Microplastics | The threat that hangs in the air

Plastic pollution not only invades the seas, but also the atmosphere. So much so that researchers are concerned about potential unsuspected effects on the environment and even… on the formation of clouds.

A dust of pollutants

In 2017, Janice Brahney explored some of the most isolated regions of the American West. In various federal protected areas, the professor of biogeochemistry at Utah State University has collected dust deposits. Its goal: to study the composition of the air far from large cities – and the potential presence of phosphorus. Surprises awaited him. In his samples, a multitude of brightly colored debris had become embedded. These fine plastic particles, called microplastics, came from everyday objects: clothing, packaging, cosmetics, etc. “I couldn’t believe my eyes,” she says. We calculated that around 4% of the dust collected was plastic. It is enormous. In places so far away, all these contaminants could not have been transported by humans. The dust collected showed alarming amounts of microplastics circulating in the atmosphere.

Little strand goes a long way

In an article published in 2020 in the journal Science, Janice Brahney’s team estimates that more than 1,000 tons of plastic are deposited each year in protected areas in the American West, the equivalent of 123 to 300 million plastic water bottles. Other researchers have found them in the Arctic, Antarctica and even in the snows near the summit of Mount Everest. “Microplastics have been in the atmosphere for quite some time now and we are noticing it quite late, fears Professor Brahney. There are relatively large concentrations of it in the air over certain regions and we don’t fully understand what that means for the environment and the climate. »

In the mood of time

Global plastic production continues to grow. It doubled between 2000 and 2019, according to the Organization for Economic Co-operation and Development (OECD), to reach more than 450 million tonnes per year. Particles detach from these plastics and travel with the winds and rains, before being redeposited on land or in the sea. From the oceans, they can be thrown back into the air, through bubbles of water. air that rises up in the waters and bursts on their surface. This is the microplastic cycle.

The power of aerosols

According to recent modeling by Janice Brahney and her colleagues, some fine microplastic particles can stay in the atmosphere for a week and circle the globe several times. In the air, they mix with dust, soot, pollen and other suspended solids and liquids. This batch of particles, called “aerosols”, has a strong influence on the climate. Some, for example, absorb solar radiation and have a warming effect, while others diffuse it and have a cooling effect. The condensation of water vapor around the aerosols is at the origin of the clouds. The greater the number of particles in suspension on which the water manages to agglutinate, the more the sky becomes covered.

A matter of properties

Until recently, microplastics were generally excluded out of hand when studying cloud formation, as these pollutants tend to be hydrophobic. But several size factors were overlooked. This is described in a recent article published in the journal Nature. “As plastic erodes, it is increasingly likely to participate in cloud formation processes,” says Denise Mitrano, professor of environmental science at ETH Zurich and co-author of the ‘article. “Some of the physicochemical characteristics of eroded plastic are similar to particles that we already knew were involved in this process. In the environment, other substances can also adhere to microplastics, making them more prone to cloud formation. Salts, sulphates and organic materials, in particular, tend to attract water vapour.

The quantity that matters

What is the current contribution of microplastics to cloud formation? How can they affect the climate? The mystery remains. To find out, it would first be necessary to better quantify these particles in the atmosphere and their importance compared to other aerosols. “In areas such as cities, there are already many other particles in the air and atmosphere,” notes Denise Mitrano. Microplastics may therefore represent only a small proportion. On the other hand, microplastics can be transported through the atmosphere to areas where there is less direct human activity releasing other particles. So in these cases microplastics may be more important. Jennifer Brahney and her colleagues’ model estimates that microplastics typically make up less than 1% of man-made aerosols that land on land, but that proportion potentially rises to more than 50% in some ocean regions.

Much more than plastics

Environment and Climate Change Canada researcher Liisa Jantunen studies microplastics in the Canadian Arctic. Beyond the potential effects of these airborne particles on the climate, she is concerned about the risks to human and animal health. “Plastic itself is associated in the literature with various inflammations in the body,” she points out. But I think what’s most concerning are all the sometimes toxic substances that are added to these plastics in their manufacture, such as flame retardants and UV protectants. »

wind of hope

Faced with the threat posed by plastic pollution, the United Nations Environment Program created an intergovernmental negotiating committee last March bringing together some 200 nations. The goal is to achieve a legally binding treaty on plastics by 2024. For Liisa Jantunen, expectations are high. “I imagine it will take some time to change the way things are done when it comes to plastic consumption,” she admits. But, you know, humanity has to try. Otherwise, it will only get worse. »