Blood pH at Higher Altitudes
The following chemical equation explains why pH decreases with increased $CO_2$:

$CO_2 + H_2O \rightleftharpoons   H^+ + HCO^{-3}$

pH changes are the reason why oxygen binds to hemoglobin in you lungs and then dissociates from hemoglobin at other parts in your body where the oxygen is low because cellular respiration turned the oxygen into carbon dioxide. The binding of oxygen to hemoglobin is in turn explained by the Bohr effect.