  This paper focusses on the mean charge per particle of monodisperse submicron
aerosol, charged by diffusion of unipolar ions in post-corona discharge. It
aims to confirm and discuss the limits of considering a single value of
Ni$\times$t to describe aerosol charging and then to present methods to control
the size-charge relation. Three aerosol chargers, with different mixings of ion
and aerosol flows are investigated. Despite comparable ion sources with
discharge currents of a few tens of $\mu$A, the size-charge relations differs
from one charger to another due to different ion-aerosol mixing conditions and
subsequent ion density along particles trajectories. Discrepancies are even
more noticeable as the particle size increases. Discharge current, velocities
of ion and aerosol flows and electric field control post-discharge ion density
in each point of the charging volume. The control of particle trajectory in
expanding unipolar ion cloud, leads to tuneable size-charge relations. Aerosol
inertia and charging dynamics, that both depends on particle size, affects the
Ni$\times$t experienced by the particle and thus the final charge of the
particle. Operating conditions to reach a constant mean charge for particles
larger than 200 nm are reported. Conclusions provide a basis to design aerosol
chargers devoted to electric mobility selection for aerosol deposition,
separation or electrical measurements especially to overcome the limits of
mobility-to-size data inversion due to multiple charge ambiguity using
diffusion chargers.
