Transport of black carbon-rich smoke from Africa to the Amazon

When forests burn those fires produce a lot of smoke. And that smoke usually contains soot, also called “black carbon”. Black carbon particles are aerosols that absorb radiation and as such can warm the Earth’s atmosphere and climate. But we still have much to learn about aerosols, their properties, and distribution in the atmosphere. One of those things is the question of how black carbon emitted from biomass burning in Africa (i.e. forests, grasslands, savannas etc.) is transported across the Atlantic and into the Amazon basin, and what role it plays there. Bruna Holanda and her co-authors tackled this by combining data from the northeastern Amazon collected with the HALO research aircraft during the ACRIDICON-CHUVA campaign in September 2014, with long-term data from ATTO.

From HALO, they spotted a layer of air with lots of black carbon at ca. 3.5 km altitude that was some 300 m thick. They were able to trace that back to fires in Southern Africa. By the time this layer of smoke reached the Brazilian coast, it had already thinned considerably. Yet it was still visible even to the naked eye. Soot particles made up 40 percent of the total number of aerosols. And they had been in the atmosphere for at least 10 days. Once the polluted air mass reached the Amazon basin, the layer sank and became broader due to strong vertical air mixing over the continent. Thus the black carbon particles are quickly entrained into the continental boundary layer. The discrete polluted layer, which had been still preserved of the coast, quickly disappeared now.

Layer of smoke with high black carbon concentration reaching the coast of Brazil.
Layer of smoke with high black carbon concentration reaching the coast of Brazil. Figure by Holanda et al. (2020), Photo by Meinrat Andreae / MPI-C.

The authors then analyzed long-term data from ATTO. They found that such polluted air layers from fires reach the Amazon basin seasonally each year. From July to September, the early part of the dry season, biomass burnings in Africa are responsible for much of the air pollution with black carbon. Later in the dry season, from October to December, fires in South America are the main source of air pollution. These aerosol particles are efficient cloud condensation nuclei (i.e. seeds for cloud formation). That means they likely play an important part in the hydrological cycle in the Amazon.

Bruna Holanda and her co-authors published the study Open Access in Atmospheric Chemistry and Physics Issue 20: Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke.

The smoke from Africa and the Amazon brings high concentrations of black carbon to ATTO. Figure altered from Holanda et al. (2020)
The smoke from Africa and the Amazon brings high concentrations of black carbon to ATTO. Figure altered from Holanda et al. (2020)

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