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ATMOSPHERIC CHEMISTRY

Gas in the Amazon rainforest accelerates cloud formation

Trees emit isoprene, which reacts with other molecules and increases the production of cloud condensation nuclei 100-fold

On descending close to the surface, aerosol particles formed by isoprene gas generate cloud condensation nuclei

Leoaraujo7 / Getty Images

A defense mechanism that has emerged and disappeared several times over the evolutionary history of different lines of plants, particularly trees in hot environments, plays a crucial role in Amazonia in one of the most important land climate parameters: the number of clouds in the sky. Probably as a way of dealing with thermal stress peaks, tree foliage emits a volatile compound—isoprene gas (C₅H₈). Under the daylight sun, isoprene degrades rapidly after being released, but under certain nocturnal conditions the compound remains in the air for longer, gaining altitude and transforming into an essential ingredient of atmospheric chemistry: a type of diffuser of the processes that result in clouds. This is the primary conclusion of two new studies conducted by international groups with the participation of Brazilians, published simultaneously in the journal Nature in December.

A molecule of the terpene family, which covers the primary volatile compounds emitted by plants, isoprene, when released by tree foliage and escaping its premature end, reacts with other gases, causing a series of reactions in the lower and higher troposphere, the most superficial layer of the Earth’s atmosphere. These interactions multiply by dozens the rate of aerosol particle formation, and then condensation nuclei, the embryo of the cloud. The mechanism not only boosts the generation of clouds over Amazonia, but also over the Atlantic Ocean, and probably other parts of the planet. Approximately two-thirds of the Earth’s surface have permanent cloud coverage, a climate aspect that can both heat up and cool down a region, while also bringing moisture through rainfall.

The articles provide a detailed look at the sequence of physical-chemical interactions between isoprene and other compounds, leading to the formation of enormous quantities of aerosol particles in the high Amazonian troposphere at altitudes between 8 and 15 kilometers (km). These particles initially measure a few nanometers (nm) in diameter, clustering with other aerosols and growing in size over time. They can be transported to different parts of the globe by atmospheric circulation, and play an important part in the origin of cloud condensation nuclei. When they reach the lower troposphere, between 1 and 2 km in altitude, nuclei at least 50 nm in diameter function as water vapor supports, which condensate (become liquid) and generate clouds. Without these nuclei there are no clouds, which can be both fair-weather and rainfall.

The discovery of these interactions initiated by the emission of isoprene, which becomes a gas at temperatures in excess of 34 degrees Celsius (ºC), explains a phenomenon observed for two decades in the Amazonian skies. At the beginning of the 2000s, researchers became aware of the existence of aerosol concentrations in the high tropical forest troposphere that were 160 times greater than those measured close to the surface. However, there was no consistent explanation for this finding until the new studies were published. “We have now solved this mystery and have demonstrated that an organic compound released by the forest—isoprene—initiates the formation process of these aerosols,” says physicist Paulo Artaxo, of the University of São Paulo’s Institute of Physics (IF-USP), coauthor of one of the studies, both referenced on the cover of Nature. “It’s very important for us to understand the cloud formation process to improve our weather and climate forecast models.”

Alexandre Affonso / Revista Pesquisa FAPESP

One of the articles is based on chemical process data obtained from overflights in Amazonia by the aircraft Halo, of the German Aerospace Center (DLR), between December 2022 and January 2023, part of the project Chemistry of the Atmosphere: Field Experiment in Brazil (CAFE-BRAZIL), a field study conducted two years ago. “We overflew for 136 hours and clocked up 89,000 km over Amazonia,” says meteorologist Luiz Augusto Machado, of IF-USP and a collaborator at the Max Planck Institute for Chemistry in Germany, who supervised all the flights and crewed on some. The Halo aircraft would take off from Amazonas state capital Manaus before sunrise to capture the physical-chemical interactions of the isoprene without the influence of sunlight, having a continuous range of 10 hours to record atmospheric parameters.

The second study was conducted at the European Organization for Nuclear Research (CERN) in Switzerland, using the CLOUD (Cosmics Leaving Outdoor Droplets) chamber. This device reproduces atmospheric conditions and processes. “CLOUD is a 3m-high stainless-steel cylinder supplied with ultraclean air in the same proportions of nitrogen and oxygen that we have in the atmosphere,” explains Brazilian meteorologist Gabriela Unfer, currently undertaking her PhD at the Leibniz Institute for Tropospheric Research in Leipzig, Germany, and the only Brazilian signing the two articles on high-altitude aerosols.

To enable the CLOUD system to simulate atmospheric conditions in a certain region of the planet, area-specific meteorological and chemical information needs to be fed into the chamber. “The field data obtained during flights in the CAFE-BRAZIL program formed the basis for the correct configurations of temperature, moisture, and gas concentration to be simulated by CLOUD. This is how we reproduced, in the laboratory, the formation of high-altitude aerosols brought about by plant isoprene emissions,” explains Unfer.

The most surprising aspect of the studies is that the aerosols formed at between 8 and 15 km in altitude owe their existence to processes that commence amongst the trees of Amazonia. “If the tropical forest continues to be stripped, this will have an impact on the aerosol production process in the high troposphere, and consequently on the formation of clouds closer to the surface,” says Micael Amore Cecchini, of the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo (IAG-USP), another author of one of the papers who participated in the CAFE-BRAZIL campaign.

Halo aircraft overflies Amazonia to take atmospheric chemistry measurementsDirk Dienhart / Max Planck Institute for Chemistry

The key to the entire process is isoprene, a volatile, colorless organic compound with a very slight aroma that may call rubber or petroleum to mind. The release of this gaseous molecule, expelled as a type of vegetation transpiration, is an evolutional mechanism that helps plants, particularly those in the tropics, to protect themselves from the negative effects of heat peaks. Close to the surface, in the low atmosphere, isoprene can last for minutes or a few hours. During sunlight hours it reacts with ozone and other compounds, and quickly disappears, almost completely, from the atmosphere.

However, isoprene molecules not destroyed, and those emitted after sundown by Amazonian trees, escape this premature end and are transported into the high atmosphere by the action of night storms. At around 15 km in altitude, where the temperature is lower than -30 ºC, isoprene does not degrade as happens close to the surface, and it interacts with other compounds. Nighttime storm rays cause the isoprene to bond to nitrogen oxide molecules, quickly forming an enormous amount of aerosol particles a few nanometers in size. “In the high atmosphere, isoprene accelerates the speed of aerosol formation by 100 times,” comments Machado. The speed of aerosol movement in the high troposphere can reach 150 km per hour. It is therefore reasonable to suggest that they migrate to regions very distant from Amazonia, where they would lose altitude and bond until they form cloud condensation nuclei between 1 and 2 km above the Earth’s surface.

It is probable that this aerosol formation mechanism at high altitudes also occurs in other regions of the globe, especially over the tropical forests of Congo, in Africa, and Southeast Asia. Isoprene is the primary volatile compound emitted by vegetation. Around 600 million tons are released into the atmosphere every year. “This type of aerosol particle formation in the high troposphere probably happens not only in Amazonia, but in all tropical forests, given that they all emit large amounts of isoprene, meteorologist Joachim Curtius, of the University of Frankfurt, and lead author of the study into the data from the CAFE-BRAZIL experiment, said in an interview for Pesquisa FAPESP. “The Amazon forest alone is responsible for more than a quarter of total isoprene emissions.”

The next step in the studies is attempting to understand in more detail how the growth and transportation of aerosols occur in the high Amazonian troposphere, and how they descend close to the Earth’s surface. “We don’t know how many of these particles are lost by collisions with other particles, and in other interactions, nor how they ‘survive’ and become condensation nuclei in the low altitudes of the tropics, where clouds form,” says Curtius. In this process, the isoprene is just the tip of the iceberg—or rather, the cloud.

The story above was published with the title “From the forest to the air” in issue 347 of january/2025.

Projects
1. Interactions between trace gases, aerosols, and clouds in the Amazon: From bioaerosol emissions to large-scale impacts (nº 23/04358-9); Grant Mechanism Thematic Project; Principal Investigator Paulo Artaxo; Investment R$4,109,920.65.
2. Correlating the organization level of convective cloud fields with hydrological and aerosol cycles in the Amazon (Cloudorg) (nº 22/13257-9); Grant Mechanism Research Grant – Young Investigator Award Program FAPESP Research on Global Climate Change (PFPMCG); Agreement Serrapilheira Institute; Principal Investigator Micael Amore Cecchini (USP); Investment R$1,471,125.05.
3. Interactions between aerosols and weather events in the Amazon (nº 21/03547-7); Grant Mechanism Master’s Fellowship; Supervisor Luiz Augusto Toledo Machado (USP); Beneficiary Gabriela Unfer; Investment R$73,911.67.

Scientific articles
CURTIUS, J. et al. Isoprene nitrates drive new particle formation in Amazon’s upper troposphere. Nature. dec. 4, 2024.
SHEN, J. et al. New particle formation from isoprene under upper-tropospheric conditions. Nature. dec. 4, 2024.

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