Accurate measurement of volcanic SO 2 flux: Determination of plume transport speed and integrated SO 2 concentration with a single device

Ground‐based measurements of volcanic sulfur dioxide fluxes are important indicators of volcanic activity, with application in hazard assessment, and understanding the impacts of volcanic emissions upon the environment and climate. These data are obtained by making traverses underneath the volcanic plume a few kilometers from source with an ultraviolet spectrometer, measuring integrated SO 2 concentrations across the plume's cross section, and multiplying by the plume's transport speed. However, plume velocities are usually derived from ground‐based anemometers, located many kilometers from the traverse route and hundreds of meters below plume altitude, complicating the experimental design and introducing large flux (can be >100%) errors. Here we present the first report of a single instrument capable of (accurate) volcanic SO 2 flux measurements. This device records integrated SO 2 concentrations and plume heights during traverses. Between traverses, two in‐plume SO 2 time series are measured from underneath the plume with the instrument, corresponding to zenith and inclined (user‐specified angle from vertical in the direction of the volcano) fields of view, respectively. The distance between the points of intersection of the two views with the plume is found on the basis of the determined plume height, and the two signals are cross‐correlated to determine the lag between them, enabling accurate derivation of the wind speed. We present flux data (with errors ≈±12%) obtained in this way at Mt. Etna during July 2004.