Volcanoes release vast amounts of gases and particles into the atmosphere. Whilst impacts from volcanic sulfur have been intensively studied, it is now acknowledged that volcanic halogens may also impact the atmosphere. It has already been shown that volcanic halogens undergo a multi-phase ozone-destroying chemistry in the troposphere. There is emerging observational evidence that recent moderate eruptions injected significant amounts of halogens into the stratosphere. In the case of a large halogen-rich eruption, this could cause large stratospheric ozone depletion alongside climate effects. It is increasingly apparent that a fully comprehensive assessment of the impacts of volcanic activity on the atmosphere and climate should not be limited to sulfur only but also include halogens. To quantify impacts from volcanic halogens requires tracing their cycle from deep subsurface to surface resulting in emissions to the atmosphere and characterising their atmospheric physico-chemical processing. However, they are still large uncertainties on key processes. By combining our expertise and innovative experimental/modelling tools across earth and atmospheric sciences, Volc-Hal-Clim will tackle long-standing issues on the fate and impacts of volcanic halogens on atmospheric composition, notably the ozone layer, and climate. There is a strong focus on volcanic bromine (alongside iodine and chlorine), that has been little studied so far but may play a potentially important role in volcanic perturbations. The project consists of 5 tasks across two work-packages. - WP1 is about deep halogen cycle and emissions. We will perform high-pressure, high-temperature experiments to characterise halogen behaviour (solubility, fluid-melt partitioning) at depth and in the shallow crustal reservoir. By developing degassing models based on these experimental data, along with melt-inclusions composition measurements, we will quantify halogen transfer from the subducting slab, up to the crust and ultimately to the surface. The predicted volcanic emissions will be evaluated against observations of halogens near volcanic sources, taking into account their processing inside the crater and on the volcano flank. This will be a key input to the atmospheric studies (WP2). - WP2 deals with the impact of volcanic halogens on atmospheric composition and climate. We will develop an imbricated multi-scale modelling system that will cover the relevant scales and phases in the atmospheric cycle of volcanic emissions: from the very local high temperature chemistry in the crater to reactive plumes dispersing at local/regional scales and finally to the global dispersion in the troposphere and stratosphere. A range of imbricated numerical models (from plume models to a global chemistry-climate model) will be used to investigate the atmospheric impacts. We will simulate the chemistry of plumes, originating from continuous emissions or recent small eruptions, to assess local/regional and global impacts in the troposphere and stratosphere. Model simulations will be evaluated against field and satellite observations, notably on selected case studies, e.g. the Ambrym (a a massive source of halogen) or Etna volcanoes. The modelled climate response will also be compared to meteorological reanalysis data. Process-oriented evaluation and analysis of sensitivity simulations will allow to disentangle the different mechanisms and assess the respective roles of halogens and sulfur including synergetic effects for selected volcanic events. This rather exploratory project will deliver key knowledge, improved volcanic degassing modelling and a unique multi-scale atmospheric modelling system validated with observations. All these elements are needed if volcanic halogens are to be accounted for in assessing the impact of volcanic activity on the Earth’s atmosphere and climate.