Volcanic gas studies are an established tool for volcanic monitoring and enhance the understanding of volcanic manifestations. The central question of the project, which we will introduce here, is whether the ratio between bromine monoxide (BrO) to sulphur dioxide (SO2) can potentially serve as indicator for volcanic activity. Both species have the advantage that they can be remotely measured by Differential Optical Absorption Spectroscopy (DOAS) at safe distances from the emission source. To answer the question above, it is of great importance to link the measurements of halogen oxides to the total emission flux of individual halogen species and to understand the influences of meteorological conditions on the formation and measurements of halogen oxides. In a second step we can evaluate the feasibility whether halogen oxide measurements can be used as an indicator of certain volcanic processes. Additionally, the impact of volcanic halogen chemistry on the atmosphere is reinvestigated in light of the novel insights. To gain a quantitative understanding of volcanic plume chemistry, gas emissions and volcanic plume composition of Mt Etna, Italy, Popocateptl, Mexico and Masaya, Nicaragua are analysed. Continuous monitoring measurements are complemented by three intensive campaigns during which DOAS is employed to characterize the concentrations of halogen oxides, their ratios to SO2 and to quantify emission fluxes. In-situ gas sampling and measurements, meteorological parameter and incident solar radiation complete the data-set.