This is particularly suitable for a model of a polluted city. It is well-adapted to the inclusion of chemical reactions and many different pollutants but it is of no use for predicting local concentrations of pollutants.

**Fig. 13**. Two boxes of a multi-box pollutant
transport model showing the matching of fluxes across inter-box boundaries.
The flux of pollutant out of Box 1 is equal to the flux of
pollutant into Box 2.