Imagine all the manufacturing industries, oil refineries, automobile emissions, and power generation activities in a typical city. Daily, these industries burn fossil fuels and produce smoke and gases containing pollutants into the atmosphere. 
See #1 in the illustration below. The pollutants, resulting from human activities travel with the wind, air currents, and storm systems and spread into the atmosphere #2. Some pollutants result from natural occurrences such as volcanic ash and natural fire emissions.

As moisture builds up in the atmosphere (rain formation processes) the moisture reacts with these atmospheric gases. Gases such as carbon dioxide, sulfur dioxide, and nitrogen oxides react with water molecules to produce sulfuric and nitric acid. Acid rain then falls #3 on land and into water bodies #4.
Typically, acid rain has a pH below 4.5 on the pH scale. Sometimes, when there are higher concentrations of pollutants, acid rain can become more acidic.

The gases sulfur dioxide and various nitrous oxides are mainly responsible for the increase in acidity. Natural sources of Sulfur Dioxide and Nitrogen oxides are from automobile combustion, fires, lightning, and volcanoes, and have an equally important effect on acid rain (pH levels) as manmade sources.