Abstract
Policy interventions and technological advances are mitigating emissions of air pollutants from motor vehicles. As a result, vehicle fleets are expected to progressively combust fuel more efficiently, with a declining ratio of carbon monoxide to carbon dioxide (CO/CO(2)) in their emissions. We assess trends in traffic combustion efficiency in Los Angeles (LA) and Salt Lake City (SLC) by measuring changes in summertime on-road CO/CO(2) between 2013 and 2021 using mobile observations. Our data show a reduction in CO/CO(2) in LA, indicating an improvement in combustion efficiency that likely resulted from stringent regulation of CO emissions. In contrast, we observed an increase in CO/CO(2) values in SLC. While slower progress in SLC compared to LA may be partially due to a later adoption of vehicle emission regulations in Utah compared to California, differing driving conditions and fleet composition may also be playing a role. This is evidenced by increased CO/CO(2) in LA during the COVID-19 pandemic, which led to faster driving speeds and changes to the fleet composition. Our results demonstrate the success of California's CO-reducing policy interventions and illustrate the impacts of traffic characteristics on vehicle combustion efficiency and air pollutant emissions.