Here's why:

* Natural Rain: Even in clean environments, rain is slightly acidic due to the natural presence of carbon dioxide (CO2) in the atmosphere. When CO2 dissolves in rainwater, it forms carbonic acid.

* Acid Rain: When pollutants like sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere, they react with water, oxygen, and other substances to form sulfuric acid (H2SO4) and nitric acid (HNO3). This acidic rain can be significantly more acidic than normal rain and can have harmful effects on the environment.

So, while carbonic acid is always present in rain, acid rain specifically refers to the presence of sulfuric and nitric acid, which are formed from air pollution.