• Observational constraint of Hg transformations in aging volcanic plumes is needed. To address the physical and chemical nature of gas-particle interactions, as well as their kinetics, simultaneous observations of speciated Hg and ancillary variables such as SO2, O3, BrO, and suspended fine particles (at least <10 µm, better <2.5 µm or <1 µm), ideally both in their concentration and composition, would be highly beneficial.