Ultrafine aerosol particles intensify rainfall in Amazon region

Study published in Science reveals that pollution particles from cities substantially affect storm cloud formation over tropical forests.

By Karina Toledo  |  Agência FAPESP – A study published on January 25 in the journal Science shows how the presence in the atmosphere of ultrafine aerosol particles (less than 50 nanometers in diameter) can intensify the cloud formation process and rainfall in the Amazon region.

According to the authors of the article, these nanoparticles have always been thought too small to play a significant role in regulating the hydrologic cycle. While this is indeed the case in polluted areas such as European and US cities, or São Paulo in Southeast Brazil, their role in the Amazon is different.

“The discovery lets us understand better how urban pollution affects the processes relating to the formation of convective storms in the Amazon and will enhance the accuracy of climate models and weather forecasting,” said Luiz Augusto Toledo Machado, a researcher at Brazil’s National Space Research Institute (INPE) and a co-author of the study.

The investigation began in 2014 and was conducted under the aegis of the Green Ocean Amazon scientific campaign (GOAmazon).

The study that has just been published was supported by FAPESP through projects led by Henrique de Melo Jorge Barbosa, a professor at the University of São Paulo’s Physics Institute (IF-USP), Paulo Artaxo, also at IF-USP, and Machado.

According to Barbosa, the data used in the article were collected in March-April 2014, during the rainy season, when there are no forest fires in the region and the only major source of pollution is Manaus, the capital of Amazonas State.

“Manaus is a city with some two million inhabitants and more than 500,000 vehicles. Its power supply comes from fossil fuel power stations. It’s a large source of pollution surrounded by pristine rainforest,” Barbosa said. “Our main experiment site was set up at Manacapuru, a small town located about 80 km from Manaus. For nearly half the year, it receives the pollution plume from the capital carried by tropical winds that blow from east to west. The rest of the time, the region receives only very clean air from the forest.”

With the help of instruments that measure atmospheric aerosol levels and calculate particle size, as well as radars that gauge the size of cloud droplets, the amount of rainfall and the speed at which vapor is borne up from the surface to the clouds, the group compared the processes of convection (the vertical movement of gases caused by heat transfer) and cloud formation when the Manaus pollution plume was or was not present over Manacapuru.

“Aerosol particles are essential to the cloud formation process because they provide a surface on which water vapor can condense,” Barbosa explained. “The droplets formed by condensation are tiny, but they grow as they collide with each other. When they become sufficiently large and heavy, they precipitate as rain.” 

Only particles more than 50 nanometers in diameter normally act as cloud condensation nuclei (CCN). According to the researchers, vapor condenses more easily on large particles because of lower surface tension – the attraction force between water molecules that enables flies to land on the surface of a pond.

“In polluted cities, and during the dry season in the Amazon, there are many particles in the atmosphere, and this intensifies the competition for the water vapor that rises from the ground. More droplets form, but they are smaller than they would be without the pollution, and they take longer to grow large enough to precipitate,” Machado said. 

As a result, he added, clouds develop mostly in the vertical direction, and ice forms in the upper portion. “Intense ice formation favors the development of thunderstorms – dense clouds with lightning flashes,” he said.

The process described by Machado is known by climate specialists as cloud invigoration. The article published in Science shows for the first time that in the Amazon, the process may also be influenced by nanoparticles.

Because relative humidity and air temperatures are very high in tropical forests, and there are few large particles in the atmosphere during the rainy season, surplus vapor also condenses on the nanoparticles, and cloud invigoration also takes place in the lower portion of clouds, where water is in a liquid state. This process of raindrop formation releases latent heat, which boosts the upward movement of the air, increasing the intensity of thunderstorms.

“For example, wind speed doubled when there were many nanoparticles in the atmosphere,” said Rodrigo Souza, a professor at the University of Amazonas (UEA) who also took part in the study.

To confirm the hypothesis based on the atmospheric data collected, the group used the Weather Research and Forecasting Model (WRF). Although WRF is a next-generation system, it fails to represent certain important aspects of the hydrologic cycle in the Amazon because its development was based on observation of the northern hemisphere.

“We had to adapt the model to our region,” said Helber Gomes, a professor at the Federal University of Alagoas (UFAL).

Filling gaps

“We never understood how these huge downpours could occur so frequently in the Amazon if the region has few cloud condensation nuclei – around 300-350 particles per cubic centimeter [São Paulo, for example, has as many as 10,000-20,000]. The reason is that we’d never considered the role of these ultrafine aerosol particles,” said Artaxo, also a co-author of the study.

The discovery, he added, shows that scientists who study tropical regions should not use only concepts developed in countries with a temperate climate. “We have to take the peculiarities of the Amazon into account,” he said. “It may be that in the past, when the global atmosphere wasn’t yet polluted by human emissions, cloud invigoration and storm intensification also occurred in other parts of the planet. But we can’t be sure until we do a lot more research.”

For Machado, the findings will change not just climate models but also how theories are formulated and atmospheric data are collected.

“Now that we’ve shown the importance of nanoparticles in the intensification of rainfall, we’ll never again study clouds the same way,” he said. “This changes the way we think about the entire process.”

The group plans to work on new data and models to see how far findings valid for the Amazon can be extrapolated to other regions of the world. “We know a huge amount of energy is needed to carry all that water vapor up to altitudes of 12-14 km. The energy comes from the Sun and is available in Amazonia,” Artaxo said.

Scientists from Brazil, the United States, Israel, China and Germany contributed to data collection and analysis. Some measurements were performed by a Grumman Gulfstream-1 research aircraft owned by the Pacific Northwest National Laboratory (PNNL) in the US.

The GOAmazon campaign was also supported by the Amazonas State Research Foundation (FAPEAM) and the US Department of Energy (DoE), as well as other partners.

The aims of the experiment, which was conducted in 2014 and 2015, included investigating the effects of urban pollution from Manaus on clouds over the Amazon and advancing knowledge of rain formation processes and the dynamics of interaction between the Amazon’s biosphere and the atmosphere. Based on the findings, the researchers plan to predict future changes in radiation balance, energy distribution, and regional climate, as well as the impact of all this on the global climate (read more at: agencia.fapesp.br/18803).

The article “Substantial convection and precipitation enhancement by ultrafine aerosol particles” can be retrieved from science.sciencemag.org/content/359/6374/411.