A New View of Wintertime Air Pollution

Wintertime smog is a severe air quality issue affecting urban regions around the world. It can be particularly acute in mountain basins of the western United States when temperature inversions trap pollution near the surface.

Now, a new study led by CIRES and NOAA researchers has found that the same processes which generate summertime ozone pollution can also trigger the formation of smoggy haze in winter.  And in an unexpected finding, their results suggest that a standard control strategy used to reduce smog could backfire in these regions. The paper was published today in the AGU journal Geophysical Research Letters.

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

Since the 1970s, pollution control regulations and cleaner technologies have steadily improved air quality in the United States. Yet some valleys in the Intermountain West still experience high levels of particulate matter (PM 2.5 ) - microscopic droplets suspended in air - during winter. In Utah’s urban Salt Lake Valley, wintertime levels of PM 2.5 exceed national air quality standards an average of 18 days per year.

Denver often has the same problem. On March 6, Denver’s Air Quality Index (AQI) reached a high of 154 at one monitoring site, exceeding the threshold for unhealthy levels of pollution on the scale set by the Environmental Protection Agency.

The problem starts when sunlight triggers chemical reactions in the soup of volatile organic carbons ( or VOCs) and nitrous oxides (NOx) that are concentrated in mountain valleys during temperature inversions, when a cap of warm air traps cold, polluted air below. In the presence of ammonia, a common fertilizer ingredient, these combine to form ammonium nitrate aerosols, which are a major component of the fine-particle or PM 2.5 pollution that develops in mountain basins.

PM 2.5 pollution is a major cause of premature death worldwide. It also affects agricultural yields, visibility, and possibly Earth’s climate.

To combat wintertime PM 2.5 pollution, scientists first needed a detailed understanding of the chemical processes that produce it. So in 2017, CIRES and NOAA researchers partnered with the University of Utah, the Utah Department of Environmental Quality, and others to measure PM 2.5 and its precursor emissions at several ground sites in and around the Salt Lake Valley.

Using the NOAA Twin Otter—a small, instrumented research airplane—the team also collected air samples throughout the pollution layer in the altitudes where particulate matter forms.

Based on the observations from the field campaign, Womack and her colleagues found that ozone and ammonium nitrate aerosol pollution are closely related, connected by the unusually named parameter “total odd oxygen.” Since the same chemical processes that form ozone pollution in the summer produce ammonium nitrate pollution in winter, strategies that have effectively controlled ozone could also limit production of ammonium nitrate.

In western valleys with high levels of ammonium nitrate aerosol, mitigation efforts have tended to focus first on controlling one component of the pollution: nitrogen oxides from burning fossil fuels. However,  researchers found this approach may actually increase ammonium nitrate pollution, at least initially. Their study suggests limiting VOCs instead could potentially be a more effective way to reduce PM 2.5 pollution.

In the western U.S., common sources of VOCs include vehicle emissions, oil and gas production and refineries, volatile chemical products like solvents, paints and cleaners, wood smoke and agriculture.

“Atmospheric scientists don’t typically look at wintertime air pollution in this way,” Womack said. “Our findings could hold true in other areas with severe winter aerosol pollution, including mountain valleys across the U.S. West and urban areas in East Asia and Europe.”

Up next for the research team is a follow-on study that will look at wintertime air pollution across the entire U.S. West.

AGU's Research Letter is available.

For more information, contact Theo Stein at NOAA Communications: theo.stein@noaa.gov