Numerical Treatment of Urban and Regional Scale Interactions in Chemistry-Transport Modelling

The physical and chemical processes that determine the distribution of air pollutants occur on a wide range of temporal and spatial scales. Multiscale models can provide finer resolution in certain key regions, e.g. around large sources. The paper focuses on some numerical aspects of modelling urban and regional scale interactions as well as on requirements on the used parameterisations in this context. Multiblock grid techniques ("two-way nesting") and implicit-explicit time integration schemes are suitable for an efficient numerical treatment of such scale interactions. In the online coupled model system LM-MUSCAT, both approaches are implemented for the chemistry-transport code. Gas phase processes, especially the formation of photooxidants, as well as the transport and the transformation of particulate matter, can be investigated. The advantages of the multiblock technique to establish the interactions between different scales in a natural way are demonstrated for one selected scenario in the Saxony area. The influence of grid resolutions on the simulation results is discussed.