Aircraft sampling in the 24 hours following the blackout found a 90 percent drop in sulfur dioxide and a 50 percent cut in ozone levels, while visibility increased by more than 25 miles, University of Maryland researchers report.
Maryland's top environmental official said the results prove what state officials have long argued — the region suffers from air pollution created elsewhere.
"It's not a model, not a meteorologist's dream. The pollution cuts actually happened," said Kendl P. Philbrick, Maryland's secretary of the environment.
The measurements were taken as part of a 13-year effort to track air pollution in the Baltimore-Washington area.
From May to September, University of Maryland researchers take measurements twice a day aboard a specially equipped airplane.
The blackout, which started with transmission lines tripping off in northern Ohio, affected all or part of eight states stretching from Michigan to New York, as well as parts of Canada. Michigan was hard hit, with southeast Michigan residents going nearly two entire days without power.
On Aug. 15, the researchers realized the blackout the night before provided a rare opportunity — to compare pollution in an area downwind of idled power plants with pollution downwind of unaffected plants.
They took air samples over central Pennsylvania — in the path of air blowing in from the blacked-out region — and compared them with air samples they had taken that day over Virginia and western Maryland.
They found sulfur dioxide levels measured over Pennsylvania were 90 percent lower and ozone levels were 50 percent lower.
"We had ideas of what the power plants were contributing to regional air quality," said Brett F. Taubman, a graduate student in chemistry who was aboard the plane that day. "This was the first opportunity to directly measure a large scale-back like this. And the results were far greater than we ever imagined."
While pollutants most closely linked to power plants were lower, soot and carbon monoxide — which are more closely associated with automobile pollution — remained steady.
The University of Maryland study is to be published in the next issue of Geophysical Research Letters.