Challenges in PM Measurement at 1 mg/mile and Tunnel Background Correction

The LEV IV FTP PM limit in the recently approved CARB ACC II regulations for passenger cars and light duty trucks will be 1 mg/mile starting in 2025. Gravimetric PM measurement at these levels is very challenging as the net mass of PM on the filter in full flow tunnel testing ranges between 8 to 32 micrograms depending on amount of dilution. This is approaching tunnel background levels which, in combination with filter handling, static charge removal and microbalance instability, compounds the uncertainty. One major source of the uncertainty at these low levels is the tunnel contamination resulting in high variability from test to test and cell to cell. This tunnel background is mostly HC artifact which cannot be easily controlled and can be significantly higher than the 5-μg CFR allowable correction limit in some test cells. Items that might affect the PM background include the type of testing being run on the tunnel prior to measuring the background such as OBD, cold and diesel testing. Quantifying the contribution of these artifacts and how they play into the overall uncertainty cannot be easily determined at this time. Regular conditioning and cleaning of the tunnel to keep the tunnel background below the 5-μg limit has shown to be crucial at these low levels. The process to drive contamination out and reduce the tunnel background below 5 μg will be described. A model is created to determine the range of net PM mass on the filter at 1 mg/mile with different CVS flowrates. The current process of tunnel background measurement and correction will be discussed. Countermeasures to reduce the uncertainty in PM measurement at 1 mg/mile PM level will be explored.