Hourly measurements of elemental carbon (EC) and organic carbon (OC) were made at Mong Kok, a roadside air quality monitoring station in Hong Kong, for a year, from May 2011 to April 2012. The monthly average EC concentrations were 3.8–4.9 μg C m<sup>−3</sup>, accounting for 9.2–17.7% of the PM<sub>2.5</sub> mass (21.5–49.7 μg m<sup>−3</sup>). The EC concentrations showed little seasonal variation and peaked twice daily, coinciding with the traffic rush hours of a day. Strong correlations were found between EC and NO<sub>x</sub> concentrations, especially during the rush hours in the morning, confirming vehicular emissions as the dominant source of EC at this site. The analysis by means of the minimum OC / EC ratio approach to determine the OC / EC ratio representative of primary vehicular emissions yields a value of 0.5 for (OC / EC)<sub>vehicle</sub>. By applying the derived (OC / EC)<sub>vehicle</sub> ratio to the data set, the monthly average vehicle-related OC was estimated to account for 17–64% of the measured OC throughout the year. Vehicle-related OC was also estimated using receptor modeling of a combined data set of hourly NO<sub>x</sub>, OC, EC and volatile organic compounds characteristic of different types of vehicular emissions. The OC<sub>vehicle</sub> estimations by the two different approaches were in good agreement. When both EC and vehicle-derived organic matter (OM) (assuming an OM-to-OC ratio of 1.4) are considered, vehicular carbonaceous aerosols contributed ~ 7.3 μg m<sup>−3</sup> to PM<sub>2.5</sub>, accounting for ~ 20% of PM<sub>2.5</sub> mass (38.3 μg m<sup>−3</sup>) during winter, when Hong Kong received significant influence of air pollutants transported from outside, and ~ 30% of PM<sub>2.5</sub> mass (28.2 μg m<sup>−3</sup>) during summertime, when local emission sources were dominant. A reduction of 3.8 μg m<sup>−3</sup> in vehicular carbonaceous aerosols was estimated during 07:00–11:00 (i.e., rush hours on weekdays) on Sundays and public holidays. This could mainly be attributed to less on-road public transportation (e.g., diesel-powered buses) in comparison with non-holidays. These multiple lines of evidence confirm local vehicular emissions as an important source of PM<sub>2.5</sub> in an urban roadside environment and suggest the importance of vehicular emission control in reducing exposure to PM<sub>2.5</sub> in busy roadside environments.