Analysis of spatial representativeness of urban monitoring stations using steady CFD-RANS simulations

Urban air quality assessment is an important task, complicated by the heterogeneities induced by urban morphology on the atmospheric features in the urban canopy layer. A network of urban monitoring stations, as used in most European cities, is often not enough because the spatial representativeness of point measurements is limited due to these heterogeneities. Hence, urban modeling can be a useful tool to complement experimental data from an urban air quality network of stations. Computational Fluid Dynamic (CFD) models solve flow and dispersion around the buildings explicitly and can provide detailed flow and dispersion patterns inside urban zones (streets, squares,...). However, the computational time required for this type of simulations is high and it is not possible to perform an unsteady CFD simulation during large time periods in order to get daily, monthly or annual averaged concentration distribution which is often required by regulatory purposes. To solve this problem, we propose a methodology based on a set of steady CFD-RANS simulations with different inlet wind directions. The emissions are modeled with a line source inside each street. In order to compute the total concentration, the predominant wind direction at each hour is taken into account, together with some factors to modulate the simulated concentrations (wind speed, hourly number of cars, length of streets). This methodology is applied to a real case (a central square in Pamplona, Spain) and it has been evaluated against experimental measurements (Parra et al., 2010, A methodology to urban air quality assessment during large time periods of winter using computational fluid dynamic models. Atmospheric Environment, doi: 10.1016/j.atmosenv.2010.03.009). Finally, we analyze the modeled concentration at the station location in comparison with the concentration in its surroundings in order to estimate the spatial representativeness of the urban monitoring station. The assessment of air quality is usually made according to urban station measurements and it is important to have an estimate of the concentration around the station to complement its measured data.