Remote measurements of volcanic gas compositions by solar occultation spectroscopy

Volcanic gases have important effects on the atmosphere and climate1,2 and are important indicators of subsurface magmatic processes3,4, but they are difficult to measure. In situ sampling on volcanoes can provide detailed information5,6,7 but is often impractical or hazardous. It is safer to apply remote techniques, for example correlation spectroscopy8, which is now widely used to estimate emission rates of sulphur dioxide; but making remote measurements of other gas species has proved more difficult. Developments in Fourier-transform infrared spectroscopy, however, have shown promise9,10,11. Here we report Fourier-transform infrared observations of volcanic plume compositions that we obtained by solar occultation at Mount Etna in 1997. We foundmolar ratios of SO2:HCl and SO2:HF to be ∼4.0 and 10, corresponding to emission rates of HCl and HF of about 8.6 and 2.2 kg s−1, respectively, confirming Mount Etna as the largest known sustained point source of these gases. Solar occultation spectroscopy has advantages over other methods as it enables measurement of plume compositions several kilometres downwind, without requiring hot rocks or lamp sources. The regular and frequent observation of volcanic gases provides a valuabletoolfor volcano surveillance, and data from plumes at different distances downwind of a volcano's summit may help us to understand the atmospheric chemistry involved in plume dispersal.