Air quality levels across the United States appear to be improving over the last decade. 

New composite NASA images show pollution levels have largely declined over the last ten years. The illustrations compare concentrations of nitrogen dioxide in the years 2005 to 2007 to readings taken from 2009 to 2011. 

Nitrogen dioxide is produced as a by-product of automobiles and coal-burning power plants. It is one of six common pollutants regulated by the Environmental Protection Agency (EPA). The chemical is a yellow-brown gas that has been known to contribute to respiratory problems. It can also contribute to the formation of other pollutants, particularly at ground level. 

Readings were taken by the Ozone Monitoring Instrument (OMI) aboard the Aura spacecraft, launched in 2004. The observatory is capable of measuring levels of ozone, aerosols and other gases in the Earth's atmosphere. 

After ten years studying pollution levels, mission managers believe they have enough data to draw accurate conclusions about pollution levels. 

New York City, the largest metropolitan area in the nation, saw a 32 percent drop in concentrations of nitrogen dioxide during the years studied. 

Population levels, as well as the number of cars on the road in the United States, has increased. Despite these trends, concentrations of nitrogen dioxide, which is often regarded as a marker for many other forms of pollution, is down. This improvement is due to technological advances, economic changes, and updated regulations, according to researchers. 

"While our air quality has certainly improved over the last few decades, there is still work to do - ozone and particulate matter are still problems," Bryan Duncan, an atmospheric scientist at the Goddard Space Flight Center, said

Chronic exposure to nitrogen dioxide can cause external damage to lungs, eyes, and skin. In high concentrations, the gas is toxic, and inhalation can be fatal. Long-term exposure is also "severely corrosive to the respiratory system," according to the chemical's material safety data sheet (MSDS). 

The EPA estimates that 142 million Americans still live in areas where levels of pollution are at unacceptable levels. The problem also extends to many other parts of the world, the agency warns. Legislators and government agencies have long depended on measurements of pollution taken from the ground to help formulate policy. New, space-based initiatives like the Aura mission, can help provide another, wider, view on the problem. 

Additional readings will be taken of pollution levels in  troposphere - the lowest layer of the atmosphere - by the airborne Discover-AQ program that will take off this summer. 

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