  Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean $a_0$ and a standard deviation $\sigma$. The wave vector dependent
scattered intensity $I(q)$ is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given $a_0$, the location $q_c$ of the
crossover decreases as $\sigma$ increases. The dependence of $q_c$ on $\sigma$
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.
