Heavy-Duty Diesel Vehicle Deterioration: Impact of Vehicle Age and Emission Control Technology on NO x Emissions

This study examines the effect of aging and deterioration on NOx emissions from heavy-duty diesel trucks (HDDTs) with different emission control technologies, including Engine Control only, exhaust gas recirculation (EGR), and selective catalytic reduction (SCR). In-use tailpipe emissions were measured from 12,023 HDDTs from 13 roadside measurement campaigns conducted in California and Utah between 2008 and 2024. Vehicle registration records and the Environmental Protection Agency’s (EPA) engine certification database were used to identify engine NOx control technology. At younger ages, SCR-equipped HDDTs generally emitted significantly less NOx than non-SCR-equipped HDDTs. However, after several years of aging, NOx emissions from SCR-equipped HDDTs were comparable to or exceeded those from non-SCR-equipped HDDTs across locations, engine manufacturers, and engine model years. Although cumulative lifetime emissions from SCR-equipped HDDTs remained lower than non-SCR HDDTs, the reduction in lifetime NOx emissions was substantially smaller than estimated by EPA’s MOtor Vehicle Emission Simulator (MOVES). MOVES predicts an 80% reduction in lifetime NOx emissions from SCR-equipped HDDTs, whereas our estimate is approximately 32%. These findings may indicate that the long-term benefits of SCR for reducing HDDT NOx emissions are smaller than expected and highlight the need for continued emissions monitoring of in-use HDDTs.