Have you ever noticed what happens when you accelerate in a stopped car that contains balloons? The balloons move to the front of the vehicle because helium is lighter than air, and the motion of the vehicle causes the heavy air to move to the back of the vehicle, which pushes the balloon forward. I propose it would be the same behavior in an elevator. The falling of the elevator car would cause the heavy air to rush to the top, forcing the balloon down toward the floor.