Stringent emission regulations have forced drastic technological improvements in diesel after treatment systems, particularly in reducing Particulate Matter (PM) emissions. Those improvements generally regard the use of Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter (DPF) and lately also the use of Selective Catalyst Reduction (SCR) systems along with improved engine control strategies for reduction of NOx emissions from these engines. Studies that have led to these technological advancements were made in controlled laboratory environment and are not representative of real world emissions from these engines or vehicles. In addition, formation and evolution of PM from these engines are extremely sensitive to overall changes in the dilution process. In light of this, the study of the exhaust plume of a heavy duty diesel vehicle operated inside a subsonic environmental wind tunnel can give us an idea of the dilution process and the representative emissions of the real world scenario.The subsonic environmental wind tunnel used for this study is capable of accommodating a full-sized heavy-duty truck and generating wind speeds in excess of 50mph. It was specifically designed and built by West Virginia University (WVU) to characterize the exhaust plume emitted of heavy duty vehicles. A 3 dimensional gantry system allows spanning the test section and sample regions in the plume with accuracy of less than 5mm. The gantry system was equipped with engine exhaust gas analyzers and Particulate Matter (PM) sizing instruments.The investigation involves three different heavy-duty Class-8 diesel vehicles equipped with after-treatment technologies, representative of legacy and modern truck fleets in the USA. The three vehicles investigated are representative of three emission regulation standards, namely a US-EPA 2007 compliant, a US-EPA 2010 compliant and a baseline vehicle without any after-treatment technologies as pre US-EPA 2007, respectively.The testing procedure includes three different vehicle speeds: idling, 20mph, and 35mph. The vehicles were tested on WVU's medium-duty chassis dynamometer, with the load applied to the truck reflecting the road load equation at the respective vehicle test speed. Wind tunnel wind speed and vehicle speed were maintained in close match during the entire test.Results show that, the cross-sectional plume area increases with increase in distance away from tailpipe. Also indicating the cooling and dilution of the exhaust begins at close proximity to the tailpipe. The rate of cooling and dilution are greatest in early stages of the dilution process for the areas with high turbulence intensity, where strong mixing phenomena occurs. On the other hand, the core of plume observes a slower cooling and dilution rate. This difference is reflected in the PM formation and evolution of these two distinct regions, as shown by the particle size distributions and number concentrations.