Quantification of SO2 emission rates from the Kilauea volcano in Hawaii by the divergence of the SO2 flux using S5P-TROPOMI satellite measurements and comparison to results from ground-based observations

With a nearly continuously effusive eruption since 1983, the Kilauea volcano (Hawaii, USA) is one of the most active volcanoes in the world. From the beginning of May till the end of August 2018, a sequence of eruptions on the Lower East Rift Zone (LERZ) caused an enhanced outbreak of volcanic gases and aerosols, releasing them into the troposphere. Since these gases and particles affect climate, environment, traffic, and health on regional to global scales, a continuous monitoring of the emission rates is essential.

As satellites provide the opportunity to observe and quantify the emissions remotely from space, their contribution to the monitoring of volcanoes is significant. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite was successfully launched by the end of 2017 and provides measurements with unprecedented level of detail at a resolution of 3.5 x 7.0 km² (3.5 x 5.5 km² since August 2019). This also allows for an accurate retrieval of trace gas species such as volcanic SO₂.  

Here, we show that the location and strength of SO₂ emissions from Kilauea can be determined by the divergence of the temporal mean SO₂ flux. This approach, which is based on the continuity equation, has been successfully demonstrated for NO_X emissions of individual power plants (Beirle et al., Sci. Adv., 2019).

The present state of our work also indicates that emission maps of SO₂ can be derived by the combination of satellite measurements and wind fields on high spatial resolution. As the divergence is highly sensitive on point sources like the erupting fissures in the 2018 Kilauea eruption, they can be localized very precisely. The obtained emission rates of about 1.5 Mt are substantially lower than the ones reported from ground-based measurements in other studies like the one from Kern et al. (Bull. Volcanol., 2020). 

We discuss several potential reasons for the discrepancies between the ground- and satellite-based observations like e.g. uncertainties of the air mass factor or possible rapid destruction of SO₂ in the presence of clouds.