By PAT BRENNAN, February 21, 2012
The hazardous gases that make up Southern California's smoggy haze might stick together like tar, not dissolve inside droplets, a new study by UC Irvine scientists shows. And while that might sound like splitting hairs, it could have profound implications for how we understand smog and forecast its effects.
The finding, by UCI chemistry professor Barbara Finlayson-Pitts and a team of researchers, might help solve the mystery of "missing" smog particles -- the huge discrepancy known to pollution regulators between the amount of some kinds of smog particles forecast by computer models, and the far larger amount seen in reality.
"Particles are extremely important in air," Finlayson-Pitt said Tuesday. "And we know they cause increased mortality and health effects in the community. They reduce visibility, and they're a huge player in the whole climate-change game." But uncertainties in the modeling of the particles, known as secondary organic aerosols, can lead to uncertainties in climate forecasts, she said, as well as in decisions about controlling air pollution. "If you ask what is going to happen in the future, if you ask, what will be the effect of various control strategies, it's important that the models get the right answer for the right reason," Finlayson-Pitts said.
The science team, which included researchers from a lab at the U.S. Department of Energy and at Portland State University, used a 26-foot-long device called an aerosol flow tube to mingle pinene, an organic gas found in Pine-Sol as well as outdoors, with nitrogen oxides and ozone. All three can be components of smog.
The scientists simulated smog buildup, then tracked the results with instruments that measure particle mass and chemical composition. They found that the previous assumptions used in computer models of smog might be incorrect.
Most models had assumed that smog-forming gases coalesced into liquid droplets, allowing the gases to dissolve -- much like carbon dioxide is dissolved in soft drinks to create their distinctive "fizz." The "fizz," in fact, is the gas escaping when you open the soda can; similarly, Finlayson-Pitts said, the gas dissolved in the droplets was believed capable of being released again.
But the research team's lab work showed something far different. "When these low-volatility gases condense, and form a particle, they don't come back out," she said. "They check in, and they can't check out." The finding parallels recent results in other labs, she said. "Independent data from independent studies suggest these particles are not liquids," she said. "They're more like semi-solid, amorphous tar -- so viscous that once something gets in, it doesn't get back out." Scientists must now find out whether the effect is widespread in the environment.
"We're not claiming this is the whole answer," she said. "This is just an intriguing first result." If the result holds up, however, it could partially account for the large underestimate of smog-particle masses found in the computer models. If the gas "comes in but doesn't go out, what you expect is that you're going to get more mass per cubic meter of air particles than you would otherwise," she said. The study was published Tuedsay in the Proceedings of the National Academy of Sciences.