Tropical rainforest emissions of the biogenic compound isoprene increase the self-cleansing power of the atmosphere, say scientists in Belgium. 

Using theoretical models, Jozef Peeters and colleagues from the Catholic University of Leuven proposed chemical mechanisms showing how the amount of hydroxyl radical - the detergent of the atmosphere - could be enhanced by isoprene oxidation. 

The hydroxyl radical is a naturally occurring compound that scrubs many manmade pollutants from our atmosphere. Popular opinion has long been that in unpolluted air, like that of a rainforest, the large amounts of isoprene emitted from plants would deplete the hydroxyl radical. So in regions with high isoprene levels you might expect to find low hydroxyl concentrations.   

Recent measurements in the Amazonian rainforest have shown the amount of hydroxyl radical present actually greatly exceeds that predicted using current atmospheric chemical models. So why is this? Peeters says it is because rather than removing the hydroxyl radical species, isoprene oxidation actually recycles it. 

Isoprene reacts with hydroxyl radicals to form OH-isoprene adducts which rapidly add oxygen yielding several hydroxy-peroxy radicals. The mechanism put forward by Peeters and the team is based on the fact that the major hydroxy-peroxy radicals can eliminate oxygen so fast that the various isomers interconvert. A hydrogen shift on one form of the hydroxy-peroxy radical regenerates the HO radical.    

'Regarding specifically the atmospheric oxidation of isoprene, experimentalists face the considerable but urgent challenge of confirming - or disproving - the new chemistry we have proposed,' says Peeters. 

'Understanding the chemistry of isoprene and other plant emitted organics is currently the leading challenge for atmospheric chemistry,' says Mat Evans, an expert in atmospheric composition modelling at the University of Leeds, UK. He adds: 'This work invokes new tools to aid our understanding and provides a unique insight.' 

Sarah Corcoran 

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