The 2022 eruption of the Hunga Tonga volcano in the South Pacific not only released vast amounts of dust, ash, and greenhouse gases but also triggered an unexpected natural process that helped reduce some of its own methane emissions. Methane is a potent greenhouse gas, trapping roughly 80 times more heat than carbon dioxide over a 20-year period. However, in the days following the eruption, satellites detected an unusual and persistent cloud of formaldehyde drifting over the region for nearly ten days—an observation that initially puzzled scientists.

Volcanoes do not typically emit formaldehyde, and the compound’s usual atmospheric lifespan is only a few hours. Researchers eventually determined that the formaldehyde cloud was a product of methane breakdown within the volcanic plume itself. This conversion occurs through a chemical reaction involving highly reactive chlorine particles generated when sunlight interacts with volcanic ash and seawater injected into the stratosphere by the eruption.

The mechanism mirrors a similar process observed in 2023 when Saharan dust carried over the Atlantic Ocean mixed with sea salt aerosol, producing chlorine radicals under sunlight. These chlorine particles then facilitated the oxidation of methane into formaldehyde. The discovery that volcanic ash can contribute to methane breakdown in the stratosphere—where conditions differ markedly from the lower atmosphere—is considered novel and unexpected.

Professor Matthew Johnson of the University of Copenhagen highlighted the significance of the finding, noting that the same chlorine-driven oxidation reaction observed with Saharan dust appears to take place in volcanic plumes at high altitudes. The Hunga Tonga event was notable for propelling large amounts of seawater into the stratosphere alongside ash, creating a unique chemical environment. Sunlight interacting with the combination of ash and saltwater generated reactive chlorine radicals, which then attacked methane released by the volcano.

Typically, methane remains in the atmosphere for 10 to 20 years before breaking down, significantly contributing to short-term global warming. The observed rapid chemical conversion linked to volcanic activity and airborne dust suggests natural processes that could influence atmospheric methane levels more dynamically than previously understood. These findings may open new avenues for addressing methane pollution and mitigating its impact on climate change, although further research is needed to assess the broader implications and scalability of such mechanisms.