In the frame of the EU-financed project BIOASH an existing model for simulating aerosol formation during combustion was improved and adapted for biomass combustion systems in order to contribute to a better understanding of aerosol formation and behavior and to the solution of ash-related problems in biomass furnaces and boilers. Subsequently, the model was applied for several test cases (test runs performed within the BIOASH project) in order to compare and evaluate modeling and measurement results. The model is a plug flow model considering gas phase modeling by means of thermodynamic equilibrium calculations and a kinetic approach for modeling gaseous sulfate formation. The main particle formation mechanisms nucleation, condensation and agglomeration as well as particle precipitation mechanisms (diffusion, thermophoresis, turbophoresis, inertial impaction and gravitational settling) as well as the condensation of ash forming vapors on boiler walls are considered. Results of modeling aerosol formation in different kinds of combustion units were compared with results from measurements regarding PSD and chemical composition of the aerosol particles at boiler outlet. The modeling results show a good match with the experimental results, which indicates the good applicability of the model. Moreover, the different properties of the aerosol particles regarding PSD and chemical composition due to different combustion and boiler technologies and different biomass fuels, which result in different formation pathways, become obvious. (C) 2007 Elsevier B.V. All rights reserved.