A mass transit vehicle including a cabin mounted to a frame and including a roof and a passenger zone defining a passenger zone length, an air cleaning unit including an air cleaner housing mounted to the roof, an air cleaning device disposed within the housing, an inlet upstream of the air cleaning device, and an outlet downstream of the air cleaning device, and a duct system disposed within the cabin and extending substantially along the passenger zone length. The air cleaning unit is operable to draw in air through the inlet, through the air cleaning device, and discharge a flow of cleaned air out of the outlet, and the duct system is in fluid communication with the outlet and operable to receive the flow of cleaned air from the outlet and direct the flow of cleaned air to the cabin substantially evenly along the passenger zone length.