Rain becomes acidic when it reacts with chemicals such as sulfur dioxide and nitrogen oxides that have been released into the atmosphere. The major reason for the increase in the occurrence of acid rain is human activity, although environmental reactions also sometimes cause acid rain.
Certain compounds can reach the higher layers of the atmosphere where they react with water, oxygen and other chemicals, leading to the creation of acidic pollutants called acid rain. Because sulfur dioxide and nitrogen oxides quickly dissolve in water and are brought to distant places by the wind, these substances reach faraway areas where they mix with precipitation, such as rain, sleet or snow. Power plants release sulfur dioxide and nitrogen oxides when they burn fossil fuels to generate electricity. They are considered the major culprits of pollution. Vehicles, such as cars and buses, also contribute to the release of the pollutants.
Some rain is naturally acidic, with a pH level of around 5. Acidic rain is neutralized by the reaction between normal precipitation and non-acidic materials, or alkaline chemicals, that exist in the atmosphere, soils, bedrocks, lakes and streams. These natural environmental reactions help reduce the impact of acid rain but may not be sufficient to counteract the human contribution.