Observation and modelling of ozone-destructive halogen chemistry in a passively degassing volcanic plume

Luke Surl* (Corresponding Author), Tjarda Roberts, Slimane Bekki

*Corresponding author for this work

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Volcanoes emit halogens into the atmosphere that undergo complex chemical cycling in plumes and cause destruction of ozone. We present a case study of the Mount Etna plume in the summer of 2012, when the volcano was passively degassing, using aircraft observations and numerical simulations with a new 3D model “WRF-Chem Volcano” (WCV), incorporating volcanic emissions and multi-phase halogen chemistry.

Measurements of SO2 – an indicator of plume intensity – and ozone were made in the plume a few tens of kilometres from Etna, revealing a strong negative correlation between ozone and SO2 levels. From these observations, using SO2 as a tracer species, we estimate a mean in-plume ozone loss rate of 1.3×10−5 molecules of O3 per second per molecule of SO2. This value is similar to observation-based estimates reported very close to Etna's vents, indicating continual ozone loss in the plume up to at least tens of kilometres downwind.

The WCV model is run with nested grids to simulate the plume close to the volcano at 1 km resolution. The focus is on the early evolution of passively degassing plumes aged less than 1 h and up to tens of kilometres downwind. The model is able to reproduce the so-called “bromine explosion”: the daytime conversion of HBr into bromine radicals that continuously cycle in the plume. These forms include the radical BrO, a species whose ratio with SO2 is commonly measured in volcanic plumes as an indicator of halogen ozone-destroying chemistry.

The species BrO is produced in the ambient-temperature chemistry, with in-plume BrO / SO2 ratios on the order of 10−4 mol/mol, similar to those observed previously in Etna plumes. Wind speed and time of day are identified as non-linear controls on this ratio. Sensitivity simulations confirm the importance of near-vent radical products from high-temperature chemistry in initiating the ambient-temperature plume halogen cycling. Heterogeneous reactions that activate bromine also activate a small fraction of the emitted chlorine; the resulting production of chlorine radical Cl strongly enhances the methane oxidation and hence the formation of formaldehyde (HCHO) in the plume.

Modelled rates of ozone depletion are found to be similar to those derived from aircraft observations. Ozone destruction in the model is controlled by the processes that recycle bromine, with about three-quarters of this recycling occurring via reactions between halogen oxide radicals. Through sensitivity simulations, a relationship between the magnitude of halogen emissions and ozone loss is established.

Volcanic halogen cycling profoundly impacts the overall plume chemistry in the model, notably hydrogen oxide radicals (HOx), nitrogen oxides (NOx), sulfur, and mercury chemistry. In the model, it depletes HOx within the plume, increasing the lifetime of SO2 and hence slowing sulfate aerosol formation. Halogen chemistry also promotes the conversion of NOx into nitric acid (HNO3). This, along with the displacement of nitrate out of background aerosols in the plume, results in enhanced HNO3 levels and an almost total depletion of NOx in the plume. The halogen–mercury model scheme is simple but includes newly identified photo-reductions of mercury halides. With this set-up, the mercury oxidation is found to be slow and in near-balance with the photo-reduction of the plume. Overall, the model findings demonstrate that halogen chemistry has to be considered for a complete understanding of sulfur, HOx, reactive nitrogen, and mercury chemistry and of the formation of sulfate particles in volcanic plumes.
Original languageEnglish
Pages (from-to)12413–12441
Number of pages29
JournalAtmospheric Chemistry and Physics
Publication statusPublished - 19 Aug 2021

Bibliographical note

Acknowledgements. As well as the financial support noted below, the authors would like to acknowledge the following. Computer modelling benefited from access to IDRIS HPC resources (GENCI allocation A007017141) and the IPSL mesoscale computing cen- tre (CICLAD: Calcul Intensif pour le CLimat, l’Atmosphère et la Dynamique). Aircraft measurements were carried out within the Global Mercury Observation System project (GMOS; http://www. gmos.eu, last access: 12 August 2021), and we acknowledge the contribution of the GMOS teams of CNR IIA, Italy, and Helmholtz- Zentrum Geesthacht, Germany. GMOS was financially supported by the European Union within the seventh framework programme (FP-7; project ENV.2010.4.1.3-2). The authors would like to thank Louis Marelle and Jennie Thomas for their assistance with the model development. The model presented in this paper is a devel- opment of work undertaken as part of Luke Surl’s PhD, which was funded by the UK’s Natural Environment Research Council and su- pervised by Deanna Donohoue and Roland von Glasow. The authors would like to thank the two anonymous referees for suggesting var- ious improvements to this paper.

Financial support. This paper is a result of a project that has re- ceived funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 800062 as well as ANR Projet de Recherche Collaborative VOLC-HAL-CLIM (Volcanic Halogens: from Deep Earth to Atmospheric Impacts), ANR-18-CE01-0018.


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