During the pressure in the alveoli is less than the atmospheric pressure

inspiration

Explanation:

Alveolar pressure is pressure of the air inside lung alveoli.

When epiglottis is opened and there is no air flowing in or out of lungs, the alveolar pressure is equals to atmospheric pressure.

During the process of inspiration, the volume of the alveoli increases which results in lung expansion and decreases the intra-alveolar pressure to the value below atmospheric pressure. This slight negative pressure change is enough to move about  500 ml volume of air into lungs in just 2 seconds. At the end of the inspiration,alveolar pressure returns to the value of atmospheric pressure,

During expiration, the opposite change occurs.