NASA released satellite images of various U.S. cities that showed marked improvement of air quality over the past decade.

The satellite images were taken by the Ozone Monitoring Instrument (OMI) onboard the NASA aura. OMI has been differentiate different aerosol types such as smoke, dust and sulfates.

NASA scientists reviewed the images gathered by OMI and found a significant reduction in the presence of nitrogen dioxide (NO2) above the U.S. cities. Nitrogen dioxide is a nasty-smelling gas emitted by cars, trucks and buses, power plants and certain kinds of equipment. Overexposure to NO2 could lead to adverse effects in the respiratory system manifested by wheezing, coughing, colds, flu and bronchitis. According to the U.S. Environmental Protection Agency (EPA), NO2 should be limited to an average of 0.053 parts per million (53 ppb) per year for it to be considered healthy.

The analysts were surprised to find that despite the increasing numbers of vehicles on the road, NO2 levels decreased in most United States cities. They compared the satellite images taken in 2005-2007 to those snapped in 2009-2011. Blue and green indicated low NO2 concentration, while orange and red indicate a high concentration.

Images of New York City showed a 32 percent reduction in NO2 levels, despite a high number of people living in the area. NYC is one the most populous states in the country that researchers often link to higher air pollution. Atlanta, meanwhile, was able to reduce air pollution levels by 42 percent due to technology enhancements in the power plants. Denver's air pollution level dropped by 22 percent during the same period.

Measuring NO2 levels is a critical step toward the development of emission-controlling policies, which could improve air quality in the U.S. overall.

"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," said Bryan Duncan, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.