Piton de la Fournaise entered an eruptive phase on the 27th of April 2018, as evidenced by large daily average SO2 degassing fluxes of ~ 200 t d -1 on the 28th (see Figure ). Daily SO2 fluxes peaked on the 30 th April with ~ 500 t d -1 and then decreased rapidly in the following days, reaching almost undetectable levels by ~May 10th. Fluxes remained low and plumes seldomly left the Enclos Fouqué caldera rim for the rest of the eruptive phase which lasted until the 1 st of June. These observations agree well with satellite-based observations from the Ozone Monitoring Instrument (OMI) and the Ozone Mapping and Profiling Suite (OMPS), as reported by the NASA Global SO2 Monitoring Homepage (). Despite large initial SO2 fluxes, ground-level insitu SO2 concentrations at the summit of Piton de la Fournaise (~1.5 km from the vent) remained initially unaffected. In contrast, in-situ SO2 concentrations already peaked on the 29 th of April at Maïdo, which is ~38.5 km away from the vent. This indicates that the plume rise was initially strong, facilitating atmospheric plume transport by winds aloft. Recently on May 10 th SO2 concentrations peaked at the summit of Piton de la Fournaise, indicating that by this time the eruptive plume rise declined enough to allow the plume to be observed at ground level in that area. On the 29 th of April 2018, i.e. early in the eruptive phase and while plume rise was still significant, we observed at Maïdo GEM depletion in transiting volcanic plumes (Figure , Figure ). GEM depletion was complete (i.e. concentrations below the instrument detection limit) in two instances, and the first complete GEM depletion event was observed before sunrise (solar zenith angle between 96° and 93°, Figure ). All GEM depletion events were clearly related to the passage of volcanic plumes, as evidenced by strongly enhanced concentrations of SO2 (up to ~900 ppbv, Figure ) and suspended particles (up to ~28 ± 8 µg m -3 for PM0.7DMPS, up to 570 ± 290 µg m -3 for PM0.95sulfate proxy, Figure ). The onset of the first GEM depletion coincided with a change in transport pathways at ~6:00 local time, as indicated by a sharp decrease in wind speed at Maïdo (from ~ 18 km h -1 to ~ 5 km h -1 , Figure ) and a decrease in observed air temperature (from ~15 C to ~10 C, Figure ). Intriguingly, no GEM depletion was observed before 6:00, even though the first strong volcanic plume had already arrived at around ~5:20. This will be discussed further below.