VOLCANIC SULFUR-DIOXIDE MEASUREMENTS FROM THE TOTAL OZONE MAPPING SPECTROMETER INSTRUMENTS

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 u se of UV wavelengths. The accuracies of TOMS sulfur dioxide retrievals , volcanic plume masses, and eruption totals under low-latitude condit ions are evaluated using radiative transfer simulations and error anal ysis. The retrieval algorithm is a simultaneous solution of the absorp tion optical depth equations including ozone and sulfur dioxide at the four shortest TOMS wavelengths and an empirical correction based on b ackground condition residuals. The retrieval algorithm reproduces mode l stratospheric sulfur dioxide plume amounts within +/-10% over most c entral 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 moderat e optical depths (0.3) produce an overestimation of the sulfur dioxide by 15-25% depending on particle size and composition. Retrievals of t ropospheric volcanic plumes are affected by the reflectivity of the un derlying surface or clouds. The precision of individual TOMS SO2 sound ings is limited by data quantization to +/-6 Dobson units. The accurac y is independent of most instrument calibration errors but depends lin early on relative SO2 absorption cross-section errors at the TOMS wave lengths. Volcanic plume mass estimates are dependent on correction of background offsets integrated over the plume area. The errors vary wit h plume mass and area, thus are highly individual. In general, they ar e least for moderate size, compact plumes. Estimates of the total mass of explosively erupted sulfur dioxide depend on extrapolation of a se ries of daily plume masses backward to the time of the eruption. Error s of 15-30% are ndt unusual. Effusive eruption total mass estimates ar e more uncertain due to difficulties in separating new from old sulfur dioxide in daily observations.