An apparatus for rotary dumping of rail cars, including a material receiving pit. A rotary rail car dumper includes a rotational frame supporting at least one baffle. The rotational frame is configured to dump a load from a rail car during rotational motion of the rotational frame between an upright position wherein the entire load is in the rail car and a dumping position wherein the load can exit the rail car. A backside airflow diverter is located beneath the rotational frame and includes a curved upper surface. A lower margin of the at least one baffle contacts the curved upper surface during at least part of the rotational motion of the rotational frame.