Ups and downs in the quest for clean air

Newly released maps reveal that U.S. air quality has markedly improved over the last decade. The evidence comes from measurements of nitrogen dioxide (NO₂ ), a pollutant emitted by vehicles and coal power plants. Yet as the United States cleans up, rapidly industrializing cities in Asia, the Middle East and Africa are spewing more and more of the yellow-brown gas.

NO₂ may not have the notoriety of carbon monoxide or lead, but it causes respiratory problems and combines with other pollutants to form ground-level ozone, a major part of smog. “You wouldn’t get unhealthy levels of ozone in cities without nitrogen dioxide,” says Bryan Duncan, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.