AIRBORNE GAS SURVEILLANCE OF CASCADES VOLCANOES

Cascades volcanoes pose challenges to gas surveillance of volcano unrest. Locations are remote, and ground access is generally difficult. Wet climates and melt from glaciers and thick winter snowpack foster hydrothermal and ground waters that can scrub acid gases (SO₂, HCl, HF) before they reach the surface, thereby masking their degassing from subsurface magma. These gases may not exhibit significant increases in emission rates until dry pathways or magma reach the surface. Background or low emissions of acid gases may thus give a false sense of security. CO₂ is more likely to give early indication of subsurface magma degassing. It is the second most abundant magmatic volatile; it is among the least soluble magmatic volatiles; and it is far less susceptible to scrubbing than SO₂, HCl, and HF. Rising H₂S emissions are also a plausible early warning, since unlike SO₂, HCl, and HF, H₂S is strongly volatilized from boiling water. Unfortunately, remote sensing of early increases in CO₂ and H₂S emissions is usually problematic, owing to high atmospheric CO₂ levels, water vapor interferences, and poor H₂S infrared absorbance. We have therefore developed an aircraft-mounted system that directly measures CO₂ and H₂S by extraction sampling of plumes. The system includes an infrared spectrometer for CO₂ and an electrochemical sensor for H₂S, in addition to a COSPEC, a high-precision barometer, a temperature probe, and a GPS receiver. Measurements are made at different altitudes along traverses orthogonal to plume (wind) direction or along orbits around a volcano if plume is not visible. A data logger records data for all gases at 1-s intervals and tags results with clock time, latitude, longitude, altitude, temperature and pressure. In-flight wind data are also acquired. Plume cross-sections are constructed with mapping software and used to calculate emission rates. Several campaigns to date show that emission rates of <200 metric tons/day (t/d) CO₂, <10 t/d H₂S, and zero SO₂ are typical, suggesting that scrubbing of acid gases, and thus edifice alteration processes, are potentially widespread at Cascades volcanoes.