Passive infrared spectroscopy of the eruption plume at Popocatepetl volcano, Mexico

Volcanic gases provide important insights into deep-Earth processes, and ga s composition and flux variations show promise as predictors of eruptive ac tivity(1-3). But data correlating gas composition with eruptions are sparse , largely because such studies have traditionally involved direct sampling inside a volcanic crater-a hazardous operation that has resulted in numerou s deaths(4,5). Crater-rim-based spectroscopy(6-9), closed-path spectroscopy of gases sampled from aircraft(10), and time-averaged studies using volati le traps(11-13) allow measurements to be taken from safer distances. But wh en a full-scale explosive eruption threatens, even these methods become dan gerous as the hazard radius expands to many kilometres. Previously, only su lphur dioxide has been reliably measurable at such large distances, using c orrelation spectroscopy(14). Here we describe techniques that extend the us eful range of passive infrared spectroscopy to monitor many gases at distan ces of over 17 km. We demonstrate the use of these techniques in a high-tem poral-resolution study of short-term compositional variations associated wi th an explosive eruption at Mexico's Popocatepetl volcano on 25-26 February 1997. We observed a steady increase in SiF4/SO2 over several days precedin g the eruption, followed by a tenfold decrease in this ratio over a few hou rs immediately afterwards.