Information Sheet for Activity 2 – Understanding the Acid Rain Problem

Information Sheet for Activity 2 – Understanding the Acid Rain Problem
Robert Angus Smith, an English chemist, first used the phrase "acid rain" in 1852 when he
noted the connection between London's polluted skies and the acidity of its rainfall.
"Acid rain" is a general name for a whole range of atmospheric acidic pollution effects
associated with precipitation including acid fog, acid mist and acid snow. Although we
associate the acid threat with rainy days, acid deposition occurs all the time, even on sunny
days. Indeed, it is not necessary to have wet conditions to have acidic pollution
effects. Some pollutants can be transported directly to the surface as gases or small particles
without being absorbed by moisture, a phenomenon known as dry deposition.
Acidic pollution can arise from both natural processes and human activities. Volcanic activity
and animal and plant decomposition are well known examples of the former. However, in
many urban and industrial areas the dominant source is from human activities. There are
four major pollutants that cause acid air pollution: sulphur dioxide, nitrogen oxides,
hydrocarbons and ozone.
Sulphur dioxide
Sulphur occurs widely in nature, in all animal and plants, and in the fossil fuels - coal, oil and
natural gas - which were formed from ancient plant and animal remains. Oil and natural gas
generally have relatively low sulphur contents, usually less than 1 percent. However, some
coals can have as much as 14 percent sulphur. When fossil fuels are burned to generate
energy the sulphur combines with the oxygen in the atmosphere to form sulphur
dioxide. Globally, about 50 percent of the SO2 levels in the atmosphere are estimated to be
of natural origin. However, in many urban and industrial areas human activity may account
for around 90 percent of the SO2 in the air.
Nitrogen oxides
A number of different kinds of nitrogen oxides exist in the atmosphere. Collectively they are
given the name NOx. Like SO2, about 50 percent of the NOx in the air on a global basis is
estimated to be from natural sources. However, in industrial areas only about 10 percent of
NOx are of 'natural' origin. The vast majority are produced when fossil fuels are
burned. The nitrogen present in the fuel and in the atmosphere combines with oxygen to
produce several different types of NOx. Coal-burning power stations and transport are the
major sources.
Hydrocarbons and ozone
Unburnt hydrocarbons emitted as a result of improper combustion of fossil fuels constitute a
major air pollutant in many urban and industrial areas. These hydrocarbons can react with
NOx in the presence of sunlight to produce ozone which can reach levels 10 times higher than
the natural background level which is generally low. This ozone then reacts with other
pollutants to form acids.
The common reactions in acid rain formation:
Sulphur dioxide + water  Sulphuric Acid
Nitrogen oxides + water  Nitric Acid
The SO2 and NOX are
transformed into acids in
the atmosphere either by
dry deposition or by wet
deposition. During dry
deposition, which usually
occurs close to the source
of the pollution, the
pollutants are deposited on
to surfaces where they then
turn into acids. With wet
deposition, which can occur
over 1,000 km from the
source, the gases become acids after coming into contact with water droplets in the
air. Generally the longer that the pollutant stays in the air the more of it will be converted
into acid. The diagram summarizes these processes.
Acidity is measured on the pH scale which runs from pH 0, the most acid, to pH 14, the most
alkaline. The midpoint of the scale, pH 7, is termed neutral. The scale is logarithmic so that
each point is ten times greater or less than the point above or below it respectively. For
example a pH of 5 is ten times more acidic than pH 6, and is 100 times more acidic than pH 7.
The scale below shows the acidity of some everyday objects on the pH scale.
Normally, rainwater from an unpolluted atmosphere will have a pH of about 5.6 or 5.7, that is,
it is naturally acidic, being about 25 times more acidic than neutral distilled water which has
a pH of 7. This is because the carbon dioxide in the air reacts with water to form a weak
carbonic acid. In addition, during thunderstorms, some nitrogen oxides are formed, which
when dissolved in water produces dilute nitric acid. However, when acidic pollutants are
present the additional sulphuric and nitric acids formed as a result of the 'acid' processes in
the atmosphere greatly reduce the pH value. Badly polluted rainwater can have a pH as low
as 2.4, about the same as lemon juice. In severe situations pH may be as low as 1.5, about the
same as the acid in a car battery, and some 10,000 times more acidic than unpolluted
William P. Cunningham, Understanding Our Environment – An Introduction, Wm. C.
Brown Publishers, 1994.
Daniel D. Chiras, Environmental Science – Action for a Sustainable Future, The
Benjamin/Cummings Publishing Company, Inc.,1994.
Eldon D. Enger and Bradley F. Smith, Environmental Science – A Study of
Interrelationships, McGraw Hill, 2000.