Wednesday, April 30, 2014

Air contaminants vanishing into walls hamper lab experiments

Models of particle pollution have
been inconsistent, researchers say.
Models trying to predict airborne particle pollutants have been inconsistent over the past decade.

Some airborne particles can vanish into the walls of laboratory chambers, which could explain discrepancies in air pollution experiments. The findings of a new study suggest that models of particle pollution have been off for about a decade.

For their tests, researchers evaporated toluene, an ingredient of car exhaust that can form secondary organic aerosols, in a Teflon chamber.

Unlike previous researchers, they added “seed particles” such as ammonium sulfate. Adding these particles increases the aerosols that form when toluene vaporizes.

When there are no seed particles, the vapors end up sticking to or dissolving into the chamber walls, said Chris Cappa, an assistant professor at the University of California, Davis, and co-author of the study published in the Proceedings of the National Academies of Science.

“The walls of these chambers act as a sponge for the vapors,” Cappa said.

The findings of a new study suggest that models of particle pollution have been off for about a decade.

Cappa said that previous lab studies have underestimated secondary organic aerosol formation by about two to four times.

These aerosols, which are a byproduct of volatile organic compounds from vehicles and the burning of fossil fuels, are a major part of fine particle pollution.

Known as PM2.5, these particles can penetrate people’s lungs and disrupt their heart.

The discovery could explain why models that have tried to predict particulate levels from emissions inventories have not jibed with levels actually measured in the air.

“Accounting for such losses has the clear potential to bring model predictions and observations of organic aerosol levels into much closer agreement,” the authors wrote.

Laboratory models are often used to estimate regional air quality. And in the past 20 years, scientists have incorporated aerosols into climate models, too, because they can scatter or absorb radiation from the sun.

Aerosols that scatter sunlight would have a cooling effect, while those that absorb it have a warming effect.

The study was limited in that only one compound was tested. However, Cappa said the results should hold true for other aerosol precursors and the researchers plan on testing more.

The experiment doesn’t mean that regional air pollution is underestimated because scientists also use observations from the atmosphere.

“It’s not quite fair to say we’ve been underestimating impact of air pollution, but from a modeling standpoint we’ve been limited in our ability to properly set up strategies for improving air quality,” Cappa said.

Source: EHN

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