The term “volcanic gas” identifies the fluid gas phase released by active volcanoes, both during eruption and quiescence. Compositionally, volcanic gases are molecular combinations of four major elements (H, C, O, and S) but also include a variety of minor (CI, F, N, He, Ne, and Ar) to trace components. Being generally water-dominated, volcanic gases show a spread of compositions, reflecting a combination of source processes (magma and its geodynamic context) and interactions with local hydrothermal or hydrologic systems (Aiuppa et al, 2016).