There is a small amount of sulfur in coal. When
the coal is burned, the sulfur enters the
atmosphere as SO2 and SO3
gas. This gas reacts with water vapor to make
sulfuric acids. The size of the droplets are VERY,
VERY tiny and they get carried into the atmosphere
to high levels, where the acid droplets can move
for thousands of miles from the smoke stacks.
These droplets dissolve in rain water and lower
the pH to make acid rain.
Acid rain happens because of chemicals in the
air combining with water to create acids. The most
common is carbon dioxide, which makes carbonic
acid with rainwater in the atmosphere. This is
entirely natural. Fortunately, carbonic acid is
such a weak acid that it doesn't do any real
damage. Pollution and volcanic eruptions often put
sulfur trioxide in the atmosphere, which becomes
sulfuric acid in combination with water, which is
a very strong acid and causes quite a lot of
damage when it happens. Nitrogen-rich pollution
can also give rise to nitric acid, which is also
strong (and bad).
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