Air quality in the United States has improved quite a bit, even over the last decade. However, while summertime pollution and haze are steadily declining, wintertime pollution levels in the eastern U.S. remain stagnant.
Now, researchers from the University of Washington have learned why, thanks to a new study published in the journal Proceedings of the National Academy of Sciences.
“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 US,” said Viral Shah, a corresponding author of the study. “We have pinpointed the chemical processes that explain the seasonal difference in response to emissions reductions.”
Both sulfate and nitrate particles can form smog and cause high levels of pollution. Sulfates come from sulfur dioxide and nitrates from nitrogen oxides.
In the United States, air quality regulations have lowered sulfur dioxide levels by 68 percent between 2007 and 2015, and nitrogen oxides levels also decreased by a third in that same time.
Despite levels decreasing, wintertime pollution levels are still higher than they are in the summer and the researchers discovered that winter climate conditions cancel out the effects of air quality regulations.
Data was collected for the study during a six-week observational survey of pollution plumes where the researchers flew through plumes in several major cities including New York City and Washington, D.C.
Focusing on pollution during the winter helped improve air quality models and show why winter pollution is unaffected by overall better air quality regulations.
“We now have a better tool to look at what is the best strategy to improve wintertime air quality on regional scales in the eastern US, and potentially other places, like Europe and Asia,” said Lyatt Jaeglé, the second author of the paper.
During the summer, sunlight stops some nitrates and sulfates from forming particles, however, during the winter cold temperatures and less sunlight results in an increase of sulfur dioxide and nitrogen oxides moving to the liquid phase and converting to sulfate and nitrate particles.
This is causing wintertime air pollution levels to decrease much slower than in the summer.
“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. “We need to make further reductions. Once the reductions become larger than the canceling effect, then winter will start behaving more like summer.”
The models in the study predict that wintertime air quality won’t catch up with summer until 2023, and the researchers say that the results call for more emissions reductions to improve pollution and air quality year-round.
Image Credit: Lyatt Jaeglé/University of Washington