Application of the Urban Climate Model PALM-4U to Investigate the Effects of the Diesel Traffic Ban on Air Quality in Stuttgart

The air pollution situation in the German city of Stuttgart is very important, as high pollutant concentrations are measured here compared to other German cities. This is mainly due to Stuttgart’s geographical location as it is in a basin covered by hills on three sides. This leads to reduced wind speeds that inhibit pollutant dispersion. One of the main contributors to the pollutant concentrations in Stuttgart is local traffic. To improve the air quality in Stuttgart, a diesel traffic ban was introduced on 1 January 2019, and is ongoing. In this study, the urban climate model PALM-4U was applied to obtain the pollutant distribution along the federal highways B14 and B27 of Stuttgart to evaluate the impact of the diesel traffic ban on air quality. The simulations were carried out in two areas of the city, namely the city center and Kaltental Valley, with domain sizes of 3.2 km × 2 km and 3.2 km × 1.6 km, respectively, and with a grid size of 10 m for each domain. The influence of traffic emissions on the air quality of Stuttgart was studied for a typical summer day. The results showed that air pollutant concentrations were highest near federal highways B14 and B27 (e.g., NO2 concentration peaks of around 200 µg/m3). Also, a significant reduction of around four times in air pollutant concentrations was observed in the study area after the diesel traffic ban was introduced.