The explosive June 1991 eruptions of Mount Pinatubo produced the largest sulfur dioxide cloud detected by the Total Ozone Mapping Spectrometer (TOMS) during its 13 years of operation: approximately 20 million tons of SO2, predominantly from the cataclysmic June 15th eruption. The SO2 cloud observed by the TOMS encircled the Earth in about 22 days (∼21 m/s); however, during the first three days the leading edge of the SO2 cloud moved with a speed that averaged ∼35 m/s. Compared to the 1982 El Chichón eruptions, Pinatubo outgassed nearly three times the amount of SO2 during its explosive phases. The main cloud straddled the equator within the first two weeks of eruption, whereas the El Chichón cloud remained primarily in the northern hemisphere. Our measurements indicate that Mount Pinatubo has produced a much larger and perhaps longer‐lasting SO2 cloud; thus, climatic responses to the Pinatubo eruption may exceed those of El Chichón.

The total ozone mapping spectrometer (TOMS), first flown on the Nimbus 7 satellite, has delivered an unanticipated set of unique information about volcanic plumes because of its contiguous spatial mapping and use of UV wavelengths. The accuracies of TOMS sulfur dioxide retrievals, volcanic plume masses, and eruption totals under low‐latitude conditions are evaluated using radiative transfer simulations and error analysis. The retrieval algorithm is a simultaneous solution of the absorption optical depth equations including ozone and sulfur dioxide at the four shortest TOMS wavelengths and an empirical correction based on background condition residuals. The retrieval algorithm reproduces model stratospheric sulfur dioxide plume amounts within ±10% over most central scan angles and moderate solar zenith angles if no aerosols or ash are present. The errors grow to 30% under large solar zenith angle conditions. Volcanic ash and sulfate aerosols in the plume in moderate optical depths (0.3) produce an overestimation of the sulfur dioxide by 15–25% depending on particle size and composition. Retrievals of tropospheric volcanic plumes are affected by the reflectivity of the underlying surface or clouds. The precision of individual TOMS SO2 soundings is limited by data quantization to ±6 Dobson units. The accuracy is independent of most instrument calibration errors but depends linearly on relative SO2 absorption cross‐section errors at the TOMS wavelengths. Volcanic plume mass estimates are dependent on correction of background offsets integrated over the plume area. The errors vary with plume mass and area, thus are highly individual. In general, they are least for moderate size, compact plumes. Estimates of the total mass of explosively erupted sulfur dioxide depend on extrapolation of a series of daily plume masses backward to the time of the eruption. Errors of 15–30% are not unusual. Effusive eruption total mass estimates are more uncertain due to difficulties in separating new from old sulfur dioxide in daily observations.

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