2019 News & Events

A New View of Wintertime Air Pollution

8 May 2019
adapted from the story by CIRES Communications

Utah winter air pollution
Air pollution in the Salt Lake region during the Utah Winter Fine Particulate Study (UWFPS). Photo: S. Brown, NOAA

A new study led by NOAA and CIRES researchers published in the AGU journal Geophysical Research Letters could help improve air quality in cities across the U.S. West. The processes that create ozone pollution in the summer can also trigger the formation of wintertime air pollution. The team's unexpected finding suggests that in the U.S. West and elsewhere, certain efforts to reduce harmful wintertime air pollution could backfire. Specifically, targeting nitrogen oxides emitted by cars and power plants could initially actually increase harmful air pollution.

"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 working in CSD and lead author of the study.

Regulations and cleaner technologies have steadily improved air quality in the United States. Yet valleys in western states still experience high levels of particulate matter (PM2.5), or microscopic droplets suspended in air, during the winter. In Utah's urban Salt Lake Valley, wintertime levels of PM2.5 exceed national air quality standards an average of 18 days per year. Denver often has the same problem in winter, when brown clouds hang over the city.

A major component of the Salt Lake Valley and Denver PM2.5 pollution is ammonium nitrate aerosol, which forms from emissions of nitrogen oxides, volatile organic compounds (VOCs), and ammonia. Those reactions happen during winter temperature inversions, when warm air aloft traps cold air below, concentrating pollutants.

To combat wintertime PM2.5 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 for the Utah Winter Fine Particulate Study (UWFPS), to measure PM2.5 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 critical altitude region where particulate matter forms.

Based on the observations from the 2017 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. The researchers found this approach may actually increase ammonium nitrate pollution, at least initially. A potentially more effective way to reduce PM2.5 pollution would be to limit VOCs, according to the new assessment.

"Atmospheric scientists typically don't 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."

PM2.5 pollution is a major cause of premature death worldwide—and besides negatively affecting human health, PM2.5 also affects agricultural yields, visibility, and possibly Earth's climate.

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

C. C. Womack, E. E. McDuffie, P. M. Edwards, R. Bares, J. A. de Gouw, K. S. Docherty, W. P. Dube, D. L. Fibiger, A. Franchin, J. B. Gilman, L. Goldberger, B. H. Lee, J. C. Lin, R. Long, A. M. Middlebrook,D. B. Millet, A. Moravek, J. G. Murphy, P. K. Quinn, T. P. Riedel, J. M. Roberts, J. A. Thornton, L. C. Valin, P. R. Veres, A. R. Whitehill, R. J. Wild, C. Warneke, B. Yuan, M. Baasandorj, S. S. Brown, An odd oxygen framework for wintertime ammonium nitrate aerosol pollution in urban areas: NOx and VOC control as mitigation strategies, Geophysical Research Letters, doi:10.1029/2019GL082028, 2019.


Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the US, it is acute in western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake City, Utah demonstrates that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, Ox,total. We show that the traditional NOx‐VOC framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake City is responsive to VOC control and, counterintuitively, not initially responsive to NOx control. We demonstrate simultaneous nitrate limitation and NOx saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution.