密歇根新闻广播 为什么环境改善进展缓慢（在线收听）

Air quality has gotten better in the U.S. over the last several decades.

But more recently, nitrogen oxide and carbon monoxide emissions have not been decreasing the way people expected.

Brian McDonald is an atmospheric scientist at the University of Colorado-Boulder, and an author of a study in the Proceedings of the National Academy of Sciences. He says data collected by ground monitoring and satellites have shown nitrogen oxide (NOx) emissions coming down quite quickly during the years 2000-2010.

"That's largely attributed to controlling emissions from power plants and passenger vehicles. And so, as you control those sources, there's less low-hanging fruit and so other sources become relatively more important," he says.

Those same monitors showed a significant slowdown in reduction of nitrogen oxides and carbon monoxide between 2011-2015.

Both carbon monoxide and nitrogen oxides contribute to ground-level ozone. That's bad for us to breathe in.

"Many areas of the U.S., especially urban areas, violate the National Ambient Air Quality standard under the Clean Air Act, for ozone," says McDonald. "One of the ways to reduce ozone is to reduce the pollutants that potentially contribute to that."

McDonald says there are multiple factors that could be leading to this slowdown.

"One is, as the power plant and automobile emissions have come down, the sources of NOx from things like industrial, residential and commercial boilers... there's also what are called off-road vehicles or diesel generators that are used in off the roadways-things like construction equipment… so those sources are becoming relatively more important," he says.

He says heavy duty diesel trucks are another factor.

"Diesel trucks have been getting cleaner, because there have been new regulations on trucks to put (on) after-treatment control technologies to reduce NOx," he says. "But I think they're still a maturing technology and so there's still gains to be made at controlling NOx from the trucks."