El Chichon: The genesis of volcanic sulfur dioxide monitoring from space Arlin Krueger a, ⁎, Nickolay Krotkov b , Simon Carn a a Joint Center for Earth Systems Technology, University of Maryland, Baltimore County,1000 Hilltop Circle, Baltimore, MD 21250, United States b Goddard Earth Sciences & Technology Center, University of Maryland, Baltimore County,1000 Hilltop Circle, Baltimore, MD 21250, United States ABSTRACT ARTICLE INFO Article history: Accepted 12 February 2008 Available online xxxx Keywords: volcanism explosive eruptions sulfur dioxide remote sensing El Chichon TOMS The 1982 eruption of El Chichon inspired a new technique for monitoring volcanic clouds. Data from the Total Ozone Mapping Spectrometer (TOMS) instrument on the Nimbus-7 satellite were used to measure sulfur dioxide in addition to ozone. For the first time precise data on the sulfur dioxide mass in even the largest explosive eruption plumes could be determined. The plumes could be tracked globally as they are carried by winds. Magmatic eruptions could be discriminated from phreatic eruptions. The data from El Chichon are reanalyzed in this paper using the latest version of the TOMS instrument calibration (V8). They show the shearing of the eruption cloud into a globe-circling band while still anchored over Mexico in three weeks. The measured sulfur dioxide mass in the initial March 28 eruption was 1.6 Tg; the April 3 eruption produced 0.3 Tg more, and the April 4 eruptions added 5.6 Tg, for a cumulative total of 7.5 Tg, in substantial agreement with estimates from prior data versions. TOMS Aerosol Index (absorbing aerosol) data show rapid fallout of dense ash east and south of the volcano in agreement with Advanced Very High Resolution Radiometer (AVHRR) ash cloud positions. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In 1982, anomalously high total ozone appeared above Mexico in data from the Total Ozone Mapping Spectrometer (TOMS) on the Nimbus-7 satellite at the same time as the eruption of El Chichon (Krueger, 1983). After 6 centuries of repose, El Chichon erupted violently on March 28 through April 4, 1982. This volcano and the eruptions are well documented in terms of chronology, petrology, stratigraphy, and motion of the ash clouds (Robock and Matson, 1983; Matson,1984; Varekamp et al., 1984; Luhr et al., 1984; Rose et al., 1984; Sigurdsson et al., 1984). The detailed behavior of the April 4 eruption gas and ash clouds was analyzed by Schneider et al. (1999) using data from TOMS and AVHRR instruments. The atmospheric effects of the eruption were reviewed by Hofmann (1987). The TOMS instrument (Heath et al., 1975; Krueger, 1989), launched in October 1978, was designed, as its name implies, to determine the spatial structure in total ozone through daily, contiguous mapping of the earth. Prior ozone data from ground stations showed high variability with time scales similar to meteorological changes, but the relations with weather were elusive because of the sparse distribution of stations. The TOMS was built with the best spatial resolution available with 1970′s technology (50 km at nadir) to resolve anticipated gradients in total ozone. The spacecraft data rate also limited the spectral coverage to 6 discrete ultraviolet wavelengths for total ozone soundings (Dave and Mateer, 1967). The contiguous mapping proved to be at least as valuable for volcanology as for atmospheric ozone. The TOMS total ozone algorithm was developed with the assumption that ozone was the only absorbing gas at near UV wavelengths (310–380 nm). Other gases were ignored because they were normally present in far lower optical depths than ozone. In 1982, the strange cloud of apparent high total ozone over Mexico (Fig. 1a) at the time of the El Chichon (small black triangle on Fig. 1a) eruption required an explanation. Krueger (1983) showed that the spectral anomalies were consistent with sulfur dioxide, making it the most likely volcanic constituent to account for the anomalous absorption. A simple scheme to separate sulfur dioxide from ozone absorption was proposed. The sulfur dioxide absorption spectrum overlaps the ozone spectrum but with different structure. Other gases, such as carbon disulfide, also absorb at these wavelengths, but would produce different spectral anomalies. Sulfur dioxide amounts were estimated from the deviation of the observed radiances from an interpolation of unperturbed radiances on either side of the volcanic cloud. Agreement at the two TOMS wavelengths within the SO 2 band was within 10%, but absolute amounts were not certain because at the time only room temperature SO 2 cross sections had been measured (Wu and Judge, 1981) while the cloud temperatures were near - 50 °C. An average column SO 2 amount over the cloud, multiplied by the cloud area obtained from the TOMS images yielded a total mass in the April 5th cloud of 3.3 Tg. New cross section data at low temperatures (McGee and Burris, 1987) are now used to produce more accurate results. With the contiguous coverage and moderate spatial resolution of TOMS it now became possible to measure the mass of sulfur dioxide in Journal of Volcanology and Geothermal Research xxx (2008) xxx-xxx ⁎ Corresponding author. Tel.: +1 410 455 8906; fax: +1 410 455 5868. E-mail address: akrueger@umbc.edu (A. Krueger). VOLGEO-03962; No of Pages 7 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.02.026 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores ARTICLE IN PRESS Please cite this article as: Krueger, A., et al., El Chichon: The genesis of volcanic sulfur dioxide monitoring from space, J. Volcanol. Geotherm. Res. (2008), doi:10.1016/j.jvolgeores.2008.02.026