Acid rain is caused by the emissions of chemical compounds into the atmosphere. When the compounds are released into the air, they combine with water molecules. The precipitation (rain, snow hail, etc.) that results is no longer H2O, but rather an acid (HNO3 or H2SO4).
Human factors contribute greatly to acid rain. One such factor is the pollution released into the atmosphere by car exhaust fumes, which mostly contain carbon monoxide.
When large factories burn fossil fuels (such as coal), they release pollution into the air, which can then be swept away by the wind and cause the precipitation to become more acidic.
When landfills decompose garbage, over ten toxic gases are released into the air, with methane as the most dangerous. Methane has proven to be more toxic than carbon dioxide.