Two GEM passive samplers were deployed between 28 th of April 2018 and 4 th of May 2018 close (~200 m distance) to the eruptive vent of the April/June 2018 eruption of Piton de la Fournaise (see Figure ). A larger number of GEM passive samplers was operating all over La Réunion Island between 2018 and 2019. This passive sampler data set is described in detail in Hoang et al. (2023) .