In-situ SO2 concentrations were measured at the summit of Piton de la Fournaise (-21.243°S; 55.709 °E; 2632 masl; ~1.5 km from the vent, see Figure ) with a Multi-GaS analyser performing 4 sets of measurements per day (two daytime, two night-time), each lasting 30 minutes. The Multi-GAS 44 is a standard volcanic gas sensing technique that is commonly used for in-situ real-time observations (at 0.1 to 1 Hz rate) of SO2 (and other major volcanic gas species like CO2, H2O, H2S) concentrations in near-vent volcanic plumes. SO2 concentrations are measured using a T3STF CiTiceL® specific electrochemical sensor from CityTecnology. SO2 degassing fluxes from Piton de la Fournaise were observed remotely with three scanning-DOAS instruments, stationed on the cliffs of the Enclos Fouqué caldera (see Figure ). The instruments are part of the Network for Observation of Volcanic and Atmospheric Change (NOVAC) and are operated almost continuously since 2007. The flux is derived from measurements of diffused solar radiation in the spectral range between 280 and 430 nm, obtained sequentially by scanning the line of sight through a flat or conical surface that intersects the volcanic plume. Spectra are then analyzed by DOAS to obtain slant column densities of SO2, BrO, and other species relative to the background. Evaluations were done in the 314-327 nm and 331-353 nm for SO2 and BrO, respectively, and included cross-sections for O3 absorption, the Ring-effect, a 5 th -degree polynomial, and generic corrections for dark current, offset, stray-light, and wavelength shifts (against a synthetic solar-atlas). The line density obtained by integrating the columns in the cross-section of the plume is multiplied by the normal component of wind velocity at plume height to derive the flux. The height and direction of the center of mass of the plume are calculated by triangulation from simultaneous measurements by two stations. For this study, we used plume velocity data from the ERA5 re-analysis product of the European Centre for Medium-Range Weather Forecasts (ECMWF), with a time resolution of 1 h. One SO2 flux measurement is obtained every 5-10 minutes, depending on light conditions. SO2 fluxes could only be accurately determined for plumes that left the rim of the caldera (Enclos Fouqué) and passed the field of view of at least 2 DOAS instruments.