Acid rain forms when airborne pollutants such as sulfur dioxide and nitrogen oxides from the burning of fossil fuels combine with water vapor in the atmosphere. It is rain with a pH lower than 5.6 compared to natural rain which has a pH of 5.6. Acid rain is damaging marble structures, changing lake pH levels so most fish and plant life cannot survive below 4.5, and breaking down tree leaves which interferes with photosynthesis. Legislation has helped reduce acid rain by requiring coal plants install scrubbers to absorb 95% of sulfur dioxide emissions before releasing it into the air.

1. ACID RAIN
Natural rain is slightly acidic, with a pH of 5.6. Acid rain is rain with a pH less
than 5.6. In some parts of the world, pH values have been reported as low as 2.6,
which is about as acidic as lemon juice or vinegar. It forms when airborne pollutants,
sulfur dioxide and nitrogen oxides, combine with water vapor in the atmosphere to
produce acids.
Although natural sources such as volcanoes and forest fires release SO2, the
primary sources of acid rain today are from the burning of fossil fuels in automobiles
and coal in industrial plants. When coal and oil are burned, the sulfur impurities
combine with oxygen in the air to produce SO2 and
SO3. The reaction of SO3 with water forms sulfuric
acid.
S + O2  SO2
SO2 + O2  SO3
SO3 + H2O  H2SO4
Nitrogen oxide forms at high temperatures
in the engines of automobiles as air containing
nitrogen and oxygen gases is burned. As NO is
emitted into air, it combines with more oxygen to
form nitrogen dioxide, then dissolves in water to form nitric acid.
N2 + O2  NO
NO + O2  NO2
NO2 + H2O  HNO3
The acids in acid rain have detrimental effects on marble and limestone
structures, lakes, and forests. Throughout the world, monuments made of marble are
deteriorating as acid rain dissolves the marble. Acid rain is changing the pH of many
lakes and streams. When the pH of a lake falls below 4.5, most fish and plant life
cannot survive. Trees and forests are susceptible to acid rain too. Acid rain breaks
down the protective waxy coating on leaves and interferes with photosynthesis.
In an effort to decrease the formation of acid rain, legislation has required a
reduction in SO2 emissions. Coal-burning plants have installed equipment called
scrubbers that absorb SO2 before it is emitted. In a smokestack, scrubbing removes
95% of the SO2 .