They may both be Olympic host cities, but Salt Lake City and Los Angeles, the major population hubs of their respective states, are many 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 more of the fuel to CO₂ instead of CO. The more CO emitted relative to CO₂, the less efficiently the fuel is being burned.