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Vehicular emissions contribute to roadside pollutants in cities, yet on-road emission estimates remain relatively uncommon. Nevertheless, they make an important contribution to exposure along Hong Kong's congested roads and street canyons. This study used kerbside microsensor-based monitors on Hennessy Road, one of the busiest roads in the north part of Hong Kong Island, to determine both concentrations and emission factors (EFs) of NOx and CO. Kerbside NOx and CO concentrations are skewed, with high concentrations representing plume segments from larger vehicles. The average EFs for each minute of traffic was determined from the pollutant concentration ratio with ΔCO 2. These related to bus frequency for NOx, and EURO 4 or lower-emission standard vehicles for CO. The 1-min mean EF NOx 7.55 g kg−1 and EF CO 12.6 g kg−1 are typical for flows for the fleet dominated by buses, but highly skewed (Weibull shape parameter ∼0.44). Individual vehicle EFs were determined from peaks in NOx, CO and CO 2 , and number plates. Leading and trailing parts of the plume segments gave similar EFs (R 2 > 0.95), suggesting this method was reasonably robust across the vehicle passage. Nevertheless, these EFs were also skewed, but the shape parameter was again ∼0.44. EF NOX for vehicle classes was: buses > goods vehicles > private cars > vans > taxis and diesel > petrol > LPG, with larger engine sizes also dominant. Differences were more difficult to assess with CO, but LPG vehicles had the highest EFs. Our EF estimates lay in the range found in previous studies but differ from fleet emissions calculated for regulatory purposes in Hong Kong (EMFAC). However, some high EMFAC emitters were also high in our estimates for individual vehicles. Vehicles may not perform on-road as inventory calculations suggest. The high EF variability found in our study implies that a large sample is required to assess the likely emissions from a vehicle fleet. • Kerbside NOx depends on bus frequency; CO on EURO emission standards. • Slope of NOx and CO against CO 2 in plumes yields individual emission factors. • Emission factors are the same from leading or trailing parts of plumes. • Individual emission factors within vehicle classes are highly skewed. [ABSTRACT FROM AUTHOR]