Palm oil plantations can cause air pollution

14 October 2009 by Tom Marshall

Planting oil palm plantations where rainforests once stood could have unplanned and unwelcome side-effects on air quality, scientists say.

Oil palm saplings

Oil palm saplings

Oil palm trees produce unusually high levels of so-called volatile organic compounds (VOCs). These react with nitrogen oxides (NOX) in the presence of sunlight to form ozone. While ozone is welcome high in the atmosphere shielding the Earth from damaging ultraviolet light, at ground level it causes breathing problems and damages plants.

The study, part of a three year project funded by NERC and published in Proceedings of the National Academy of Sciences, compared an area of oil palm plantation with a nearby area of protected rain forest in Malaysia's Danum Valley on the island of Borneo. The scientists took measurements from towers over the canopy in both environments, as well as using sensors on the NERC BAE 146 research aircraft.

"Our measurements show that the conversion of rainforest to oil palm plantation substantially increases VOC and NOX concentrations," they write. Plants like oil palms produce VOCs, in particular the chemical isoprene, as a defence against environmental stress. Meanwhile the industrialisation and heavier use of fertiliser on plantations cause NOX levels to rise. "Our paper is about what happens when you replace rainforest with oil palm plantation," says Professor Nick Hewitt, an atmospheric chemist at Lancaster University and the paper's lead author. "One of the consequences is that the plantation landscape produces much more VOCs than the natural forest, and this can be detrimental to air quality."

At present, ozone concentrations are low in the areas the scientists studied - lower than is typical in the UK, for example. But this could change if care isn't taken to limit emissions of nitrogen oxides in and around oil palm plantations. These emissions come mostly from burning fossil fuels and from nitrogen-based fertilisers.

Based on their measurements, the researchers modelled the effect of increasing NOX in the region on ground-level ozone. They found that in an untouched rainforest landscape, increasing NOX concentrations to around 10 parts per billion (ppb) - equivalent to typical rural levels in the Northern hemisphere - could push ozone levels up from around 12ppb at present to around 50ppb as an eight-hour average. This is close to the World Health Organization's threshold value for ozone for acceptable air quality.

But because oil palm plantations emit far more VOCs than the same area of rainforest, an equivalent increase in NOX emissions in an area dominated by plantations would cause ozone concentrations to rise to around 120ppb, a level known to be dangerous to human health.

The problem is that when rainforest is converted to plantation, an unsullied landscape with no infrastructure becomes an industrialised landscape crisscrossed by roads and dotted with villages, processing plants and other facilities. This means NOX emissions are expected to rise at the same time as VOCs emissions do.

The increased ozone concentrations this would cause would damage plants and so harm crop production in the region, possibly endangering food security; eventually they could even affect the production of the oil palm plantations themselves. Although NOX emissions are currently quite low, the paper's authors warn that plans need to be put into place to keep them that way.

"NOX levels are still low, but the palm plantations will increase VOC emissions, and our paper is an early warning that we need to be very careful about managing reactive nitrogen emissions to control NOX in the atmosphere," Hewitt explains. Measures to do this might range from putting catalytic converters on cars and machinery to limiting the use of fossil fuels and artificial fertilisers.

An alternative to managing emissions of nitrogen oxides could be to genetically engineer oil palms to reduce or eliminate their isoprene output. Scientists have already managed to achieve this with poplar trees. But this came at a cost; the trees were more susceptible to damage from high temperatures. And even if it were possible, creating isoprene-free oil palm trees is a long-term project that won't be possible for years.

"Without NOX emissions controls or genetic modification, palm oil production... will incur significant future costs in terms of human health effects and crop yield reductions," the paper's authors write. "The utility of palm oil as an environmentally friendly fuel therefore may be severely time-limited."

Ironically, the area given over to oil palm plantations in the tropics is growing partly because the oil has been presented as a potential source of bioenergy that could help us reduce our dependence on fossil fuels, and hence as a way to cut greenhouse gas emissions.

The other factor behind increasing demand for palm oil is dietary change, particularly in China, where cooking oil use is growing quickly. Palm oil is also an extremely common ingredient of many consumer goods. It's a cheap vegetable oil, and finds its way into everything from margarine to soap and skin lotion.