Real-world exhaust temperature declines and NOx hotspots during urban heavy-duty truck operation

Heavy-duty diesel trucks are major contributors to urban air pollution, yet the real-world operating conditions that trigger episodic nitrogen oxide (NOx) emission surges remain insufficiently understood. This study examines how exhaust temperature declines during in-use truck operation generate NOx emission hotspots beyond conventional cold-start conditions. Using second-by-second operating and emissions data from five selected heavy-duty trucks operating in California, we identified 8,959 temperature-decline events associated with low-load conditions, including idling, low-speed driving, deceleration, and downhill coasting. Across these events, exhaust temperature reductions were strongly associated with lower NOx conversion efficiency and higher tailpipe NOx emissions. When exhaust temperature dropped below 200 °C, conversion efficiency fell below 75%, and tailpipe emissions approached engine-out levels. Prolonged idling was especially important: most idling events occurred on urban non-freeway roads, where low-load operation weakened SCR performance and produced elevated tailpipe NOx emissions. These findings suggest that standard drive cycles and emission inventories may underrepresent real-world heavy-duty diesel truck NOx burdens when they do not fully capture non-cold-start high-emission events associated with running-phase exhaust temperature declines, particularly during idling and other low-load urban operations. An exploratory spatial analysis further indicates that these emissions may be concentrated in pollution-burdened communities, underscoring the need for targeted mitigation. The results highlight the importance of reducing low-load operation, improving idling management, and incorporating real-world SCR thermal behavior into emissions assessment and freight decarbonization strategies.