Effects of Ambient Temperature on NOx Emissions from Heavy-Duty Diesel Vehicles Measured in Utah

This study investigates the effects of ambient temperature on NOx (NO + NO2) emissions from model year 2011 and later heavy-duty (HD) diesel vehicles. Emission measurements were collected in Perry, Utah, using the Fuel Efficiency Automobile Test (FEAT) remote sensing device. Data were limited to model year 2011 and later to focus on vehicles likely equipped with selective catalytic reduction (SCR) systems, which control tailpipe NOx emissions and are shown to be temperature sensitive. HD diesel vehicles measured in the winter of 2020 had consistently higher NOx emissions than those measured in the summer of 2023, most significantly for vehicles aged 0 to 3. A non-linear model fit to the data that accounts for age effects, predicts fleet-average NOx emissions to be two times higher at colder ambient temperatures (−4.4 °C, 24 °F) than warmer ambient temperatures (28.1 °C, 82.5 °F). The temperature effect from this study supports temperature effects observed in other studies measuring real-world emissions from HD diesel vehicles. One possible improvement to the accuracy of NOx emission inventories could be including a temperature effect for SCR-equipped HD diesel vehicles.