This GEM lifetime is conservative, as SO2 was typically lower in the dilute plume. It was also found to be robust towards variations in O3 (GEM lifetime varies < 1 hr if O3 was 10 times lower; and remains unchanged at higher O3 in the plume). At the observed low levels of BrO, bromine oxidation could thus explain at best ~8% GEM oxidation over 3 hours, and ~18% over 7 hours, even in the daytime plume. This is congruent with a recent study 14 that modelled radical halogen chemistry in volcanic plumes from Mount Etna and, coupled to a gas-phase mercury scheme, obtained very low net GEM oxidation within the plume, even at BrO/SO2 ratios of > 1 •10 -4 (i.e. about an order of magnitude higher than observed here).