It is well known that dispersion of gaseous pollutants in the atmosphere can be experimentally investigated on scale models in wind tunnels and hydraulic flumes. Typical problems that can be handled with these techniques are industrial smoke emissions, as for example from chimneys of thermal power plants, and natural gas emission, for example from geothermal or volcanic sources. Wind tunnels are mainly operated on relatively small scale models, to study dispersion at short range distances, while hydraulic flumes can also work on large scale models on which atmospheric flows are simulated over extended sites. Here, we refer to the latter technique but the concepts could be extended to the former one. We propose a method for measuring concentrations in a plane section of a spreading flow by an optical-digital apparatus over a large (0.5 m × 1.0 m) investigation area. A stream of fresh water is used as a hydraulic flume to simulate wind flow and alcohol-water or salt-water solutions are employed to reproduce pollutant gasses, respectively lighter or heavier than air. It is usual to seed the solution with a dye (fluorescein or rhodamine) so as to visualize the spreading pollutant plume.