EC framework directive 96/62/EC and its daughter directives allow the use of models to evaluate ambientair quality. The model system M-SYS has been developed for this purpose. M-SYS employs a hierarchy of mesoscale and microscale models for both, meteorology and chemistry, to calculate air quality metrics corresponding to the limit values of the EC daughter directives. Corresponding to the directives the model system delivers maps showing concentration distributions in different spatial resolution for the different subjects of protection. M-SYS results show the resolution dependence of maximum values. Concentration fields show a large variability on spatial scales smaller than the average spacing of air quality stations. These heterogeneities cannot be resolved by the operational air quality network. Commonly used interpolation techniques can also not capture these spatial patterns as long as they lack information on the small-scale variability of wind, turbulence, and chemical conversion. At traffic-orientated sites in the urban obstacle layer, particularly large heterogeneities are found which require the use of three-dimensional obstacle resolving models.
