A new study conducted by atmospheric scientists at the University of Utah, in collaboration with the federal National Oceanic and Atmospheric Administration, has revealed a consistent decrease in the emissions of two significant pollutants from vehicles in Salt Lake City over the past two decades. The study also notes that carbon dioxide emissions, a greenhouse gas associated with climate change, have remained relatively stable. The findings are detailed in the journal Atmospheric Environment.
The research team, led by John Lin, director of the Wilkes Center for Climate Science & Policy and a faculty member in the Department of Atmospheric Sciences, meticulously analyzed continuous pollution data from three strategic locations in Utah's largest city. Their aim was to identify trends that could assist Utah in addressing its air quality challenges.
Between 2005 and 2023, the study focused on three pollutants that are closely associated with human activities: nitrogen oxides (NOx), carbon monoxide (CO), and carbon dioxide (CO2). These pollutants are typically released simultaneously when fossil fuels are combusted, particularly in vehicles, residential heating systems, and industrial processes.
The results of the study indicate that advancements in emission regulations and vehicle technology have successfully reduced the output of harmful gases from automobile exhaust systems.
"Catalytic converters have proliferated, and the new cars are so much cleaner than what we had before," Lin remarked.
Additionally, the study indicates that the sources of pollution are becoming increasingly diversified. In the past, vehicular emissions were the primary contributors, but now other sources—including industrial activities, home heating, and off-road vehicles—are becoming more significant, according to the lead author, Haley Humble, who is a graduate student in Lin's lab.
The study’s key findings are as follows:
- Emissions of NOx and CO have demonstrated a significant decline over the years.
- CO2 emissions, however, have not shown a substantial decrease, suggesting that total fuel consumption has not significantly diminished, despite improvements in emissions per unit of fuel.
- The rate of improvement in emissions has decelerated over time, implying that while pollution levels are still declining, they are doing so at a slower pace than previously observed.
- The COVID-19 pandemic has caused a notable and enduring change, with traffic levels remaining reduced on city streets even after the lifting of restrictions.
"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 noted.
Home to one of the world’s oldest urban CO2 observation networks, established in 2001, Salt Lake City provides a unique dataset that enhances the research effort, according to Lin.
Humble analyzed emissions data collected from monitoring equipment located at Hawthorne Elementary School on 700 East—a prominent thoroughfare; at the Browning building on the University of Utah campus, situated on the city's elevated East side; and in the Rose Park neighborhood on the Westside, close to both the Salt Lake City International Airport and various industrial sites.
The monitoring equipment consistently measured levels of the three gases, accounting for variations by time of day and days of the week. It is noteworthy that summertime data was excluded from the analysis, as the CO2 levels were significantly influenced by the CO2 absorption capabilities of the city’s trees during the growing season.
To focus on local pollution sources, the researchers examined "enhancements," which indicate how much higher pollutant levels are compared to baseline background levels. They also compared the ratios of NOx and CO to CO2, an approach that aids in identifying specific emission sources. For instance, diesel engines emit higher levels of NOx but lower CO levels compared to CO2 than their gasoline counterparts.
"If we have more diesel traffic relative percentage-wise to gasoline traffic, it's going to lower the observed CO-to-CO2 ratio," Humble explained.
The research team also observed a gradual convergence between weekday and weekend NOx-to-CO2 and CO-to-CO2 ratios, which reflects a shift in the contributions from non-traffic sources relative to on-road emissions, potentially linked to federal diesel-emission regulations.
"When we start partitioning the weekday and weekends, we begin to see the smaller emission sectors assuming significant roles, with the relative contribution percentages shifting," Humble said. "For example, we would find a 14% contribution from off-road vehicle CO emissions on weekdays, which would change and show a different 14% contribution from another source on weekends."
Publication details
Haley Humble et al, Long-term urban emission trends in Salt Lake City: Examining CO, CO2, and NOX enhancements, Atmospheric Environment (2026). DOI: 10.1016/j.atmosenv.2026.121883
Journal information: Atmospheric Environment
Key concepts: air quality field monitoring stations carbon flux greenhouse gases
Provided by University of Utah
Citation: What's driving Salt Lake City's downward emissions trends? (2026, April 1) retrieved 1 April 2026 from https://phys.org/news/2026-04-salt-lake-city-downward-emissions.html
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