Clouds over the Amazon basin: photo taken during a research flight (credit: Philip Holzbeck/Max Planck Institute of Chemistry)
Published on 02/27/2025
By Luciana Constantino | Agência FAPESP – An international group of researchers, with the participation of Brazilians in particular, has succeeded for the first time in unraveling the physiochemical mechanism that explains the complex system of rain formation in the Amazon, which influences the global climate. It involves the production of aerosol nanoparticles, electrical discharges and chemical reactions at high altitudes that occur between night and day, resulting in a kind of aerosol “machine” that produces clouds.
The research, published on the cover of the journal Nature, describes the mechanisms by which isoprene – a gas emitted by vegetation as part of its metabolism – is transported into the layer of the atmosphere above the Earth’s surface near the tropopause during nighttime storms. A series of chemical reactions triggered by solar radiation produces large quantities of aerosols that form clouds. This particle production is accelerated by reactions with nitrogen oxides produced by electrical discharges in the upper atmosphere in clouds dominated by ice crystals.
Previously, scientists had already identified the particles on another expedition, but not the full physiochemical mechanism. It was thought that isoprene would not reach the upper layers of the atmosphere because it would react along the way, being highly reactive and quickly broken down by sunlight. With the discovery of these new mechanisms, it will be possible to improve models of the Earth’s system, which are fundamental tools for simulating climate and understanding the present and future functioning of the planet.
To arrive at the result, the group used material obtained during the scientific experiment CAFE-Brazil, which stands for Chemistry of the Atmosphere: Field Experiment in Brazil. The experiment, the only one of its kind, made several flights over the Amazon basin between December 2022 and January 2023 at an altitude of 14 kilometers (km), which is twice the height of Aconcagua, the highest point in South America. The total flight time was 136 hours, covering a distance of 89,000 km – more than two complete circumnavigations of the equator.
CAFE-Brazil research aircraft after takeoff (photo: Dirk Dienhart/Instituto de Química Max Planck)
“One of the highlights of this work is seeing how the Amazon has a symbiosis of complex mechanisms and important phenomena that operate within a delicate ecosystem balance. Preserving this balance allows us to maintain the climatic conditions we know today. Changes such as those caused by climate change or deforestation can have unexpected effects that haven’t yet been studied,” one of the Brazilian authors of the study, Professor Luiz Augusto Toledo Machado, explained to Agência FAPESP.
A researcher at the Physics Institute of the University of São Paulo (IF-USP) and collaborator at the Chemistry Department of the Max Planck Institute in Germany, Machado says that the result opens up a broad horizon for analyzing the impact of global warming on the climate, the environment and the ecosystem.
For Paulo Artaxo, coordinator of the USP Center for Sustainable Amazon Studies (CEAS), professor at IF-USP and co-author of the article, the results allow for more reliable modeling that can include mechanisms from a physiochemical and biological point of view.
“Isoprene emissions depend on the standing forest. They don’t occur when native vegetation is replaced by pasture or soy crops. With deforestation, this particle production mechanism is destroyed, reducing cloud formation and precipitation. This is what we call negative feedback in the overall climate system because deforestation significantly reduces precipitation by reducing evapotranspiration and particle production, which depend on isoprene emissions,” says Artaxo.
In October 2024, MapBiomas – a collaborative network of NGOs, universities and technology startups that maps land cover and land use in Brazil – published a study based on satellite imagery showing that cattle ranching was the main purpose of deforestation in the Amazon between 1985 and 2023. During this period, this area increased by more than 363%, from about 12.7 million hectares to 59 million hectares. As a result, 14% of the Amazon had become pastureland by 2023.
The mechanism
The forest gives off very distinctive aromas. These are gases known as volatile organic compounds (VOCs), including terpenes – a group of substances found in tree resins and essential oils – and isoprene. It is estimated that the world’s forests release more than 500 million tons of isoprene into the atmosphere every year, a quarter of which comes from the Amazon.
In the Amazon rainforest, isoprene is emitted during the day because it depends on sunlight. It was thought that the gas didn’t reach the upper layers of the atmosphere because it would be destroyed within a few hours by highly reactive hydroxyl radicals. “We’ve now established that this is partly true. There’s still a considerable amount of isoprene at night. A significant part of these molecules can be transported to higher layers of the atmosphere,” says corresponding author, Joachim Curtius, a professor at Goethe University in Frankfurt, Germany.
During the night, tropical storms over the forest help transport gases such as isoprene to higher layers through intense convection. Like a vacuum cleaner, this process is driven by rising air currents, especially in regions of high humidity and trapped heat. The gases combine with nitrogen compounds from lightning in the upper atmosphere.
In the highest areas, between 8 and 15 km above sea level, temperatures reach minus 60 °C. About two hours after sunrise, hydroxyl radicals, which also form at these altitudes, react with isoprene to form organic nitrates, different compounds from those found at ground level. This produces high concentrations of aerosol nanoparticles, with more than 50,000 per cubic centimeter.
These particles grow over time, are transported over long distances, and can act as cloud condensation nuclei. They influence the global hydrological cycle, the radiation balance and climate. The formation mechanisms of these organic nitrogen compounds will now be incorporated into climate models to improve rainfall prediction, especially in tropical regions.
FAPESP supports the study through a Thematic Project linked to the Research Program on Global Climate Change (RPGCC), led by Machado and another by Artaxo, as well as four other projects (23-04358-9, 21/03547-7, 22/01780-9 and 22/13257-9).
In addition to this research, the same issue of Nature contains another study carried out by part of the research team, which deals with the new formation of particles from isoprene in the upper troposphere. They reproduced the conditions present at these altitudes in experimental chambers and analyzed in detail the reactions triggered by sunlight.
The expedition
Several research flights carried out as part of the CAFE-Brazil experiment contributed to the generation of altitude profiles for different gases. It was possible to measure air masses transported into the upper troposphere and the differences between daytime and nighttime situations.
Professor Machado, who participated in information gathering in the Amazon, says that the flights lasted up to 12 hours. “We would fly through the night. We noticed that the particles formed in the morning. That’s why we left at dawn. The teams would go to the airport to fly, while I and other researchers would stay in the operations room, monitoring and giving guidance on the forecasts and where the rainfall was. I also flew into the clouds to measure isoprene. It was very exciting,” he reports.
Machado, left, during one of the flights (photo: personal collection)
The working base was set up in Manaus, the capital of the Brazilian state of Amazonas. The flights were carried out with the HALO (High Altitude and LOng range research aircraft), a research aircraft for long distances (more than 8,000 km), high altitudes (up to 15.5 km) and large payloads (up to 3 tons). The experiment involved a partnership between the Goethe University in Frankfurt, the Max Planck Institute for Chemistry (Germany), the Brazilian National Institute for Space Research (INPE) – which was responsible for licensing the scientific expedition coordinated by researcher Dirceu Herdies, also the author of the article – the National Institute for Amazonian Research (INPA) and USP.
CAFE-Brazil team based in Manaus (photo: press release)
Recently, another study led by Machado was published in the journal Nature Geoscience, showing that the forest is capable of producing aerosols on its own, which, induced by the rain itself, trigger a process of new cloud formation and precipitation.
The articles “Isoprene nitrates drive new particle formation in Amazon's upper troposphere” and “New particle formation from isoprene in the upper troposphere” can be read at www.nature.com/articles/s41586-024-08192-4 and www.nature.com/articles/s41586-024-08196-0, respectively.
Source: https://agencia.fapesp.br/54089
