Recent measurements and modeling of primary exhaust particulate matter (PM) emissions from both gasoline and diesel-powered motor vehicles suggest that many vehicles produce PM at rates substantially higher than assumed in the current EPA PM emission factor model, known as “PART5.” The discrepancy between actual versus modeled PM emissions is generally attributed to inadequate emissions data and outdated assumptions in the PART5 model. This paper presents a study with the objective of developing an in-house tool (a modified PART5 model) for the Texas Natural Resource Conservation Commission (TNRCC) to use for estimating motor vehicle exhaust PM emissions in Texas. The work included chassis dynamometer emissions testing on several heavy-duty diesel vehicles at the Southwest Research Institute (SwRI), analysis of the exhaust PM emissions and other regulated pollutants (i.e., HC,CO,NOx), review of related studies and exhaust PM emission data obtained from literature of similar types of light and heavy-duty vehicle tests, a review of the current PART5 model, and analysis of the associated emission deterioration rates. Exhaust PM emissions data obtained from the vehicle testing at SwRI and other similar studies (covering a relatively large number and wide range of vehicles) were merged, and finally, used to modify the PART5 model. The modified model, which was named PART5-TX1, was then used to estimate new exhaust PM emission factors for in-use motor vehicles. Modifications to the model are briefly described, along with emissions test results from the heavy-duty diesel-powered vehicles tested at SwRI. Readers interested in a detailed understanding of the techniques used to modify the PART5 model are referred to the final project report to TNRCC (Eastern Research Group 2000).

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