https://phys.org/news/2018-07-eastern-air-pollution-stagnant-winter.html

"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 corresponding author Viral Shah, who did the work as part of his UW doctorate in atmospheric sciences. "We have pinpointed the chemical processes that explain the seasonal difference in response to emissions reductions."

The study, published the week of July 23 in the Proceedings of the National Academy of Sciences, shows that the particles follow different pathways in the winter.