Ultrafine aerosol particles intensify rainfall in Amazon region

“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).

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Image credit: FAPESP.