NASA has released some encouraging images showing a marked decline in air pollution over much of the eastern U.S., including Washington, D.C. and the I-95 corridor since 2005.

The images show the change in concentration of nitrogen dioxide, a pollutant linked to adverse effects on the respiratory system. NASA provides this helpful background information:

Nitrogen dioxide is one of the six common pollutants regulated by the U.S. 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.

Here’s an animation portraying the decline in nitrogen dioxide concentrations over the Northeast between 2005 and 2011:

This animation is a zoomed-in version of the one above:

Here’s a close-up view of the I-95 corridor from Washington, D.C. to New York City, illustrating nitrogen dioxide concentrations averaged over 2005-2007 (left) vs. 2009-2011 (right)

Philadelphia has experienced a 26 percent decrease in nitrogen dioxide between the 2005-2007 and 2009-2011 period, NASA calculates.

Declines in air pollution were also noted in the western U.S, including Denver, Los Angeles and San Francisco.

All of the data used to documents these air quality improvements was attained from NASA’s Ozone Monitoring Instrument (OMI) aboard its Aura satellite.

The data trends measured from space mirror improvements in air quality observed on the ground. In recent years, Washington, D.C. has seen a notable drop in the number of ozone exceedance days, in which air quality falls below federal health standards.

Environmental scientists say additional reductions in air pollution are needed.

“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,” says Bryan Duncan, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.