Multi-decadal satellite measurements of global volcanic degassing

Satellite instruments have been providing measurements of global volcanic emissions of sulfur dioxide (SO 2 ) since 1978, based on observations in the ultraviolet (UV), infrared (IR) and microwave spectral bands. We review recent advances in satellite remote sensing of volcanic gases, focusing on increased instrument sensitivity to tropospheric SO 2 emissions and techniques to determine volcanic plume altitude. A synthesis of ~ 36 years of global UV, IR and microwave satellite measurements yields an updated assessment of the volcanic SO 2 flux to the upper troposphere and lower stratosphere (UTLS) between 1978 and 2014 (~ 1–2 Tg/yr). The present availability of multiple UV and IR satellite SO 2 products provides increased confidence in calculated SO 2 loadings for many eruptions. We examine the temporal and latitudinal distribution of volcanic SO 2 emissions and reassess the relationship between eruptive SO 2 discharge and eruption magnitude, finding a first-order correlation between SO 2 emission and volcanic explosivity index (VEI), but with significant scatter. Based on the observed SO 2 -VEI relation, we estimate the fraction of eruptive SO 2 emissions released by the smallest eruptions (~ 0.48 Tg/yr), which is not recorded by satellite observations. A detailed breakdown of the sources of measured SO 2 emissions reveals intuitively expected correlations between eruption frequency, SO 2 loading and volcanic degassing style. We discuss new constraints on e-folding times for SO 2 removal in volcanic plumes, and highlight recent measurements of volcanic hydrogen chloride (HCl) injections into the UTLS. An analysis of passive volcanic emissions of SO 2 detected in Ozone Monitoring Instrument (OMI) SO 2 data since 2004 provides new insight into the location and stability of the dominant sources of volcanic SO 2 over the past decade. Since volcanic SO 2 emissions constitute a random, highly variable perturbation to the atmosphere-climate system, continued monitoring of volcanic SO 2 emissions from space by multiple UV and IR instruments to extend the current multi-decadal record is essential, and near-global, geostationary measurements of SO 2 may be available by the end of the current decade.