Reconstruction of flux and altitude of volcanic SO2 emissions from IASI satellite observations: implications for volcanological and atmospherical studies.
Résumé
Volcanic SO2 degassing is a crucial indicator of the sub-surface volcanic activity, which is widely
used today for volcano monitoring and hazard assessment purposes. Volcanic SO2 is also important
regarding atmospherical studies. More easily detectable from space, SO2 can be used as a proxy of
the presence of ash to anticipate air traffic issues caused by explosive eruptions. Moreover, volcanic
SO2 strongly impacts air quality but also climate following its conversion to radiatively-active
sulphate aerosols. However, the accurate assessment of these various impacts is currently hampered
by the poor knowledge of volcanic SO2 emissions, which can substantially vary with time, in terms
of flux and altitude.
To fulfil this need, we propose a strategy relying on satellite observations, which consequently allows
for monitoring the eruptive activity of any remote volcano. The method consists in assimilating
snapshots of the SO2 load, provided every ~12 hours by IASI, in an inversion scheme that involves
the use of a chemistry-transport model to describe the dispersion of SO2 released in the atmosphere.
Applied on Eyjafjallajökull (Iceland) and Etna (Italy) eruption case-studies, this procedure allows for
retrospectively reconstructing both the flux and altitude of the SO2 emissions with an hourly
resolution. We show the improvement gained in the simulations and forecasts of the location and
mass load of volcanic SO2 clouds using such a detailed reconstruction of emissions.
For calibration-validation purpose, we compared our satellite-derived time-series of the SO2 flux
with ground-based observations available on Etna. This comparison indicates a good agreement
during ash-poor phases of the eruption. However, large discrepancies are observed during the ashrich
paroxysmal phase as a result of enhanced plume opacity affecting ground-based ultraviolet
spectroscopic retrievals. Therefore, the SO2 emission rate derived from the ground is underestimated
by almost one order of magnitude. This result calls for the necessity to revisit currently available
inventories of the global budget of sulfur released by volcanoes, because they heavily rely on groundbased
observations. It also shows that volcano observatories cannot rely solely on ground-based
spectroscopical observations for the monitoring of ash-rich explosive eruptions.
Moreover, we will discuss the assimilation procedure of recently-developed IASI products which
deliver snapshots of the volcanic SO2 cloud altitude. Such improvement renders the inversion
procedure, used for the reconstruction of the altitude of emissions, independent of the wind shear
prerequisite.
Eventually, building on our accurate source of precursory SO2 gas emissions, we can explore the
formation and lifecycle of sulphate aerosols in volcanic plumes. Remote sensing of tropospheric
sulphate aerosols from moderate eruptions is not straightforward. However, we will show how a
combination of chemistry-transport modelling and space-borne CALIOP lidar observations allows
for tracking these aerosols despite their small concentration. The latest promising developments from
high-resolution infrared sounders will bring further constraints on sulphate aerosol load and
characteristics in dispersed volcanic clouds.