The estimated contribution of on-road heavy-duty diesel vehicles to the total NOx emission in California was ~32% in 2016, which is considerably higher than the US average (~16-18% in the past decade).
This work used on-board NOx sensor data to assess the real-world nitrogen oxides (NOx) emissions from heavy-duty diesel vehicles equipped with Selective Catalytic Reduction (SCR) in California.
Although substantial progress has been made in controlling NOx emissions from heavy-duty diesel vehicles since 2010, a large fraction of the fleet could have much higher than expected in-use NOx emissions due to insufficient SCR performance. Potential reasons for insufficient SCR performance include low operation temperature, catalyst malfunction, and insufficient dosing of diesel exhaust fluid (DEF).
Results will help understand in-use NOx emissions from heavy-duty diesel vehicles, and inform the revision of heavy-duty OBD requirements. It also helps to identify specific vehicle types that may be malfunctioning on-road. For example, real-world emissions data helped lead to the recall of Cummins engine aftertreatment devices.