Flying for research is a lot different than flying commercially.

“We were usually flying like 500 to 1000 feet above the ground. Low enough that with our phones we were getting notifications to connect to people’s wifi from their houses,” said Carrie Womack, a research scientist in atmospheric chemistry at the National Oceanic and Atmospheric Administration.

The accommodations weren’t quite first class either:

“It’s very crowded. So inside the cabin it was only about five feet tall so all of us were standing like this [crouched over] except for the one person who was only five feet tall who can stand up straight. And we crammed this plane full of instruments.”

The joys of research in the field! Womack, did note that “from just a flying perspective it was a lot of fun because … it wasn’t turbulent at all so we were just flying along in this plane looking at this pollution, dipping in and out of it. But mostly staying in it.”


The team of researchers standing in front of their decked-out aircraft, a NOAA Twin Otter. (Photo credit: Carrie Womack)

Womack is the lead author of a study published today on wintertime air quality in Salt Lake City. The pollution she was referencing is surprisingly common for a seemingly pristine mountain city. Salt Lake City is so prone to major episodes of pollution that it exceeds EPA air quality standards an average of 18 times a year.

“The motivating thing for us was that Salt Lake City has what is often the worst air quality in the US,” said Womack. “Which is surprising to some people because you think of it as this sort of western valley—it’s not LA, it’s not New York—how could they be having the worst air quality in the US?”

The pollution problems in Salt Lake City are tied into what are known as “persistent cold air pools.” These are pockets of cold air in the atmosphere that hang out over a particular region, acting like an invisible cap. Typically, air warms over the course of the day and any pollution near the surface will gradually mix in with the air above it. But when these cold air pockets are in place, the air near the ground stays trapped at the surface.

“It no longer dilutes the pollutants from the surface away, they just stay contained and it’s like being in a bowl and putting a lid on the top,” explained Womack. “So then people keep driving and heating their houses and these emissions continue to build up, they’re not going anywhere.”

Pollution continues to accumulate at ground level for as long as these pockets of cold air are in place. Salt Lake City is particularly susceptible to this problem due to its location in a valley surrounded by mountains. Once a cold snap hits, it’s more likely to stick around.


“You know it’s this beautiful city and when it’s clear there’s mountains in all directions and so we flew in at the beginning of one pollution episode and you could see mountains all over and by two days later you couldn’t see them anymore. They were just gone.”—Carrie Womack (Photo credit: Carrie Womack)

The overarching pollution problem in Salt Lake City is tied to tiny particles known as PM2.5. High levels of PM2.5 produce hazy skies and are hazardous to human health as they can penetrate deep into the lungs. Womack and her team collected samples from the skies and compared them with measurements from the ground to understand how ammonium nitrate aerosols, one of the main contributors to PM2.5, are formed.

By studying the components of ammonium nitrate aerosols, the researchers hope to inform cities and states how to better manage their pollution outbreaks by enforcing emissions standards.

“There’s lots of ingredients that go into this ammonium nitrate and what they’re not sure of is what does it make sense to target first,” said Womack.

For example, if ingredient A combines with ingredient B to produce ammonium nitrate in a one-to-one ratio, “it makes more sense to target the one that you have less of. Let’s say you have two of A and five of B. You can still only make two of the product, whereas if you have one of A and five of B you can only make one.” The key is to find which ingredient is stopping more ammonium nitrate from being created and reduce its levels even further.


(Photo credit: Carrie Womack)

Using their suite of measurements and sophisticated computer techniques, the team made a surprising discovery. While pollution mitigation efforts often focus on controlling nitrogen oxides—produced in the burning of fossil fuels—the team found that this may actually lead to increased ammonium nitrate pollution, to a point at least. In other words, controlling the number of cars on the road or power plant emissions might not be the best option.

Instead, it may be more effective to control volatile organic compounds, a different class of pollutants, to control ammonium nitrate production. Common sources of these compounds include vehicle emissions, fossil fuel production, wood smoke, agriculture, and chemical products such as paint and solvents.

“It is sort of interesting that ammonium nitrate—which is a molecule that has no carbons in it—could be limited by [volatile organic compounds], which are mostly carbon,” said Womack. “It’s a little counterintuitive, but once you just work through the chemistry it makes a lot of sense because of the mechanism.”

Womack explained why she is excited about this research in a CIRES press release: “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.”

So the next time you’re crammed into a tiny seat in economy between two people who like to pop your personal bubble, remember that some scientists do it on purpose so we might all breathe a little easier.

By Ryan Harp

Posted by Science Buffs

A CU Boulder STEM Blog

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