What causes ocean acidification?

There is more CO2 dissolving into the ocean. The ocean’s average pH is now around 8.1 , which is basic (or alkaline), but as the ocean continues to absorb more CO2, the pH decreases and the ocean becomes more acidic.

What about the air in the ocean? Does it produce acid?

It is not really the acid but it comes from the water evaporating from the ocean going to the clouds then going down when it rains. Rain absorbs some pollutants from the air but it is more likely because of the rocks and pH levels that is why it is acidic.