How Acid Rain Develops, Spreads, and Destroys
Acid rain is environmentally damaging rainfall that occurs after fossil fuels
burn, releasing nitrogen and sulphur oxides into the atmosphere. Acid rain,
simply stated, increases the acidity level of waterways because these nitrogen
and sulphur oxides combine with the airs normal rainfall. Acid rain is a
silent threat because its effects, although slow, are cumulative. This analysis
explains the cause, the distribution cycle, and the effects of acid rain.
Most research shows that power plants burning oil or coal are the primary
cause of acid rain. The burnt fuel is not completely expended, and some residue
enters the atmosphere. Although this residue contains several potentially toxic
elements, sulphur oxide and, to a lesser extent, nitrogen oxide are the major
problem, because they are transformed when they combine with moisture. This
chemical reaction forms sulphur dioxide and nitric acid, which then rain down to
Once fossil fuels have been burned, their usefulness is over. Unfortunately,
it is here that the acid rain problem begins. Fossil fuels contain a number of
elements that are released during combustion. Two of these, sulphur oxide and
nitrogen oxide, combine with normal moisture to produce sulphuric acid and
nitric acid. The released gases undergo a chemical change as they combine with
atmospheric ozone and water vapour. The resulting rain or snowfall is more
acidic than normal precipitation.
Acid level is measured by pH readings. The pH scale runs from 0 through 14 —
a pH of 7 is considered neutral. (Distilled water has a pH of 7.) Numbers below
7 indicate increasing alkalinity. (Household ammonia has a pH of 11.) Numbers
below 7 indicate increasing acidity. Movement in either direction on the pH
scale, however, means multiplying by 10. Lemon juice, which has a pH value of 2,
is 10 times more acidic than apples which have a pH of 3, and is 1000 times more
acidic than carrots, which have a pH of 5.
Because of carbon dioxide (an acidic substance) normally present in air,
unaffected rainfall has a pH of 5.6. At this time the pH of precipitation in the
northeastern United States and Canada is between 4.5 and 4. In Massachusetts,
rain and snowfall have an average reading of 4.1. A pH reading below 5 is
considered to be abnormally acidic, and therefore a threat to aquatic
Although it might seem that areas containing power plants would be most
severely affected, acid rain can in fact travel thousands of miles from its
source. Stack gases escape and drift with the wind currents. The sulphur and
nitrogen oxides are thus able to travel great distances before they return to
earth as acid rain.
For an average of two to five days after emission, the gases follow the
prevailing winds far from the point of origin. Estimates show that about 50
percent of the acid rain that affects Canada originates in the United States; at
the same time, 15 to 20 percent of the US acid rain originates in Canada.
The tendency of stack gases to drift makes acid rain a widespread menace.
More than 200 lakes in the Adirondacks, hundreds of miles from any industrial
centre, are unable to support life because of their water has become so acidic.
Acid rain causes damage wherever it falls. It erodes various types of
building rock such as limestone, marble, and mortar, which are gradually eaten
away by the constant bathing in acid. Damage to buildings, houses, monuments,
statues, and cars is widespread. Some priceless monuments and carvings already
have been destroyed, and even trees of some varieties are dying in large
More important, however, is acid rain damage to waterways in the affected
areas. Because of its high acidity, acid rain is dramatically lowers the H in
lakes and streams. Although its effect is not immediate, acid rain eventually
can make a waterway so acidic that it dies. In areas with natural acid-buffering
elements such as limestone, the dilute acid has less effect. The northeastern
United States and Canada, however, lack this natural protection, and so are
The pH level in an affected waterway drops so low that some species cease to
reproduce. In fact, a pH level of 5.1 to 5.4 means that fisheries are
threatened; once a waterway reaches a pH level of 4.5, no fish reproduction
occurs. Because each creature is part of the overall food chain, loss of one
element in the chain disrupts the whole cycle.
In the northeastern United States and Canada, the acidity problem is
compounded by the run-off from acid snow. During the cold winter months, acid
snow sits with little melting, so that by spring thaw, the acid released is
greatly concentrated. Aluminum and other heavy metals normally present in soil
are also released by acid rain and run-off. These toxic substances leach into
waterways in heavy concentrations, affecting fish in all stages of development.
Acid rain develops from nitrogen and sulphur oxides emitted by industries and
power plants burning fossil fuels. In the atmosphere, these oxides combine with
ozone and water to form acid rain: precipitation with a lower than average pH.
This acid precipitation returns to earth many miles form its source, severely
damaging waterways that lack natural buffering agents. The northeastern United
States and Canada are the most severely affected areas in North America.