How would you summarize the ACROSS project?

Christopher Cantrell: The primary objective is a field study in the summer of 2022 to observe the composition of the atmosphere, and our main task now is to plan this campaign and secure funding and partners. The air exiting Paris carries along emissions from industries, automobiles and energy production, which undergo a variety of chemical and physical processes leading to their transformation into many products.

We want to look at the reactions in the atmosphere when these pollutants reach the surrounding forest and react with the molecules produced by the activity of trees. We already have a good understanding of the individual processes in urban and biogenic air masses, but we lack a specific field study on the chemistry of the mixtures, from which findings we expect to improve our models and understanding.

What will you be looking at in the atmosphere?

CC : There are different kind of models for the various chemical processes happening in the atmosphere, and our first goal is to get a better understanding of how compounds are transformed in the atmosphere. When making energy with fossil fuels we are releasing carbon compounds that are oxydized in the atmosphere and can be converted into the particulate phase.

Many questions remain related to the oxidation of organic compounds, the formation of secondary organic aerosols and the budgets of HOx radicals. We want to see what happens in a mixed environment between urban and biogenic air, because the molecules released by trees perhaps reacts in ways that change the properties of the gas-phase and aerosol substances.

The last field study of this size and complexity was more than 10 years and recent advances in measurement capacities allow a more detailed inquiry into the mechanisms describing the degradation of organic coumpounds and other interesting aspects of atmospheric composition. Eventually, we’d like to have an atmospheric model that could accurately predict the location and timing of pollution events. This requires detailed understanding combination of weather processes and chemistry of the atmosphere.

What kinds of measurements will be done?

CC : We are planning a combination of ground site and aircraft measurements, to have horizontal as well as vertical data. A 40 meters tall tower is set up in the forest of Rambouillet, we also are collaborating with another measurement site in Orleans and the existing air quality measurement network around Paris. We also have access to a French ATR-42 aircraft able to follow air masses we want to study as they are carried by the wind. We will use these are state-of-the-art measurements along with numerical models to test our current understanding of atmospheric chemical and physical processes.

Why choose the summer 22?

CC : Chemical processes are more active in the summer for various reasons. It is the period of the year with the most sunlight per day and generally higher temperatures which drives photochemical processes and affects the rates of chemical reactions. Considering the past few years we also expect heatwaves and spikes in ozone production which could increase the effects of this air mass mixing on atmospheric chemical processes. While the processes are important for daytime, there are also interesting chemical interactions happening at night which we will also study. It concerns different molecules but similar processes that can oxidize organic compounds and lead to them being incorporated into the particulate phase.

What are the next steps in the project?

CC : We are now in the planning phase of the field study. We are looking to secure funding and gather partners for measurements. We are discussing with the ONF (Office National des Forêts) for instance, to come to an agreement to use one of their towers that was used to survey wild fires but is no longer used for this purpose.

We are organizing a variety of logistics, for example to bring our equipment to the top of the tower and providing internet access to check the status of our instrument from a distance. We are also looking for new partners because there are more measurements that could benefit the project We are still expecting to have our field study in motion in the summer of 2022 and hope that the sanitary situation will not prevent it.

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Make Our Planet Great Again (MOPGA) is an initiative started by the French president Emmanuel Macron as a response to the withdrawal of the United Sates of America from the Paris Agreement. This call for projects encourages international researchers to join forces with France in the research to fight climate change.

The ACROSS Project.