Measuring Real-World Emissions from the On-Road Heavy-Duty Truck Fleet

This page last reviewed April 3, 2015


Diesel engines are a major source of air pollution in California. The Air Resources Board (ARB) Truck and Bus Regulation will accelerate the introduction of exhaust after-treatment devices to reduce emissions of particulate matter (PM) and oxides of nitrogen (NOx) from on-road diesel engines. The rule applies first to 1996-1999 model year engines, which must be equipped with diesel particulate filters (DPFs) or replaced by January 2012. As of January 2016, all heavy-duty diesel trucks will be equipped with DPFs, and all pre-1996 engines will be replaced with newer engines. The accelerated engine replacement/retrofit program is a critical element of Californiaís air pollution control efforts over the next decade. Along with the accelerated emissions reduction benefits, there may be some unintended consequences of using after-treatment control devices, such as concomitant increases in ultra fine particle (UFP) and ammonia (NH3) emissions, and in the nitrogen dioxide (NO2) to NOx emission ratio. Also, there is little data available currently to predict the durability of new and retrofit emission control devices. It is therefore imperative to measure in-use emissions of vehicles on the road in order to quantity the effects on emissions of truck retrofits and replacements.

To evaluate emissions benefits of the Truck and Bus Regulation, the evolution in pollutant emissions distributions will be examined through field sampling campaigns at the Caldecott tunnel in 2014, 2015 and 2017. Emissions changes will be quantified for specific segments of the truck fleet. This will be accomplished by matching the emission results for individual trucks to truck attributes using ARBís Truck and Bus registry, such as engine model year and, if available, type and installation date of DPF. With these data, it may be possible to assess the durability and the failure rates of exhaust after-treatment control devices. Additionally, the investigators will assess interactive effects among pollutants present in diesel exhaust, including effects of lower particle mass on UFP number and size distribution, and the extent to which expected increases in the NO2/NOx emission ratio are offset by lower NOx mass emission rates.