Particle size distributions from a series of experiments involving the photo-oxidation of iron pentacarbonyl [Fe(CO)(5)] in ozone at atmospheric pressure and 295 K are reported for a range of initial reactant concentrations, varying photolysis rates and particle growth times. These data sets were used to test a model which describes the formation of FeO3 in the gas phase, followed by clustering to produce primary Fe2O3 particles. These subsequently coagulate to form fractal-like structures as a result of magnetic dipole coupling of the primary particles. For the smallest size, spherical particles, Smoluchowski theory was used to determine a coagulation constant (k(S)) of 7.0 x 10(-10) cm(3) s(-1), indicating a primary particle diameter of 6.6 nm, in very good agreement with the optimised value used in the particle growth model for this system. Finally, these findings are used in discussion of the formation and growth of Fe2O3 'meteoric smoke' particles in the upper atmosphere. (C) 2010 Elsevier Ltd. All rights reserved.