Due to the common stop-and-go driving style, the low temperature of vehicular exhaust gas in the urban driving cycle is a major cause of air pollution in the Seoul Metropolitan Area. We herein investigate the characteristics of regulated (NOx, PM, CO, and non-methane hydrocarbons (NMHC)) and unregulated (volatile organic compounds (VOCs), aldehydes, and polycyclic aromatic hydrocarbons (PAHs)) air pollutants emitted from heavy duty diesel trucks and buses equipped with different after-treatment systems (diesel particulate filter (DPF) + exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) in urban conditions. NOx emissions depended on the combustion and working temperature of the SCR catalysts, and PM emissions were low. Alkanes dominated the non-methane volatile organic compound (NMVOC) emissions, 43–59% of which resulted from the low efficiency of the oxidation catalyst for alkane. The after-treatment system and the engine start conditions influenced the chemical components of the NMVOC emissions due to incomplete combustion and the evaporation of liquid fuel. Formaldehyde comprised the largest portion of the aldehydes, whereas PAH emissions remained largely undetected. Furthermore, formaldehyde was the largest contributor to the NMHCs, forming 14–29%. The results of this study will aid in establishing a system for calculating hazardous air pollutants emitted by vehicles in Korea.