ossible to calculate the average emission for a much larger area than those considered in previous research," Artaxo said. "As a result, we could see that natural biogenic emissions are far greater than we supposed."
The researchers made another discovery that they consider surprising: Isoprene emissions vary sharply with terrain elevation and increase at higher altitudes. At a terrain elevation of 30 meters, for example, the isoprene emission flux was 6 milligrams per square meter per hour (mg/m2/h), while at an elevation of 100 meters, it was about 14 mg/m2/h.
"The Brazilian Amazon region is mostly low elevation. In the areas overflown by the research aircraft, there were minor terrain undulations, and we were able to observe a significant increase in emissions in the higher areas," Artaxo said. 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 that 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).
"Although it rains a lot in Amazonia, studies have shown that in some areas, the water table falls well below the surface in the dry season. There are plants with very deep roots, capable of collecting water 10 to 20 meters underground," Artaxo said. Isoprene is one of the volatile organic compounds (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 byproducts, 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," Artaxo said. "Depending on the situation, these OH radicals can prolong or shorten the half-life of methane, with implications for the balance of greenhouse gases."
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."