Over three decades ago, the existence of an “Ozone Hole” above the Antarctic caused scientists to start fretting about the effects of man-made gases, eventually resulting in the Montreal Protocol that banned the use of CFCs responsible for ozone depletion.
Today there’s a whole new focus of concern about ozone and the volatile gases responsible for its creation. But this time investigations are going on in the Amazon, far from Antarctica, and the agents are natural, not human.
New results from Brazil’s blue-ribbon research project into the climatic effects of the Amazon forest have revealed that the forest produces huge quantities of Isoprene, a volatile organic compound (VOC) known to be a precursor chemical for ozone. These gases indirectly influence the balance of greenhouse gases in the atmosphere.
In fact, the forest is producing over three times as much isoprene had been previously thought. The results will now be fed into global climate modelling.
Isoprene is one of the VOCs naturally emitted by vegetation in the Amazon. Along with other VOCs, it is a source of the secondary organic aerosols that form cloud condensation nuclei and that help to regulate the hydrologic cycle in the region.
Isoprene decomposition in the atmosphere gives rise to various by-products, such as hydroxyl radicals (OH). Under certain conditions, this molecule reacts with atmospheric oxygen (O2) to form ozone (O3), one of the gases responsible for the greenhouse effect. High concentrations of ozone can irritate plant stomata, the pores used in gas exchange and transpiration. Irritation of the stomata hinders photosynthesis and the assimilation of carbon by plants.
Furthermore, hydroxyl radicals control atmospheric oxidation of methane, another important greenhouse gas,. Depending on the situation, these OH radicals can prolong or shorten the half-life of methane, with implications for the balance of greenhouse gases.
The discoveries made by the team of scientists at Brazil’s GOAmazon (Green Ocean Amazon) Project were published on May 23 in Nature Communications. Lead scientist and co-author of the study is Paulo Artaxo, a Professor at the University of São Paulo’s Physics Institute (IF-USP) who is now Brazil’s best-known climate scientist and the foremost exponent of the Amazon’s influence on world weather patterns. (You can read an earlier article about GOAmazon by clicking here.)
The new findings made by Prof. Artaxo and his team are a result of researchers being able to sample gases emitted by the forest at very much higher altitudes, thank to the use of specially converted jet aircraft. Previously, readings were taken from towers above the forest (such as this one) , or estimated by satellites. The key study is called “GOAmazon: interactions of the urban plume of Manaus with biogenic forest emissions in Amazonia”. Much of the focus, too, had been on the effect that human-induced pollution was having on rainfall patterns and weather systems in Amazonia (you can read an article about GOAmazon by clicking here).
But, thanks to the loan of a Grumman Gulfstream-1, a research aircraft capable of flying at 6,000m, or nearly 30,000ft, and owned by the Pacific Northwest National Laboratory (PNNL) in the United States, researchers were able to show the effect of gases produced by the forest – and not a result of man’s presence.
The airborne measurements were made in 2014 and 2015 during both the rainy and the dry seasons; the measurements were subsequently compared with data collected at ground level.
“With measurements taken at 4,000m, it was possible to calculate the average emission for a much larger area than those considered in previous research,” Prof. Artaxo said. “As a result, we could see that natural biogenic emissions are far greater than we supposed.”
One telltale sign is that, although the Amazon basin is flat and variations in ground elevation are very small, the atmospheric concentrations of isoprene at different altitudes closely track the rise and fall of the ground.
Of course, the difference in flora types between the low-lying, waterlogged regions and the higher-drier areas, may also result in a different mix of gases being thrown off by vegetation, prof Artaxo notes.
The researchers found this “surprising”: isoprene emissions vary sharply with terrain elevation and increase at higher altitudes. At a terrain elevation of 30m, for example, the isoprene emission flux was 6 milligrams per square meter per hour (mg/m2/h), while at an elevation of 100m, it was about 14 mg/m2/h.
The researchers are not yet confident that they can fully explain this variation in emissions, which they observed during both the wet and the dry seasons. Their article proposes two hypotheses, which will need to be tested in future experiments.
One of the possibilities is that plant species in low-lying areas, which are often waterlogged, are different from those found at higher altitudes; isoprene emission levels may vary according to the predominant plant species.
The other hypothesis is that, at higher elevations, plants release more isoprene in response to water stress (because there is less water at higher elevations).
According to Artaxo, the Amazon forest was already considered the world’s largest source of isoprene even before these new discoveries. “These findings reinforce the importance of this ecosystem to the regulation of the planet’s tropical atmospheric chemistry,” he said. “Now, we need to include the results in global climate models to see exactly what effect these new values for isoprene emissions have on the climate.”
Launched in 2014, GOAmazon is investigating the effects of urban pollution from Manaus on cloud formation in the Amazon, among other phenomena. The project also aims to extend knowledge of rain formation processes and the dynamics of interaction between the Amazon’s biosphere and the atmosphere.
The GOAmazon consortium is funded by the US Department of Energy (DoE), FAPESP, and the Amazonas State Research Funding Agency (FAPEAM), among other partners (read more at: agencia.fapesp.br/18803).
The article “Airborne observations reveal elevational gradient in tropical forest isoprene emissions” (doi:10.1038/ncomms15541) by Dasa Gu, Paulo Artaxo, Alex Guenther Hu et al. can be read at: nature.com/articles/ncomms15541).
You can read a detailed article by Brazilian journalist Karina Toledo by clicking here.