Oil palm plantation. Credit: Michael Thirnbeck

(Phys.org)New models are being developed to predict how changing land use in the tropics could affect future climate, air quality and crop production.

Ozone has a dual personality best described as "good up high, bad nearby": the atmospheric gas is both vital and potentially fatal for our health. High in the stratosphere, the gas filters sunlight and protects us from the damaging effects of ultraviolet light. At ground level, however, it causes respiratory problems and damages crops.

'Bad' ozone is formed by the reaction of sunlight on gases emitted from fossil fuel combustion, and its concentration is predicted to continue to rise unless global emissions can be reduced. But ozone levels are not only affected by emissions from cars, power stations and industrial processes; they are also affected by emissions of volatile organic compounds (VOCs), such as isoprene, by plants.

"It's a complicated mechanism," explained Professor John Pyle from Cambridge's Department of Chemistry. "In pristine conditions, such as in the tropical rainforests, VOCs can reduce ozone levels. However, in the presence of oxides of nitrogen (NOx), which are pollutants produced during combustion of fossil fuels, VOCs can increase ozone levels."

In recent years, atmospheric chemists such as Pyle have been concerned that widespread changes in land-use in the tropics could have a dramatic impact on the formation of ozone, tipping the balance towards ozone production rather than destruction. Tropical rainforests currently account for over half of the world's forests and are biodiversity hotspots, but clearing for biofuels, crops and livestock is having a dramatic effect on their extent, with estimates suggesting that as much as 1.5 acres of rainforest are destroyed every second.

"Among the most widespread of tropical crops being planted in the cleared rainforests is the oil palm. In Malaysia, for instance, in just four decades the percentage of land covered by oil palm plantations has risen from 1% to 13% to meet an increasing demand for bioenergy and palm-oil-based consumer goods," added Pyle. "Is this change in land use resulting in unwelcome side-effects on ground-level ozone?"

Understanding this uncertainty has been a major focus of his team's research. By knitting together expertise in atmospheric chemistry with state-of-the-art climate models, the researchers aim to predict future concentrations of surface ozone with changing industrial emissions and land-use, from now until the end of this century.

"Our models rely on solving a set of differential equations that describe how reactants in the ozone pathway turn into products," explained researcher Dr Alex Archibald. "The complexity is potentially enormous. If we were to take into account all of the reactions of gases in the atmosphere, we would need to consider something like tens of millions of reactions. In reality, models can't cope with this level of complexity and so part of our work has been to determine the sensitivity of our models depending on the number of reactions we include."

PhD student Oliver Squire has been testing this sensitivity by comparing a range of commonly used chemistry mechanisms within the climate model. "The sign and magnitude of the ozone change due to a change in isoprene emissions in tropical regions show a strong dependence on the number of isoprene reactions included," he said. "This highlights the importance of correctly simplifying the full complexity of the atmosphere's chemistry".

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Climate chemistry and the tropics