A team is developing a new method for assessing the ability of the atmosphere to remove methane, a potent greenhouse gas

Glenn Wolfe of UMBC and his collaborators conducted a new study that explains how scientists understand the fate of methane, a potent greenhouse gas, in the Earth's atmosphere.

Of the greenhouse gases, methane has the third largest overall effect on climate after carbon dioxide and water vapor. And the longer it stays in the atmosphere, the more heat it holds. This is why it is essential that climate models correctly represent the lifetime of methane before it is broken down. This happens when a molecule of methane reacts with a hydroxyl radical – an oxygen atom bonded to a hydrogen atom, represented by OH – as part of a process called oxidation. Hydroxyl radicals also destroy other harmful air pollutants.

"OH is really the most central oxidizing agent in the lower atmosphere, controlling the life of almost all reactive gases," says Wolfe, assistant research professor at the Joint Center for Earth's Technology Systems. ; UMBC. However, "overall, we do not have a way to measure OH directly." Better yet, it is clear that current climate models are struggling to properly simulate OH. With existing methods, scientists can deduce the OH on a coarse scale, but there is little information on where, when and why variations in the OH.

New search published in Proceedings of the National Academy of Sciences and led by Wolfe puts scientists on the path to change that. Wolfe and his colleagues have developed a unique way to deduce how global OH concentrations vary over time and in different regions. A better understanding of OH levels can help scientists understand how the highs and lows of global methane levels are due to changing emissions, such as oil and natural gas production or wetlands, compared to at varying OH levels.

A flying laboratory

NASA satellites have measured atmospheric concentrations of formaldehyde for more than 15 years. Wolfe's new research supports these data, as well as new observations gathered during NASA's recent atmospheric tomography (ATOM) mission. ATom has completed four tours around the world, sampling air with the help of a NASA research aircraft.

This "flying laboratory", as described by Wolfe, has collected data on the atmospheric levels of formaldehyde and OH that illustrate a remarkably simple relationship between the two gases. This did not surprise scientists because formaldehyde is a major byproduct of methane oxidation, but this study provides the first concrete observation of the correlation between formaldehyde and OH. The results also showed that the concentrations of formaldehyde measured by the aircraft were consistent with those measured by the satellites. This will allow the Wolfe team and others to utilize existing satellite data to infer OH levels across most of the atmosphere.

"Thus, airborne measurements give you a ground truth that this relationship exists," says Wolfe, "and satellite measurements allow you to extend this relationship around the world."

Wolfe is however the first to recognize that the work of improving global models is far from complete. The aircraft measured OH and formaldehyde levels on the high seas, where air chemistry is relatively simple. It would be more complicated on a forest and even more on a city.

Although the relationship established by the researchers provides a solid foundation, while most of the Earth's air floats over the oceans, further work is needed to see how the OH levels differ in more complex environments. Potentially, different data from existing NASA satellites, such as those that track urban emissions or forest fires, could be useful.

Wolfe hopes to continue fine-tuning this work, which he says is "at the heart of the chemistry and climate research communities, and they are very interested in getting the OH right."

Make the right choices

The present study took into account the seasonal variations of OH, by analyzing the measures taken in February and August. "Seasonality is one aspect of this study that is important," says Wolfe, "because the latitude where OH is at its maximum is moving." Taking into account seasonal changes in OH concentrations, or even multi-year changes caused by phenomena such as El Niño and La Niña, could be an angle to explore in an attempt to improve global climate models.

Further analysis of global OH levels using satellite data validated by aerial data could also help scientists to refine their models. "You can use spatial variability and seasonality to understand, at the process level, what leads to OH, and then ask whether the model is working properly or not," says Wolfe. "The idea is to be able to explore all these features, for which we had no data before."

This new research is a step in the process of improving our understanding of the global climate, even as it evolves rapidly. To understand more precisely how, for example, the reduction of methane emissions would affect the climate and how fast could even influence political decisions.

"It's not perfect, it takes work," says Wolfe. "But the potential is there."