In the Amazon region, downdrafts bring aerosol particles from higher altitudes to the atmospheric layer where clouds form.
In a joint study international scientists investigated how clouds and hence rain develops over the Amazon tropical rainforest. They found out that the rain is generating further rain by downdrafts entraining fine particles from the upper atmospheric layers to the layer where clouds are formed. As, in this area, there are no cloud condensation particles from anthropogenic pollution, these findings are important to help understand how precipitation developed in preindustrial times.
Rain clouds above the Amazon rainforest. Downdrafts accompanying precipitation entrain smaller particles from higher altitudes to the atmospheric boundary layer where clouds are formed.
Christopher Pöhlker, MPI for Chemistry
For clouds to form in the atmosphere, moisture alone is not sufficient: there must also be aerosol particles that serve as condensation nuclei for the cloud droplets. Some particles such as mineral dust, sea salt or pollen are directly emitted from the ground and held aloft by normal air circulation. However, the majority of particles is formed from atmospheric gas molecules and starts out at a size of only a few millionths of a millimeter (nanometers). They subsequently grow to a diameter where they can serve as condensation nuclei for fog, clouds, and precipitation.
For densely populated and polluted areas, the mechanism behind new particle formation is very well known: emissions from fossil fuel burning release sulfur dioxide, which then forms particles of sulfuric acid. This is not the case in the Amazon rain forest, one of the most pristine regions of the world. Here, the sulfuric acid particles from anthropogenic sources are missing and the atmosphere in the rainy season is almost as clear as in preindustrial times. Therefore, an international team of scientists investigated the origin of aerosol particles at a very remote location in the natural forest. This will help answer the question how cloud formation and precipitation worked in preindustrial times.
Among the research team were scientists from the Max Planck Institutes for Chemistry, Mainz, and Biogeochemistry, Jena, Germany. They could show that downdrafts above the rain forest are responsible for the transport of the finest particles down to lower layers. These downdrafts form in the presence of precipitation and thunderstorms.
They entrain the smaller particles from the upper troposphere at a height of several kilometers downwards and feed the boundary layer close to the ground surface with aerosol particles. Out of this layer, which is only one to two kilometers high, grow the clouds that produce the rain that the forest needs. “In the rain forest, the nursery for the smaller aerosol particles is not in the lower part of the atmosphere as it is the case in polluted areas, but at much higher altitudes,” comments Christopher Pöhlker, scientific group leader at Max Planck Institute for Chemistry. The measurement results are published in the current edition of the scientific journal Nature.
In this article, the scientists explain how they measured the size and concentration of the aerosol particles above the tropical rain forest. They took advantage of the Amazon Tall Tower Observatory (ATTO) located at about 150 km northeast of Manaus in the middle of the primeval forest. This measurement station is jointly managed by the Brazil Institute for Amazon Research (INPA) and the Max Planck Society. At a height of 60 meters, well above the tree canopy, the research team collected air samples that were analyzed for the number and size of aerosol particles. The particle distribution at this level is very representative for the distribution in the atmospheric boundary layer, because the air is well mixed throughout this layer.
In addition, aerosol sampling and analysis was carried out at several heights and times on board the research aircraft Gulfstream-1 of the U.S. Department of Energy. The scientists observed that the concentration of the smallest particles, with a diameter smaller than 50 nanometers, was very high in the upper troposphere. At the ground station, they registered a substantial increase in the smaller particles after rain or storm events. While the average particle size was around 100 nanometers before the precipitation events, it dropped to below 50 nanometers directly after the rain. This is explained by the downdrafts that accompany rainy and stormy weather conditions and entrain the smaller particles from the upper troposphere to the near-ground boundary layer. Here, they can grow to larger particles, which can serve as condensation nuclei for cloud formation. One can say that rain makes for even more rain in the Amazon.
Next, the researchers are curious about the precise origin of the smaller aerosol particles in the upper troposphere. However, they already have some clues from still unpublished measurements performed by the German HALO research aircraft over the Amazon: “These particles probably derive from chemical reactions in the upper troposphere,” states Meinrat O. Andreae, director at the Max Planck Institute for Chemistry in Mainz. “If we can prove this hypothesis, we will make substantial progress in understanding how precipitation processes worked before humans irreversibly altered our environment and especially the atmosphere,” he adds with a view towards the Anthropocene.
The results of this study performed by researchers from Brazil, China, Germany, Finland, Sweden and the USA have another important impact: Aerosols play an important role in climate modeling and represent a major uncertainty factor in the computer models, also because it is not well understood how aerosols form under natural conditions. These new findings on the origin of the small particles above the tropical rain forest will help improve predictions on climate change as well.
Jian Wang, Radovan Krejci, Scott Giangrande, Chongai Kuang, Henrique M. J. Barbosa, Joel Brito, Samara Carbone, Xuguang Chi, Jennifer Comstock, Florian Ditas, Jost Lavric, Hanna E. Manninen, Fan Mei, Daniel Moran-Zuloaga, Christopher Pöhlker, Mira L. Pöhlker, Jorge Saturno, Beat Schmid, Rodrigo A. F. Souza, Stephen R. Springston, Jason M. Tomlinson, Tami Toto, David Walter, Daniela Wimmer, James N. Smith, Markku Kulmala, Luiz A. T. Machado, Paulo Artaxo, Meinrat O. Andreae, Tuukka Petäjä, and Scot T. Martin:
Vertical transport during rainfall sustain aerosol concentration in Amazon boundary layer, Nature, 2016, doi: 10.1038/nature19819
Contact at MPI for Chemistry, Mainz:
Prof. Dr. Meinrat O. Andreae
Phone: +49 (0)6131-305 6000
Dr. Christopher Pöhlker
Phone: +49 (0)6131-305 4800
Contact at MPI for Biogeochemistry, Jena:
Dr. Jošt Lavrič
Phone: +49 (0)3641-57 6368
Dr. Susanne Benner | Max-Planck-Institut für Chemie
Microscope measures muscle weakness
16.11.2018 | Friedrich-Alexander-Universität Erlangen-Nürnberg
Good preparation is half the digestion
16.11.2018 | Max-Planck-Institut für Stoffwechselforschung
Researchers at the University of New Hampshire have captured a difficult-to-view singular event involving "magnetic reconnection"--the process by which sparse particles and energy around Earth collide producing a quick but mighty explosion--in the Earth's magnetotail, the magnetic environment that trails behind the planet.
Magnetic reconnection has remained a bit of a mystery to scientists. They know it exists and have documented the effects that the energy explosions can...
Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.
Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...
Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.
In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...
On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.
When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure
Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...
09.11.2018 | Event News
06.11.2018 | Event News
23.10.2018 | Event News
16.11.2018 | Health and Medicine
16.11.2018 | Life Sciences
16.11.2018 | Life Sciences