Ups and downs in the quest for clean air

By Andrew Grant, 12:44 PM August 7, 2014

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...

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