FRANKFURT/MAINZ/HELSINKI/LEIPZIG. Who hasn’t enjoyed the aromatic scent in the air when walking through the woods on a summer’s day? Partly responsible for this typical smell are terpenes, a group of substances found in tree resins and essential oils. The primary and most abundant molecule is isoprene. Plants worldwide are estimated to release 500 to 600 million tons of isoprene into the surrounding atmosphere each year, accounting for about half the total emissions of gaseous organic compounds from plants. “The Amazon rainforest alone is responsible for over a quarter of these emissions,” explains atmospheric researcher Professor Joachim Curtius from Goethe University Frankfurt.

So far, it was thought that the isoprene in the Amazon basin degrades rapidly and does not reach higher atmospheric layers. This is because hydroxyl radicals form in the atmosphere close to the ground during the day when the sun shines. They are highly reactive and destroy the isoprene molecules within hours. “However, we have now established that this is only partly true,” says Curtius. “There are still considerable amounts of isoprene in the rainforest at night, and a substantial proportion of these molecules can be transported to higher atmospheric layers.”

Thunderstorms act like vacuum cleaners

Responsible for this are tropical thunderstorms that brew over the rainforest at night. They pull the isoprene up like a vacuum cleaner and transport it to an altitude of between 8 and 15 kilometers. As soon as the sun rises, hydroxyl radicals form, which react with the isoprene. But at the extremely low temperatures that prevail at these high altitudes, the rainforest molecules are transformed into compounds different from those near the ground. They bind with nitrogen oxides produced by lightning during the thunderstorm. Many of these molecules can then cluster to form aerosol particles of just a few nanometers. These particles, in turn, grow over time and then serve as condensation nuclei for water vapor – they thus play an important role in cloud formation in the tropics.

“We were able to shed light on these processes with the help of research flights that started two hours before sunrise and then continued through the day,” explains Professor Jos Lelieveld. He is director at the Max Planck Institute for Chemistry in Mainz and also head of the CAFE-Brazil research project (Chemistry of the Atmosphere: Field Experiment in Brazil), in which an international research team was collecting data on the chemical processes in the atmosphere over the Amazon rainforest. “We were able to detect considerable amounts of isoprene in the air flowing out of the thunderstorms at high altitude, from which new aerosol particles rapidly formed after several chemical reactions.”

Possible influence on the cloud formation over the ocean

Curtius and Lelieveld are not only partners in CAFE-Brazil but also involved in the CLOUD consortium, in which over 20 research groups study climate-relevant chemical processes in the atmosphere. They reproduce the conditions that prevail at this altitude in the aerosol and cloud experiment chamber at CERN in Geneva. With the help of this simulation chamber, they analyze in detail which reactions are triggered by sunlight. “In this way, we were able to determine exactly the rate at which the aerosol particles form from the isoprene products,” explains atmospheric researcher Dr. Xu-Cheng He, who is in charge of the isoprene experiments. “Interestingly, it emerged that even extremely small amounts of sulfuric acid and iodine oxoacids commonly present in the atmosphere are sufficient to accelerate the formation of the aerosol particles by a factor of 100. These molecules may, therefore, jointly influence marine cloud formation – a critically uncertain process in climate projections.”

Sulfuric acid forms in the atmosphere from various sulfurous substances. It can result, above all, from the reaction of sulfur dioxide with hydroxyl radicals. Within the CLOUD experiment, the Frankfurt research group was responsible for measuring the extremely low concentrations of sulfuric acid, and the Mainz team measured the hydroxy radicals.

The winds that prevail at high altitudes above the Amazon rainforest can transport the particles that form from isoprene up to thousands of kilometers away from the sources. This means they may influence cloud formation at great distances. As clouds, depending on their type and height, both shield solar radiation and prevent heat from being radiated into space, they play a crucial role in the climate. The researchers, therefore, expect that their findings will contribute to improving climate models.

It also follows from the CAFE-Brazil project results that continued deforestation of the Amazon rainforest could affect the climate in two respects. “On the one hand, greenhouse gases are released because the forest no longer stores carbon dioxide,” says Curtius. “On the other hand, clearing the forest impacts both the water cycle and isoprene emissions, further propelling climate change.”

Background information:
CAFE-Brazil: Research in and high above the Amazon rainforest (5th Dec. 2024)
https://aktuelles.uni-frankfurt.de/news-in-brief/research-in-and-high-above-the-amazon-rainforest/
https://www.mpic.de/5299698/cafe-brazil-kampagne

Ocean sunshade: How clouds influence climate change (Forschung Frankfurt 2.2021)
https://www.goethe-university-frankfurt.de/118615101.pdf