Up to two-thirds of the soot over the central Amazon rainforest originates in Africa.
This is the finding of a study published in Nature Communications Earth and Environment.
Researchers led by the Max Planck Institute for Chemistry in Mainz, Germany, and the University of São Paulo distinguished soot particles based on their properties and assigned them to their sources.
They found that bushfires and burning savannas in northern and southern Africa contribute significantly to air pollution in central Amazon throughout the year and thus also play an important role in the atmospheric radiation budget and the hydrological cycle.
This is caused by efficient transatlantic particle transport through the atmosphere.
The Amazon 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 fine particles is very low.
The situation is completely different in the dry season when numerous deforestation fires burn in the Amazon rainforest because an “arc of deforestation” eats into it from the south.
As a result, soot and other emissions from the fires significantly reduce air quality at this time, and the air is no better in the central Amazon than in European urban areas.
Thus, the concentration of soot particles in the atmosphere above the sea of leaves fluctuates between very low and very high.
For the first time, a team of researchers has now investigated the sources of the soot particles.
They made a surprising discovery: a large proportion of the particles do not originate in South America but have traveled some 10,000 kilometers across the Atlantic from Africa with air masses and originate from natural bushfires, slash-and-burn agriculture, and the burning of biomass for cooking, for example.
“Smoke from Africa is found in large proportions over the rainforest for most of the year – we didn’t expect that,” says Bruna Holanda, a doctoral student at the Max Planck Institute for Chemistry who led the study.
“We had estimated the fraction from Africa at 5%, maybe 15%. But it was 60% 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.
SOOT PARTICLES FROM AFRICA AND SOUTH AMERICA DIFFER PHYSICALLY AND CHEMICALLY
To attribute soot over the Amazon to different sources, the scientists analyzed soot particles in the air at the Amazon Tall Tower Observatory (ATTO) over 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 Africa burns mostly grasslands, savannas, and open forests.
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.
In this way, the scientists also determined that a particularly large amount of smoke flows from Africa to the Amazon twice a year.
First, winds repeatedly bring soot together with Saharan dust into the area during the rainy season from January to March.
During this period, an average of 60% of the soot particles over the Amazon originate from African fires.
At this time, the air is particularly clean because there is hardly any slash-and-burn agriculture in the area.
But the smoke from Africa sometimes makes the air as dirty during this time of year as it is during the dry season.
On the other hand, a lot of soot from Africa can be observed in the central Amazon during the dry season from August to November.
In contrast to the rainy season, there are many natural and human-made fires regionally during this time, especially in the drier regions of the Amazon basin.
Regional fires account for about two-thirds of the soot load in much of the Amazon.
But as much as one-third of the soot comes from Africa, adding to already serious pollution levels.
SMOKE AFFECTS CLIMATE AND THE HYDROLOGICAL 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 highly radiatively active, absorbing significantly more solar radiation than they reflect, and thus are more likely to retain heat in the Earth 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 and affect the water balance.
“Our results can help improve climate and Earth system models that have so far inadequately reflected 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 been burning 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, in recent years are unprecedented and may have serious consequences for regional and global climate change,” Pöhlker concludes.
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