Assessment of mesoscale and microscale simulations of a NO₂ episode supported by traffic modelling at microscopic level

The aim of this work is to simulate with high accuracy an episode of high NO2 pollution during December 2016 in the city of Madrid (Spain). For this purpose, a multiscale modelling system has been implemented that includes a mesoscale model (WRF/Chem) up to a horizontal resolution of 1 km and a computational fluid dynamics model (CFD; MICROSYS) with a resolution of 5 m. For the calculation of traffic emissions, a traffic simulation has been previously conducted with the SUMO microscopic model, calibrated from data measured with traffic counters. We show a substantial improvement in the results obtained with 5 m resolution with respect to those of 1 km, reproducing very closely the daily peaks of NO₂ concentrations since a very detailed traffic emission source is being used and the CFD reproduces the interactions between the air flow and the buildings. The modelling system presented can be used as a tool to evaluate different emission reduction strategies at street level, as it would allow to have an orientation on their effectiveness without having to implement them.