All objects change velocity as they move toward earth at a rate of 9.8m s due to which factor of gravity?

Its 9.8 m/s2. Gravity is a force. The object starts falling on the earth's surface initially increasing its velocity but at some point the velocity becomes constant. Newton's third law can be applied to explain this. The force of gravity initially exceeds the buoyancy of air and that is when the body's fall is accelerated. But at some height the 2 forces equalize. Then the body falls at a constant velocity and this is called the drag velocity.