An emissions tale of two cities: Salt Lake City vs. Los Angeles

They may both be Olympic host cities, but Salt Lake City and Los Angeles, the major population hubs of their respective states, are much different places. However, they both experience poor air quality and share valley topography that traps pollutants during weather inversions.

Utah and Southern California differ sharply in their approaches to this problem, with the latter implementing more stringent regulations and fuel standards aimed at reducing emissions from motor vehicles. New research from the University of Utah, in collaboration with University of California scientists, shows California's earlier adoption of stricter rules may have helped lower concentrations of one pollutant—carbon monoxide, or CO—on LA freeways.

“We wanted to see empirically how emission characteristics have changed in these two cities over time,” said co-author John Lin, a Utah professor of atmospheric sciences. The research was initiated by Francesca Hopkins, a professor of climate change and sustainability at UC Riverside, and conducted with colleagues at UC Irvine.

The study relied on measurements taken by mobile labs 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 Los Angeles over the next two summers to observe the effect of the Covid pandemic.

The study especially focused on the ratios of CO to CO₂ (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 Tmore of the fuel to CO₂ instead of CO. The more CO emitted relative to CO₂, the less efficiently the fuel is being burned.

“The CO-to-CO₂ ratios appear to increase in Salt Lake, but they decreased in LA, and of course the question is ‘Why?’” said Lin, who serves as associate director of the U's Wilkes Center for Climate Science & Policy. “What's different between Salt Lake and LA? A lot of things are different. One thing proposed in the paper is the regulation and how much earlier it was adopted in Los Angeles. Salt Lake adopted Tier Three fuel standards not too long after LA but the fuel wasn't available in Utah until 2020.”

Lin offered a caveat arising from a yet-to-be-published study, led by one of his graduate students and based on parallel data gathered by stationary monitoring equipment from the Utah Division of Air Quality housed at Hawthorne Elementary School on 700 East, one of Salt Lake City’s busiest streets about a mile east on Interstate 15. Between 2013 and 2019, CO-to-CO₂ ratios recorded at this spot declined.

Breathing carbon monoxide, or CO, impairs the human body’s ability to circulate the oxygen needed for survival. Indoor CO pollution can literally asphyxiate a person. Outdoor concentrations rarely reach such a toxic level, but 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. The agency set the 8-hour threshold for ambient levels of CO at 9 parts per million (ppm), or 35 ppm for 1 hour.

“There are other emissions we could look at, like the ratio of NOx to CO₂," Lin said. "Carbon dioxide is the common denominator because it's what you get from burning fossil fuels and, and the engine undergoes ‘clean combustion’, when you convert most of the fuel into CO₂ and water.  I put clean in quotes because CO₂ is a greenhouse gas that leads to climate change”

Motor vehicles are the main anthropogenic source of CO in the atmosphere. Nationally, 38% of anthropogenic CO originates from on-road mobile sources, but that portion is higher for Los Angles and Salt Lake counties, 47% and 56%, respectively, according to the new study.

These metro areas’ transportation fleets are different, with Salt Lake having a higher proportion of trucks, while passenger cars dominate in Los Angeles, and Salt Lake’s cars tend to be older, with a mean age of 9 years. And speed limits are higher in Utah. During the study period, in 2015, Salt Lake City freeway speed limits increased by 5 to 10 mph, which may explain some of the increase in CO emissions.

On the emission control side, California adopted mandates for cleaner vehicles and fuels several years ahead of Utah, as well as offered incentives to drivers to replace their older vehicles. Tellingly, the researchers discovered CO ratios increased in the follow-up testing on LA freeways in the summers of 2020 and 2021 when speeds increased as traffic thinned in the wake of pandemic shutdowns.

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The study titled, “Contrasting Summertime Trends in Vehicle Combustion Efficiency in Los Angeles, CA and Salt Lake City, UT,” was published in the Jan. 21 edition of Environmental Science & Technology. University of Utah authors include Susan Bush and former laboratory specialist Ryan Bares, now an environmental scientist with the Utah Division of Air Quality. The lead author is former UC Irvine graduate student Cindy Yañez, who is now with the California Air Resources Board.