Following decades of progress in air quality, reductions in pollutant emission in the United States have slowed dramatically since 2011, an international study said Monday.
A significant slowdown in decreasing U.S. emissions of nitrogen oxides and carbon monoxide for 2011-2015 is showed in the Proceedings of the National Academy of Sciences, which says: "This observed slowdown in emission reductions is significantly different from the trend expected."
Nitrogen oxides and carbon monoxide contribute to ground-level ozone, or smog, a pollutant that is harmful to human health and the environment.
The study used both ground-based measurements and satellite data, and compared them with the estimates of the U.S. Environmental Protection Agency (EPA) on the level of pollutants released by vehicles, factories and other sources.
Nitrogen oxide concentrations dropped by 7 percent yearly from 2005 to 2009, but fell by only 1.7 percent annually from 2011 to 2015, indicating a 76-percent slowdown, much bigger than the EPA's estimate of 16 percent, according to the study.
The discrepancy between observations and EPA's estimate can be explained by the growing emissions from industrial and residential sources, commercial boilers and off-road vehicles, as well as slower-than-expected decreases in on-road diesel emissions.