Evaluation of solid particle number and black carbon for very low particulate matter emissions standards in light-duty vehicles

ABSTRACT

To reliably measure at the low particulate matter (PM) levels needed to meet California’s Low Emission Vehicle (LEV III) 3- and 1-mg/mile particulate matter (PM) standards, various approaches other than gravimetric measurement have been suggested for testing purposes. In this work, a feasibility study of solid particle number (SPN, d50 = 23 nm) and black carbon (BC) as alternatives to gravimetric PM mass was conducted, based on the relationship of these two metrics to gravimetric PM mass, as well as the variability of each of these metrics. More than 150 Federal Test Procedure (FTP-75) or Supplemental Federal Test Procedure (US06) tests were conducted on 46 light-duty vehicles, including port-fuel-injected and direct-injected gasoline vehicles, as well as several light-duty diesel vehicles equipped with diesel particle filters (LDD/DPF). For FTP tests, emission variability of gravimetric PM mass was found to be slightly less than that of either SPN or BC, whereas the opposite was observed for US06 tests. Emission variability of PM mass for LDD/DPF was higher than that of both SPN and BC, primarily because of higher PM mass measurement uncertainties (background and precision) near or below 0.1 mg/mile. While strong correlations were observed from both SPN and BC to PM mass, the slopes are dependent on engine technologies and driving cycles, and the proportionality between the metrics can vary over the course of the test. Replacement of the LEV III PM mass emission standard with one other measurement metric may imperil the effectiveness of emission reduction, as a correlation-based relationship may evolve over future technologies for meeting stringent greenhouse standards.

Implications: Solid particle number and black carbon were suggested in place of PM mass for the California LEV III 1-mg/mile FTP standard. Their equivalence, proportionality, and emission variability in comparison to PM mass, based on a large light-duty vehicle fleet examined, are dependent on engine technologies and driving cycles. Such empirical derived correlations exhibit the limitation of using these metrics for enforcement and certification standards as vehicle combustion and after-treatment technologies advance.

Acknowledgment

The authors thank the CARB management and staff members who conducted emissions testing. The authors also thank David Eiges, Satya Sardar, Shiyan Chen, Pippin Mader, Bruce Frodin, Shiou-mei Huang, David Quiros, Michael Kamboures, and Mang Zhang for consultation on data validation, as well as Michael McCarthy and Alberto Ayala for providing policy direction and careful review of this paper.

Disclaimer

The statements and opinions expressed in this paper are solely the authors’ and do not represent the official position of CARB. The mention of trade names, products, and organizations does not constitute endorsement or recommendation for use. CARB is a department of the California Environmental Protection Agency. CARB’s mission is to promote and protect public health, welfare, and ecological resources through effective reduction of air pollutants while recognizing and considering effects on the economy. CARB oversees all air pollution control efforts in California to attain and maintain health-based air quality standards.

Additional information

Notes on contributors

M.-C. Oliver Chang

M.-C. Oliver Chang is the manager and J. Erin Shields is a staff member of the Aerosol Analysis and Methods Evaluation Section of the California Air Resources Board.

J. Erin Shields

M.-C. Oliver Chang is the manager and J. Erin Shields is a staff member of the Aerosol Analysis and Methods Evaluation Section of the California Air Resources Board.