California Air Resources Board

Vehicles emit inhalable particles from two major sources: the exhaust system, which has been extensively characterized and regulated; and non-exhaust sources including brake wear, tire and road wear, clutch wear and road dust resuspension. The non-exhaust sources have not been regulated because they are difficult to measure and control. However, with increasingly stringent standards for exhaust emissions, the non-exhaust fraction has become increasingly important. Model predictions (both MOVES and EMFAC) suggest that traffic-related emissions of both PM_2.5 and PM₁₀ will eventually be dominated by non-exhaust sources.

Additionally, there is concern that exposure to these particles may increase in California because proposed regional land use and transportation plans may lead to denser cities and a higher proximity of people to major roadways. Given the increased relevance of non-exhaust emissions, new studies are needed to better estimate their magnitude and to assess the potential to control them. But before an effective method to control these emissions can be devised, a greater understanding of their physical and compositional characteristics as well as overall emissions is needed.

The study of non-exhaust sources is highly complex and each of the individual sources listed above has a wide range of controlling factors.

The Vehicle Emissions Research Program has been tasked with researching this topic, initiating external research contracts and performing in-house studies, to better understand the individual non-exhaust sources. We are also in the process of collaborating with various other agencies and organizations to help understand the scope of this problem. Our research strategy can be broadly summarized as follows:

One of our primary goals is to provide updated information for CARB's EMFAC model and to gain a better understanding of the most important factors affecting the emissions from non-exhaust sources. We also want to better understand the impact of this pollution source on communities living near roadways. Below is a list of projects currently studying the major non-exhaust sources.

Research Efforts

Measure brake wear emissions under controlled conditions and under different driving conditions (17RD016)

CARB awarded Eastern Research Group (ERG) and LINK Engineering a contract to derive speed-dependent emission factors using a brake dynamometer which minimizes PM losses and has zero interferences. Six light-duty vehicles, including both passenger cars and trucks, were tested on this dynamometer. ERG/LINK varied brake materials, vehicle loads, and driving behavior to explore variations and define the most important parameters affecting brake wear PM.

This project began in the Spring of 2018 and was recently completed. Major findings include:

• Vehicle-level brake wear PM emission rates vary from 3.3 to 13.6 mg/mile
• Metallic brake pad materials tended to emit larger particles and greater overall mass
• A California Brake Dynamometer Cycle was developed, based on in-use LDV activity data, for use on the brake dynamometer

The results of this project have been used to constrain brake wear emissions rates in EFMAC2021. More information on this study can be found in its final report (link), its Research Seminar (link), and in the following journal articles:

• Agudelo, C., Vedula, R.T., Capecelatro, J., Wang, Q.: Design of Experiments for Effects and Interactions during Brake Emissions Testing Using High-Fidelity Computational Fluid Dynamics, SAE Technical Paper 2019-01-2139 (2019), doi:10.4271/2019-01-2139.
• Agudelo, C., Vedula, R., Collier, S., and Stanard, A.: Brake Particulate Emissions Measurements for Six Light-Duty Vehicles Using Inertia Dynamometer Testing, SAE Int. J. Adv. & Curr. Prac. in Mobility (2021), 3 (2), 944 – 1019, doi:10.4271/2020-01-1637.

Brake Wear in Particulate Matter Emission Modeling (CalTrans Project No. 65A0703)

CARB has also collaborated on a CalTrans-led project that characterized brake wear emissions from heavy-duty vehicles (HDVs). This project both quantified brake wear PM emission rates from HDVs and analyzed the chemical composition of this PM. CARB staff contributed to this effort by serving as technical experts. The final report is now available. These results have already been used to constrain HD brake wear emission rates in EMFAC2021, and will be used in real-world source apportionment studies that estimate the portion of ambient PM that originates as HDV brake wear.

• Brake Wear in Particulate Matter Emission Modeling (ERG & LINK)

Real-World Tire and Brake Wear Emissions (18RD017)

CARB has awarded the University of California, Riverside (UCR) a research contract to investigate the impact of non-exhaust emissions on near-roadway environments. UCR has assembled a team of experts to sample roadside emissions in high detail in order to investigate the impact of brake wear, tire wear, and road dust as it relates to other vehicle emissions such as tailpipe emissions and background sources in Southern California. In early 2020, they collected samples at sites characterized by different fleet mixes and made real-time as well as time-integrated particulate matter (PM) measurements (below). This has allowed the researchers to collect both physical and compositional information of PM in high detail which can now be analyzed to distinguish sources.

The results will be analyzed to derive emission factors as a function of traffic conditions and will be fed into a dispersion model to better understand the impact of non-exhaust emissions on downwind communities that are impacted by traffic pollution. This project was kicked off in April of 2019 and will end by early 2022.

More information on this study can be found in its final report (link), and its Research Seminar (link).

In-house brake wear measurements

As part of CARB's efforts to better characterize non-exhaust particulate matter, staff are working at the Haagen-Smit Laboratory to adapt a unique emissions testing platform to measure brake wear PM. 

Staff utilized one of the running loss sealed housing for evaporative determination (RL-SHED) chambers. The RL-SHED is a sealed chamber featuring a full chassis dynamometer where vehicle exhaust is isolated from the inside of the chamber, and can be used to observe non-exhaust sources of PM, particularly from brake wear. A similar study was performed previously and the results of that study are available here.

Results showed that PM measured in the chamber integrated over whole drive cycles depended on number of braking events per mile driven. Real-time braking behavior showed high PM events highly dependent on braking force. Although these results are preliminary, they indicate that this facility can be used in the future to gain a better understanding of this source of emissions. Future work will include more instrumentation and multiple vehicles tested.

Collaboration with the European Joint Research Centre

Program (PMP) and recently it was tasked with defining a drive cycle to be used with a brake dynamometer which simulates typical braking behavior in light-duty vehicles. A second task includes defining a standardized method for measuring particulate matter (PM) from an enclosed brake dynamometer.

The results from both of these tasks will be made publicly available in the future and will set the standard for laboratory measurements of brake wear PM which will be used by various international stakeholders including industry and academic researchers. CARB is collaborating with the JRC on this project, by contributing both technical expertise and sharing lessons learned from similar projects in California. Specifically, CARB staff have provided information on the brake dynamometer study discussed above, including the California Brake Dynamometer Cycle developed by that study.