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Quantification of volcanic halogen impacts in the troposphere through WRF-Chem modelling, satellite and in-situ observations

Periodic Reporting for period 1 - WCVolcano (Quantification of volcanic halogen impacts in the troposphere through WRF-Chem modelling, satellite and in-situ observations)

Reporting period: 2019-01-03 to 2021-01-02

"This project considers what volcanoes put into our atmosphere. There are two major compelling motivations for this:
The first is that the vast quantities of gases and particles ejected by volcanoes change our atmosphere. Volcanoes put. To fully understand the chemistry of the air, it is necessary to understand what volcanoes are emitting, and what happens to these emissions after they mix with air. On geological timescales volcanoes have formed and changed the atmosphere and shaped the landscape. Large eruptions can alter the climate, while even comparatively small eruptions can be significant for local populations.
Secondly, monitoring the material that comes out of a volcano is an important tool for monitoring its activity. Changes in the quantity and chemical composition of a volcano’s output can tell us about sub-surface processes, this information can potentially be used for predicting future activity.
This type of monitoring can be performed by instruments on satellites, including TROPOMI on board the recently launched S5P. Data from such sources is of particular importance for volcanoes which have little ground-based monitoring, more likely for volcanoes that are remote and/or in less economically developed regions.

This project focuses on a few particular chemicals that are present in volcanic plumes – chemicals containing bromine and/or chlorine atoms. These chemicals undergo complex series of reactions when volcanic gases mix with air. One significant impact of these reactions is that ozone – an important gas for atmospheric chemistry – is consumed. The current level of scientific understanding of these processes in volcanic plumes is moderate, although the important chemical reactions are mostly known, there are several unknowns as to what effects this chemistry has in the “real world”.

Only one bromine chemical – bromine monoxide (BrO) - can be easily detected by satellites and other remote techniques. Our current understanding of plume chemistry is insufficient to determine what fraction of bromine is present as BrO plume. Simuilar difficulties underlie observations of two oxidise chlorine chemicals. This limits the usefulness of these measurements for volcano monitoring.

The objective of this project is to produce a computer modelling system for the atmospheric chemistry of volcanoes, including the chemistry of halogens. This model needs to be versatile, so that emissions from almost any volcano could be simulated. We intend to use this model to make robust assessments of the chemistry occurring within plumes. As much as possible, these conclusions we make about the chemistry are to be *quantitative* rather than simply *qualitative*.
The first implementations will be a case study on a well-studied volcano that will provide opportunities to verify the performance of the model. We also wish to show how the model can be used in combination with observations from TROPOMI. We shall show the model has ""skill"" by predicting for a past event similar numbers to those from observations. After proving this skill, we will move on to ""unobservable"" aspects of volcanic plume chemistry – such as the bromine-containing chemicals other than BrO.

As well as using the model ourselves, we hope to make a tool that the scientific community can use and further adapt for further research."
"We have made a computer model which we call WRF-Chem Volcano [WCV]. It is a version of a pre-existing model called ""WRF-Chem"", expanded to include bromine, chlorine, and mercury chemistry and several interactions of these systems. This expansion is enough to answer the scientific questions of the project. Part of this is a system that allows a user of the model to set the location (latitude, longitude, and altitude) as well as the magnitude and chemical composition of volcanic emissions. This allows the system to be easily used for any volcano (except for some very large eruptions).
The model code has been made freely available publicly so that any person worldwide can use it for their research.

Colleagues have acquired TROPOMI satellite data relating to the Christmas 2018 eruption of Mount Etna volcano. We ran a computer model of this plume, using this data to make sure our model eruption was of a similar magnitude to the real one. We evaluated the model predictions regarding BrO against the observations and used this to improve the model.

We also acquired some ozone and SO2 data collected from an aircraft that flew through Mount Etna's plume in the Summer of 2012. This data had not been previously analysed scientifically. This data was found to be very useful and we decided to form the main case study of the project around this.
We ran WCV to simulate the same volcanic plume that was measured by this aircraft. A lot of work went into visualising and analysing the model output. This included maps and graphs. We also developed a tool to work out the strength of all the different chemical reactions - this allowed us to make a highly detailed assessment of the halogen chemistry cycling in the plume.

The ""main"" model run used as inputs a ""best guess"" as to the volcanic emissions. We also ran several model runs with different emissions. We compared how the output changed as a result of variations in the input.

The most significant means of disseminating the results has been the writing of a detailed scientific paper, which has been submitted to an open-access journal (publications are free to all to read without subscription or payment). We have also presented the ongoing progress of our work at several national and international conferences. These included workshop we arranged in Paris for European scientists working in the volcanic halogens field.

We anticipated doing two field campaigns during the project in order to collect data regarding volcanic mercury. However COVID-19-related travel restrictions meant that the second campaign has been delayed, and first campaign's data is insufficient in quantity for productive analysis by itself."
The ozone data from Mount Etna plume that we analysed expands a very small set of in-plume ozone measurements. It also, uniquely, can be connected to simultaneous near-vent measurements. This analysis allows us to identify continual processes in in-plume halogen-ozone chemistry beginning almost immediately after emission, something that was not possible from previous data.
Our analysis of the halogen chemistry in Mount Etna's plume is the most detailed assessment of these processes in the early plume. We quantify the relationship between halogens and ozone in degassing (non-explosive) plumes, and show that halogen chemistry needs to be considered for an accurate assessment of many chemical systems. As we are able to determine the reaction rate of each individual reaction, we can determine the processes that are occurring to cycle halogens in unprecedented detail (see Figure).
We have also implemented recently-discovered mercury chemistry kinetics into a volcanic plume model for the first time, and found that these have the potential to significantly change the mercury chemistry within the plume.
One important finding was a dependence of in-plume BrO levels on plume density and the time-of-day that observations are made. These results should allow for a refinement of the use of BrO measurements as a volcano monitoring tool.
Bromine cycle in a part of the volcanic plume. Line widths are proportional to the flux of bromine.