ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-5-417-2005 The two-way nested global chemistry-transport zoom model TM5: algorithm and applications Krol M. 1 Houweling S. 2 Bregman B. 3 van den Broek M. 2 3 Segers A. 3 van Velthoven P. 3 Peters W. 4 Dentener F. 5 Bergamaschi P. 5 Institute for Marine and Atmospheric Research Utrecht, The Netherlands Space Research Organisation Netherlands, Utrecht, The Netherlands Royal Netherlands Meteorological Institute, de Bilt, The Netherlands National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory, Boulder, USA Joint Research Centre, Institute for Environment and Sustainability, Ispra, Italy 10 02 2005 5 2 417 432 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/5/417/2005/acp-5-417-2005.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/417/2005/acp-5-417-2005.pdf

This paper describes the global chemistry Transport Model, version 5 (TM5) which allows two-way nested zooming. The model is used for global studies which require high resolution regionally but can work on a coarser resolution globally. The zoom algorithm introduces refinement in both space and time in some predefined regions. Boundary conditions of the zoom region are provided by a coarser parent grid and the results of the zoom area are communicated back to the parent. A case study using <sup>222</sup>Rn measurements that were taken during the MINOS campaign reveals the advantages of local zooming. As a next step, it is investigated to what extent simulated concentrations over Europe are influenced by using an additional zoom domain over North America. An artificial ozone-like tracer is introduced with a lifetime of twenty days and simplified non-linear chemistry. The concentration differences at Mace Head (Ireland) are generally smaller than 10%, much smaller than the effects of the resolution enhancement over Europe. Thus, coarsening of resolution at some distance of a sampling station seems allowed. However, it is also noted that the budgets of the tracers change considerably due to resolution dependencies of, for instance, vertical transport. Due to the two-way nested algorithm, TM5 offers a consistent tool to study the effects of grid refinement on global atmospheric chemistry issues like intercontinental transport of air pollution.