This paper presents a comprehensive set of ultrafine particles (UFPs) emission factors (EFs) for heavy duty vehicles (HDVs) as a function of vehicle flow rate, speed, and mode of operation (free flow and congestion) using 664 measurements of UFPs, carbon dioxide (CO2), meteorology and traffic conditions near a major roadway (average daily traffic 300,000 day–1). 5-min samples were collected for 2 to 3 hour time period on 60 days between 2015 and 2018. The average traffic-induced concentration of UFPs was 11,300 pt cm–3 for free flow and 12,400 pt cm–3 for congestion. Results demonstrate that HDVs produce significantly more dispersion (30x) than light duty vehicles (LDVs). The additional dispersion from HDVs results in the minimum pollutant concentrations occurring at the highest vehicle flow rate. EFs for UFPs are determined using inverse modeling based on the calculated CO2 dispersion. This eliminates the need to rely on air-quality models to estimate dispersion. The EFs for HDVs range from 4 × 1014 to 20 × 1014 (pt km–1 veh–1). The variations in EFs are correlated with variations in vehicle flow rate and speed. The average UFP EFs for HDVs are significantly higher (3x) for congestion compared to free flow. UFP EFs for HDVs are more sensitive to speed in congestion compare to in free flow conditions. Thus, even a moderate increase in HDVs speed or mitigation of congestion will have a significant impact on lowering UFP concentrations.