Pollutant plays buffering role in atmosphere self-cleaning

The hydroxyl radical (OH) is a very reactive and short-lived molecule that contains one hydrogen atom and one oxygen atom. It is a crucial agent in the self-cleaning actions of the Earth's atmosphere. It helps degrade volatile organic compounds, such as methane and isoprene which are contributing to climate change. However, little is known about the chemistry involved in that process.

A team led by Domenico Taraborrelli, a chemist at the Max Planck Institute for Chemistry in Mainz, Germany, which includes Johannes Lelieveld at King Saud University, Riyadh, propose a mechanism for this oxidation process, publishing their results in Nature Geoscience.

They propose a complex relationship between isoprene and OH. While oxidation of isoprene leads to a reduction in OH radicals — which are used up in the reaction – the process creates a cascade reaction that leads to the formation of more OH radicals. The system both sustains and limits itself, according to Taraborrelli.

When isoprene is oxidised, it produces unsaturated hydroperoxy aldehydes, which then react with OH, using it up in the process. However, exposure to light causes photolysis of hydroperoxy aldehydes which releases OH. These two processes compete and balance each other, giving isoprene a buffering role in regulating OH radicals.

When the team analysed real data collected above a pristine region of the Amazon forest and from moderately polluted areas, they found that their proposed oxidation mechanism accurately predicted the actual OH concentrations in the atmosphere.

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