On a hot summer day, plumes of nitric oxide and nitrogen dioxide--collectively known as NOx--billow forth from coal-fired power plants in the eastern U.S. Along with urban concentrations of their transportation counterparts--think the tailpipes of cars and trucks--such plumes can be readily detected from space via orbiting satellites. And such space-based data reveals that NOx emissions, from power plants at least, are on the decline.

Si-Wan Kim of NOAA's Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder and her colleagues collected satellite observations of such coal-burning power plants in the six states of the upper Midwest. By 2005, emissions from the largest such plants in the Ohio River valley had declined, dropping overall NOx in the atmosphere 38 percent below 1999 levels, according to the satellites' measurement of such plumes. This decline follows a recent federal law that requires cuts in such smog-forming emissions by setting an overall cap for the entire power sector and allowing individual power plants to trade pollution permits to meet it--a so-called cap-and-trade system.

Because such power plants account for roughly 25 percent of total U.S. emissions of the smog-forming gases, that should be good news for clean air in the eastern half of the country. But the satellites showed little decline in such pollution in the northeast U.S. "Power plants account for less than 20 percent of total NOx emissions in this region," Kim explains. Even a "50 percent reduction of power plant NOx emissions means less than 10 percent overall decrease in the total NOx budget." In other words, pollution spewed from tailpipes in the urban Northeast dwarfs--and swallows up--the reductions produced by cleaning up belching coal smokestacks in the Midwest.

The satellite data confirms measurements taken by so-called continuous emission monitors on the smokestacks themselves as well as data gathered by airplane, analyzed earlier this year by NOAA's Gregory Frost and others. Ground-level ozone, otherwise known as smog, has dropped in areas near the largest power plants by as much as 16 percent. Because smog has been linked to a variety of health effects, including heart disease, any drop--no matter where it is located or how widely it spreads--brings benefits. But, as the researchers write in the paper presenting the findings in the November 29 Geophysical Research Letters, "these findings suggest that further substantive reductions in eastern U.S. [ground-level] NOx levels will require decreases in mobile source NOx emissions."