High above the Amazon rainforest there are strange accumulations of particles. Since the 1980s, this phenomenon has defied explanation, and had researchers searching for the culprit. Now an international collaboration of researchers identified a new suspect: isoprene.
Isoprene is a molecule emitted by plants, in particular by species found in the rainforest, and is the most abundant, non-methane hydrocarbon emitted into the atmosphere.
Researchers wanted to know whether the release of isoprene could be the basis for the formation of the particles above the Amazon. “The high concentrations of aerosol particles in the upper troposphere over the amazon are of great significance for the climate system,” said Jiali Shen, a postdoctoral researcher at the Institute for Atmospheric and Earth System Research at the University of Helsinki.
The troposphere is the lowest layer of the atmosphere and extends from the Earth’s surface upward roughly 18km. Here, particles block incoming radiation or trap outgoing radiation, and are the initial seeds for newly forming clouds.
Shen and her colleagues were inspired to test whether isoprene was forming the unidentified particles when an unrelated group of researchers reported that nighttime isoprene emissions in the rainforests were higher than expected.
Taking a look above the clouds
According to Shen, her colleague Xu-Cheng He suggested a series of experiments that could reveal whether isoprene, once emitted into the atmosphere, changes and forms particles. For this they would need to expose isoprene to the same conditions found in the troposphere such as low temperatures and mixtures of other molecules that are emitted into the troposhpere.
To test the hypothesis, Shen and teams of researchers from Europe and the United States enlisted the use of a highly specialized experimental chamber at CERN, the famous physics research facility located in Switzerland.
While it is known for its massive particle accelerator, CERN also houses what is called the CLOUD chamber. “The CLOUD chamber at CERN is one of the cleanest and most sophisticated chambers in the world for investigating aerosol formation under atmospherically relevant conditions,” explained Shen.
They investigated whether oxygenated isoprene, a compound formed when isoprene is oxidized in the atmosphere, could form new particles under the same conditions found in the troposphere.
Their tests confirmed that the isoprene compounds nucleate, meaning they cluster together with other molecules to form aerosol particles, on their own at conditions typical of the troposphere without the aid of other vapours.
“Furthermore, the nucleation rates are significantly enhanced by extremely low concentrations of sulfuric acid or iodine oxoacids,” said Shen. Both of which are are naturally produced by planktons, algae, and other marine sources and present in the troposphere. This processes of clustering by oxygenated isoprene compounds explains the mysterious particle accumulations overtop the Amazon.
“The mechanism revealed in this study offers valuable insights into aerosol particle formation in the upper troposphere, enhancing our understanding of natural aerosol processes,” said Shen.
This data improves global climate models by more accurately representation the behavior of aerosols in the atmosphere. “Ultimately, this enhanced understanding has the potential to improve the accuracy of historical climate simulations, refining predictions of past and future climate behavior by advancing our understanding of aerosol formation,” Shen said.
In a press release, Katrianne Lehtipalo, another researcher at the University of Helsinki involved with the study said, “This demonstrates how emissions from trees can have far-reaching effects on cloud formation and potentially on the global climate.” Adding, “This type of fundamental research is crucial for improving our understanding of climate processes and our ability to predict and mitigate climate change.”
Reference: Jiali Shen, et al. New particle formation from isoprene under upper-tropospheric conditions, Nature (2024). DOI: 10.1038/s41586-024-08196-0
Feature image credit: Renting C on Unsplash
