) in conjunction with radiative transfer mod els to retrieve properties of ash clouds including the optical depth and ef fective particle radius, R-eff. Using the T-matrix method for computing the scattering properties of randomly oriented spheroids, we have tested the s ensitivity of the TOMS volcanic ash retrievals to particle shape. For the c ase of the August 19, 1992 Mt. Spurr ash cloud and the TOMS observational g eometry, 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 o ther hand, the optical depth will be overestimated by as much as 25%. In te rms of the total mass of the cloud, the compensating errors in R-eff and op tical depth tend to produce a 5-20% underestimation. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.