The California Air Resources Board (CARB) has tested the utility of the Model 3090 Engine Exhaust Particle Sizer (EEPS™) by TSI in measuring pre- and post-trap particulate matter (PM) emissions from a medium-duty truck. Pre- and post-trap measurements are used to evaluate the effect of engine operation on PM emissions and trap effectiveness.Because of mounting evidence that ultrafine (UF) particles are harmful, regulatory agencies are investigating new and promising instrumentation for improved characterization of such particles in emissions. This is especially true for fast-response instruments that can be used to size-resolve real-time UF emissions from prominent sources such as diesel engines. The EEPS uses diffusion charging, electrical mobility segregation, and electrometers. It is designed for the number measurement of transient aerosols in the size range of 5.6 to 560 nm. It collects 10 measurements per second at a flow rate of 10 lpm.We sampled the emissions from a model year 2000 Isuzu medium-heavy-duty delivery truck. The truck was retrofitted with a Johnson Matthey Continuously Regenerative Trap (CRT™) and fueled by ultra low sulfur diesel (ULSD). Pre- and post-trap partial-flow samples of raw exhaust were taken using a double-stage Dekati diluter. Primary and secondary dilution ratios of approximately 8 each resulted in a total dilution of 64:1. The vehicle was operated over steady-state (50 mph cruise and idle) and transient (CBD and NYBC) duty cycles.The EEPS measurements suggest that without a trap, the size of the particles emitted by the diesel engine is highly dependent on the driving cycle. Emissions of particles smaller than 120 nm were greatest during accelerations and emissions of particles larger than 250 nm were greatest during decelerations and idle. Bursts of particularly high emissions of particles smaller than 10 nm were seen immediately after accelerations.The pre-trap size distribution was bi-modal during both steady state and transient engine operation. Peak concentrations were observed at or below 10 nm and at 60 nm at concentrations, uncorrected for dilution, of 105-106 N/cc. The post-trap size distribution and concentration did not change significantly as a function of driving cycles. The post-trap size distribution was bi-modal with peaks at 10 and 30 nm and peak concentration of 102 N/cc.The EEPS measurements showed substantial effectiveness of the CRT in reducing both transient and steady-state UF particle emissions over the entire size range. In the current study the CRT reduced the total number concentrations by two to three orders of magnitude.