The development and application of chemistry transport models has a long tradition in and outside Europe. RIVM and TNO have independently developed models to calculate the dispersion and chemical transformation of air pollutants in the lower troposphere over Europe. The two models are the TNO model LOTOS (Builtjes, 1992; Schaap et al., 2004a) and the RIVM model EUROS (de Leeuw and van Rheineck Leyssius, 1990; van Loon, 1994, 1995; Matthijsen et al., 2002).