Here is some information I pulled out from an Education.com science fair project (http://www.education.com/science-fair/article/acid-rain-erosion/) about acid rain: “All rain has some acid. However, in places where large amounts of smoke are released into the air, gases such as sulfur dioxide and nitrogen dioxide are released from smoke into the air. In the air, these acidic gases become water droplets that make up clouds. Then these acids are released in the form of rain back to the earth. Acid rain harms the environment by killing seeds and eggs of many plants and animals. It also wears away rocks."
I also suggest looking at these other science fair projects concerning acid rain. The background information and bibliography sources are good places to look to find out more information about acid rain: