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Study finds significant slowdown in decrease of NOx emissions in US; implications for air quality management

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, according to a new study by an international team of researchers.

In an open-access paper published in Proceedings of the National Academy of Sciences (PNAS), the researchers suggested that 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. The findings have implications for air quality management.

Slowdown in air quality improvement. Estimates by the Environmental Protection Agency indicate that levels of nitrogen oxides have continued their decline over the past decade. But top-down emission estimates using satellite measurements show that the decline has slowed significantly in recent years. (Figure by Zhe Jiang, redrawn by Simmi Sinha, UCAR. Click to enlarge.

The 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 US Environmental Protection Agency.

We were surprised by the discrepancy between the estimates of emissions and the actual measurements of pollutants in the atmosphere. These results show that meeting future air quality standards for ozone pollution will be more challenging than previously thought.

—Zhe Jiang, lead author

Jiang, who conducted much of the research during a postdoctoral fellowship at NCAR, is now with the University of Science and Technology of China.

Nitrogen oxides and carbon monoxide contribute to the formation of ground-level ozone, a pollutant that is harmful to human health and the environment. Levels of the pollutants have declined significantly since passage of the 1970 Clean Air Act, which spurred development of emission-reducing technologies, such as catalytic converters on automobiles and low nitrogen oxide burners at power plants.

A number of cities and outlying areas in the United States, however, remain out of compliance with EPA standards for ozone, which the agency made more stringent in 2015.

EPA emission estimates are based on monitored readings or engineering calculations of pollutants emitted by vehicles, factories, or other sources.

To obtain a fuller picture of national pollution levels, Jiang and his co-authors turned to satellite instruments that measure levels of nitrogen oxides and carbon monoxide. They analyzed these atmospheric observations with advanced computer simulations and statistical analyses, both to quantify pollutant concentrations and to map their concentrations across the contiguous United States. They then corroborated their findings with observations from air quality monitoring stations that measure local pollution levels.

The results showed that emission reductions slowed down significantly in the five-year period from 2011 to 2015 compared to 2005 to 2009. Whereas nitrogen oxide concentrations dropped by 7% yearly from 2005 to 2009, they declined by just 1.7% yearly from 2011 to 2015—a 76% slowdown. Those findings contrast with EPA emission inventories, which put the slowdown at only 16% during the same time period.

Similarly, the study showed that carbon monoxide levels have declined much more slowly in recent years.

The research team originally thought that emissions from Asia could be playing a role, but this was not supported by the data. The measurements showed that the slowdown in improved air pollution levels was particularly pronounced in the Eastern United States, one of several signs that the pollutants were not coming in from overseas.

The authors concluded that some of the reasons for the discrepancy for nitrogen oxides may be:

  • The decreasing relative contributions of gasoline cars to the pollutant, due to the ongoing effectiveness of three-way catalytic converters;

  • The increasing relative emissions of nitrogen oxides from such sources as industrial, residential, and commercial boilers and off-road vehicles; and

  • Slower-than-expected reductions in emissions by heavy-duty diesel trucks that have newer (and still maturing) catalytic converter technologies.

The study concluded that the slowdown in carbon monoxide, which is largely emitted by cars, is likely due to the large gains that have already been achieved by equipping cars with three-way catalytic converters.

As you become effective at controlling emissions from cars and power plants, the other sources become more important and there’s less information about them.

—co-author Brian McDonald, a scientist with the National Oceanic and Atmospheric Administration and the Cooperative Institute for Research in Environmental Sciences

The authors said that follow-up research, combining EPA inventories with a new generation of increasingly sophisticated satellite instruments, would lead to a more detailed understanding about how pollution is changing in response to emission controls.

This work highlights the importance of satellite and model inversion technologies to monitor changes in pollutant emissions and interpret the effects of regulations and economic activities.

—Jiang et al.

The research was funded primarily by NASA, the National Oceanic and Atmospheric Administration, the University of Colorado Boulder, and the National Science Foundation, which sponsors NCAR.


  • Zhe Jiang, Brian C. McDonald, Helen Worden, John R. Worden, Kazuyuki Miyazaki, Zhen Qu, Daven K. Henze, Dylan B. A. Jones, Avelino F. Arellano, Emily V. Fischer, Liye Zhu, K. Folkert Boersma (2018) “Unexpected slowdown of US pollutant emission reduction in the past decade” Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1801191115



You in the US must now (again) blame the diesel cars and VW. Er... I just realized that VW does not sell diesel cars anymore in the USA.


Plenty of diesel delivery trucks and big rigs to convert to PHEV/BEV.


With more and more vehicles and heavy gas guzzling SUVs and Pick Ups on the roads, air pollution-GHG and particles emission will increase.

Ford announced that it will stop making and selling small (more efficient cars) in North America (to better the profit bottom line with large SUVs and Pick Ups)?

The majority may have to buy cleaner electrified and more efficient ICEVs from Europe and Asia. (we have been using better built more efficient vehicles by Toyota for the last 30+ years and will continue to do so).


I don't see how this study sheds any new light on this topic.

Based on EPA's monitoring data, which are publicly available, the rate of decline in ambient NO2 and CO levels has been known to be slowing. Even then, ambient NO2 levels have declined by 20% between 2010 and 2016, while ambient CO levels have declined by 14%.

Although no monitoring data are available, anthropogenic VOC emissions have declined by 10% between 2010 and 2016 according to EPA.


Not so sure that EPA's stats are exact.
The latest switch to larger heavier gas guzzling SUVs/Pick Ups will increase emissions.
Satellite measurements may be more accurate on a long term/wider basis?


Surface-based monitors are mostly concentrated in urban areas, so satellite measurements may provide much more data in rural/non-urban areas, but determination of attainment with the U.S. NAAQS is based solely on the surface-based monitoring data last I knew.

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