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.

LOTOS model
The LOTOS (Long Term Ozone Simulation) model originates from the US UAM(Urban Airshed Model). In the early 1970's, it were Steven Reynolds and colleagues in the group of John Seinfeld of Systems Applications International (SAI), who made the pioneering attempts at photochemical air quality modeling. These efforts resulted in the UAM model, a local air quality model which was firstly designed to investigate ozone formation over Los Angelos (US). Back then, the UAM model was focused on ozone in episodic situations in urbanized areas. The present day UAM air quality modeling system is one of the most widely used photochemical air quality models in the world.
In a coorporation with SAI and TNO, the UAM was modified for application over the Netherlands and its surroundings. Around 1980, TNO started a coorporation with the FU Berlin (Free University of Berlin, meteorology department) to apply UAM for parts of Germany. UAM was extended to cover larger areas and was subsequently called RTM (Regional Transport Model). RTM was the direct predecessor of LOTOS. RTM, originially designed for the description of air quality in episodic situations, was further developed for application to longer time scales. This was done with preservation of the original UAM/RTM model features such as the representation of the mixing height - therewith making the now called LOTOS model unique in its existence.
The LOTOS model being first only focussed on ozone (Builtjes, 1992) was extended around 1995 to incorporate also aerosols.

EUROS model
The EUROS model (European Ozone Simulation) was developed at RIVM (de Leeuw and van Rheineck Leyssius, 1990; van Loon, 1994, 1995; Matthijsen et al., 2002).

LOTOS and EUROS were originally developed and used as photo-oxidant models (Builtjes, 1992; Hass et al., 1997; Hammingh et al, 2001, Roemer, 2003). During the last years attention was given to simulate the inorganic secondary aerosols SO4, NH4 and NO3. (Schaap et al., 2004a; Erisman and Schaap, 2004; Matthijsen et al., 2002) and carbonaceous aerosols (Schaap et al., 2004b). The EUROS model also contains the possibility to perform simulations for persistent organic compounds (Jacobs and van Pul, 1996).
Since the two models had a similar structure and comparable application areas. Hence, based on strategic and practical reasoning, RIVM/MNP and TNO agreed to collaborate on the development of a single chemistry transport model: LOTOS-EUROS. During 2004 the two models were unified which resulted in a LOTOS-EUROS version 1.0 (Schaap et al., 2005).