Unexpected slowdown of US pollutant emission reduction in the past decade

Emissions of nitrogen oxides (NOₓ) have a large impact on air quality and climate change as precursors in the formation of ozone and secondary aerosols. The authors find that NOₓ emissions have not been decreasing as expected in recent years (2011–2015) when comparing top-down estimates from satellites and surface NO₂ measurements to the trends predicted from the US Environmental Protection Agency’s emission inventory data. The discrepancy can be explained by the growing relative contribution of industrial, area, and off-road mobile sources of emissions, decreasing relative contribution of on-road gasoline vehicles, and slower than expected decreases in on-road diesel NOₓ emissions, with implications for air-quality management.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 the authors 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. The authors 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.


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  • Accession Number: 01670331
  • Record Type: Publication
  • Files: TRIS
  • Created Date: May 15 2018 2:21PM