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Air quality in urban environments depends on numerous processes and phenomena, which cover a wide range of length scales, from the scale of the street to that of the region. In order to reproduce such a wide range of length scales, a modelling system based on nested models has been developed to compute atmospheric pollution in the Greater Lyon region. Regional scale calculations are performed using the models SAIMM for meteorological conditions, and UAM-V for pollutant transport; the calculations are performed for a domain measuring 208 km square, with a resolution of 4 km. Urban scale calculations are performed using the code MERCURE, on a domain measuring 32 km2, with a resolution of 500 m. More detailed local scale calculations are performed using ADMS-3 and SIRANE. This system has been used to simulate NOX concentrations for a 24 h period during the field measurement campaign ELO (June 1999) and the results are compared with measured data for that period. The comparison demonstrates the value of an approach based on nested models; in particular, the inclusion of a local scale simulation improves the accuracy of the calculations for near field locations. The local scale model needs to be chosen according to the source type and the characteristics of the domain. [Copyright &y& Elsevier]