Modelling reactive pollutants dispersion in an urban hot-spot in summer conditions using a CFD model coupled with meteorological mesoscale and chemistry-transport models

Air quality assessment requires detailed studies about urban air pollution. In a city, the interaction between atmosphere and urban morphology induces complex flow pattern which leads to irregular spatial distributions of pollutants in the streets. In addition, the influence of chemical reactions and the variability of air temperature and wind make difficult to reproduce the pollutants dispersion at microscale during long periods, especially in summer conditions. The aim of this work is to simulate reactive pollutants dispersion in summer conditions in an urban hot-spot using a computational fluid dynamics (CFD) model. In summer, the high solar radiation and the high levels of O3 concentration requires introducing a chemical system in the CFD modelling. Focusing on studying the NO2 dispersion, the photostationary state is implemented in the CFD simulation. The detailed boundary conditions are obtained from the outputs of other microscale and mesoscale models. The vertical profiles of wind speed, turbulence and temperature are derived from a meteorological mesoscale simulation (WRF) and the background concentration of pollutants is obtained from a chemistry-transport (CMAQ) simulation. Additionally, the emissions of traffic-related pollutants are implemented into the CFD simulation based on the results of a microscale traffic model. Finally, the time evolution of meteorological variables and pollutants concentration resulting from the CFD simulation are thoroughly evaluated with measurements registered at several points from an experimental campaign carried out in the research area (in the framework of TECNAIRE project (S2013/MAE-2972)). Regarding the air quality assessment, the deviation of NO2 concentration including chemical reactions in the CFD simulation is quantified in comparison with a non-reactive pollutant. Likewise, the differences of assuming neutral atmospheric conditions instead of using profiles from mesoscale model for this period are analyzed through the time average concentration map at pedestrian level. In this way, the improvements included in the CFD modelling and the conclusions obtained from this analysis provide information on how to simulate reactive pollutant dispersion in an urban hot-spot in summer conditions