Here’s why Salt Lake City has higher per-vehicle carbon monoxide emissions than LA
Mar 4, 2025, 8:37 PM | Updated: 9:02 pm
Traffic moves on I-15 in Salt Lake City. New research from University of Utah and University of California scientists shows lower per-vehicle concentrations of carbon monoxide emissions on LA freeways compared to Salt Lake freeways. (Spenser Heaps, Deseret News)
(Spenser Heaps, Deseret News)
SALT LAKE CITY — Los Angeles and Salt Lake City: two cities with many differences. But the valley topography in which each city lies is very similar, providing the right conditions to trap pollutants and exacerbate poor air quality.
While they face that similar problem, particularly during winter months, the two locations differ sharply in their approaches to addressing it. Over the past decade, California has implemented more stringent regulations and fuel standards aimed at reducing emissions from motor vehicles than its neighbor to the east.
California’s earlier implementation of those regulations seems to have paid off. New research from University of Utah and University of California scientists shows lower per-vehicle concentrations of carbon monoxide emissions on LA freeways compared to Salt Lake freeways.
“What we found, I would say, was a little surprising. Per-vehicle, the emissions appear to be worse in Salt Lake, compared to LA,” said John Lin, professor of atmospheric sciences at the U. and associate director of the Wilkes Center for Climate Science and Policy.
Mobile labs measured carbon monoxide
The study harnessed emissions measurements taken by mobile labs — similar to those used to take pictures and map routes for Google Maps — that drove up and down LA and Salt Lake freeways for a few weeks in the summers of 2013 and 2019, with follow-up data gathering in LA over the next two summers to observe the effect of the COVID-19 pandemic.
“Given the six years or so between 2013 and 2019, I was hoping things would have improved a bit more in Salt Lake City. But actually, with respect to carbon monoxide, the pollution seems to have gotten a bit worse in terms of emissions from vehicles on highways,” said Lin, who also co-authored the study.
Particularly, the study focused on the ratios of CO to carbon dioxide observed by the mobile labs. These two gasses are co-emitted from fossil fuel combustion, and their ratio is an indicator of the efficiency of that combustion since efficient internal combustion engines would convert more of the fuel to CO2 instead of CO. Essentially, the more CO emitted relative to CO2, the less efficiently the fuel is being burned.
Looking at the emissions from motor vehicles was especially relevant, as they are the main source of anthropogenic, or human-caused, CO in the atmosphere. And while outdoor CO concentrations rarely reach the toxic level of indoor concentrations, elevated levels can harm those with heart disease, making it difficult for their organs to get enough oxygen, according to the U.S. Environmental Protection Agency.
Nationally, 38% of anthropogenic CO originates from on-road mobile sources; however, the study shows that portion is higher for Los Angeles and Salt Lake counties, at 47% and 56%, respectively.
“The CO-to-CO2 ratios appear to increase in Salt Lake, but they decreased in LA and, of course, the question is ‘why?'” Lin said.
That reasoning likely boils down to a few factors, according to Lin and the study.
What are the factors?
One is the difference in transportation fleets in Salt Lake City and LA, with the former having a higher proportion of trucks while passenger cars dominate in Los Angeles. Salt Lake’s cars also tend to be older, with an average age of 9 years. Another factor is the aforementioned mandates for cleaner vehicles and fuels implemented by California before Utah, coupled with incentives for drivers to replace their older vehicles.
Lin also mentioned a final factor that contributed to the results, which he said could be seen as “a little counterintuitive.”
“I-15 freeway for instance, during that time period from 2013 to 2019, the speed limit increased. There’s some suspicion that vehicles, once you get really high speeds, can actually emit more rather than less,” Lin said. “Vehicle speeds appear to matter.”
To that point, University of California researchers discovered that CO ratios increased in follow-up testing on LA freeways in the summers of 2020 and 2021, when speeds increased with thinning traffic in the wake of pandemic shutdowns.
Lin also noted an important caveat to that discussion from a yet-to-be-published study led by one of his graduate students. That study is based on parallel data gathered by stationary monitoring equipment from the Utah Division of Air Quality housed at the now-closed Hawthorne Elementary School, 1948 S. 700 East, on one of Salt Lake City’s busiest streets about a mile east on I-15.
Between 2013 and 2019, CO-to-CO2 ratios recorded at Hawthorne Elementary declined, further reinforcing Lin’s self-described “speculation” that speed plays a role in the emission totals.
“Vehicles are driving more like 40 to 50 mph, which might be kind of a sweet spot for internal combustion engines,” he said.
Looking ahead, Lin said he’d love to do more research into what happened in Salt Lake’s freeways, as far as CO emissions, after 2019 — especially with more of the valley’s workforce working from home after the pandemic.
“There’s really interesting patterns emerging as we move forward, so this is a very dynamic picture in terms of our air and how clean it is,” Lin said.
The full study can be found here.
Related: Risk of carbon monoxide poisoning rises in cooler months
