Upon arrival at Maïdo, we expect the plume to be strongly mixed and diluted by background air. To estimate kinetic rate constants, we assume that the dilution occurred near-instantly (compared to the plume transport time of several hours) after emission from the vent, after which the plume continued travelling without much further dilution until arrival at Maïdo. With this simplification, the mean interaction time between GEM and plume constituents can be set equal to the plume transport time. It should be noted that this simplification likely overestimates the average interaction time between entrained background GEM and plume constituents because real plume dilution might have occurred in several steps or more gradually during the transport. For example, background GEM entrained into the plume shortly before arrival at Maïdo would have had a much shorter interaction time with plume constituents than estimated here. The transport time might also overestimate the interaction time because reactive chemicals or surface adsorption sites on particles might not be instantly available after plume emission but form over time.