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.