Tree scent helps to create particles in the atmosphere
25 January 2010, by Sara Coelho
Organic compounds produced by forests, such as the chemicals that give pine trees their smell, play a key role on the formation of tiny atmospheric particles known as aerosols. The findings may have implications for predictions of future climate change as they change what we know about atmospheric aerosols.
Aerosol particles are important for climate models because 'they absorb and reflect sunlight and they change the properties of clouds,' says Dr Dominick Spracklen, a NERC fellow from the University of Leeds.
Some aerosols in the atmosphere are emitted by natural processes, such as salt from sea spray, dust blown up by desert winds or microscopic ash released by forest fires. But little was known about how aerosols produced in the atmosphere actually form.
Spracklen and colleagues bridged the gap with a set of experiments performed in a smog chamber, where scientists can mix different types of compounds and gases at variable temperatures. 'The rate of aerosol formation depends on the concentration of the gases and the conditions we use in the experiment,' explains Spracklen.
The team found that to match the rate of aerosol formation observed in nature, they need sulphuric acid and organic compounds, such as the molecules that give pine trees their characteristic smell.
'This means that organic compounds emitted by forests are likely to alter the way that particles form in the atmosphere,' says Spracklen.
Although most climate models include data from atmospheric aerosols, they focus on particles emitted by the sea, deserts or combustion, not the aerosols produced in the atmosphere with sulphuric acid and organic compounds. 'This is because until now it was very hard to get the conditions in the lab to match what we see in the atmosphere,' he says.
The role of organic compounds in aerosol formation 'is a potential negative climate feedback that needs to be studied further,' adds Spracklen.
Everyone who has walked through a pine forest in the summer knows that pines smell more when it's hot than when it's winter. More organic compounds in the atmosphere mean that more aerosols are produced when the temperatures are higher. With more particles up in the air, more sunlight is reflected back into space. This could be a way to protect the Earth from warming.
It's not clear why trees emit odour-rich compounds, but 'they could be a way that the Earth's biosphere helps regulate climate.'
The results, reported last week in the Proceedings of the National Academy of Sciences, come from the first set of experiments that successfully matches the rate of aerosol formation in nature. The lab work was done at the Paul Scherrer Institute in Switzerland.
Axel Metzger, Bart Verheggen, Josef Dommen, Jonathan Duplissy, Andre S. H. Prevot, Ernest Weingartner, Ilona Riipinen, Markku Kulmala, Dominick V. Spracklen, Kenneth S. Carslaw, and Urs Baltensperger.
Atmospheric Chemistry Special Feature: Evidence for the role of organics in aerosol particle formation under atmospheric conditions. Proceedings of the National Academy of Sciences, published online before print January 19, 2010, doi:10.1073/pnas.0911330107
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