Emissions of two major pollutants have steadily decreased on Salt Lake City roads over the past two decades, while levels of carbon dioxide emissions, a related gas blamed for climate change, remained steady, according to a new study by University of Utah atmospheric scientists conducted in partnership with the federal National Oceanic and Atmospheric Administration.
The team led by John Lin, scientific director of the Wilkes Center for Climate Science & Policy and a faculty member in the Department of Atmospheric Sciences, analyzed pollution data recorded continuously at three locations in Utah's biggest city and documented trends that could help Utah tackle its air quality problems.
The study tracked emissions between 2005 and 2023, focusing on three gases linked to human activity: nitrogen oxides (NOx), carbon monoxide (CO) and carbon dioxide (CO₂). These gases are co-emitted when fossil fuels are burned, especially in cars, trucks, homes and industrial processes.
The findings offer proof that emission standards and vehicle improvements have helped curb the amount of unhealthy gases spewing from tailpipes.
"Catalytic converters have proliferated, and the new cars are so much cleaner than what we had before," Lin said.
The study also shows that pollution sources are becoming more mixed. While vehicle emissions used to dominate, other sources, such as industry, residential heating and off-road equipment, are becoming relatively more prevalent, according to lead author Haley Humble, a graduate student in Lin's lab.
The findings, appearing in the journal Atmospheric Environment, documented the following:
- NOx and CO emissions have dropped significantly over time.
- CO₂, however, has not decreased much overall, suggesting that total fuel use hasn't fallen significantly, even as pollution per unit of fuel has improved.
- Over time, the rate of improvement has slowed, meaning Utah is still reducing pollution, but not as quickly as before.
- The COVID-19 pandemic established a clear, lasting shift, with reduced traffic levels persisting on city streets even after lockdowns ended.
"There's been so much more flexibility since the pandemic, with the ability to work from home or when you can come into the office," Humble said.
The rapidly growing Salt Lake Valley is home to one of the world's longest running urban CO₂ observation networks, established in 2001, resulting in a valuable one-of-a-kind dataset, according to Lin.
Humble analyzed emissions data recorded by equipment housed at Hawthorne Elementary School on 700 East, a major north-south thoroughfare through the city; atop the Browning building on the U campus on the city's elevated Eastside; and in the Westside neighborhood of Rose Park near the Salt Lake City International Airport and industrial sites.
Equipment continuously recorded levels of the three gases, which were tracked by time of day and days of the week. Summertime data was excluded from the analysis because photosynthetic activity of the city's trees, which takes in CO₂, had a pronounced effect on carbon dioxide levels relative to other gases, Humble noted.
To isolate local pollution, the researchers looked at "enhancements," that is how much higher pollutant levels were compared to background levels. They also compared levels of NOx and CO relative to CO₂. This ratio approach helps identify emission sources. Diesel engines, for example, emit more NOx than gasoline engines, but less CO relative to CO2.
"If we have more diesel traffic relative percentage-wise to gasoline traffic, it's going to lower the observed CO-to-CO₂ ratio," Humble said.
The team also registered a gradual convergence in weekday and weekend NOx-to-CO₂ and CO-to-CO₂ ratios, reflecting increasing contributions from non-traffic sources relative to on-road emissions, and possibly the influence of federal diesel-emissions controls.
"When we start partitioning the weekday and weekends, we start to see the smaller emission sectors having really nuanced roles where the relative percentage contribution was shifting," Humble said. "For example, we would have a 14% contribution of off-road vehicle CO emissions on the weekdays, and then it would flip, and then we would have a different 14% contribution of another emission source on weekends."
The study, titled "Long-Term Urban Emission Trends in Salt Lake City: Examining CO, CO₂, and NOx Enhancements," is to appear in a forthcoming edition of Atmospheric Environment. It was posted online Feb. 19. Co-authors include Derek Mallia, Maria Garcia and Megan Ostlie of the U's Department of Atmospheric Sciences, and Brian McDonald and Colin Harkins of the National Oceanic and Atmospheric Administration. Funding was provided by the NOAA's AC4 Program and key support was provided by the U's Center for High Performance Computing.
Banner image: Morning rush hour traffic on Salt Lake City's 700 East by Hawthorne Elementary School. Photo credit: Brian Maffly
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