On-Road Measurement of Light-Duty Gasoline and Heavy-Duty Diesel Vehicle Emissions

This page last reviewed February 7, 2012


Gasoline and diesel engines remain as a significant source of air pollution in California, the U.S., and worldwide (Sawyer et al., 2000). Emissions from these engines give rise to a range of air quality problems and human health concerns (Lloyd and Cackette, 2001). In addition to contributing to local and regional air pollution problems, vehicle exhaust emissions contribute to climate change. Motor vehicles are responsible for 35% of California CO2 emissions (CEC, 2006), the greenhouse gas responsible for the greatest amount of global warming. NOX is a precursor to tropospheric ozone, which also contributes to global warming. PM has direct and indirect effects on radiative forcing, leading to both global warming and cooling; the direct effect of BC emissions is positive forcing (IPCC, 2007).

An unintended consequence of catalytic converter use on light-duty motor vehicles has been increased emissions of ammonia due to over-reduction of nitrogen oxides (Fraser and Cass,1998; Kean et al., 2000; Durbin et al., 2004; Emmenegger et al., 2004). Ammonia is the primary alkaline gas in the atmosphere, and an important precursor to secondary particle formation. For some vehicles, emissions of ammonia exceed the emissions of other regulated compounds, though Durbin et al. found that ammonia emission rates are lower for newer technology vehicles. While probably decreasing, the rate of change in fleet-average ammonia emissions remains unclear.

More stringent emission standards apply to new heavy-duty diesel engines sold starting in 2007; ultra-low sulfur diesel fuel was introduced in 2006 to facilitate use of post-combustion exhaust treatment devices. Past diesel engine emission control efforts have relied on modifications to fuel injection system pressure and fuel injection timing. In contrast, new engines will be equipped with continuously regenerating traps (CRT), also known as diesel particulate filters (DPF). NOX present in diesel exhaust is deliberately converted to NO2 using an oxidation catalyst, then the NO2 is used to oxidize collected soot particles, so the accumulated carbon particles on the filter can be removed to permit long-term continued use of the exhaust filter. NO2 emissions may increase using this approach, which is an issue of regulatory and public health concern. Emission control options for NOX include increased exhaust gas recirculation (EGR), selective catalytic reduction (SCR) systems, and absorbers that store NOx while the system is operating with excess oxygen, with intermittent operation in NOX reduction mode to eliminate stored NOX.

On-road measurements provide data that are complementary to what can be measured using chassis dynamometers in the laboratory. While fuel and test conditions in the laboratory can be carefully controlled, it is expensive and time consuming to test large numbers of vehicles. As both LD and HD vehicle emissions become increasingly skewed and dominated by emissions from a small number of gross-polluting vehicles, a large random vehicle sample becomes increasingly important to arrive at a robust estimate of population mean emission rates. Tunnel sampling provides fleet-average emission rates from large samples of vehicles as they are driven on the road.



  • Harley, (March, 2009). On-Road Measurement of Light-Duty Gasoline and Heavy-Duty Diesel Vehicle Emissions. ARB Research Final Report (PDF 7,431KB)

Publications for Peer-reviewed Journals

  • Dallmann et al., (2011). Effects of Diesel Particle Filter Retrofits and Accelerated Fleet Turnover on Drayage Truck Emissions at the Port of Oakland. Environmental Science and Technology. Link