African smoke over the Amazon

The Brazilian rainforest is one of the few continental areas in the world with clean air. However, this only applies during the rainy season, when particulate concentrations are very low. During the dry season, it’s a different story: lots of deforestation fires occur inside the Amazon rainforest, as a “deforestation arc” eats away at the rainforest from the south. The soot and other emissions from the fires have caused a drastic reduction in air quality this time of year. Air quality in the central Amazon is currently no better than in urban Europe. The concentration of soot particles in the atmosphere above the forest canopy fluctuates between very low and very high.

For the first time, the research team investigated the origins of soot particles. They made a startling discovery: a large number of the particles did not originate in South America; instead, they travel with air masses some 10,000 kilometers from Africa over the Atlantic, originating from natural bush fires, slash-and-burn practices, and burning of biomass, such as for cooking. “Smoke from Africa can be found most of the year in large numbers over the rainforest – we didn’t expect this,” said Bruna Holanda, who led the research as a doctoral researcher at the Max Planck Institute for Chemistry. “We estimate the amount of smoke coming from Africa is around 5 or maybe 15 percent. It turns out, sometimes it goes up to 60 percent.” According to atmospheric physicists, this value indicates the efficiency of atmospheric transport of soot and aerosol particles through air masses from Africa to South America.

The soot particles from Africa and South America are physically and chemically different from one another

To link soot over the Amazon to multiple sources, researchers analyzed soot particles in the air over the Amazon over two years at the Amazon Tall Tower Observatory (ATTO). The research unit is located in an almost untouched region of the central Amazon and, among other facilities, has an observation tower 325 meters high.

The team identified two common types of soot: soot particles from Africa were much larger than those from the Amazon region and exhibited lower concentrations of organic matter. Researchers attribute this to the fact that in Africa, the burned areas are mostly grasslands, savannas and open woodlands. Dryer fuel is more likely to produce an icy combustion and more soot particles. In contrast, South American fires occur in dense, moist forests. This moist fuel causes smoldering combustion, which produces soot with a greater concentration of organic matter. Using meteorological data such as main wind fields and satellite imagery, where clouds of smoke are occasionally visible, Holanda and his colleagues then determined the source of each smoke.

In this way, the researchers also determined that there are two periods of the year when large amounts of smoke spread from Africa to the Amazon: first, during the rainy season from January to March, winds consistently carry soot mixed with Saharan dust into the region. So far, an average of 60 percent of soot particles over the Amazon have come from African fires. The air is actually very clean during the rainy season, as there are almost no slash-and-burn fires in the area. However, sometimes the smoke from Africa makes the air in this season as dirty as during the dry season. Secondly, during the dry season from August to November, a lot of soot from Africa can be observed in the Central Amazon. In contrast to the rainy season, at this time there are many natural and man-made fires in the region, especially in the dry areas of the Amazon basin. Elsewhere in the Amazon, regional fires account for about two-thirds of soot pollution. However, a third of the soot in the region comes from Africa, exacerbating already severe levels of air pollution.

Smog affects climate and the water cycle

Soot and other aerosol particles absorb and scatter sunlight, which affects the radiation or energy balance of our earth and climate. Soot particles are particularly active against radiation, as they absorb far more sunlight than they reflect, thereby retaining heat in the Earth’s system. Dust and soot particles also serve as condensation nuclei in the appearance of cloud droplets. Thus, they influence cloud formation and precipitation; in this way, they also impact the water budget.

“Our results can help improve models of the climate and Earth systems, which until now poorly reflected the components of the African smog”, explained Christopher Pöhlker, group leader at the Max Planck Institute of Chemistry. According to him, the efficiency of transport also shows that African smog had reached South America in pre-industrial times, because African vegetation that is prone to fires is thought to have been burning for tens of thousands of years. “We suspect that soot has long played an important role in soil fertilization and forest formation in the Amazon region, as well as in the carbon and water cycles,” continues the atmospheric chemist. However, previously positive effects like this can now become detrimental. “The rate of deforestation, the number of fires and soot produced in previous years is unprecedented and could have serious consequences for regional and global climate change”, summarizes Pöhlker.