Step-by-step explanation:
Ok so water is H2O and cabon dioxide in the air is CO2, so the water goes through the carbon dioxide and makes acid rain H2SO4.
Now we have our limestone which is CaCO3.
What happens is that the acid breaks apart our limestone into Ca2+ and CO3 2-. This then reforms into Calcium bicarbonate Ca(CO3)2.
Calcium bicarbonate is soluble in water and is hence washed away by the rain eroding the limestone.