U.S. Environmental Protection AgencyOffice of Research and Development

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Size and Morphology of Particulate Emissions from Real-World Motor VehiclesParik Deshmukha, John. Bangb, Amar Kumbharc, Rich Baldaufd,e : Chad Baileye presentinga Arcadis Inc., Durham, NC, USAb North Carolina Central University, Durham, NC, USAc University of North Carolina-Chapel Hill, Chapel Hill, NC, USAD U.S. Environmental Protection Agency, Office of Research & Development, Durham, NC, USAe U.S. Environmental Protection Agency, Office of Transportation & Air Quality, Ann Arbor, MI, USA

Abstract

As motor vehicle emission standards continue to reduce tailpipe exhaust, brake and tire wear has become a more important pollution source, especially for airborne particles. While information exists on coarse mode particulate emissions from brake and tire wear, far less has been reported on the full size distribution of particulates released from brake and tire abrasion. Growing interest in using nano-materials for brake pad and tire tread manufacturing further raises interest in the size of particles emitted from these sources. For this study, a sampling system collected real-time, integrated filter, and transmission electron microscopy (TEM) grid samples for brake and tire wear from an all-electric vehicle operating under real-world driving conditions. The use of an electric vehicle provided particulate sampling of brake and tire wear without concerns for contamination from the test vehicle’s exhaust emissions. The study consisted of two field monitoring campaigns: sampling on existing highway and arterial roads in an urban area and sampling on a large vehicle performance track. The results identified particle emissions across a wide range of sizes, from nano- to coarse-mode. Different driving conditions (acceleration, deceleration and steady-state) resulted in the largest differences in emissions across all size ranges.

An all-electric vehicle provided the platform for collecting brake and tire wear particulate samples during real-world driving. The use of an electric vehicle eliminated the possibility of self-contamination of nano-sized combustion particles from the engine’s exhaust into the brake and tire sampling systems. The electric vehicle was equipped with filter collection systems and real-time particle counting instrumentation. The sampling system collected particulate matter in the nano- to micrometer-size range using PM10 cyclones at the sample inlet followed by real-time instrumentation measuring particle size and number concentrations from 5.6 nanometers (nm) to 20 microns (um). A second sampling train collected integrated TEM grid samples from the moving vehicle using two size-separation instruments which collected samples separately and in tandem. The table below lists all the instruments used in this study.

Methods

Continuous PM measurements provided insight on average and real-time emissions from differing tire types and brake pads during varying vehicle operations. The table below provides average PM number and mass measurement percentages from the EEPS and APS units collected during the performance track testing (see figure below for speed/location map). For all three tires, ultrafine particles accounted for over 90% of the number of particles emitted. Fine and coarse particles dominated the PM mass concentration values, with over 99 percent of the mass in these size ranges. However, 75% of the PM mass concentrations occurred in the fine size range for tire brands B and C while tire brand A had over 75 percent of the PM mass in the coarse size range.

Particulate Mass and Number Emissions Particle Morphology

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Transmission Electron Microscopy (TEM) provided a method to investigate the morphology of ultrafine particles emitted during tire and brake wear. For this study, the TEM results primarily focused on the identification of carbon nano-materials which may be used in tire and brake manufacturing to determine the potential presence in wear emissions. TEM results confirmed nano-particle emissions from tire and brake wear. These particles were often found on the TEM grid as part of larger particle clusters with diameters exceeding 1 micron. These primarily carbon compounds also contained metallic elements as part of the particle cluster.

PHOTO PHOTO

PHOTO

The sampling inlet faced the direction of vehicle travel to satisfy the primary condition of isokinetic sampling, not requiring oncoming particles emitted by the brake and/or tire wear to make a turn to enter the sampling probe. Since the vehicle speed varied during the real-world driving operations, isokinetic flow rates could not be maintained during the entire sampling period, a limitation for coarse particulate sampling results. For sampling brake and tire wear, the inlets were placed in close proximity to the contact surfaces; the tire and road surface for tire wear and brake pad and brake rotor surface for brake wear. For collecting tire wear, the sample probe was placed close to the front passenger side tire in the wheel well while, for collecting brake wear particles, the sample probe was placed in close proximity to the brake pad-rotor area using fixtures on the front passenger side to hold the sampling line in place securely. A schematic of the sampling system is shown in the figure to the right.

Measurement Frequency Instrument

Particle number concentration (size range 5.6-560 nm, 32 channels)

1 sEngine Exhaust Particle Sizer (Model 3090, TSI, Inc., Shoreview, Minnesota, USA)

Particle number concentration (size range 0.5-20 µm, 52 channels)

1 sAerodynamic Particle Sizer (Model 3321, TSI, Inc., Shoreview, Minnesota, USA)

Longitude and latitude 1 sGlobal positioning system (Crescent R100, Hemisphere GPS, Scottsdale, Arizona, USA)

Video of route <1 sWebcam (Logitech, Silicon Valley, California, USA)

Gravimetric Mass Filters Run length

47mm Teflon Filters/Filter Packs (URG, Chapel Hill, North Carolina, USA)

TEM Grid Samples Run length

Nano-ID™ Select (Particle Measuring Systems, Boulder, Colorado, USA)

TEM Grid SamplesRun Length

Thermal Precipitator (Design from Bang, et.al, 2009)

Type PM Concentration Number % PM Concentration Mass %

Ultrafine Fine Coarse Ultrafine Fine Coarse

Tires

A 90.20 9.71 0.09 0.06 21.66 78.27

B 93.27 6.66 0.06 0.24 77.89 21.86

C 94.18 5.81 0.01 0.02 81.41 18.57

Brakes

M97.48 2.50 0.012 0.313 59.81 39.88

NM99.59 0.39 0.012 0.069 44.71 55.22

The real-time EEPS and APS measurements provided information on brake and tire wear emissions under changing vehicle operations. The performance track offered controlled conditions that allowed vehicle operations to be binned into three categories: acceleration (vehicle speed increases from stationary to ~40 mph), deceleration (vehicle speed decreases from ~40 mph to stationary), and coast (vehicle speed is maintained with little to no acceleration or braking). These conditions represent only a portion of the total test period. Ultrafine particles dominated the number concentrations for all three tires during all three operating conditions, although the ultrafine and fine number concentrations were comparable for one tire and condition (Tire B, Coast). This same tire also had higher fine number concentrations for the acceleration condition than the other two tires. The coarse size bin dominated the mass concentrations for most of the drive conditions with an exception of higher fine particle mass concentrations under deceleration conditions for two of the tires (Tires A and C). Ultrafine particles dominate the PM number concentrations for the non-metallic brake pads for all three driving conditions. However, the fine size bin dominates the PM number concentrations under acceleration and coast conditions for the metallic brake pads. The mass concentrations also differ significantly between metallic and non-metallic brake pads with the coarse size bin dominant for non-metallic brake pads and the fine size bin dominant for the metallic brake pads.

Drive Condition

PM Concentration Number % PM Concentration Mass %

Ultrafine Fine Coarse Ultrafine Fine CoarseTire AAcceleration 98.65 1.28 0.06 0.014 20.13 79.85Deceleration 85.67 14.33 0.001 4.11 51.48 44.41Coast 82.37 17.08 0.55 0.02 16.71 83.27Tire B Acceleration 87.73 12.12 0.15 0.021 18.26 81.71Deceleration 99.33 0.67 0.004 1.30 18.53 80.16Coast 51.42 48.16 0.42 0.015 18.27 81.71Tire CAcceleration 97.60 2.37 0.03 0.074 17.27 82.66Deceleration 81.48 18.52 0.001 3.87 61.11 35.02Coast 93.23 6.71 0.06 0.06 18.38 81.56

Drive Condition

PM Concentration Number % PM Concentration Mass %

Ultrafine Fine Coarse Ultrafine Fine CoarseNon-Metallic BrakeAcceleration 81.96 17.59 0.44 0.012 40.95 59.04Deceleration 99.88 0.12 0.002 0.683 36.94 62.38Coast 81.96 17.59 0.44 0.059 40.38 59.56Metallic BrakeAcceleration 0.442 94.14 5.41 0.015 64.41 35.57Deceleration 85.64 13.74 0.614 4.33 63.82 31.84Coast 0.473 93.14 6.386 0.013 53.70 46.28

Metallic brake pad sample: The figure above shows a tubular nanostructure on a 100nm scale captured by the Nano-ID Select. This tubular structure appears to have several CNOs or rolled up graphene sheets forming an onion-like structure along its surface (shown in the 20nm scale image). The rolled up grapheme sheets can be seen on the highest magnification (10nm scale image).

Non-metallic brake pad sample: Tubular nanostructures can be seen on the 50nm scale as part of a larger particle cluster. Magnified scales show the tubular nanostructure and graphitic sheets arranged as layers in a tubular structure (20nm and 10nm scales).

Tire sample: The 100nm scale shows an agglomerate particle structure. A number of circular nano structures appear embedded along the surface of this agglomerate. The 10 nm scale image shows a nano-onion captured from a tire wear sample.