Early evolution of a stratospheric volcanic eruption cloud as observed with TOMS and AVHRR

This paper is a detailed study of remote sensing data from the total ozone mapping spectrometer (TOMS) and the advanced very high resolution radiomete r (AVHRR) satellite detectors, of the 1982 eruption of El Chichon, Mexico. The volcanic cloud/atmosphere interactions in the first four days of this e ruption were investigated by combining ultraviolet retrievals to estimate t he mass of sulfur dioxide in the volcanic cloud [Krueger et al., 1995] with thermal infrared retrievals of the size, optical depth, and mass of fine-g rained (1- 10 mu m radius) volcanic ash [Wen and Rose, 1994]. Our study pro vides the first direct evidence of gravitational separation of ash from a s tratospheric, gas-rich, plinian eruption column and documents the marked di fferences in residence times of volcanic ash and sulfur dioxide in volcanic clouds. The eruption column reached as high as 32 km [Carey and Sigurdsson , 1986] and was injected into an atmosphere with a strong wind shear, which allowed for an observation of the separation of sulfur dioxide and volcani c ash. The upper, more sulfur dioxide-rich part of the cloud was transporte d to the west in the stratosphere, while the fine-grained ash traveled to t he south in the troposphere. The mass of sulfur dioxide released was estima ted at 7.1 x 10(9) kg with the mass decreasing by approximately 4% 1 day af ter the peak. The mass of fine-grained volcanic ash detected was estimated at 6.5 x 10(9) kg, amounting to about 0.7% of the estimated mass of the ash which fell out in the mapped ash blanket close to the volcano. Over the fo llowing days, 98% of this remaining fine ash was removed from the volcanic cloud, and the effective radius of ash in the volcanic cloud decreased from about 8 mu m to about 4 mu m.