We thus hypothesize that the GEM depletions observed here were mainly caused by gas-particle interactions, considering the strongly elevated concentrations of sulfate-rich and highly acidic particles in the volcanic plumes (see Figure ), and the relatively long interaction time (several hours) between particles and gas-phase Hg before plume arrival at Maïdo. In the following sections, we explore heterogeneous GEM oxidation and GEM uptake by particles as possible manifestations of such a gas-particle interaction. However, we highlight that we have no suitable high-resolution observations that allow us to determine whether the "missing" GEM was found in the gas phase (in the form of GOM) or in the particulate phase (in the form of PBM) so that a definite answer about the underlying nature of interaction cannot be achieved here.