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Forest emissions driven by biological clock

Tree burps A new study has found circadian rhythms control volatile gas emissions from plants, shedding light on an atmospheric modelling conundrum.

The international research led by Dr Nick Hewitt from the University of Lancaster, studied forest emissions of isoprene.

Isoprene is a naturally occurring volatile gas given off by many plant species, including rainforest trees, eucalyptus and oil palms.

While isoprene helps protect plants from environmental stresses, and herbivory, its chemical reactions in the presence of nitrogen oxides (NOx) in the atmosphere form ground level ozone, says a study co-author Dr Malcolm Possell, an expert in biogenic volatile organic compounds (VOCs) at the University of Sydney.

"Isoprene is one of thousands of different compounds given off by plants.

"It is, however extremely important as it is estimated to make up between 40 and 45 percent of the global VOC emissions budget."

The study, which was published today in Nature Geoscience , looked at the factors controlling the chemical compound and particle formation in a natural rainforest area and at an oil palm plantation in Sabah, in Malaysian Borneo.

Circadian rhythms

To explore the effects of isoprene production on ground level ozone production the researchers took measurements from the leaf and branch level to within and above the canopy.

They were surprised to find that isoprene emissions are controlled by circadian rhythms at the canopy level.

"Many plant processes, such as photosynthesis and the opening and closing of stomata (pores) in the leaves, are controlled by this natural clock," explains Possell.

"While we know this happens at the leaf and twig scale, we didn't expect to see it operating at canopy level".

It was previously thought that circadian rhythms would be undetectable in the canopy because of the impact of atmospheric chemical processes and the presence of so many species, each with their own 'clock', making it difficult to detect an overall pattern.

But this study found that circadian rhythms operate over a very large footprint and control isoprene emissions.

When they factored circadian control into global models of atmospheric chemistry and transport, the researchers found ozone concentrations to be more in line with what has been observed, rather than what the models had predicted.

"The research has given us a more complete understanding of the processes contributing to isoprene emissions," says Possell. "We are now better able to predict ozone concentrations especially in isoprene sensitive regions of the world."

Isoprene:NOx ratios

Previous studies have found a strong relationship between the amount of isoprene being emitted, the availability of NOx and formation of ozone. Small amounts of either isoprene or NOx relative to the other, can significantly limit ozone formation.

"Most NOx is a product of combustion processes, meaning the concentration is higher in urban areas," explains Possell.

Whether ozone levels go up or down depends on the level of background pollution, says Dr Alexander Archibald, an atmospheric chemist from the University of Cambridge, who was not involved in the study.

"In remotely, lightly polluted regions, circadian control raises ozone levels. But in warm, heavily polluted regions … circadian rhythms reduce ozone levels," writes Archibald in a separate commentary in the journal Nature Geoscience.

"The significance of this circadian clock for atmospheric chemistry is likely to prove greatest in regions that are strongly influenced by isoprene emissions."

The study was undertaken as part of the Oxidant and Particle Photochemical Processes (or OP3) Project by several international research institutions and funded by the Natural Environment Research Council (NERC) in the UK.