Unexpected slowdown of US pollutant emission reduction in the past decade

Proc Natl Acad Sci U S A. 2018 May 15;115(20):5099-5104. doi: 10.1073/pnas.1801191115. Epub 2018 Apr 30.


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 (NO x ) 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 NO x 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.

Keywords: decadal scale variation; emission regulations; nitrogen oxides.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Air Pollutants / analysis*
  • Air Pollution / analysis*
  • Carbon Monoxide / analysis*
  • Environmental Monitoring / standards*
  • Gasoline
  • Humans
  • Nitrogen Oxides / analysis*
  • Particulate Matter / analysis*
  • United States
  • Vehicle Emissions / analysis*


  • Air Pollutants
  • Gasoline
  • Nitrogen Oxides
  • Particulate Matter
  • Vehicle Emissions
  • Carbon Monoxide