The development of a multi-scale modelling system for evaluation of urban NOx levels in Modena (Italy)

In order to support environmental policies, epidemiological studies and urban mobility planning, a multi-scale modelling system was developed to provide hourly NO_x (NO + NO₂) concentration fields at a building-resolving scale in the urban area of Modena, a city in the middle of the Po Valley (Italy). The modelling system relied on two different models: the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), which is able to compute concentration fields over regional domain by considering specific emission scenarios, and Parallel Micro SWIFT SPRAY (PMSS), a Lagrangian particle model accounting for dispersion phenomena within the urban area. PMSS was used to simulate at building-scale resolution the NO_x dispersion produced by urban traffic flows in the city of Modena. Conversely, WRF-Chem was selected to estimate the NO_x  background concentrations over three nested domains with resolution of 15, 3 and 1 km in order to take into account emissions both at regional and local scale by excluding traffic emissions sources over the city of Modena. The estimation of traffic emissions in the urban area of Modena was based on a bottom-up approach relying on the Emission Factors suggested by the European Monitoring and Evaluation Programme (EMEP/EEA) and traffic fluxes estimated by the PTV VISUM model. By contrast, other anthropogenic emissions were taken from the TNO-MACC III inventory at the scales resolved by the WRF-Chem model.

Simulation was performed between 28 October and 8 November 2016, the same period whereby a direct vehicle flow measurement campaign was carried out continuously, with 4 Doppler radar counters in a four-lane road in Modena, to reproduce the hourly modulation rates of the emissions. The performances of the model chain were finally assessed by comparing modelled NO_x concentrations with observations at two air quality monitoring stations located inside the urban domain.

Simulated and observed NO_x hourly concentrations exhibit a large agreement, in particular for urban traffic site where detailed traffic emissions estimation (real traffic modulation combined with a bottom-up approach) proved to be very successful in reproducing the observed NO_x pattern. At the urban background station, notwithstanding a general underestimation of the observed concentrations (more pronounced than at the urban traffic site), the analysis of hourly daily modelled concentrations shows that PMSS combined with WRF-Chem provided a daily pattern in line with observations. These features highlight the strength of this modelling chain in representing urban air quality, in particular at traffic sites, whose concentration levels make them the most critical area of the city; characteristics that chemical transport models alone cannot express, due to the coarser resolution to which they operate and to their inability to reproduce street canyons and urban structures.