Simulation of NOx and ultrafine particles in a street canyon in Stockholm, Sweden

Abstract A computational fluid dynamic (CFD) model has been used to assess the concentrations of NO x and particle number in a street canyon in Stockholm with a high traffic volume. Comparisons of a simulated 11-week long time series of NO x with measurements (both sides of the street, urban background excluded) show good agreement, especially if emissions are distributed to be three times higher along the side of the street where the traffic is uphill, as compared to the downhill side. The simulation of number concentrations of inert particles indicates a similar asymmetry in emissions. A month-long measurement of particle size distribution (7–450 nm) at street level indicates that the ratio of nucleation size mode particle (7–20 nm) to total particle number (7–450 nm) is decreasing for increased particle surface area. Given the strong dominance of the locally generated particles over the urban background, this is interpreted as a local change in the size distribution. The results of a monodisperse aerosol dynamic model, coupled to the CFD model that simulates also the turbulence generated by vehicle movements, show that coagulation and deposition may reduce total particle inside the canyon with approximately 30% during low wind speeds. Most of the removal occurs shortly after emission, before the particles reach the leeward curb-side. Losses between the leeward curb-side and other locations in the street, e.g. roof levels, is estimated to be smaller, less than 10%. Coagulation is the dominating removal process under low wind speed conditions and deposition for higher wind speeds, the summed removal being smaller for high wind velocities. Deposition is enhanced over the road surface due to the velocities generated by vehicle movements. Although coagulation and deposition removal is most effective on the smallest ultrafine particles, this effect is not sufficient to explain the observed change in size distribution. It is suggested that also the formation of particles in the exhaust plumes is influenced by a larger particle surface area in the ambient air.