The Amazon rainforest as a cloud machine: How thunderstorms and plant transpiration produce condensation nuclei

Two studies with the participation of Goethe University Frankfurt, Max Planck Institute for Chemistry, University of Helsinki, and Leibniz Institute for Tropospheric Research, and Brazilian partner institutions, shed light on new climate mechanism

Goethe University Frankfurt

Rain — image:
Heavy showers occur over the rainforest again and again.
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Credit: Philip Holzbeck, MPI for Chemistry, Germany

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

Publications:

Joachim Curtius et al.: Isoprene nitrates drive new particle formation in Amazon’s upper troposphere. Nature (2024), DOI: https://doi.org/10.1038/s41586-024-08192-4

Jiali Shen et al.: New particle formation from isoprene in the upper troposphere. Nature (2024), DOI: https://doi.org/10.1038/s41586-024-08196-0

roject’s research aircraft shortly after take-off.

Credit

Dirk Dienhart, MPI for Chemistry

Clouds over the Amazon basin, taken during a research flight.

The instruments and measurement data are checked on board the research aircraft by scientists Gabriela Unfer (left) and Zaneta Hamryszczak.
Credit
Philip Holzbeck, MPI for Chemistry, Germany

Journal

Nature

DOI

10.1038/s41586-024-08192-4

Method of Research

Experimental study

Subject of Research

Not applicable

Article Title

Isoprene nitrates drive new particle formation in Amazon’s upper troposphere

Article Publication Date

4-Dec-2024

Rainforest emissions linked with new particle formation at high altitudes

Recent research has identified that isoprene, a naturally occurring organic compound emitted by vegetation, significantly contributes to the formation of new particles in the upper troposphere. These findings enhance our understanding of atmospheric process

University of Helsinki

Rainforest emissions linked with new particle formation at high altitudes — image:
With experiments performed in the CLOUD chamber at CERN, researchers aimed to determine whether isoprene oxygenated organic molecules could form new particles under upper-tropospheric conditions. Xu-Cheng He, Jiali Shen and Katrianne Lehtipalo in CERN.
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Credit: Xu-Cheng He

Why are there so many newly formed aerosol particles in the upper troposphere over tropical regions such as the Amazon? Tropical forests play an important role in global climate regulation; however, the high concentrations of new particles above these areas have puzzled atmospheric scientists for the past 20 years.

An international study, led by researchers at the University of Helsinki, shows that the answer may be isoprene. Isoprene is the most abundant non-methane hydrocarbon emitted into the atmosphere, primarily by vegetation.

The study, just published in Nature, investigated the formation of new particles from isoprene in the upper troposphere. The troposphere is the lowest layer of the atmosphere, spanning from ground level to an altitude of 18 km at the equator. With experiments performed in the CLOUD chamber at CERN, researchers aimed to determine whether isoprene oxygenated organic molecules (IP-OOM), which are compounds that form when isoprene is oxidized in the atmosphere, could form new particles under upper-tropospheric conditions such as temperatures below -30°C.

They also explored how factors, such as temperature, the presence of trace acids, and nitrogen oxides, affect this process.

Isoprene can drive rapid particle formation

The researchers found that isoprene oxygenated organic molecules can rapidly form new particles under upper-tropospheric conditions. Previously, isoprene was thought to have negligible ability to form particles; however, this study showed that isoprene can drive rapid particle formation under certain conditions.

“Our key finding is that the presence of extremely low concentrations of sulphuric acid or iodine oxoacids dramatically enhances particle formation, accelerating it up to 100 times faster compared to when only isoprene oxygenated organics are present. These findings can explain the high particle number concentrations observed at high altitudes over tropical regions such as the Amazon”, explains Jiali Shen, postdoctoral researcher at the Institute for Atmospheric and Earth System Research (INAR), University of Helsinki.

Increasing understanding of cloud formation and climate

Aerosol particles are important for the climate because they scatter and absorb incoming solar radiation and seed cloud droplets by acting as cloud condensation nuclei. These newly published findings could have significant implications for our understanding of cloud formation and climate.

“This research connects the abundant isoprene emissions from tropical rainforests to particle formation in the upper troposphere, highlighting a new aspect of the interaction between forests and the atmosphere. These results may lead to improvements in atmospheric chemistry and climate models, potentially enhancing our ability to predict climate change and its impacts”, says Xu-Cheng He, one of the lead investigators in the study.

“This study underscores the complex interactions between forests, the atmosphere, and climate. This demonstrates how emissions from trees can have far-reaching effects on cloud formation and potentially on the global climate. This type of fundamental research is crucial for improving our understanding of climate processes and our ability to predict and mitigate climate change”, says Professor Katrianne Lehtipalo from the University of Helsinki.

Schematic of new particle formation from isoprene in the upper troposphere. Isoprene emitted by forests is efficiently transported to the upper troposphere at night via deep convective clouds. During daylight, the oxidation products of isoprene drive the formation of high particle number concentrations, which subsequently influence cloud formation and climate.