A decade of global volcanic SO2 emissions measured from space

Abstract

The global flux of sulfur dioxide (SO₂) emitted by passive volcanic degassing is a key parameter that constrains the fluxes of other volcanic gases (including carbon dioxide, CO₂) and toxic trace metals (e.g., mercury). It is also a required input for atmospheric chemistry and climate models, since it impacts the tropospheric burden of sulfate aerosol, a major climate-forcing species. Despite its significance, an inventory of passive volcanic degassing is very difficult to produce, due largely to the patchy spatial and temporal coverage of ground-based SO₂ measurements. We report here the first volcanic SO₂ emissions inventory derived from global, coincident satellite measurements, made by the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite in 2005-2015. The OMI measurements permit estimation of SO₂ emissions from over 90 volcanoes, including new constraints on fluxes from Indonesia, Papua New Guinea, the Aleutian Islands, the Kuril Islands and Kamchatka. On average over the past decade, the volcanic SO₂ sources consistently detected from space have discharged a total of ~63 kt/day SO₂ during passive degassing, or ~23 ± 2 Tg/yr. We find that ~30% of the sources show significant decadal trends in SO₂ emissions, with positive trends observed at multiple volcanoes in some regions including Vanuatu, southern Japan, Peru and Chile.

Figure 1

Mean SO₂ columns (in Dobson Units [DU]; 1 DU = 2.69 × 10¹⁶ molecules cm⁻²) for 2005–2007 over (a) the Aleutian Islands (USA) and (b) Indonesia. The volcanic SO₂ sources (including paired sources) are labeled. The Aleutian map also shows locations of explosive eruptions since 2005 (red triangles), with symbol size proportional to total SO₂ emission. The Indonesian map also shows anthropogenic SO₂ sources in Singapore and central Sulawesi, but does not show volcanic SO₂ emissions from Sinabung, Rinjani and Sangeang Api, which first appeared after 2007. Maps were generated using Interactive Data Language (IDL) version 8.5.1 (http://www.harrisgeospatial.com/).

Figure 2. OMI-derived annual mean SO₂ fluxes in 2005–2015 for the ten strongest volcanic SO₂ sources (including paired sources) in the inventory.

Plots are titled with the volcanic source name and rank, and the trend (slope) and linear correlation coefficient (r) of an error-weighted linear regression fit of the annual mean SO₂ fluxes. Each plot shows the annual mean SO₂ fluxes (solid black line), mean SO₂ flux in 2015 (labeled red dot), linear regression trend line (dashed orange line), decadal mean SO₂ flux (horizontal red line), ±1 standard deviation of the decadal mean SO₂ flux (gray band), and an independent estimate of SO₂ flux (horizontal dashed blue line) from a recent compilation or another source. Here, SO₂ flux data for Etna and Popocatepetl are from refs and , respectively. If no independent measurements are available, the plot is labeled with ‘N/A’. See Supplementary Figures (Figs S9–S16) for similar plots for all other sources.

Figure 3. Total, global eruptive and passive volcanic SO₂ emissions (in Tg; 1 Tg = 10¹² g) in 2004–2016.

Eruptive emissions are derived from [10] and recent updates; passive emissions are from the inventory described here. Passive volcanic SO₂ emissions in 2016 are assumed to continue at the mean annual rate observed in 2005–2015.

Figure 4. Cumulative frequency – SO₂ flux plot for all volcanic SO₂ sources detected by OMI.

Symbol color indicates whether the CO₂/SO₂ ratio of the volcanic gases has been measured as of October 2016. Information on availability of CO₂/SO₂ ratios is from E. Hauri (DCO-DECADE, pers. comm.).

Figure 5

(a) Decadal trends in SO₂ emissions measured at 32 volcanic SO₂ sources showing a significant linear correlation coefficient from a weighted linear regression fit (r ≤ −0.5 or r ≤ 0.5). Plots are ranked in order of calculated SO₂ flux trend (i.e., the slope of the linear fit) from negative to positive values. Hence, cold and warm colors indicate sources showing a significant reduction or increase in SO₂ emissions over the 11-year period of measurements, respectively. Each individual plot shows the annual mean SO₂ fluxes for 2005–2015 (white-gray line), the decadal mean SO₂ flux (red line) and the annual mean SO₂ flux in 2015 (labeled red dot) for each source; axis labels are omitted for clarity. The vertical scale on each plot extends from zero to the maximum measured SO₂ flux. For more detailed time-series plots, see Fig. 2 and Supplementary Figures (Figs S9–S16); (b) Location map of the 32 volcanic SO₂ sources, colored based on SO₂ flux trend in 2005–2015 (also see Supplementary Figure S1). Map generated using Interactive Data Language (IDL) version 8.5.1 (http://www.harrisgeospatial.com/).