16th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes

The influence of vegetation on the concentrations of traffic pollutants in urban street canyons is examined using numerical simulations with the CFD code OpenFOAM. This CFD approach is validated against literature wind tunnel data on traffic pollutant dispersion in street canyons. The impact of avenues of trees is simulated for a variety of vegetation types and the full range of approaching wind directions at 15° interval. All these results are combined in meteostatistical analysis, including effects of seasonal leaf loss, to determine the annual average effect of trees in street canyons. This analysis is performed for two pollutants, elemental carbon (EC) and PM10, using background concentrations and emission strengths for the city of Antwerp, Belgium. The results show that due to the presence of trees the annual average pollutant concentrations increase with about 8% (range of 1% to 13%) for EC and with about 1.4% (range of 0.2 to 2.6%) for PM10. The study also indicates that this annual effect is considerably less than earlier estimates which are generally based on steady state results based on a specific set of governing conditions (1 wind direction, full leafed trees, peak hour traffic emissions). This implies that the adverse air quality effect of street canyon trees may not be as harmful as previously assumed.