Project at a Glance
Title: Evaluation of the proposed new European methodology for determination of particle number emissions and its potential in California for in-use screening.
Principal Investigator / Author(s): Durbin, Dr. Thomas D
Contractor: UC Riverside
Contract Number: 05-320
Research Program Area: Emissions Monitoring & Control
Topic Areas: Mobile Sources & Fuels
The European countries have developed new methodologies for the measurement of “solid” particle number above a size cut threshold of 23 nanometers (nm) to compliment gravimetric measurements at low particle levels typically found with engines and vehicles equipped with diesel particle filters (DPFs). This methodology provides significant improvements in measurement sensitivity, but should be fully understood in terms of the representativeness of the particles, which are only solid and > 23 nm in size, their adverse health effects, and the repeatability of the measurement. The California Air Resources Board (CARB) sought to enhance the understanding of the PMP methodology and to develop well-informed opinions regarding its application. The specific objective of this study was to critically evaluate the proposed PMP method for determining “solid” particle number emissions from heavy-duty vehicles in the laboratory and during over-the-road driving. For this program, testing was conducted on the chassis dynamometer at the CARB heavy-duty vehicle emissions laboratory in Los Angeles and over the road with the CE-CERT mobile emissions laboratory (MEL). One or two PMP dilutions systems for measuring solid particle number were tested and compared directly with filter-based PM measurements on two heavy-duty trucks equipped with a DPF. These PMP dilution systems met the latest specifications and design criteria for setting up the PMP system, but were not characterized for the calibration criteria incorporated into the final PMP methodology. As such, they can not be considered to be fully compliant with the most recent PMP requirements. A full suite of other particle measurements and instruments were also used in conjunction with this testing including a TSI EEPS, a Cambustion DMS, and TSI CPCs with lower size cuts ranging from 3 to 23 nm, a Dekati DMM, and a DustTrak. The test cycles included a 50 mph cruise, UDDS, idle, and some European driving schedules. This program was conducted in collaboration with other particle emission experts from Europe and the United States (US), including European counterparts who have direct, hands on experience with the PMP protocol.
Particle number offers the potential to more readily characterize emissions from DPF-equipped engines and vehicles. The emission level and variability between the different PMP particle number counts and the PM mass depended on the cycle, sampling location and time, the specific test instrument, and other experimental conditions. The advantages of particle number measurements in these respects was clearly seen for the on-road testing, but were less clear for the laboratory measurements, where longer tests produced higher/quantifiable PM mass levels and particle number outliers were found. Outlier tests were observed in the laboratory tests that appear to be real events that may be masked in the mass measurement. Thus, statistical techniques for the removal of outlier tests for particle number should be considered to provide for the most consistent and repeatable measurements. The particle number measurements provided the greatest advantages at typical emission levels for current wall-flow DPFs, which are often only a small percentage of the certification value. At these levels, the mass measurements typically show greater scatter, are usually below the quantification limit, and can be impacted heavily by artifact formation. At higher mass levels, such as levels closer to the actual limit of the 2007 standard for heavy-duty engines, the quantification accuracy and repeatability of the gravimetric PM measurements improve, so the advantages of the PMP system would not be as significant. Nucleation particles were found to form under a variety of testing conditions when the temperature of the aftertreatment device exceeded a ‘critical’ temperature, which was typically in excess of 300°C. Sulfate was an important portion of the chemical make-up of the nucleation particles, suggesting that SO2 to SO3 conversion over the catalyst played an important role in their formation. The level of nucleation increased significantly under the most aggressive, on-road testing conditions. In order to better characterize the PMP system, further study is suggested to understand the chemical composition and nature of the particles measured below the PMP, the potential impacts of utilizing a smaller size cut for counting particles below the PMP, and the impacts of the different design elements of the PMP. Testing should also be expanded to include a wider range of vehicles and aftertreatment systems.
For questions regarding research reports, contact: Heather Choi at (916) 322-3893
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