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Study finds that current approach to limiting wintertime pollution may initially backfire

The processes that create ozone pollution in the summer can also trigger the formation of wintertime air pollution, according to a new study from researchers at the University of Colorado Boulder and NOAA, in partnership with the University of Utah. The team’s unexpected finding, published in the journal Geophysical Research Letters, suggests that in the US West and elsewhere, certain efforts to reduce harmful wintertime air pollution could backfire.

In the US, Europe and Asia, PM is severe in urban areas in the winter when ammonium nitrate, NH4NO3, comprises an appreciable fraction of the total PM mass. A key control strategy is to reduce emissions of the limiting reagent. Using measurements from a recent field campaign in the Salt Lake Valley, Utah, which experiences high PM levels in winter, we demonstrate that emission control strategies can be evaluated using the same framework commonly used to control ozone, another common pollutant that occurs at high levels in urban areas only in the summer.

We show that initial control of the NOx precursor is ineffective at reducing NH4NO3 aerosol in the SLV, while initial control of volatile organic compounds (VOC), which is not a direct precursor for either nitrate or ammonium, is effective due to its influence on oxidation cycles. This finding differs from many mitigation strategies in the western US, and may also be relevant to other regions in Europe and Asia which experience high wintertime PM.

—Womack et al.

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.

—Caroline Womack, a CIRES scientist working in the NOAA Earth System Research Laboratory and lead author

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) 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, researchers from the Cooperative Institute for Research in Environmental Sciences (CIRES) and NOAA partnered with the University of Utah, the Utah Department of Environmental Quality, and others to measure PM2.5 and its precursor emissions at several 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, 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 field campaign, Womack, Lin and their 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.

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.

—Caroline Womack

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


  • Womack, C. C., McDuffie, E. E., Edwards, P. M., Bares, R., Gouw, J. A., Docherty, K. S., et al. ( 2019). “An odd oxygen framework for wintertime ammonium nitrate aerosol pollution in urban areas: NOx and VOC control as mitigation strategies.” Geophysical Research Letters, 46. doi: 10.1029/2019GL082028



Well, it seems to be similar to abatement of the formation of summer ozone. You have to decrease VOCs. If you decrease NOx, while keeping VOC constant, it might get worse. The so-called "ozone weekend effect" is familiar to many on this forum, I presume. During weekends, when HD traffic (major NOx contributor) is low but LD traffic does not decrease that much, ozone levels are higher than during weekdays. A paradox, if you like! The formation of secondary PM seems to respond in a similar way. I wonder why there is so little effort to control VOCs (i.e. hydrocarbons, HC) from traffic. Particularly during cold starts in cold climate, HC emissions from gasoline cars can increase by more than one order of magnitude compared "normal" ambient temperatures. Today, most of the focus seems to be on abatement of NOx (and PM/PN, to some extent in the EU) but HC (particularly off-cyle) is more or less neglected.


In addition to the disbenefits of a regulatory strategy that reduces NOx emissions relatively more than VOC/HC emissions, as seen in this study and ozone weekend effect studies, reducing ambient NOx levels relatively more than ambient VOC levels results in more secondary organic aerosol (SOA) formation, based on another recent study -

It's becoming increasingly clear that the regulatory focus on NOx emissions almost exclusively couldn't be more misguided.


Yes, Peter, it's quite the conundrum isn't it?

Reducing VOCs reminds me of one of the earliest pollution-control schemes, the exhaust manifold reactor.  It was an afterburner which used injected air to burn off the excess HC from a rich combustion process which prevented NOx formation.

Today everything has a three-way catalyst.  Perhaps it's time to go back to another old scheme, the electrically-preheated catalyst.  Achieving immediate light-off and even a surge of heat as the excess fuel from cold-start hits it would probably do a lot to address that issue.


Hopefully electric motors will reduce the need to modify exhaust gasses by replacing the engines.


It seems as though Austria has found a practical solution to reducing winter-time pollution. They are mandating that all new homes / buildings etc. are prohibited to install oil heating systems. Oil heaters that need to be replaced in older homes must be substituted by respective alternatives. These may be gas heaters, heat pumps and other sustainable sources. Anyway, it's a start in the right direction.


Recent SEER 32 heat pumps operate effectively to about -30C. Even with a below normal cold winter, our new heat pumps maintained inside temps at a constant +22C without the use of the installed baseboard resistive heaters, normally electronically set at +20C.

The e-energy saving was an average close to 30% for most days specially when overnight heating was reduced to +20/21C by the heat pumps and +18/19C for resistive baseboard heaters. Those high SEER heat pumps are also very effective as A/Cs during the hot season.

Similar heat pumps can/could reduce e-energy consumption and extend range of most HEVs-PHEVs-BEVs and FCEVs.

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