Atmospheric recycling

A team of researchers at the Max Planck Institute for Chemistry in Germany has found unexpectedly high concentrations of the atmosphere’s so-called ‘detergent’ — hydroxyl radicals — over the pristine Amazon forests of Suriname, Guyana and Guyane (French Guiana). Jos Lelieveld and colleagues’ results, reported in Nature this week, illustrate the remarkable balance that the biosphere maintains with its atmospheric environment in the absence of human influences.

Hydroxyl radicals, produced by the action of solar ultraviolet radiation on ozone, remove gases in the atmosphere known as volatile organic compounds (VOCs) by oxidation reactions.

Tropical rainforests release huge amounts of VOCs, including the hydrocarbon isoprene. Previously, isoprene — in unpolluted air — was assumed to deplete the levels of hydroxyl radicals and reduce the capacity of the atmosphere to remove particular greenhouse and toxic gases. However, natural VOC emissions also serve important biological functions for the forest.

Based on measurements atmospheric trace gases from an aircraft over remote areas of the Amazon and the tropical Atlantic Ocean, laboratory measurements and numerical modeling, Lelieveld and colleagues suggest that natural oxidation of the forest’s biogenic compounds actually enables the atmosphere’s hydroxyl radicals to be recycled. However, they note that further laboratory studies are “necessary to explore the chemical mechanism responsible for hydroxyl radical recycling in more detail”.

In a related ‘News and Views’ article in Nature, Alex Guenther of the US National Center for Atmospheric Research also suggests that that field studies in less pristine regions are needed to determine the broader relevance of such oxidation processes.