With addition of smog, trees may generate air pollution

April 26, 2013

In 2004 scientists found that isoprene, a compound produced by trees to protect their leaves from oxidation and temperature fluctuations, plays a role in the production of particulate matter, tiny particles that can cause lung damage, asthma, and other health problems. The finding — which led some to argue that forests are worsening air pollution rather than helping mitigate it — was incomplete however. Researchers didn't fully understand how isoprene and air pollution was linked.

A new study sheds some light on the matter. Researchers writing in the Proceedings of the National Academy of Sciences describe how isoprene, once exposed to the sun, reacts with nitrogen oxide produced by vehicle exhaust and coal plants to produce particulate matter. In other words how smog transforms a seemingly benign byproduct of forests into something that is damaging for people and the planet.

"Isoprene evolved to protect trees and plants, but because of the presence of nitrogen oxides, it is involved in producing this negative effect on health and the environment," said study co-author Jason Surratt of the University of North Carolina at Chapel Hill. "It’s the interaction between these natural and man-made emissions that produces this air pollution, smog and fine particulate matter – and now we know one reason for how it happens."

Fig. 1. Proposed mechanism for SOA formation from isoprene photooxidation in the presence of NOx.

The authors say the findings could help develop better models for predicting air quality and therefore issuing public health alerts when air pollution levels are high.

The results suggest that reducing smog may be twice as beneficial as previously thought since the effort could cut both local nitrogen oxide levels and the production of particulate matter from isoprene.

"We certainly can't cut down all the trees, but we can work on reducing these man-made emissions to cut down the production of fine particulate matter," said Surratt.

CITATION Ying-Hsuan Lin et al (2013). Epoxide as a precursor to secondary organic aerosol formation from isoprene photooxidation in the presence of nitrogen oxides. PNAS http://www.pnas.org/content/110/17/6718

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With addition of smog, trees may generate air pollution.