From AMS Glossary
The downward movement of pollutants or smoke plumes immediately to the lee of flow obstacles, such as buildings, bluffs, or smokestacks.
This downwash is caused by wake turbulence or lee cavity circulations generated by the obstacle and brings higher-concentration pollutants down toward the ground. Stack downwash occurs when the average vertical effluent velocity w0 out of the top of the smokestack is less than 1.5 times the mean wind speed M at stack top. The average height decrease of the pollutant plume centerline is of order 2D[(w0/M) - 1.5], where D is the diameter of the top of the smokestack.