Where c is the GEM concentration observed at Maïdo upon plume arrival (in ng m -3 ), c0 is the initial concentration in the diluted plume before interaction with particles (in ng m -3 ), t is the mean interaction time (in h, see methods), 𝑃𝑀 is the aerosol mass in the plume (in µg m air -3 ), and "k" is a first-order reaction rate constant (in µg -1 m air 3 h -1 ).