Volcanic Halogens: from Deep Earth to Atmospheric Impacts – VOLC-HAL-CLIM

The project brings together leading expertise in: HT-HP methods including pioneering experiments on bromine and iodine at depth, an internationally unique EQUIPEX facility that couples HT-HP experiments with on line gas sampling, degassing modelling, in atmospheric measurements with mini-sensors, and in atmospheric modelling from plume to global scales, including recent developments in the atmospheric processing of volcanic halogens and in chemistry-climate impacts.

We will capitalize, inter alia, on a petrological experimental facility rather unique in Europe that is being developed at Orléans (PLANEX, Planète experimentation : simulation et analyse in situ en conditions extrêmes). This facility will enable to extract and measure directly ‘on-line’ the HP-HT degassed fluids. One of the major advances will be the acquisition of new experimental data that will specifically constrain the halogen fluid/melt partitioning in degassing models, a critical but still highly unconstrained process.

Experimental Methods include:
HP-HT (High-Pressure, High-Temperature) experiments, XRF, EXAFS, Paris-Edinburgh Press, Quenched sample and online gas analysis, Melt-Inclusions
Field-observation Methods include:
Mini-sensors and portable field instruments, model data will also be compared to: aircraft, balloon and surface datasets, satellite datasets including high-resolution TROPOMI BrO/SO2
Numerical Model Methods include:
CHOS-Halogens degassing model, Hot-crater and cooled-plume PlumeChem models, WRF-Chem with nested grids for regional scale, Global-scale MOCAGE CTM with sub-grid parameterisation and IPSL CCM LMDZ-REPROBUS.

- A new kinetics-based model of the high-temperature chemistry of volcanic gases entering the atmosphere as an advancement over previous thermodynamic approach (Roberts et al., 2019)
- First results from the development of WRFChem regional model for volcanic halogen chemistry (EGU 2020 presentation, paper in prep.)
- Development of volcanic halogen chemistry in MOCAGE global/regional model and first results of simulations
- First long simulations of the new stratospheric chemistry-climate model LMDZ6A-REPROBUS
-High-Temperature High-Pressure laboratory experiments show that bromine partitioning in the fluid-melt system is dependent on all investigated parameters and that bromine strongly favours fluid to silicate melts with D_Br ^(f/m) that increases drastically with melt silica content (EMPG presentation, paper in prep)
-The study of the behaviour of iodine is currently in progress focusing on a case study in Ethiopia where field work and geochronological data provide new constraints on the timing, evolution an characteristics of the Central sector off the MER (Franceschini et al. in press).

This project will deliver key knowledge on halogens behaviour in magma and degassing and an observation-validated multi-scale modeling system as a framework for the eventual study of halogen impacts from volcano emissions globally, and from major volcanic activity in the past.

WP1) Constrain experimentally the behaviour of halogens in magma, from production to emission.

Tasks 1 and 2 will quantify experimentally at HTHP (high-temperature, high-pressure) the halogens (Br, I) behavior in magma during their production, storage and transfer toward the surface and implement the findings into a numerical model of magma degassing. This degassing model will be used in conjunction with analyses of eruption samples (MI) to, finally, constrain the chemical composition of volcanic gases as they reach the open vent and are emitted to the atmosphere. The results feed into WP2 Task 3 that quantifies emissions and their near-source processing by field-observations, and subsequently into Task 4 and 5 atmospheric modelling.

WP2) Evaluate the atmospheric physico-chemical processing and impacts of volcanic halogens.

A key challenge in evaluating the regional and global impacts of volcanic halogens is to be able to describe the relevant processes associated with each specific phase and scale, from the hot crater to the plume, then to the regional and finally global scales. No single model can cover all the different processes and scales. For this reason, our modelling strategy relies on a multiscale chain of coupled state-of-the-art numerical models: hot chemistry module, plume chemistry model, regional chemistry-transport model and global chemistry-transport/chemistry-climate models. WP2 aims to develop such a multi-scale modelling system and use it to assess the impact of volcanic halogens, for selected case studies. The model system focuses on Br with Cl that are recognized to have major chemistry-climate impacts.

Boucher, O., …S. Bekki, et al., Presentation and evaluation of the IPSL-CM6A-LR climate model, Journal of Advances in Modeling Earth System, in press, doi:10.1029/2019MS002010, 2020.

Lurton, T., …S. Bekki, et al., Journal of Advances in Modeling Earth System, 12, in press, doi:10.1029/2019MS001940, 2020.

Roberts T., Dayma G., Oppenheimer C. et al., Reaction rates control high-temperature chemistry of volcanic gases in air, Frontiers in Earth Science, 7, 154, doi.org/10.3389/feart.2019.00154

Whitty R., …Roberts T.J., et al., Spatial and Temporal Variations in SO2 and PM2.5 Levels from 2007-2018 Kilauea Volcano, Hawai’i, Front. Earth Sci., 25 February 2020, doi.org/10.3389/feart.2020.00036

Clyne M., …M. Khodri, W. Ball, S. Bekki, S. Dhomse, N. Lebas, et al., Model physics and chemistry causing intermodel disagreement within the VolMIP-Tambora Interactive Stratospheric Aerosol ensemble, Atmos. Chem. Phys. Discus., doi.org/10.5194/acp-2020-883

Franceschini, Z., Cioni, R., Scaillet, S., Corti, G., Sani, F., Isola, I., Mazzarini, F., Duval, F., Erbello, A.,Muluneh, A., Brune, S. 2020. Recent volcano-tectonic activity of the Ririba rift and the evolution of rifting in South Ethiopia Journal of Volcanology and Geothermal Research , in press

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.