Given anticipated significant increases in the proportion of electric drive vehicles (EDVs), including Hybrid Electric Vehicles (HEVs), Plug-In Hybrid Electric Vehicles (PHEVs), and Battery Electric Vehicles (BEVs) in the passenger and heavy-duty vehicle fleet to achieve a more sustainable transportation system, it is critical that we have reliable models of HEV and PHEV tailpipe emissions behavior during real-world operation of these vehicles. The proposed research addresses a lack of real-world data on the detailed behavior of HEV technology in response to real driving conditions (road conditions and weather). The University of Vermont Transportation Air Quality Laboratory (UVM TAQLab) previously collected a unique dataset of HEV emissions and vehicle operating parameters during on-road driving through all seasons. The proposed study aims to develop quantitative understanding of the relationships between hybrid system energy management and emissions for a HEV passenger car. Specifically, the study will fill two knowledge gaps: (1) the detailed effects of environmental conditions (ambient temperature changes) and road characteristics (road grade and average link speed), on the HEV electric motor/battery-Internal Combustion Engines (ICE) “power split” during real-world driving; and (2) how this “power split” affects the magnitude and spatial distribution of exhaust particle emissions from HEVs. Ultimately, the researchers will develop a quantitative understanding of the factors relating instantaneous energy use to ultrafine particle tailpipe emissions.