A wall assembly for mixing polluted air with less polluted air to provide moderately polluted air. The wall assembly includes means for dividing air from the roadway region into a lower part and an upper part, and means for permitting at least a portion of the upper part to flow substantially in one or more flow directions toward the leeward region. The wall assembly also includes means for directing the lower part substantially upwardly in a direction substantially transverse to the flow direction to intersect with the upper part and to mix the polluted air with said less polluted air, to provide the moderately polluted air proximal to the leeward area.