Quantifying SO2 emissions from the 2021 eruption of Fagradalsfjall, Iceland, with TROPOMI and PlumeTraj

Effusive eruptions are a significant source of volcanic volatile species, injecting various reactive and climate altering products into the atmosphere, while low-level emissions can be hazardous to human health due to the degradation of local or regional air quality. Quantification of the flux and composition of these emissions also offers an insight into the magmatic processes driving the eruption. These factors mean that gas flux measurements are a key monitoring tool for managing the response to such eruptions. The usual target species for gas flux measurements is sulphur dioxide (SO₂) due to its high concentration in volcanic emissions but low ambient concentration, and its ability to be measured with UV and IR spectroscopy from both ground and space.

Fagradalsfjall volcano, Iceland, underwent an effusive eruption between March – September 2021, emitting over 100 million m³ of lava and producing significant SO₂ emissions. The eruption progressed through several distinct phases in eruptive style, with different surface activity and gas emission behaviour for each. Satellite instruments have not traditionally been used for monitoring emissions from effusive eruptions such as this, as they often lack the spatial or temporal resolution to detect and quantify low-level effusive emissions. However, the launch of ESA’s Sentinel-5P, carrying the TROPOMI instrument, in October 2017 opened the door for such measurements, offering a step change in sensitivity to tropospheric emissions over previous missions.

Here, we will present measurements of altitude- and time-resolved SO₂ fluxes from Fagradalsfjall by combining TROPOMI observations with a back-trajectory analysis toolkit called PlumeTraj. We compare the emissions with other geophysical monitoring streams throughout the eruption and explore changes across the different phases of the eruption. This will demonstrate the ability of TROPOMI and PlumeTraj for quantifying intra-day, low-level SO₂ emissions and highlight the potential insight these measurements can provide for future effusive eruptions.