Water vapour in the atmosphere, become liquid in the clouds. CO2 is dissolved into these small droplets, and they become aqueous (oooh, 4 vowels in a row...), reacting with H2O... makes H2CO3
Thus more acidic.
It goes naturally as CO2 is always in the atmosphere at around 0.01% highly depending on location.
btw, which code refers to the subtext 2? In your post w/ the CO2...