The Amazon rainforest as a cloud machine: How thunderstorms and plant transpiration produce condensation nuclei
Frankfurt/Mainz/Helsinki/Leipzig,
04.12.2024
– Markus Bernards (Goethe-Universität Frankfurt)
Two studies with the participation of Goethe University Frankfurt, Max Planck Institute for Chemistry, University of Helsinki, and Leibniz Institute for Tropospheric Research, together with Brazilian partner institutions, shed light on a new mechanism that affects the climate.
The rainforest in the Amazon basin transpires vast amounts of gaseous isoprene. Until now, it was assumed that this molecule is not transported far up into the atmosphere, as it rapidly declines when exposed to light conditions. The CAFE-Brazil measurement campaign provided data for two studies – now published as the Nature cover story – which demonstrate, however, that nocturnal thunderstorms transport the isoprene to an altitude of up to 15 kilometers. There, it reacts to form chemical compounds capable of forming vast numbers of new aerosol particles. These grow further and contribute to cloud formation as condensation nuclei. This mechanism is likely to affect the climate, too.
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.”
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
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
Further information
Prof. Dr. Joachim Curtius
Institut für Atmosphäre und Umwelt
Goethe-Universität Frankfurt
Tel: +49 (0)69 798-40258
curtius[at]iau.uni-frankfurt.de
Website: https://www.uni-frankfurt.de/44625400/Joachim_Curtius
Prof. Dr. Jos Lelieveld
Max-Planck-Institut für Chemie, Mainz
Tel: +49 (0)6131 3054040
jos.lelieveld@mpic.de
Dr. Xu-Cheng He
Institute for Atmospheric and Earth System Research/Physics
University of Helsinki
Tel. +358294150284
xucheng.he@helsinki.fi
Prof. Mira Pöhlker
Leibniz-Institut für Troposphärenforschung (TROPOS), Leipzig
Tel. +49 341 2717-7431, -7468
poehlker@tropos.de, unfer[at]tropos.de
Prof. Dr. Luiz Augusto Toledo Machado
Instituto de Física
Universidade de São Paulo
lmachado[at]if.usp.br
The research aircraft HALO (High Altitude and Long Range Research Aircraft) is a joint initiative of German environmental and climate research institutions. HALO is funded by grants from the German Federal Ministry of Education and Research (BMBF), the German Research Foundation (DFG), the Helmholtz Association, the Max Planck Society (MPG), the Leibniz Association, the Free State of Bavaria, the Karlsruhe Institute of Technology (KIT), the Research Center Jülich (FZJ) and the German Aerospace Center (DLR). DLR is both the owner and operator of the aircraft.
The Leibniz Institute for Tropospheric Research (TROPOS) is a member of the Leibniz Association, which unites 96 independent research institutions. Their focus ranges from the natural, engineering and environmental sciences to economics, spatial and social sciences and the humanities. Leibniz Institutes are dedicated to socially, economically and ecologically relevant issues.
They conduct knowledge- and application-oriented research, including in the overarching Leibniz Research Alliances, are or maintain scientific infrastructures and offer research-based services. The Leibniz Association focuses on knowledge transfer, especially with the Leibniz Research Museums. It advises and informs politics, science, business and the public.
Leibniz institutions maintain close cooperation with universities - including in the form of the Leibniz ScienceCampi, with industry and other partners in Germany and abroad. They are subject to a transparent and independent review process. Due to their national importance, the federal and state governments jointly fund the institutes of the Leibniz Association. The Leibniz Institutes employ around 20,500 people, including 11,500 scientists.
The total budget of the institutes is 2 billion euros. They are jointly financed by the federal and state governments. The basic funding of the Leibniz Institute for Tropospheric Research (TROPOS) is provided by the Federal Ministry of Education and Research (BMBF) and the Saxon State Ministry of Science and the Arts (SMWK). The institute is co-financed by tax revenue on the basis of the budget approved by the Saxon State Parliament.
http://www.leibniz-gemeinschaft.de
https://www.bmbf.de/
https://www.smwk.sachsen.de/
