Biomass burning leads to a decrease in rain

Soot and other aerosols from biomass burning can influence regional and global weather and climate because it influences radiation, clouds, and precipitation. PhD student Lixia Liu in the Minerva Research Group led by Yafang Cheng at the Max Planck Institute of Chemistry in Mainz and her colleagues studied how this affects the Amazon Basin during the dry season. While there are many different interactions between biomass burning aerosols and climate, they found that they overall lead to fewer and weaker rain events in the Amazon rainforest.

Biomass burning aerosols

Smoke from biomass burning rises into the atmosphere over the central Amazon basin. © NASA
Smoke from biomass burning rises into the atmosphere over the central Amazon basin. © NASA

In the Amazon rainforest, the dry season typically lasts from July to October. During this time, there are often many fires, both from deforestation and agricultural practices. When trees, shrubs or other biomass burns, it produces soot, also known as Black Carbon, and a lighter variety of particles called Brown Carbon. These particles are transported high into the atmosphere.

Black and Brown Carbon are aerosols, that is, particles suspended in the air. As such they can scatter and absorb sunlight. This in turn can trigger changes in the energy budget at the surface. Scientists refer to these processes as aerosol-radiation interactions (ARIs). But these aerosols can also act as seeds for cloud formation, called cloud condensation nuclei. This affects the number of cloud droplets in the atmosphere, which may influence a range of atmospheric processes, including the cloud formation itself. These processes are called aerosol-cloud interactions (ACIs).

Analyzing their effects in the Amazon

ARIs and ACIs may overlap or counterbalance each other. Lixia Liu and her colleagues now wanted to disentangle the two processes to figure out how they affect the regional climate individually and compare their relative significance at different biomass burning emission intensities. To do so, they used computer simulations, in which they could simulate different scenarios with or without the ARIs.

The team found that how many aerosols there are influences which processes dominate. Aerosol-Cloud interactions are more important when the air is relatively clean. This may lead to a modest cooling of the atmosphere. Aerosol-Radiation interactions dominate when there are lots of aerosols, which may lead to strong warming in the atmosphere.

Clouds over the rainforest at ATTO / © Paulo Brando
Clouds over the rainforest at ATTO / © Paulo Brando

More fires, less rain

However, in all scenarios adding aerosols from biomass burning to the atmosphere resulted in less rain in the region. ACIs play an important role in it, especially with low and moderate biomass burning. Through acting as cloud condensation nuclei, biomass burning aerosols lead to more, but smaller cloud droplets. This slows down the cloud formation process and therefore thwarts rain formation. But ARIs amplify this and would even overrun the ACIs at high biomass burning emission because the soot particles absorb sunlight and thus warm the atmosphere and dimming the surface. This reduces atmospheric convection, which would lead to the afternoon rainstorms that are so typical for the tropics. With reduced convection, it is less likely to rain.

Climate change and the loss of forests due to deforestation already lead to drier and more prolonged Amazonian dry seasons. These effects from biomass burning threaten to further amplify droughts in the Amazon rainforest.

Liu et al. published “Impact of biomass burning aerosols on radiation, clouds, and precipitation over the Amazon: relative importance of aerosol-cloud and aerosol–radiation interactions” Open Access in Atmos. Chem. Phys.

Similar articles

GRAEGOR Detektoreinheit im Laborcontainer. © Robbie Ramsay / University of Edinburgh

Ramsay et al. measured inorganic trace gases such as ammonia and nitric acid and aerosols in the dry season at ATTO. They are to serve as baseline values for their concentration and fluxes in the atmosphere and are a first step in deciphering exchange processes of inorganic trace gases between the Amazon rainforest and the atmosphere.

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 in their new study published in ACP.

Graphical Abstract

In a new study, Dr. Haijie Tong and co-authors studied a subset of PM2.5, the so-called highly oxygenated molecules (HOMs) and its relationship with radical yield and aerosol oxidative potential. They analyzed fine particulate matter in the air in multiple locations. This including the highly polluted megacity Bejing and in the pristine rainforest at ATTO. They wanted to get insights into the chemical characteristic and evolutions of these HOM particles. In particular, they wanted to find out more about the potential of HOMs to form free radicals. These are highly reactive species with unpaired electrons.

Wu et al. collected and analyzed aerosols in two locations: the city of Manaus, a large urban area in Brazil, and the ATTO site in the heart of the forest. The aerosol compositions varied largly. At ATTO most aerosols were emitted by the forest itself, while in Manaus, anthropogenic aerosols were very common. The results were published in ACP.

Saturno et al. analyzed the concentration of black and brown carbon in the atmosphere above the Amazon. They found that the dry season is characterized by lots of biomass burnings, which produce a lot of black and brown carbon. But they also sound significant interannual variations. The results were published in ACP.

Mira Pöhlker and her team continuously measured aerosols and their properties in the atmosphere at the 80 m tower at ATTO, thereby created the first such long-term record in the Amazon. They analyzed the data in two subsequent paper. The second was now published in ACP.

Moran-Zuloaga et al. analyzed the coarse fraction of aerosols every 5 minutes for over 3 years at ATTO. They found that the composition remains fairly constant throughout the year, except for a short period in the wet season when Saharan dust occurs regularly. They published their results in ACP.