The University of Tennessee and Oak Ridge National Laboratory conducted a 5-month-long air monitoring study at the Watt Road interchange on I-40 in Knoxville, Tennessee, where 20,000 heavy-duty trucks travel the Interstate each day. In addition, three large truck stops are situated at this interchange, where as many as 400 trucks idle engines at night. As a result, high levels of PM2.5 have been measured near the interchange, often exceeding National Ambient Air Quality Standards. This paper presents the results of the air monitoring study, illustrating average hourly patterns of PM2.5 resulting from diesel truck emissions on the Interstate and at the truck stops. Most of the PM2.5 concentrations detected occurred during the night, when the largest contribution of emissions was from idling trucks rather than trucks on the Interstate. A nearby background air monitoring site was used to identify the contribution of regional PM2.5 emissions, which were also a significant factor in the concentrations measured at the site. The relative contributions of regional background, local truck idling, and trucks on the Interstate to local PM2.5 concentrations are presented and discussed in the paper. The results indicate the potential significance of diesel truck idling emissions to the occurrence of hot spots of high PM2.5 concentrations near large truck stops, ports, or border crossings. The significance of truck idling emissions is similar to the findings of other studies.