The dynamical behaviour of atmospheric aerosols is characterized by a wide range of timescales, especially due to the fast growth of small particles through condensation/evaporation. The resulting models are particularly stiff and their numerical simulation still remains a challenge. Some techniques based on the assumption of condensation/evaporation equilibrium are usually advocated in order to circumvent these difficulties. The objective of this article is to investigate the different approaches on a theoretical basis. The appropriate framework is the theory of reduction for slow/fast models, for which many tools are already available. The application to aerosol modelling leads to the so-called hybrid methods. We also propose a hierarchy of reduced models and compare some numerical algorithms in a box model simulation for inorganic species. (C) 2005 Elsevier Ltd. All rights reserved.