Volcanic SO2 emissions in the post-Pinatubo era – how quiescent is quiescent?

Abstract:

The volcanically ‘quiescent’ period since the 1991 Pinatubo eruption, and consequent clean stratosphere, has provided a unique opportunity to assess the impact of frequent small-to-moderate explosive volcanic eruptions (Volcanic Explosivity Index [VEI] <5) on stratospheric sulfate aerosol abundance. Accurate satellite measurements of volcanic SO₂ and aerosol emissions and their injection altitude are key to this assessment, as inputs for prognostic aerosol microphysical models that simulate aerosol evolution. The post-Pinatubo era has seen major advances in passive (UV, IR, microwave) and active (radar, LiDAR) satellite remote sensing assets to measure volcanic emissions. These datasets have been used to analyze the fate of volcanic emissions from initial SO₂ injection to derived stratospheric aerosol in unprecedented detail after several recent eruptions.

Whilst recent decades are considered volcanically quiescent, this refers largely to the lower frequency of major explosive eruptions (VEI 5+) and does not necessarily apply to eruptions of lesser magnitude. The latter are rarely discernible in proxy records (e.g., ice cores), though they may contribute to the ‘background’ flux of stratospheric sulfate aerosol, which is often assumed to be constant. A current challenge in volcano science is extrapolating fluxes of volcanic gases (SO₂ and CO₂) measured during the geologically brief instrumental era back into the past, to assess the atmospheric impact of past volcanic activity and its role in volatile cycles.

Here, we report efforts to extrapolate volcanic SO₂ emissions data back in time by scaling recent satellite measurements to VEIs reported in eruption databases. We also highlight recent work to quantify all volcanic and anthropogenic SO₂ sources detectable using UV satellite observations since 1978. This comprehensive catalog of sources of SO₂ emissions to the boundary layer and free troposphere paves the way towards a more complete understanding of the spatial and temporal variability of tropospheric SO₂ burdens, and hence cross-tropopause transport of aerosol and aerosol precursors that may contribute to background stratospheric aerosol. Anthropogenic SO₂ emissions inventories also provide insight into potential ice core contamination from non-volcanic sources.