African smoke over the Amazon

More soot particles enter the central Amazon rainforest from brush fires in Africa than from regional fires at certain times.

Up to two-thirds of the soot over the central Amazon originates in Africa. This is the result of a study now published in Nature Communications Earth and Environment. Researchers led by the Max Planck Institute for Chemistry in Mainz and the University of Sao Paulo distinguished soot particles based on their properties and assigned them to their sources. They found that bushfires and burning savannahs in northern and southern Africa contribute significantly to air pollution in central Amazonia throughout the year, and thus also play an important role in the atmospheric radiation budget and the hydrological cycle. This is a result of efficient transatlantic particle transport through the atmosphere.

The Brazilian rainforest is considered one of the few continental areas in the world with clean air. However, this is only true during the rainy season, when the concentration of particulate matter is very low at times. The situation is completely different in the dry season. Then numerous deforestation fires burn in the Amazon rainforest, because the “arc of deforestation” spreads into it from the south. As a result, soot and other emissions from the fires significantly reduce air quality at this time. During this time, the air in the central Amazon is no better than in European metropolitan areas. The concentration of soot particles in the atmosphere above the canopy thus fluctuates between very low and very high.

For the first time, a team of researchers has now investigated the sources of these soot particles. And they made a surprising discovery: a large proportion of the particles do not originate in South America, but have traveled some 10,000 kilometers with air masses from Africa across the Atlantic and originate from natural bush fires, slash-and-burn agriculture and the burning of biomass for cooking, for example. “Smoke from Africa is present in large proportions over the rainforest for most of the year – we didn’t expect that,” says Bruna Holanda, who led the study as a doctoral student at the Max Planck Institute for Chemistry. “We had estimated the fraction from Africa at 5, maybe 15 percent. But it was actually 60 percent at times.” According to the atmospheric physicist, this value proves how efficient the atmospheric transport of soot and aerosol particles is with air masses from Africa to South America.

The satellite image shows that large areas of the Amazon are covered with smoke, some from regional fires, but some fires in Africa, coming in from the Atlantic. The green outline marks the extend of the Amazon rainforest and the red circle the location of ATTO. The blue lines indicate the main wind field at ATTO during the time the image was taken. © Holanda et al. (2023)

Soot particles from Africa and South America differ physically and chemically

In order to attribute the soot over the Amazon to different sources, the scientists analyzed soot particles in the air at the Amazon Tall Tower Observatory (ATTO) over a period of two years. The research site is located in a nearly pristine region in the central Amazon and includes a 325-meter tall measurement tower.

The team found two predominant types of soot: soot particles from Africa were significantly larger than those from the Amazon and had lower organic material content. The researchers attribute this to the fact that in Africa, it is primarily grasslands, savannas and open forests that burn. The drier fuels result in more flaming combustion and more soot particles. South American fires, on the other hand, occur in dense and moist forests. The wetter fuels are more likely to result in smoldering fires that produce soot with larger organic fractions. Holanda and her colleagues then used meteorological data such as the main wind field and satellite images, which sometimes even show the smoke plumes, to determine the particular source of the smoke.

This is how the scientists also determined that twice a year, particularly large amounts of smoke from Africa travel to the Amazon: During the rainy season from January to March, winds repeatedly bring soot together with Saharan dust into the area. During this time, an average of 60 percent of the soot particles over the Amazon originate from African fires. At this time, the air is normally particularly clean because there is hardly any slash-and-burn agriculture in the region. But the smoke from Africa sometimes leaves the air as dirty as it is during the dry season, even at this time of year. In the dry season, from August to November, there is also a lot of soot from Africa in the central Amazon. In contrast to the rainy season, there are many natural and man-made fires regionally during this time, especially in the drier regions of the Amazon basin. In much of the Amazon, regional fires account for about two-thirds of the soot load. But as much as one-third of the soot comes from Africa, adding to the already serious pollution levels.

Smoke influences the climate and the water cycle

Soot and other aerosol particles absorb and scatter sunlight, thus affecting the Earth’s radiation or energy balance and our climate. Soot particles in particular are very radiatively active, as they absorb significantly more solar radiation than they reflect, and thus are more likely to retain heat in the Earth’s system. However, dust and soot particles also serve as condensation nuclei in the formation of cloud droplets. Therefore, they influence the formation of clouds and precipitation, so they also affect the water balance.

“Our results can help improve climate and Earth system models that have so far inadequately accounted for the African smoke component,” explains Christopher Pöhlker, group leader at the Max Planck Institute for Chemistry. He says the efficiency of transport also suggests that African smoke reached South America in pre-industrial times, as fire-prone African vegetation has likely burned seasonally for tens of thousands of years. “We suspect that soot has long played an important role in soil fertilization and thus forest formation in the Amazon, as well as in the carbon and water cycles,” adds the atmospheric chemist. However, such positive effects of the past could now turn into the opposite. “The rates of deforestation and the number of fires, as well as the resulting soot, have been unprecedented in recent years and may have serious consequences for regional and global climate change,” Pöhlker concludes.

Holanda et al. published the study “African biomass burning affects aerosol cycling over the Amazon.” Open Access in the journal Nature Commun Earth Environ. DOI: 10.1038/s43247-023-00795-5

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