  Based on the photometric properties of NGC 6822 we derive a robust star
formation history. Adopting this history we compute 15 models of galactic
chemical evolution. All of them match present-day photometric properties. The
dark halo mass in all models evolves according to the mass assembly history
predicted by a $\Lambda$CDM cosmology. We model the evolution of the baryonic
mass aggregation history in this cosmological context assuming that part of the
gas available for accretion never falls into the system due to two different
physical processes. For seven models we assume that during accretion the
universal baryon fraction is reduced by reionization only. The best model of
this first group, a complex model with an early outflow, fits the observed
gaseous mass, and the O/H, C/O, and Fe/O present-day values. This model
requires a lower upper mass limit for the IMF than that of the solar vicinity,
in agreement with recent results by other authors. We have also computed eight
models where, in addition to reionization, the accreted baryon fraction is
reduced by large-scale shock heating.The best model of this series, that also
requires and early outflow and a lower upper mass limit for the IMF,can
marginally fit the gaseous mass and the O/H and C/O observed values.
