The simplest pollutant distribution is due to diffusion in the absence of wind.
The above figure illustrates the concept of periodic scrubbing to contain emissions on the road.
Barriers have the effect of trapping pollution on the roadway and slowing the spread off-road. Wind normal to the road causes re-circulation of emissions and traps high concentrations at or near barriers.
Below are iso-plots of pollutant concentrations for barriers of different heights.