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Highway Barriers Stifle Sound, Sight, and Soot

December 15, 2009

Barrier and instruments NOAA scientists used in the highway study.

High resolution (credit: NOAA)

Barrier and instruments NOAA scientists used in the highway study.

Highway barriers erected along roadways to block the sound and sight of traffic for the adjoining neighborhoods may also be reducing the amount of pollutants, such as soot from diesel exhaust, reaching area residents.

In a study by NOAA and the U.S. Environmental Protection Agency, researchers released harmless “tracers” – gases that act as a stand-in for vehicle-related toxic pollutants such as carbon monoxide and heavy metals, and volatile organic compounds such as benzene -- so scientists can “trace” their movement through the air.

The study, the first to systematically and comprehensively investigate the role of atmospheric stability in real world conditions on the movement of pollutants near highway barriers, is now online and will appear in  a January 2010 print edition of Atmospheric Environment.

“While the barriers block the noise and view of hundreds of vehicles whizzing by, we found that they also reduce high concentrations of pollutants from those vehicles by lifting and channeling them away from the adjoining areas, often a residential area,” said Dennis Finn, lead author and a research meteorologist at NOAA’s Air Resources Laboratory in Idaho Falls, Idaho.

A large body of research shows a variety of human health effects such as respiratory disease, cardiovascular illness, and cancer in individuals living or working near heavily trafficked roadways. It is difficult to measure accurately and isolate the effect of highway barriers on the transport and dispersion of the pollutants that cause these health effects in real-world environments with a wide range of atmospheric conditions.

Researchers were able to conduct tracer studies in unstable, neutral and stable atmospheric conditions in tightly controlled circumstances, to quantify the effects of roadside barriers on pollutant dispersion. Atmospheric stability is a measure of top-to-bottom mixing in the atmosphere. The atmosphere is stable when the coldest air is at ground level. When there is no significant difference between temperatures in the top and bottom layers, conditions are neutral. Like a pan of boiling water, an unstable atmosphere roils as warm air rises from ground level.

“We also found that the barriers tended to trap pollutants in the area of the roadway itself, especially at night in low wind speed conditions,” Finn said. “The amount of pollutants was much higher on roadway areas flanked by barriers than in areas without them.”

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