Carbonic Acid Buffer?

[Gabriel Wolf 3I]

I was a bit confused about the explanation given for CO2 making rain more acidic. I understand that it combines with H2O then gives off H, but in the textbook it says that it will be protonated at low pH and deprotonated at high pH. Then figure 6E.2 shows that the pKa1 of carbonic acid is 6.37, which is pretty close to 7. If carbonic acid acts as a base at pH lower than 6.37 and acts as an acid at pH higher than 10.25, how does it make rain more acidic?

[Zechuan Rao 1K]

For weak acid, it will also have a weak conjugate base. That been said, H2CO3 is itself an acid, and more of it will decrease the pH by donating one of one H+. When CO2 is dissolved in water, it is making H2CO3, an acid, not HCO3- nor CO3 2-. Which form is predominant only tells you the form favored in a certain pH. The lower the pH, the more likely its base form will take an H+ and convert it back into its acid form. But acid is still acid, and more of it will shift the equilibrium and make the H+ concentration higher or more acidic.