Two groundbreaking studies involving researchers from Goethe-University Frankfurt, the Max Planck Institute for Chemistry, the University of Helsinki, the Leibniz Institute for Tropospheric Research and collaboration in Brazil have discovered a new climate mechanism. The Amazon rainforest releases large amounts of gaseous isoprene through plant transpiration. Previously, scientists thought isoprene couldn't make it into the atmosphere over long distances because it breaks down quickly when exposed to sunlight. However, data from the CAFE-Brazil measurement campaign, presented in Nature's cover story, reveal a different insight.
Studies have shown that nighttime thunderstorms can carry isoprene up to 15 kilometers into the air. At these high altitudes, isoprene reacts to form compounds that create large amounts of new aerosol particles. These particles continue to grow and become condensation nuclei, promoting cloud formation. This process may affect climate, highlighting the complex interactions between rainforest ecosystems and atmospheric dynamics.
Who hasn’t enjoyed the aromatic scent that fills the air during a summer stroll through the woods? Part of the reason for this typical smell is terpenes, a class of substances found in tree resins and essential oils. The main and most abundant molecule is isoprene. It is estimated that plants around the world release 500 to 600 million tons of isoprene into the surrounding atmosphere every year, accounting for about half of the total emissions of gaseous organic compounds from plants. Professor Joachim Curtius, an atmospheric researcher from Goethe University Frankfurt, explains: "The Amazon rainforest alone is responsible for more than a quarter of isoprene emissions."
Until now, it has been thought that isoprene in the Amazon degrades quickly and does not enter the upper atmosphere. This is because hydroxyl radicals are formed in the atmosphere near the ground when the sun shines during the day. They are highly reactive and can destroy isoprene molecules within hours. "However, we have now shown that this is only part of the truth. There are still considerable amounts of isoprene in the rainforest at night, and a large proportion of these molecules can be transported higher into the atmosphere," Curtius said.
The cause is tropical thunderstorms brewing over the rainforest at night. They suck up isoprene like a vacuum cleaner and send it to an altitude of 8 to 15 kilometers. As soon as the sun rises, hydroxyl radicals form and react with isoprene. However, at extremely low temperatures at high altitudes, rainforest molecules convert into different compounds than those found at ground level. They combine with nitrogen oxides produced by lightning in thunderstorms. Many of these molecules clump together to form aerosol particles of a few nanometers. In turn, these particles grow in size over time and then become condensation nuclei for water vapor - thus playing an important role in the formation of clouds in the tropics.
Professor Jos Lelieveld explains: "We were able to reveal these processes through a research flight that started two hours before sunrise and continued throughout the day. We were able to detect large amounts of isoprene in the air flowing out of high-altitude thunderstorms, and after several chemical reactions, new aerosols Particles form quickly." He is director of the Max Planck Institute for Chemistry in Mainz and director of the CAFE-Brazil research project (Atmospheric Chemistry: Brazilian Field Experiment), in which an international research team is collecting data on atmospheric chemical processes over the Amazon rainforest. "
Curtius and Lelieveld are not only partners in CAFE-Brazil, but are also involved in the CLOUD consortium, which has more than 20 research groups studying climate-related chemical processes in the atmosphere. They recreated conditions at this altitude in the Aerosol and Cloud Experimental Chamber at CERN in Geneva. With the help of this simulation chamber, they analyzed in detail which reactions are triggered by sunlight.
Dr. He Xucheng, an atmospheric researcher responsible for the isoprene experiment, explained: "With this method, we were able to accurately determine the rate at which isoprene products form aerosol particles. Interestingly, the study found that even trace amounts of sulfuric acid and iodine oxides, commonly found in the atmosphere, are enough to speed up the formation of aerosol particles by a factor of 100. Therefore, these molecules may collectively influence the formation of ocean clouds - a process that is extremely uncertain in climate predictions."
Sulfuric acid is formed in the atmosphere from various sulfur-containing substances. It is mainly produced by the reaction of sulfur dioxide and hydroxyl radicals. In the CLOUD experiment, the Frankfurt research group measured very low concentrations of sulfuric acid, while the Mainz group measured hydroxyl radicals.
High-altitude winds over the Amazon rainforest can carry isoprene-forming particles thousands of kilometers from their source. This means they may influence the formation of distant clouds. Because clouds vary in type and height, they play a vital role in climate by both blocking solar radiation and preventing heat from radiating into space. Therefore, the researchers hope their findings will help improve climate models.
The results from the CAFE-Brazil project also suggest that continued deforestation of the Amazon rainforest could affect the climate in two ways. "On the one hand, because forests no longer store carbon dioxide, greenhouse gases are released. On the other hand, clearing forests affects the water cycle and isoprene emissions, further driving climate change," Curtius said.
Compiled from /ScitechDaily
DOI:10.1038/s41586-024-08192-4