California Air Resources Board

Principal Investigator/Author(s): Sandeep Kishan

Contractor: Eastern Research Group, Inc

Contract Number: 22RD002

Relevant CARB Programs: Mobile Source Emissions

Topic Areas: Non-Exhaust Emissions

Research Summary:

The California Air Resources Board (CARB) is tasked with protecting the public from the harmful effects of air pollution and developing programs and actions to fight climate change. As tailpipe PM emissions from on-road vehicles have decreased significantly over time, non-exhaust PM emissions such as airborne brake- and tire-wear emissions have increased and their relative contributions to regional PM emissions and impact on nearby communities have increased. It is projected that airborne brake- and tire-wear PM emissions will increase as transportation vehicle miles traveled (VMT) increases while tailpipe emissions have been and are expected to continue to decrease due to regulatory and incentive programs in California.

Brake and tire materials, driving behaviors, and roadway conditions (speed, traffic, road geometry, and pavement types) can affect chemical and physical characteristics of brake- and tire-wear PM emissions. Airborne brake-wear and tire-wear PM is generated during vehicle deceleration and acceleration when shear forces increase between friction material and disk and between tire and road, respectively. Brake- and tire-wear PM emissions are known to contain heavy metals that potentially cause adverse health effect. Recent CARB analysis has revealed that major inorganic contents include Fe, Cu, Zn, and Ti for California LDV brake-wear and Si, Al, Ca, Ti, S, K, Cu, Fe, Pb, Mg, Te, Se, and Cd for tire-wear PM (Contract #18RD017). Advanced technologies such as regenerative braking and low-rolling-resistance tires are rapidly entering the fleet for the purpose of reducing PM and greenhouse gas emissions from vehicles. Based on brake-dynamometer tests, regenerative braking technology can decrease braking PM emissions by 50% for hybrid electric vehicles and by 75% for battery electric vehicles compared to conventional internal combustion engine vehicles. However, the reductions observed from the laboratory-based study was unable to account for the large number of braking events that are less than 3 seconds that occur during real world driving. The uncertainty of the laboratory results needs to be investigated by comparing to brake-wear PM emissions measured during real-world on-road driving conditions. For tire-wear emissions, there is neither a commonly accepted nor standardized test method. It is necessary to develop a test method for measuring airborne PM emissions from conventional vehicle and electric vehicle tires in real-world driving conditions that reflect tire types, vehicle types, traffic conditions, roadway geometry, pavement types and materials, etc. In this research, real-world on-road brake- and tire-wear PM emissions will be measured to characterize California LDV and HDV non-exhaust PM emissions. The results of this study inform relevant programs, including the emission inventory.

The objectives of this study are to measure and analyze real-world brake and tire-wear particulate matter (PM) emissions, to understand the emission impact of moving to electric vehicles, and to inform relevant health impact studies and programs. California representative light-duty vehicles (LDV) and heavy-duty vehicles (HDV) with different power sources (gasoline, diesel, and electricity) will be tested in this study. This study will require developing innovative methods to measure real-world brake- and tire-wear PM as well as particulate number (PN) count and size distributions. Measurement uncertainties reported from the laboratory tests (CARB Contract #17RD016 and Caltrans Project #65A0703) will be investigated by comparing the test results to real-world brake-wear PM and PN that will be measured from this study.