We find that the GEM depletions adjust well to a first-order relationship, for both PM0.7DMPS (Figure ) and PM0.95sulfate proxy (supplementary Figure ). Assuming a higher GEM/SO2 ER from Piton de la Fournaise and consequently higher GEM scavenging yields higher kinetic rate constants (i.e. a steeper slope of the fit) (Figure ). It also improves the goodness of fit (R 2 ), mainly because the two clear outliers before 5:55 AM local time, when observed GEM appeared unaffected by the first volcanic plume passage ("A" and "B" in Figure ; see also Figure ), move closer to the line of best fit. We hypothesize that these outliers result from a competition between volcanic GEM emission and GEM scavenging in the plume, i.e. GEM depletion at first plume arrival might have been masked by large volcanic GEM emissions that raised GEM concentrations in the freshly diluted plume significantly above the background. More generally, considering that GEM/SO2 ERs from a single vent can vary within minutes 8 , some of the scatter around the lines of best fit might be attributable to a fluctuating ER during the event.