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HARD NOX: NOT ALL EMISSIONS ARE EQUAL
Size of emissions, location of power plants affect ozone production
When it comes to producing the ozone that's a major component of smog, not all nitrogen oxide emissions are equal. Size matters, it turns out, and so does location. These findings, from detailed measurements within exhaust plumes of coal-fired power plants, have major implications for air-pollution control strategies.
Ozone is produced in the troposphere when volatile organic compounds (VOCs) stew in sunlight with nitrogen oxides--NO and NO2, collectively known as NOx--from combustion sources. Controls on VOCs have had some effect on reducing smog levels. But NOx levels are also an important factor, so EPA has issued regulations limiting NOx emissions from about 400 U.S. power plants.
Atmospheric scientists now report that reducing NOx emissions at certain plants would get you more bang for the buck than if cuts were made at other plants [Science, 292, 719 (2001)]. Research chemist Thomas B. Ryerson at the National Oceanic & Atmospheric Administration's Aeronomy Laboratory in Boulder, Colo., and colleagues find that, all else being equal, power plant plumes with lower concentrations of NOx produce more ozone per unit of NOx emitted than plumes with higher NOx concentrations. And power plants located near forests that release significant quantities of hydrocarbons create proportionally more ozone than plants located away from such natural VOC sources.
"Theoretical calculations and photochemical models have been predicting this kind of behavior, but our aircraft measurements nailed down a lot of uncertainties," Ryerson tells C&EN. "We were surprised to see such a large dependency on these factors."
The findings are good news, Ryerson notes, in that they identify features that can be easily addressed in NOx control strategies. For example: "From the standpoint of air quality, forests aren't the best place for power plants."
|NIX ON NOX Capping emissions from several smaller power plants may be more effective than equivalent reductions from one large plant.
COURTESY OF TOM RYERSON
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