Based on the results from a 6-week monitoring campaign in an area close to a major highway north of Stockholm, Sweden, NOx emission factors representative for vehicle speeds of 100-120 km per h were determined to 0.61 g/veh,km for light duty and to 7.1 g/veh,km for heavy duty vehicles. The corresponding factors for particle number were 1.4 x 10(14) and 52 x 10(14) particles/veh,km, determined for an ambient temperature interval of +7 to +17 degrees C. The removal effects of coagulation and dry deposition on total number concentrations were assessed by numerical model simulations. Velocity and turbulence fields, including those produced by the vehicles, were simulated in a Computational Fluid Dynamics (CFD) model. Coagulation was found to be of little importance over the first 100 m downwind of the highway. The high friction velocities over the road surface created by vehicle movements enhanced deposition locally, contributing to the removal of approximately 10% of the particles originally emitted. Beyond a point 10 m downwind of the highway the removal rate was low and the ultrafine particles were almost inert while being advected over the next hundred meters. As a consequence, it seems reasonable to use monitored data from stations close to highways to estimate emission factors for particle number, assuming that the particles are inert. Those "effective" emission factors should be applicable for urban models with a larger spatial resolution.