White paper

California’s Clean Diesel Program

The California Air Resources Board (CARB) has been engaged since the 1960s in improving the health of citizens by reducing air pollution. This white paper summarizes the efforts CARB has made and plans to make to reduce emissions from diesel engines of all types. Specifically, it describes steps taken to develop plans and strategies to combat pollution from vehicles, off-road equipment and marine vessels, enforcement efforts taken to ensure compliance, and programs developed to accelerate the replacement of older, high-emitting diesel engines. In addition, it discusses CARB’s efforts to initiate the inevitable need to transform to zero-emission propulsion and identifies incentives needed to accelerate the transition.

For governments that have begun more recently or are just starting to develop diesel emission control programs, the path to eliminating the adverse health effects of diesel emissions can be shorter because many of the steps California has already taken, and programs can focus on requiring use of the most advanced technologies. Based on CARB’s experience reducing diesel and other mobile sources of emissions, the following lessons are relevant to current and future emission control efforts:

An effective emission control program requires clear legal authority. A single implementing agency needs to be empowered to take actions to achieve a clearly stated goal, such as meeting health-based air quality standards, and reducing climate emissions by a specified percentage. These goals must include dates by which they must be achieved. Legal authority to enforce regulations and impose fines is also necessary to make rules fair and affective. The penalty for failing to meet rules and regulations should also be enough to deter noncompliance.

Technology-forcing emission standards for new vehicles are effective. Technology-forcing regulation challenges engine and vehicle manufacturers and developers of emission control technologies to invent and demonstrate the feasibility of the most effective systems. Such requirements also establish the necessary lead time to commercialize new technologies. In a few instances where problems with implementation occurred in California, the affected manufacturer and CARB were able to develop a solution, most often by allowing more time for development rather than weakening the emissions standard.

An active in-use compliance program is essential. Manufacturers successfully comply with new engine and vehicle emission standards and are permitted to sell their products. However, the actual reduction of emissions on the road usually falls short of expectations due to design flaws or use of operating modes that are not emphasized during pre-sale compliance testing. A solution to these causes is an active compliance program based on testing in-use vehicles. A portable emissions monitoring system (PEMS) recording vehicle emissions while a truck is in operation can identify engine families suspected of poor emissions-control durability. Installation on a random sample of trucks can also identify operational modes where emissions are high and lead to regulatory improvements. In addition, such testing can identify malfunctioning control systems or detect tampering.

Accelerate turnover of old diesel vehicles for new, low-emission vehicles. The adverse effects of diesel pollution on public health, especially for those living adjacent to or near areas of high truck traffic, need to be remedied sooner, not later. One way of accomplishing this is to adopt programs that incentivize or require retiring older vehicles and replacing them with new ones. As current-model trucks are more than 90% cleaner, turnover can quickly reduce emissions and improve air quality. CARB has used both incentives, such as grants to help reduce the price of new trucks, and a requirement that in 2023 no truck operating in California can be older than a 2010 model. While retrofits can accelerate emission reductions, installing a retrofit device on an old engine can create problems. If feasible, the accelerated-turnover approach is more beneficial to the truck owner due to improved reliability and fuel efficiency.

Zero emissions are essential. Successfully abating climate change requires GHG emissions to rapidly approach zero. For mobile sources, CARB has concluded that ZEVs such as battery electric and hydrogen fuel cells are the best approach to addressing the climate problem and eliminating urban pollutants. However, the full transition to zero-emissions will take several decades, so efforts to reduce combustion-engine NOx, PM, and CO2 emissions need to continue.