ABSTRACT |
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Major volcanic eruptions provide excellent opportunities to
understand atmospheric processes by comparison of the theoretical
atmospheric responses to the observed ones. Two recent major volcanic
eruptions, the El Chichon and Mt. Pinatubo volcanoes, have attracted
great attention. However, there are still unanswered interesting
questions, especially during the early stages of the volcanic eruption,
mainly due to the lack of observational data.
A new algorithm that retrieves column amounts of SO2 and ozone, and aerosol signal from the radiances measured by TOMS (Total Ozone Mapping Spectrometer) has been developed. The new algorithm uses a two dimensional look-up table approach that utilizes the differential absorption of sulfur dioxide. To increase the accuracy, new sets of pair wavelengths are chosen. An extensive error analysis shows that the aerosol is the most significant error source followed by uncertainties in the absorption cross sections of ozone and sulfur dioxide. The aerosol signal in the measured radiance is corrected using the characteristics of the radiance residuals. The retrieval error due to the uncertainties in the aerosol parameters is studied. The overall error in the retrieved ozone and sulfur dioxide is better than 10 %. The new algorithm is applied to the stratospheric volcanic cloud for the first 40 days after the eruption of the El Chichon and Mt. Pinatubo volcanoes. The volcanic dust cloud and the gaseous cloud move in different directions for the El Chichon volcano, but in the same direction for the Mt. Pinatubo volcano. The initial emission of sulfur is in the form of sulfur dioxide for the Mt. Pinatubo, but the amount of sulfur dioxide for El Chichon increases for the first four days, indicating that the initial sulfur is in the form of hydrogen sulfide. The total sulfur dioxide masses injected by the El Chichon and Mt. Pinatubo volcanoes are about 6 Mt and 14 Mt, respectively. The chemical lifetime of gas species within the volcanic cloud, undergoing dynamical mixing and chemical conversion, can be estimated from the total mass change with time. The estimated chemical lifetime of sulfur dioxide is the same for both eruptions, about 39 days. The initial clouds for both eruptions contain little ozone, indicating the tropospheric origin of gases within the cloud. The ozone within the volcanic cloud approaches the ambient value in about four to five days, with the rate for El Chichon being faster than for Mt. Pinatubo. The faster rate is attributed to the oxidation process for hydrogen sulfide to sulfur dioxide, which produces an ozone molecule. |