The Meteorological Synthesizing Centre-West (MSC-W) of the European Monitoring and Evaluation Programme (EMEP) has been performing model calculations in support of the Convention on Long Range Transboundary Air Pollution (CLRTAP) for more than 30 yr. The EMEP MSC-W chemical transport model is still one of the key tools within 5 European air pollution policy assessments.

Traditionally, the EMEP model has covered all of Europe with a resolution of about 50× 50 km 2, and extending vertically from ground level to the tropopause (100hPa). The model has undergone substantial development in recent years, and is now applied on scales ranging from local (ca. 5km grid size) to global (with 1 degree resolution). 10 The model is used to simulate photo-oxidants and both inorganic and organic aerosols. In 2008 the EMEP model was released for the first time as public domain code, along with all required input data for model runs for one year. Since then, many changes have been made to the model physics, and input data. The second release of the EMEP MSC-W model became available in mid 2011, and a new release is targeted for 15 early 2012. This publication is intended to document this third release of the EMEP MSC-W model. The model formulations are given, along with details of input data-sets which are used, and brief background on some of the choices made in the formulation are presented. The model code itself is available at www. emep. int, along with the data required to run for a full year over Europe. 20