The partial pressure of oxygen (PO2) in alveolar air is lower than atmospheric air because as atmospheric air enters the alveoli, it mixes with residual alveolar gases containing higher levels of CO2 and water vapor. The presence of water vapor and CO2 reduces the fraction of oxygen, thus decreasing its partial pressure. Additionally, oxygen is continuously absorbed into the pulmonary capillary blood, further lowering alveolar PO2. This gradient is vital for the diffusion of oxygen from alveoli into blood.

Reference: Respiratory Physiology: The Essentials, John B. West, 9th Edition.