A balloon filled with helium is used to lift measuring instruments high above the Earth. At ground level, the pressure of the helium in the balloon is 1.0 × 10⁵ Pa. The volume of the helium is 9.6 m³. The balloon is released and it rises quickly through the atmosphere. The volume of the helium increases. The temperature of the helium remains constant.

Explain why the pressure in the balloon decreases as the balloon rises. You should refer to helium atoms in your answer.