Air Pollution Underestimated For A Decade Due to Escaped Vapors Hiding Behind Lab Walls
Environmentalists have been having problems reconciling their readings of air pollution in the laboratory with heir readings in the atmosphere for at least a decade due to a flaw in laboratory testing, according to a study published Monday in the Proceedings of the National Academy of Sciences' Online Early Edition. The flaw - which showed up on readings of a major component of auto emissions - would be laughable if the repercussions weren't so grave: Vapors formed from reactions of more volatile compounds have been hiding within laboratory chamber walls before chemists have had a chance to measure them, environmental engineers at the University of California at Davis and chemical engineers at the California Institute of Technology, Pasadena, wrote in their study. These vapors can condense and cause particles in the atmosphere to grow, by forming secondary organic aerosols (SOA), which play a key role in air pollution and smog. And the underestimation has been significant, co-author John H. Seinfeld, a chemical engineer at the California Institute of Technology, told the International Science Times.
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"Such vapor losses can lead to substantially underestimated SOA formation, by factors as much as 4," Dr. Seinfeld said. "Our results show that the effect of vapor wall loss on SOA yields can be substantial," by factors of approximately 1.1 to 4.2."
Reconciling laboratory measurements of SOAs with its measurements in the atmosphere is key not just to understanding air quality in the United States, but in the world at large. "By virtue of its ubiquitous prevalence in atmospheric particles, SOA has important impacts on climate and on urban and regional air quality," Dr. Seinfeld wrote. "Quantitative simulations have consistently under-predicted the global concentrations of secondary organic aerosols relative to observations." Those simulations have been going on for a decade, he added.
"Earlier efforts recognized the importance of accounting for direct loss of particles to the chamber walls," Christopher D. Cappa, an associate professor at the Department of Civil and Environmental Engineering, University of California at Davis told the International Science Times in an email interview. "There have been some efforts in the past to account for losses of vapors, although these only accounted for sticking of vapors to wall-deposited particles, and not to the chamber walls themselves as we have done in this study," Dr. Cappa said.
The study measured SOA formed from the atmospheric chemical degradation of the volatile compound, toluene. Though toluene by itself is not likely to cause environmental harm at levels normally found in the environment, the Office of Pollution Prevention and Toxics of the U.S. Environmental Protection Agency said in a 1994 report, the most recent exclusively on the subject that's available, it can contribute to the formation of photochemical smog when it reacts with other volatile organic carbon substances in air. (That finding, though, is based on underreported ascertainment of the chemical's levels.)
Toluene (also called methyl benzene) is a flammable liquid that occurs naturally in petroleum crude oil - the largest source of toluene. In 1994, the U.S. oil industry generating up to a billion pounds of the compound. Toluene-rich crude oil is added directly to gasoline and then emitted into the air through the cars' exhaust. As a component of high-octane fuel, it's also is emitted by jet planes. Other industries use it to manufacture explosives, spray paints, paint strippers, antifreeze, adhesives, printing ink, and even cosmetics and perfumes.
Once exposed to air, toluene evaporates and then breaks down into other chemicals. Because it is a liquid that does not bind well to soil, toluene can move through the ground with ease to enter groundwater.
The environmental and chemical engineers conducted a series of experiments at Caltech in a 24-cubic meter environmental chamber with walls made of thin Teflon. In these experiments, toluene is added to the chamber and then exposed to light from black lights, which leads to the chemical breakdown of the toluene to produce other vapors that can ultimately condense to form SOA particles if they are not first lost to the walls."
The environmental and chemical engineers conducted a series of experiments at Caltech in a 24-cubic meter environmental chamber with walls made of thin Teflon, where they exposed toluene to black lights, leading to its chemical breakdown to produce other vapors that condensed to form SOA particles - if they were not first lost to the walls, that is. The chemical engineers then counted the number, size and distribution of the toluene particles.
The hiding-within-the walls problem has contributed to its under-prediction of SOA in climate and air quality models. SOA impacts air quality and climate and makes up a major fraction of particulate matter in the atmosphere. Yet SOA concentrations have been significantly underestimated in regional air quality models.
The next step, according to Seinfeld, is the assessment for other SOA precursors to be tested, to see to what extent they're being absorbed by chamber walls so that climate scientists can "make "better predictions for the future."
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