Observations and retrieval of volcanic SO2 emissions from the Sakurajima volcano, Japan, using temperature-stabilised Car-DOAS

Volcanic eruptions in recent years, including the long-lasting eruption of Cumbre Vieja, La Palma (September-December 2021) and the fissure eruption at Reykjanes Peninsula, Iceland (December 2023, currently ongoing), have put a spotlight on the severe impacts volcanic gas emissions can have on human activities and the environment. Observations of such volcanic degassing were the target of the Volcanic Emissions Observation and Modeling (VOLCOM) field campaign, carried out on Sakurajima volcano, Japan, during November 2023, chosen due to the volcano’s long-lasting activity (currently ongoing since 1955) and surrounding population (>1 million residents in the surrounding 20 km). VOLCOM provided an excellent opportunity to employ and test a well-known remote sensing methodology (Car-DOAS). To reduce the noise level in Car-DOAS measurements, we incorporated a temperature stabilization device. The temperature stabilized-Avantes spectrometer was mounted on a vehicle equipped with a UV bandpass filter. The planning of the measurements was supported by a dedicated high-resolution meteorological forecasting approach using the Weather Research and Forecasting (WRF) and FALL3D models. With the presented setup, SO2 emission rates from the vent were monitored, and strong signals (SCD >1018 molecules cm-2) were observed. High concentrations of SO2 in the plume, however, can make proper retrieval of the signal challenging as the strong absorption of SO2 pushes the DOAS method to its limits. It was seen that shifting the fit window depending on the SO2 signals ensures that the weak absorber assumption remains valid, which is used to create a composite product featuring multiple fit windows. Based on the composite product, the passive emissions during that period were estimated using a mass-balance approach with the forecasted wind field over the volcano. Proper retrieval of the SO2 signal will help us establish a new dataset for the volcano and allow us to gain insight into the chemistry and environmental impacts of volcanic gas emissions.