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You are only allowed to ask one question at a time so I deleted some of the extra material from your question. Acid rain is any rainwater or vapor that has a lower pH than normal. That is to say that the water contains an unusual excess of protons (H+). Regular, natural cloud vapor is slightly acidic due to the fact that carbon dioxide naturally dissolves in water to form carbonic acid. The equation is shown below:
CO2 + H2O --> H2CO3
Carbonic acid is a weak acid, though, and doesn't lower the pH of the water too much.
The two main chemicals that cause acid rain are sulfur dioxide (SO2) and nitrogen dioxide (NO2). SO2 can be oxidized by oxygen to SO3, which in turn combines with water to form sulfuric acid (H2SO4). The equations are shown below:
SO2 + 1/2 O2 --> SO3
SO3 + H2O --> H2SO4
Similarly, nitrogen dioxide combines with water to form nitric acid (HNO3). The equation is shown below:
NO2 + H2O --> HNO3
The big problem here is that sulfuric acid and nitric acid are both very strong acids. A small amount of each acid can greatly affect the pH of atmospheric water vapor.
A major natural source of SO2 is from volcanic eruptions/activity. A major natural source of NO2 is from lightning in the atmosphere (it oxidizes nitrogen gas which is 80% of the atmosphere). But man-made activities also can produce these gasses. The biggest man-made sources of these gasses are the burning of coal for electricity and automobile exhaust. We have managed to greatly reduce acid rain production in the US and Europe, though, by treating the gasses produced from these sources. The introduction of the catalytic converter on cars greatly reduces SO2 and NO2 emissions. Similarly, chemical scrubbers in the smokestacks of coal furnaces remove much of the SO2 produced from the burning of sulfur rich coal. The SO2 is converted to sulfate salts which can then be sold and used commercially.
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