Volcanic Halogens Processing and Impacts in the Troposphere and Stratosphere

International audienceVolcanoes release vast quantities of gases, including halogens (e.g. HBr, HCl) as well as sulfur (SO2). Volcanic SO2 is oxidized into sulfate aerosols in the atmosphere that impact climate and favor the destruction of stratospheric ozone. Volcanic halogen emissions can also be oxidized in the plume to form reactive halogens (including BrO, OClO). The near-source plume processing is an important control on whether volcanic halogens become activated to destroy tropospheric ozone (Roberts, Geosciences, 2018). In addition, satellite observations show that volcanic halogens (e.g. HCl) can be co-injected with SO2 into the stratosphere, with potential for further chemistry-climate impacts. Building on earlier box and 1D model studies we have performed 3D regional and global atmospheric model investigations of the processing and atmospheric impacts from volcanic halogens released from present-day degassing/eruption case studies. In the troposphere: Jourdain et al. ACP (2016) simulate the local-regional scale processing of a large continuous release of halogens and sulfur from Ambrym volcano in 2005. The model predicts conversion of emitted HBr into reactive bromine species (BrO, HOBr, Br, Br2, BrCl, BrONO2) downwind from the volcano, reproducing reported measurements of plume BrO. As well as predicting a strong depletion of tropospheric ozone regionally, the study highlights the potential for volcanic reactive halogens to be convectively transported into the stratosphere. In the stratosphere: Lurton et al. ACP (2018) simulate the chemical processing and aerosol microphysics following the injection of volcanic SO2 and HCl by the 2009 Sarychev Peak eruption, using a global earth system model. Stratospheric ozone depletion is more severe by about forty percent in the model simulation that includes the HCl co-injection than for the SO2-only case. Both models identify effects of coupled SO2-halogens processing: volcanic halogen chemistry depletes oxidants (ozone, HOx, NOx) and consequently slows the oxidation of SO2 into sulfate aerosols, whilst volcanic sulfate aerosols have a key role in the multi-phase processing of volcanic halogens. Understanding this synergy is crucial to quantifying the impacts of volcanic halogen-sulfur emissions in the present day and also from past volcanic events