These use an equation such as (24), (47) or (57) to predict the
concentration of pollutant emitted by a short-lived or continuous point or
line source. They are particularly useful for studying the effects of a single
source. They are less well adapted to problems involving many sources (e.g.
a whole city) or problems where chemical reactions of pollutants are
important.
8.2 Single Box Models
These models define a box enclosing the region of interest and for that
box consider
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.
8.3 Multi-Box Models
These are similar to single box models except that the fluxes of pollutant
into each box are derived from the fluxes out of the adjacent box
(see Fig. 13).
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.
8.4 Numerical Solution of the Concentration Equation
This is the most general approach but is usually the most expensive
(see §10 for more details).
of pollutant out of Box 1 is equal to the flux 
of
pollutant into Box 2.