News - April 10, 2008

Rainforest better at cleaning air than thought

The cleansing effect of the atmosphere in tropical areas is much greater than previously assumed. Scientists measured a surprisingly high concentration of hydroxyl above the rainforest in Surinam. Hydroxyl is crucial for breaking down many gases in the atmosphere, including greenhouse gases. Nature publishes an article on this discovery on 10 April.

Hydroxyl is a radical that breaks down greenhouse gases such as methane by forming chemical compounds with them. The substance’s function was already known, says Laurens Ganzeveld of the Earth Systems Science and Climate Change group. ‘We also call hydroxyl the washing powder of the atmosphere.’ But it is only recently that scientists have been able to measure the exact concentration of hydroxyl in the air. This is due to its short lifetime: ‘It is formed quickly, but because it reacts so quickly with other substances, it disappears again very rapidly.’

Calculations of atmospheric hydroxyl concentration were based on estimates and models. Thanks to developments in measuring equipment, however, researchers at the German Max Planck Institute for Chemistry were able to measure the concentrations of the radical from a small aircraft flying above the tropical rainforest. ‘The measurements show that we have always underestimated the concentration, and quite considerably,’ says Ganzeveld, who took part in the research.

Researchers also assumed that the hydroxyl radicals in the rainforest reacted with the large quantities of reactive hydrocarbons produced by plants, resulting in fewer radicals to react with other gases. The measurements taken from the aircraft show that new hydroxyl is formed during the reactions with hydrocarbons. According to Ganzeveld, this is the ‘missing piece in the puzzle of tropical atmospheric chemistry’ and explains the high concentrations.

‘It appears that the tropics are much more capable of cleaning up atmospheric pollution that we thought,’ says Ganzeveld. The discovery also has consequences for climate change predictions. Disturbances in the rainforest such as deforestation result in far less natural purification of the atmosphere that has been assumed. ‘We’ll have to adjust our model analyses of the effects of deforestation on climate, chemistry and air pollution,’ Ganzeveld concludes.