The increased complexity of atmospheric aerosols makes the characterisation of their chemical composition and sources a very difficult task. These aerosols, including a large variety of organic and inorganic particles in terms of size and composition are found in urban indoor and outdoor environments. As these particles get into the respiratory tract via inhalation, they may cause potential hazards to human health with a degree of harmfulness that is still not very well known. Motivated by this, the URBAN-AEROSOL project focused on the chemical characterisation of the particulate matter, which is considered as an atmospheric pollutant. Thereby, researchers developed a micro-environmental modelling tool with increased capabilities for running full gas/aerosol processes and advanced gas phase photochemistry. On the basis on the well-known MAPS1.2 aerosol model (NCAR, USA), this model is written in Fotran (g77) with the addition of novel subroutines for distinguishing between indoor-outdoor environments. More specifically, the aerosol module involves 21 aerosol species, aerosol chemistry and inorganic condensation/evaporation - organic condensation. This innovation employs a hybrid fixed/moving aerosol bin structure that uses inorganics (sulphate, nitrate, ammonium, chloride, sodium, hydrogen ion), organics (primary and 8 secondary), water, elemental carbon and dissolved gases. Hence, it displays increased sensitivity and specificity for aerosol species. The advanced tool may be applied in indoor/outdoor investigations on the physico-chemical features of particulate matter and precursor gaseous species. Within the context of this project such research studies will be made in several European urban areas under various meteorological conditions and during different weekly and daily cycles. The results combined with dosimetry measurements are expected to provide a better understanding of the complex relationship between the internal deposition of particulate matter in humans and pollution sources.