Air quality has largely improved over the past several decades in the United States, but those gains have slowed substantially since 2011, an international study said Monday.

The report in the Proceedings of the National Academy of Sciences found a stark difference between estimates and reality when it came to nitrogen oxides and carbon monoxide, which contribute to ground-level ozone, or smog, from 2011-2015.

"We were surprised by the discrepancy between the estimates of emissions and the actual measurements of pollutants in the atmosphere," said lead author Zhe Jiang, who conducted the research while on a postdoctoral fellowship at the US National Center for Atmospheric Research and is now with the University of Science and Technology of China.

"These results show that meeting future air quality standards for ozone pollution will be more challenging than previously thought."

The US Environmental Protection Agency made ozone standards stricter in 2015, under former US president Barack Obama.

The study used both ground-based measurements and satellite data, and compared them to EPA estimates on the level of pollutants released by vehicles, factories and other sources.

"Emission reductions slowed down dramatically in the five-year period from 2011 to 2015 compared to 2005 to 2009," said the report.

Nitrogen oxide concentrations dropped by seven percent yearly from 2005 to 2009, but declined by just 1.7 percent yearly from 2011 to 2015, representing a 76 percent slowdown.

The EPA's estimates put the slowdown at only 16 percent during the same period.

Carbon monoxide levels -- which largely come from vehicle exhaust -- also declined more slowly in recent years.

Reasons for the discrepancy may include rising emissions from industrial and residential sources, commercial boilers and off-road vehicles, along with slower-than-expected reductions in emissions by heavy-duty diesel trucks, according to the report.

Researchers found several signs that the problem was US-based and not due to emissions from China, which is the world's largest polluter, including indications that the slowdown in improved air pollution levels "was particularly pronounced in the eastern United States."

Pollutants have been dropping since passage of the 1970 Clean Air Act, which led to new technologies that cut emissions, including catalytic converters on automobiles and low nitrogen oxide burners at power plants.

In early April, President Donald Trump's administration announced it would roll back Obama-era pollution and fuel efficiency rules for cars and light trucks, saying they were too stringent.

Trump withdrew the United States from the global Paris Agreement on climate change and has defended coal-fired power plants, which contribute to global warming.

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