Researchers report trends in emissions of nitrogen oxides (NOx) over the past decade. NOx emissions, a precursor of ozone, have declined substantially in the United States over the last several decades as a result of air pollution regulations. However, technological improvements and changing human activity patterns can affect pollutant emissions in complex ways. Zhe Jiang and colleagues evaluated trends in NOx emissions in the continental United States from 2005 through 2015 using top-down estimates from satellite measurements, surface NO2 measurements, and bottom-up estimates from the Environmental Protection Agency's (EPA) emissions inventory data. Top-down estimates revealed a 76% slowdown in the rate of emissions reduction, from -7% per year during 2005-2009 to -1.7% per year during 2011-2015. Surface NO2 measurements showed a similar slowdown. This slowdown was inconsistent with the EPA's bottom-up emissions estimates, which predicted a 16% slowdown during the same time period. The authors found that much of the discrepancy could be explained by increasing relative contributions of industrial, area, and off-road vehicle emissions sources, decreasing relative contributions of on-road gasoline vehicles, and slower-than-expected reductions in on-road diesel emissions. NOx control the formation of surface ozone pollution, the reduction of which has also slowed during 2005-2015. The results suggest challenges to meeting future air quality standards for ozone, according to the authors.
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Article #18-01191: "Unexpected slowdown of US pollutant emission reduction in the past decade," by Zhe Jiang et al.
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Journal
Proceedings of the National Academy of Sciences