Study Sheds Light on Wintertime Air Pollution

The processes that cause ozone pollution in the summer can also cause the formation of wintertime air pollution, according to a new research from scientists at the University of Colorado Boulder and NOAA, in collaboration with the University of Utah. The team’s surprising finding shows that in the U.S. West and elsewhere, some efforts to decrease harmful wintertime air pollution could fail.

In particular, targeting nitrogen oxides released by cars and power plants could initially actually increase detrimental air pollution, the scientists stated in their new paper, published recently in the journal Geophysical Research Letters.

This insight arose from some of the most extensive measurements of the chemistry behind our wintertime pollution problem.

John Lin, Professor of Atmospheric Sciences and Study Co-Author, University of Utah

“This is contrary to what is typically assumed and suggests a new way to mitigate this type of pollution in Salt Lake City, Denver and beyond,” said Caroline Womack, a CIRES scientist employed in the NOAA Earth System Research Laboratory and the study’s lead author.

Protocols and cleaner technologies have progressively enhanced air quality in the US. But still during the winter, valleys in Western states still endure high levels of particulate matter (PM_2.5), or microscopic droplets suspended in air. In Utah’s urban Salt Lake Valley, wintertime levels of PM2.5 surpass the country’s air quality standards an average of 18 days per year. Denver frequently undergoes the same problem in winter, when brown clouds cover the city.

A big component of the Salt Lake Valley and Denver PM_2.5 pollution is ammonium nitrate aerosol, which develops from emissions of volatile organic compounds (VOCs), nitrogen oxides, and ammonia. Those reactions occur during winter temperature inversions, when warm air aloft captures cold air below, concentrating pollutants.

To fight wintertime PM_2.5 pollution, researchers primarily needed a comprehensive understanding of the chemical processes that cause it. So in 2017, scientists from the Cooperative Institute for Research in Environmental Sciences (CIRES) and NOAA joined with the University of Utah, the Utah Department of Environmental Quality, and others to measure PM_2.5 and its precursor emissions at a number of ground sites in and around the Salt Lake Valley, including a site on top of the College of Mines and Earth Sciences’ William Browning Building on the U campus. Using the NOAA Twin Otter—a small, instrumented research airplane—the researchers also gathered air samples from all over the pollution layer in the critical altitude region where particulate matter forms.

Founded on the observations from the field survey, Womack, Lin, and their colleagues learned that ozone and ammonium nitrate aerosol pollution are closely related, connected by the strangely named parameter “total odd oxygen.” Since the same chemical processes that cause ozone pollution in the summer create ammonium nitrate pollution in winter, policies that have successfully controlled ozone could also curtail production of ammonium nitrate.

In western valleys having high levels of ammonium nitrate aerosol, mitigation attempts have mostly focused first on controlling one component of the pollution: nitrogen oxides from burning fossil fuels. The scientists discovered this approach may, in fact, raise ammonium nitrate pollution, at least at the outset. A potentially more operative way to minimize PM_2.5 pollution would be to restrict VOCs, according to the new evaluation.

“No one has looked at air pollution in winter before in this way. Our findings could hold true in other areas with severe winter aerosol pollution, including mountain valleys across the US West and urban areas in East Asia, and Europe,” said Womack. PM_2.5 pollution is a big cause of premature death globally—and in addition to negatively impacting human health, PM_2.5 also disturbs visibility, agricultural yields, and perhaps Earth’s climate.

Going forward, the research team will be involved in a follow-on study under planning that will explore wintertime air pollution across the whole U.S. West.

Watch a time-lapse of one of the Twin Otter’s flights around the Salt Lake Valley