Plume velocity determination for volcanic SO2 flux measurements

Abstract

Ground based volcanic SO2 fluxes provide important insights into the behaviour of volcanoes, and their impacts upon the atmosphere. In order to compute a flux, the plume transport speed, and direction, must be known. In practice these are typically assumed to equal, respectively: (A) a ground based anemometer reading, and (B) the bearing of the vector between the volcanic gas source and the position on the downwind plume cross‐section where the gas concentration is highest. However, use of these proxies is open to question, and they can introduce large errors (possibly > 100%), thereby significantly reducing the utility of the derived fluxes. Here we present direct spectroscopic measurements of volcanic plume velocity; the data were obtained using three ultraviolet spectrometers, at Masaya volcano, Nicaragua, during January 2004. We estimate that flux measurements with overall error budgets < 10% are readily achievable with this approach.