Computer simulation helps understanding the transport of suspended particles in the atmosphere

A study developed in Brazil and presented during FAPESP Week France aims at elucidating the behavior of the so-called aerosols, which have an important influence over climate, agriculture, and human health.

By Heitor Shimizu, from Lyon  |  Agência FAPESP – The smoke that leaves truck exhausts, factory chimneys, or fires in fields or forests is easily visible. Similarly, sand storms can be observed from kilometers away. However, the compounds in these two types of formations usually go unnoticed, and a large amount of the particles that circulate everywhere are unseen.

The tiny solids or liquids suspended in the air are called particulate matter. This material is only micrometers (thousandths of millimeters) in size. Particulate matter in the air, whether in the form of dust, fog, or smoke, is called aerosol. The sources of aerosols can be natural or anthropogenic, such as pollution.

These suspended microscopic particles play an important role in climate and rains and can affect human health. Because of this, they are the object of studies by scientists in various countries, and Brazil is at the front line of this research, with projects such as GoAmazon, with FAPESP’s support (read more at agencia.fapesp.br/29665).

One of the speakers at FAPESP Week France, held in Lyon and Paris until November 27th, Livia Freire, a researcher at the Institute of Mathematical and Computing Sciences of the University of São Paulo (ICMC-USP), has been developing computer simulations to try to broaden the knowledge on the transport of aerosols.

“I’m developing a study funded by FAPESP whose aim is to understand the behavior of the particles that circulate in the atmosphere. These are various types of tiny particles, which we cannot see but which can have a significant impact on our lives, on health, on agriculture, on climate,” she said to Agência FAPESP.

“Our interest lies in knowing how these particles are transported by atmospheric flows, which are very complicated movements to simulate and understand, because they are turbulent. We are developing numerical models that simulate flows in the atmosphere and how they transport particles. The aim is to obtain simple equations that researchers in other areas can use to understand particle concentrations in the atmosphere,” she said.

The researcher explained that to predict the behavior of particles, such as their concentration at a particular time and location, it is necessary to understand the turbulent flows present in the region that corresponds to the first hundreds of meters of the atmosphere, known as the atmospheric boundary layer. “This is a region that concentrates all the energy, gas, and particle exchanges between the atmosphere and the elements that compose the planet’s surface,” she said.

“The problem of turbulent flows is very complex since it involves various scales, ranging from the scale of the atmosphere itself to other very small ones, such as the turbulent vortices that transport particles. To simulate that on a computer, we need to represent all these scales, which means a significant increase in computing costs. It is a big challenge to represent all the different components in the atmosphere in a computer system with a viable cost,” said Freire.

Large-Eddy Simulation

The researcher mentioned that the atmospheric boundary layer has turbulent flows whose most faithful computational representation is obtained using a technique called Large-Eddy Simulation (LES).

“Due to its complex nature, the study of turbulence is based on the use of numerical simulations combined with experimental data analysis. For atmospheric flows, the use of the LES technique provides important indicators regarding the unique behavior of turbulence, and significant progress has been made in developing models for material and energy transport in the atmosphere in simplified conditions,” she said.

“For example, the average concentration of fine particles emitted from a flat and naked soil region can be represented by a simple flow-profile ratio, a result found based on the use of LES,” she said.

According to Freire, advances in computational power capacities offer an opportunity to investigate more complex problems, such as particle transport in the presence of forests and cities.

“We are using LES, an advanced numerical tool, to develop new models that enable us to explain the transport of particles in the atmosphere. This can increase our understanding and our ability to predict their role in the environment,” she said.

In the research with LES, Freire has also been working with professors Leandro Franco de Souza, from ICMC-USP, and Amauri Pereira de Oliveira from USP’s Institute of Astronomy, Geophysics, and Atmospheric Sciences, and with David Richter, from the University of Notre Dame.

The FAPESP Week France symposium is taking place between November 21st and 27th, thanks to a partnership between FAPESP and the Universities of Lyon and Paris, both in France. Read other news about the event at www.fapesp.br/week2019/france/.