Even the smallest particles of pollution alter the rain cycle in the Amazon

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Even the finest particles of pollution impact the mechanisms of cloud formation and development and alter the rainfall regime.

A study carried out in the city of Manaus showed that, through a chemical process known as oxidation, small aerosols expelled by factories or car exhausts, for example, grow very quickly, reaching size up to 400 times greater. And this interferes with the formation of raindrops.

“Understanding the mechanisms of cloud formation and rainfall in the Amazon is a major challenge due to the complexity of non-linear physical-chemical processes in the atmosphere”, explains Paulo Artaxo, professor at the Physics Institute of the University of São Paulo (IF-USP) and author of the study published Wednesday (12) in Science Advances.

The discovery by Brazilian and North American researchers increases the accuracy of models and mathematical simulations of climate change.

“These pollution nanoparticles [com menos de dez nanômetros] used to be neglected in atmospheric calculations and models. Attention was focused on particles with more than one hundred nanometers, as they are the ones that act as the condensation nucleus of clouds. [onde o vapor de água condensa e forma gotículas] and change the pattern of rainfall”, explains Luiz Augusto Machado, professor at IF-USP and co-author of the article.

“With this study we showed that, along their trajectory in the atmosphere, the smaller particles are oxidized and grow rapidly until they reach the necessary size to become a condensation nucleus.”

The data were collected with the help of planes that flew over the Manaus region at low altitude, covering about one hundred kilometers of the pollution plume produced in the metropolis between the years 2014 and 2015.

The work was supported by Fapesp (Fundação de Amparo à Pesquisa do Estado de São Paulo) through the scientific campaign Green Ocean Amazon (GoAmazon) and a Thematic Project –both linked to the PFPMCG (Fapesp Research Program on Global Climate Change).

“Little was known about the role of these nanoparticles in rainfall. It turns out that the Manaus region is a unique place in the world, an open-air laboratory. It is a megacity surrounded by forest and far from other cities. understand how a metropolis modifies an environment similar to that of the pre-industrial era”, says Machado.

Aerosols are particles (solid or liquid) suspended in the air. They can be produced naturally by the forest, as primary particles, or secondarily in the atmosphere from gaseous precursors (VOCs) emitted by forests (secondary organic aerosols), for example, or –as investigated in this study– by human activities, such as burning fossil fuels.

Machado explains that aerosols with less than ten nanometers, when released in the region of Manaus by vehicle exhausts, by the industry or during the generation of electric energy, form a kind of plume of pollution that travels towards the southwest (because of the winds). The researchers estimated that it is during this path that the particles grow rapidly.

“It is very difficult to assess the effect of particulate matter on rain, as there are a large number of atmospheric variables that interfere in this relationship. Therefore, we compared the pollution line with the surrounding areas, which are outside the pollution plume”, he says. .

“What we notice is that this particulate is growing in size quickly. At 10 km away from Manaus it is already bigger and at 30 km it is possible that it has already reached sufficient size to become a condensation nucleus, interfering with the formation of droplets rain”, he adds.

variable impact

Cloud formation mechanisms are complex and depend on many atmospheric parameters. In the case of small aerosols, they will interfere with the condensation of raindrops. However, depending on the atmospheric condition and, above all, the formation of clouds at any given moment, the rains can be intensified or reduced.

Machado explains that, as there is a lot of particulate matter, when the pollution plume comes into contact with a cloud, there is competition for the water vapor present there – reducing the size of the droplets.

“For the rain to fall, the drops must have a certain size. This is what we call the terminal velocity of the drop, which needs to be smaller than the air movement that is rising. Otherwise, the cloud gets a lot of a small drop and the rain doesn’t fall”, he explains.

However, Machado points out, if a very strong vertical wind occurs, it can take this large amount of droplets to a higher altitude, forming ice particles, which can generate an intense storm.

“We realized that, as this particulate matter grows, it becomes a condensation nucleus. When it encounters a small, weak cloud [nuvem quente], it rains little”, he explains.

“The aerosol reduces precipitation. But if the cloud gains power and becomes a cumulonimbus [de grande desenvolvimento vertical], for example, aerosols increase precipitation. In other words, even these small particles of pollution have an influence on the formation of rain.”

According to the researchers, the project should continue in an expanded form, capturing new data. The team will carry out this year the Cafe-Brasil experiment (Chemistry of the Atmosphere: Field Experiment in Brazil) with the help of a German aircraft that can fly at an altitude of 15 km.

Artaxo explains that similar studies using remote sensing are also being carried out at the ATTO tower, 325 meters high, in the middle of the Amazon rainforest.

“In this study that we are now publishing, we collected the data through low-altitude flights. [4.000 metros]. The German aircraft that we are going to use for our next collections is one of the most sophisticated laboratory planes that exist”, says Artaxo.

“In this way, we will be able to carry out an experiment to understand fundamental physicochemical issues in the production of aerosols, clouds and precipitation that still remain a mystery to us”, he concludes.

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