The blackout that left 50 million North Americans without power in August 2003 had an unexpected benefit - the air became cleaner.
As power plants were turned down in south-east Canada and the north-east and mid-west US, levels of pollutants fell, says meteorologist Russell Dickerson.
His team from the University of Maryland in College Park flew an aircraft over the middle of the blackout zone 24 hours after the power had gone down. "This was a unique opportunity to explore what would happen to air quality if power station emissions were reduced," he says.
The team compared pollution levels over Pennsylvania with those on a similar hot, sunny day the year before. While there was no significant difference in levels of pollutants associated solely with traffic, other pollutants linked with power stations fell dramatically.
Sulphur dioxide levels decreased by 90 per cent, there was around half the amount of ozone and visibility increased by 40 kilometres.
Oil, coal and gas-burning power plants emit sulphur dioxide, nitrogen oxides and particulate matter into the atmosphere as waste, where they go on to form acid rain and smog that can drift hundreds of kilometres from the source.
Prevailing winds in Pennsylvania, for example, tend to blow whatever is in the air towards nearby cities on the east coast.
Measurements from the US Environmental Protection Agency show that during the blackout, emissions from affected plants dropped by two-thirds or more. However, measuring the effect of this on air quality was difficult because many ground-based stations were also blacked out, being powered by electricity.
The team's results should allow researchers to improve the sophisticated models already used for tracking atmospheric pollution from different sources. It is also likely to provide ammunition for environmental groups that are calling for stricter regulation of emissions from power plants.
The research was presented at the spring meeting of the American Geophysical Union in Montreal, Canada