2018 News & Events

U.S. Gains in Air Pollution are Slowing Down

30 April 2018
adapted slightly from the story by CIRES Communications

I5 northbound
Los Angeles region traffic congestion.

A new study indicates challenges of meeting ozone goals. After decades of progress in cleaning up air quality, U.S. improvements for two key air pollutants have slowed significantly in recent years, new research concludes. The unexpected finding indicates that it may be more difficult than previously realized for the nation to achieve its goal of decreased ozone pollution, scientists said.

"Although our air is healthier than it used to be in the 80s and 90s, air quality in the U.S. is not progressing as quickly as we thought," said National Center for Atmospheric Research (NCAR) scientist Helen Worden, a co-author of the paper published in the Proceedings of the National Academy of Sciences. "The gains are starting to slow down."

The study, by an international team of researchers, analyzed extensive satellite and ground-based measurements of nitrogen oxides and carbon monoxide. They found that levels of pollutants that can contribute to the formation of ground-level ozone, or smog, have failed to continue a fairly steady decline as estimated by the U.S. Environmental Protection Agency.

CSD's Brian McDonald is a co-author on this study. More information from the NCAR UCAR News Release.

Jiang, Z., B.C. McDonald, H. Worden, J.R. Worden, K. Miyazaki, Z. Qu, D.K. Henze, D.B.A. Jones, A.F. Arellano, E.V. Fischer, L. Zhu, and K.F. Boersma, Unexpected slowdown of US pollutant emission reduction in the past decade, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1801191115, 2018.


Ground and satellite observations show that air pollution regulations in the United States (US) have resulted in substantial reductions in emissions and corresponding improvements in air quality over the last several decades. However, large uncertainties remain in evaluating how recent regulations affect different emission sectors and pollutant trends. Here we show a significant slowdown in decreasing US emissions of nitrogen oxides (NOx) and carbon monoxide (CO) for 2011–2015 using satellite and surface measurements. This observed slowdown in emission reductions is significantly different from the trend expected using US Environmental Protection Agency (EPA) bottom-up inventories and impedes compliance with local and federal agency air-quality goals. We find that the difference between observations and EPA's NOx emission estimates could be explained by: (i) growing relative contributions of industrial, area, and off-road sources, (ii) decreasing relative contributions of on-road gasoline, and (iii) slower than expected decreases in on-road diesel emissions.