If we assume insignificant SO2 scavenging in the plume during the transport from the vent to Maïdo, we can parametrize 𝐺𝐸𝑀 𝑣𝑜𝑙𝑐𝑎𝑛𝑖𝑐 as the product of the SO2 concentration observed in the plume and a GEM/SO2 emission ratio (ER) from the vent (Eq.1). While gas-phase SO2 is certainly oxidized within volcanic plumes , this occurs, outside clouds, reportedly at a rate of ~1-2% h -1 at solar noon . Assuming a similar SO2 oxidation rate in the plumes observed here, which encountered mostly cloudless conditions, we would expect that > 90% of the emitted SO2 remained in the plume after transport times relevant here (~3-7 hours), making SO2 a fairly conservative tracer for the present purpose.