Recently, there are discussions about whether current sampling and measurement practices for the regulated gravimetric PM measurement are sufficiently accurate in quantifying PM at the proposed 3 and 1 mg/mi emission standards for light-duty vehicles. In this study, a series of modifications were made to the existing gravimetric PM measurement method, aiming to preserve the integrity of the method while increasing the robustness and decreasing the testing variability. The experiments were conducted with a Higher (~2 mg/mile) and a Lower (0.1-0.2 mg/mile) PM Source Vehicle over the Federal Test Procedure (FTP) and US06 cycles, providing PM emissions with various solid/semi-volatile compositions and size distributions. The results showed the suggested modifications, i.e., increased filter face velocities (from 100 to 150 cm/s) and combined filters (single filter vs. 3/4 filters), could increase the collected filter mass without introducing statistically significant differences in the measured PM mass emission rates. No statistically significant improvements were seen in the measurement variability with the Higher PM Source Vehicle. For the Lower PM Source Vehicle; however, the 4-phase cumulative filter showed a statistically significant reduction in PM mass measurement variability, while not impacting the measured PM mass emissions, but these improvements must be weighed against the increased testing costs/time required for the longer test time.
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.
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