Acid rain has been around for a long time. It was first noticed around the 17 century during the industrialization period. Scientists have been paying attention to "acid rain" since that time and the effects it has had on the plants, animals, humans etc. Acid rain is very dangerous as most people know. First off the term is not totally correct and scientists prefer the terminology acid depositions. The reason these scientists believe that it is called acid depositions instead of acid rain is simply because the acid which was formed by pollution can in fact return to the earth as either solid or a gas and not only in the form of rain.
Acid depositions can come in rain, fog or even snow. Industries, factories, vehicles, aerosol cans, etc are all causes for "acid rain". This is because all of these things increase the level of sulphur dioxide and nitrogen in the air we breathe in. These chemicals will later transform into sulphuric acid as well as nitric acid which in the long run will come down in the form of "acid rain". Electric companies have to burn a lot of coal, which contains a very low amount of sulphur, in a day's time which will increase the level of sulphur dioxide in the atmosphere. But the electric companies are not the only one's to blame; a lot of other companies and industries have to process raw ore (a mineral or a numerous amount of minerals which a valuable or constituent can be profitably mined or extracted) so they can obtain copper, zinc, and nickel.
The problem with that is that copper, zinc, and nickel are also causes of acid rain. Most pollution, if not all, is caused by humans in the formed of vehicles, industries etc. The accidents in which man has no control over such as volcanic eruptions, lightning, decay etc are a cause for the different pollutants in the atmosphere but nothing can be done to avoid this. But this is not a great risk to the atmosphere and us when one considers that 90% of the sulphur dioxide in our atmosphere and 95% of nitrogen oxides is caused by man. Which leaves about 10% and 5% respectively are caused by nature which is very minimal. The biggest problem about "acid rain" is that it does not stay where it was created. Since acid rain takes many forms, it will travel everywhere and create itself somewhere else thru the aid of wind, water, etc.
The United States is well known to be a big part of the pollutants in the world; for example is it believed that 50% of the "acid rain" in Canada is caused by the pollution in America. It is important to know that pollution is not the only cause of "acid rain". If you take rainfall as an example, even if there is no pollution at all in the atmosphere, the rain itself is still acidic. The rain in rainfalls has a pH of roughly 6.0. The cause of this is because there is carbon dioxide in our atmosphere and when it mixes with water it created carbonic acid. Since this is not a perfect world, there will always be pollution in the world and because of these pollutants it affects the pH level and makes it drop. When the pH level reaches below 5.5 it is classified as "acid rain". The lowest ever recorded pH level was around 2 - 2.5.