Nitrogen Dioxide Source Attribution for Urban and Regional Background Locations Across Germany

It is important to understand the sources causing exposure to nitrogen dioxide. Previous studies on nitrogen dioxide (NO2) source attribution have largely focused on local urban scales. This study aims to assess the source contributions to NO2 levels at regional and urban background locations in Germany. For this purpose, we used the chemistry-transport model LOTOS-EUROS. Road transport was identified as the largest contributor, particularly in urban background settings (up to 59% in major cities), with larger shares from light-duty vehicles than from heavy-duty vehicles. Modelled contributions from traffic on highways exceed those from urban roads in the urban background. This study also highlights contributions from shipping, agriculture, energy, and industry, which vary significantly from region to region. Transboundary contributions also play a role, particularly near the border. The model performance has been validated by comparison with ground-based observations from the federal state networks and the Federal Environmental Agency. The comparison to the observations showed an underestimation of NO2 concentrations in cities, hinting at shortcomings in the spatial allocation of the emissions. The observed difference between the NO2 levels in Berlin and those in the rural background showed a large sensitivity to ambient temperature, which was not reproduced by the model. These results indicate that the way the traffic emissions are described, including the temperature influence, needs to be updated.