Summers Less Smoggy Now, But Winter Air Hasn't Improved

Air quality improvements in the United States have been significant but largely limited to summer. Now, researchers say they know why.

For decades, summer air was the worst of the year, replete with haze-containing particles that cause asthma, lung cancer and other illnesses.

But overall lower levels of emissions from power plants and vehicles have paid off in the hot weather, the study authors said.

"In the past 10 years or so, the summer air pollution levels have decreased rapidly, whereas the winter air pollution levels have not. Air quality in summer is now almost the same as in winter in the eastern U.S.," said study corresponding author Viral Shah.

"We have pinpointed the chemical processes that explain the seasonal difference in response to emissions reductions," he said in a University of Washington news release. Shah did the work as part of his doctorate in atmospheric sciences.

The researchers' analysis of 2015 data showed that particles that form smog follow different pathways in the winter.

The findings suggest that further reductions in emissions of sulfur and nitrogen oxides are needed to improve wintertime air quality in the eastern United States and other cold regions.

"It's not that the reductions aren't working. It's just that the reductions have a canceling effect, and the canceling effect has a set strength," said Shah, who is now a postdoctoral researcher at Harvard University.

"We need to make further reductions. Once the reductions become larger than the canceling effect, then winter will start behaving more like summer," Shah explained.

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