Volcanoes are messy things, what with all that ash, water vapor, sulfur, and greenhouse gases polluting the atmosphere.
As one of the largest blasts seen in modern history, the 2022 Hunga Tonga-Hunga Ha’apai eruption in the South Pacific could be considered one of the messiest. At least the colossal blast had the good graces to clean up after itself, according to a surprising discovery by a team of researchers from across Europe.
A recently published investigation led by Maarten van Herpen, a physicist from Acacia Impact Innovation BV in the Netherlands, used satellite data to uncover evidence of methane breaking down in the volcano’s plume, high in the stratosphere.
“It is known that volcanoes emit methane during eruptions, but until now it was not known that volcanic ash is also capable of partially cleaning up this pollution,” says van Herpen.
Methane is a rather notorious greenhouse gas, with a heat-trapping potency some 28 times that of carbon dioxide. It’s with some small fortune, then, that the molecule breaks down comparatively readily, combining with ozone to form CO2 and water within around a decade of its release.
While the most concerning sources of emission include fossil fuel industries and fermentation involving livestock and landfill, volcanoes contribute a small amount of the gas by breaking down organic material in the planet’s crust or through inorganic geological reactions and ejecting it into the atmosphere during eruptions.
Researchers typically monitor the emission and breakdown of methane using satellites that detect signature infrared reflections from the surface. Over the dark, open ocean, this isn’t possible, forcing scientists to get creative.
Knowing that methane temporarily oxidises to form formaldehyde, which has a long-wave ultraviolet signature, van Herpen and his team hunt for this short-lived molecule in the plume of the Hunga Tonga-Hunga Ha’apai eruption.
“When we analysed the satellite images, we were surprised to see a cloud with a record-high concentration of formaldehyde,” says van Herpen. “We were able to track the cloud for 10 days, all the way to South America.”
By the team’s estimates, around 300,000 metric tons of methane – the equivalent of emissions from 2 million cows – was released from the eruption. It was being removed at a rate of around 900 metric tons a day.
Given that formaldehyde disintegrates into water and carbon dioxide in just a few hours, some mysterious process must have been churning it out of the plume’s methane.
The solution, the team claimed, was in another discovery made by van Herpen and his colleagues half a world away, just a few years prior.
In 2023, they published the results of a modeling study that showed sunlight could release chlorine from sea spray when combined with iron-bearing dust blown from the Sahara. This chlorine, they argued, could react with methane to form hydrochloric acid.
Was it possible that the same reaction was taking place in the ash and seawater spewed up by the Hunga Tonga-Hunga Ha’apai eruption?
If so, models predicting the global methane budget may need a bit of tweaking.
“We now know that atmospheric dust – for example, from a volcanic eruption – impacts the methane budget, meaning the budget of how much methane is added to the atmosphere and how much is removed,” says University of Copenhagen atmospheric chemist Matthew Johnson.
“Because dust has not previously been taken into account, it is important that we correct the data on which these estimates are based.”
This research was published in Nature Communications.
Source: University of Copenhagen
Fact-checked by Bronwyn Thompson