Credit: (Figure by Zhe Jiang, redrawn by Simmi Sinha, UCAR.)
Changes in Nitrogen Dioxide (NO2) levels. These satellite observations show the changes in NO2 during two recent periods. From 2005 to 2009, NO2 levels declined sharply across much of the United States, as shown in the areas of darkest blue. But in more recent years, NO2 has declined more slowly and even increased slightly in some areas (light red). The total column NO2 observations (1 unit = 1000 trillion molecules per square centimeter) are from the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite.