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| Image from the original article |
The images show an improvement in air-pollution conditions across parts of the northeastern United States, from Boston to Richmond, between 2005 (left image) and 2011 (right image).
Nitrogen dioxide is a light-brown gas that can contribute to serious health problems. The article describes it as a useful proxy for air pollution because it participates in the formation of other atmospheric pollutants associated with respiratory and cardiovascular problems.
Nitrogen dioxide is produced primarily through combustion, including gasoline burned in vehicle engines and coal burned in power plants.
In the United States, the article attributes the reduction in air pollution—including atmospheric nitrogen dioxide—despite population growth and an increasing number of vehicles on the roads to stricter regulations affecting automobile and power-generation industries, technological improvements over the preceding decade, and economic changes.