SO 2 depletion in tropospheric volcanic plumes

Ground based remote sensing techniques are used to measure volcanic SO 2 fluxes in efforts to characterise volcanic activity. As these measurements are made several km from source there is the potential for in‐plume chemical transformation of SO 2 to sulphate aerosol (conversion rates are dependent on meteorological conditions), complicating interpretation of observed SO 2 flux trends. In contrast to anthropogenic plumes, SO 2 lifetimes are poorly constrained for tropospheric volcanic plumes, where the few previous loss rate estimates vary widely (from ≪1 to >99% per hour). We report experiments conducted on the boundary layer plume of Masaya volcano, Nicaragua during the dry season. We found that SO 2 fluxes showed negligible variation with plume age or diurnal variations in temperature, relative humidity and insolation, providing confirmation that remote SO 2 flux measurements (typically of ≈500–2000 s old plumes) are reliable proxies for source emissions for ash free tropospheric plumes not emitted into cloud or fog.