The chemical and physical properties of exhaust particlesproduced by a Caterpillar 3176 C-12 heavy duty dieselengine equipped with a catalytic trap (CRT) are reported.The engine was operated at 600 Nm and 1500 rpm,using fuels containing 15 and 49 ppm sulfur. A two-stagedilution tunnel designed to simulate the reactions that occurwhen hot combustion products mix with cooler atmosphericair was used. Particle size distributions were measuredusing a scanning mobility particle sizer (SMPS) and nano-scanning mobility particle sizer (nano SMPS); a nano-micro-orifice uniform deposit impactor (nano MOUDI)collected size-resolved samples for gravimetric and chemicalanalysis. A nanometer tandem differential mobilityanalyzer (nano TDMA) was used to measure the volatilityand hygroscopicity of 4-15 nm particles. These measurements confirm that the particles consisted primarily of sulfates.