With the reduction in PM emission standards for light duty vehicles to 3 mg/mi for current Federal and California standards and subsequently to 1 mg/mi in 2025 for California, the required PM measurements are approaching the detection limits of the gravimetric method. A “filter survey” was conducted with 11 laboratories, representing industry, agencies, research institutes, and academic institutions to analyze the accuracy of the current gravimetric filter measurement method under controlled conditions. The reference filter variability, measured within a given day over periods as short as an hour, ranged from 0.61 μg to 2 μg to 5.0 μg for the 5th, 50th, 95th percentiles (n > 40,000 weights, 317 reference objects), with a laboratory average of 2.5 μg. Reference filters were found to gain approximately 0.01 to 0.56 μg per day (50th percentile) and 0.5 to 1.8 μg per day (95th percentile) with an average of 4.1 μg for the laboratories, which suggests a gas-phase adsorption artifact because metal reference objects did not gain any weight. Tunnel blank biases (n = 615) were much higher than the reference filter bias and had a range from 1.1, 2.8, and 13.0 μg, for the 5th, 50th, and 95th percentiles, with an average of 4.1 μg. Robotically weighed filters showed lower reference filter variability, but expectedly, there were no significant advantages for weighing tunnel blanks. The higher tunnel blank compared to the reference blank suggests that the sample collection system is a relatively significant contamination source. The uncertainties associated with filter weighing for tunnel blanks were generally less than the 5 μg tunnel blank correction allowed under 40 CFR 1066.
Bibliographical noteFunding Information:
We acknowledge Caterpillar, Chrysler, Cummins, Ford, John Deere, General Motors, Environmental Protection Agency at Ann Arbor, Environmental Protection Agency at Research Triangle Park, California Air Resources Board, and West Virginia University for their participation to make this study possible. Authors also acknowledge Coordinating Research Council and California Air Resources Board for funding under E-99 project.
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- Blank filter