Sulphur dioxide levels in the lower atmosphere plummeted 90% during last year's blackout.© Photodisc

The power blackout that hit eastern North America last summer may have made it darker at night, but during the day it increased visibility by up to 40 kilometres. US researchers have found that air pollution plummeted far more than they would have expected as the power plants shut down.

The concentration of ozone was halved while sulphur dioxide levels fell by 90%, clearing the haze that usually sits over Pennsylvania. "We expected to see some decreases," says Russell Dickerson of the University of Maryland in College Park, "but the reduction was bigger than we expected."

The power failure that struck the electricity grids on 14 August 2003 was one of the largest in North American history. It forced over 100 power plants to stop operating in the northeast United States and southeast Canada. A day into the blackout, which left around 50 million people without electricity for nearly 30 hours, Dickerson and his team realised that the crisis presented an unprecedented opportunity to study the effects that power plants have on air pollution.

The Maryland team uses a light aircraft to measure the amounts of various pollutants in the lower atmosphere. On 15 August they flew it over Selinsgrove, an area of rural Pennsylvania within the blackout zone and downwind of several big power plants. On the same day, they made measurements over Cumberland, Maryland, in an area where power plants were still operating.

Most of the electricity for the US is generated in this part of the country, along the Ohio River. "These are among the largest, oldest, most polluting plants in the United States," says Dickerson. The air pollution they create ultimately ends up over Baltimore and New York.

Among the main pollutants from coal-fired power plants are nitrogen oxides and sulphur dioxide. Nitrogen oxides react with other compounds in the air to produce ozone, a prime component of smog and a cause of respiratory problems. Sulphur dioxide creates acid rain and can also be transformed into tiny particles of sulphate, which create atmospheric haze.

Cutting through the haze

The researchers have now published their results in Geophysical Research Letters¹. Compared with an August day in 2002, the concentration of ozone over Selinsgrove during the blackout was reduced by about half. At Cumberland, ozone levels were unaffected.

The change in sulphur dioxide was even more dramatic: it fell by over 90%, helping to reduce the obscuring effects of haze by around 70%. What particularly surprised the researchers is that these reductions were larger than would have been expected on the basis of the estimated emissions from power plants: the plants are thought to contribute only 20% of the nitrogen oxides and 70% of the sulphur dioxide in US air.

Dickerson suspects the results could show that pollutants injected from smokestacks a few hundred metres high have a bigger impact on air quality than those released at ground level, for example from motor vehicle exhausts.

But he adds that the shutdown of other industries during the blackout may have contributed to the drop in pollution. Most major industries have their own power generators, which are independent of the grid, but some factories were forced to stop operating during the power cut.

The good news, Dickerson says, is that many of the dirty old power plants on the Ohio River are scheduled for big reductions in nitrogen oxide and sulphur dioxide emissions over the next couple of years. It now seems this could be even more beneficial to air quality than had been anticipated. 

• References

  1. Marufu L. T., et al. Geophysical Research Letters, 31. L13106 (2004). | Article |