An aerosol is a colloid suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke In a suburb of smoggy Los Angeles, University of California experts are providing a likely answer to a sticky scientific problem. A growing body of research has shown that computer models used by federal regulators for decades significantly underestimate the quantity of organic aerosols, a major component of dangerous smog and the largest unknown in climate-change calculations. Organic aerosols may act as a transport vehicle of organics and other water insoluble compounds into the atmosphere.
The term smog, a contraction of smoke and fog, was introduced in 1905 by Dr. H.A. des Vœux to describe the mixture of soot, sulfuric acid, and other pollutants in emissions from coal furnaces in London. During the winter of 1952, London experienced a very severe smog event during a unusual cold spell that resulted in approximately 12,000 additional deaths in the city. This episode resulted in significant legislation to regulate and limit the underlying causes of the smog.
To help solve the organic aerosol mystery, the UCI team injected common ingredients of household cleansers and outdoor air into a long, metal aerosol flow tube and mixed them evenly through the shower head to form smog compounds. It had been assumed that the aerosols dissolve quickly into liquid droplets. But the researchers found that the pollutant gases get sucked deep into stubborn aerosol particles from which they cannot escape.
"They check in, and they don't check out. The material does not readily evaporate and may live longer and grow faster in total mass than previously thought," says chemistry professor and AirUCI director Barbara Finlayson-Pitts. "This is consistent with related studies showing that smog particles may be an extremely viscous tar." The organic aerosol seems similar to a small ball of sticky tar.
The UCI findings, published in the Proceedings of the National Academy of Sciences, could significantly affect air pollution control strategies. Models long used by the U.S. Environmental Protection Agency and others include far lower amounts of organic aerosols than have been detected in field studies.
"You can't have a lot of confidence in the predicted levels right now," says Perraud, lead author on the new paper. "It's extremely important, because if the models do a bad job of predicting particles, we may be underestimating the effects on the public."
UCI engineer and co-author Donald Dabdub, who designs sophisticated atmospheric simulations, agrees, calling the research results "critical to the modeling community."
"Currently, atmospheric assumptions do not account for a tar-like behavior inside particles," he says. "This will help us close the well-known gap between predictions and field observations of organic aerosols."
The UCI group worked with collaborators at Pacific Northwest National Laboratory, Portland State University and elsewhere. Alla Zelenyuk of PNNL traveled to Irvine from Washington with the unique, 900-pound instrument she designed known as SPLAT (a single particle mass spectrometer). Zelenyuk, who has flown over Alaska capturing soot from Asia, says she was glad to share the equipment for a few weeks to help ensure that dangerous pollution is properly understood and predicted.
While UCI monitors can record large numbers of particles, SPLAT can measure in real time the evaporation rate and other characteristics of a single 50-nanometer particle.
For further information: http://www.ucop.edu/sciencetoday/article/27205