Nitrogen dioxide is one of the six common pollutants regulated by the US Environmental Protection Agency (EPA) to protect human health. Alone it can impact the respiratory system, but it also contributes to the formation of other pollutants including ground-level ozone and particulates, which also carry adverse health effects. The gas is produced primarily during the combustion of gasoline in vehicle engines and coal in power plants. It’s also a good proxy for the presence of air pollution in general.

Nitrogen dioxide pollution, averaged yearly from 2005-2011, has decreased across the United States. Image Credit: NASA Goddard’s Scientific Visualization Studio/T. Schindler.

Air pollution has decreased even though population and the number of cars on the roads have increased. The shift is the result of regulations, technology improvements and economic changes, scientists say.

The new NASA images also take a close up look at the Ohio River Valley, Northeast Corridor, and some populous US cities. They show how nitrogen dioxide concentrations during spring and summer months, averaged from 2005-2007, compare to the average from 2009-2011.

Measurements of nitrogen dioxide from OMI depict the concentration of the gas throughout a column of air in the troposphere, Earth’s lowest atmospheric layer. The images are color-coded: Blue and green denote lower concentrations and orange and red denote higher concentrations, ranging from 1x1015 to 5x1015 molecules per square centimeter, respectively.

Another ongoing effort by NASA to study air quality is Discover-AQ, a multi-year airborne mission flying this summer in Denver to learn more about how the wide range of air pollutants viewed from satellites relates to what's happening close to the ground where people live and breathe. The mission flew previously in 2011 over Baltimore, Maryland and Washington, DC; in 2013 over the San Joaquin Valley, California; and in 2013 over Houston, Texas.

Images were composed by NASA’s Scientific Visualization Studio based on data and input provided by atmospheric scientists Yasuko Yoshida, Lok Lamsal, and Bryan Duncan, all of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. More before-and-after images are available on flickr.