The downtown Dallas, Texas (USA) skyline from a levee along the Trinity River. Facing southeast. Credit: drumguy8800/Wikipedia

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. "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.

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

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

The study will be published next week in the Proceedings of the National Academy of Sciences. 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.

Revealing the slowdown

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 dramatically in the five-year period from 2011 to 2015 compared to 2005 to 2009. Whereas nitrogen oxide concentrations dropped by 7 percent yearly from 2005 to 2009, they declined by just 1.7 percent yearly from 2011 to 2015—a 76 percent slowdown. Those findings contrast with EPA emission inventories, which put the slowdown at only 16 percent 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," said 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.

"The top-down satellite measurements and the inventories provide complementary data that will enable us to get better estimates of the emission sources," McDonald said. "It will be useful to learn more about why the discrepancies exist and why the trend toward better air quality is slowing down."

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More information: Zhe Jiang el al., "Unexpected slowdown of US pollutant emission reduction in the past decade," PNAS (2018). Journal information: Proceedings of the National Academy of Sciences Zhe Jiang el al., "Unexpected slowdown of US pollutant emission reduction in the past decade,"(2018). www.pnas.org/cgi/doi/10.1073/pnas.1801191115