CHAPTER ONE
INTRODUCTION
1.1    OVERVIEW
Acid rain is a wide spread term used to describe all forms of acid precipitation (rain, snow, hail and fog) Atmospheric pollutants particularly oxides of sulphur, oxides of Nitrogen and carbonic acid can cause precipitation to become more acidic when converted to sulphuric and Nitric acids hence the term acid rain.
Acid deposition, acid rain and acid precipitation all relate to the chemistry of air pollution and moisture in the atmosphere. Scientists generally use the term acid deposition but all three terms relate to the same issue (acid precipitation, Acid deposition and Acid rain). The term acid rain was first used by (Robert Augus Smith, 1870) a scientist working in the 1870s. The problem of acid rain is hence not a new one but the nature of the problem has changed from being a local problem for towns and cities to being an international problem (Johnson, 1972).
In Smith’s (1870) time, acid rain fall was both in towns and cities whilst today pollution can be transported thousand of kilometers due to the introduction of tall chimneys dispersing pollutants high into the atmosphere. Precipitation is naturally acidic because of carbon dioxide in the atmosphere. The burning of fossil fuels such as (coal, oil and gases) produces sulphur dioxide and Nitrogen oxides which can increase the acidity of rain or other precipitation. Sources of sulphur dioxide and oxide of Nitrogen may be natural such as volcanoes, oceans, biological decay and forest fires, or may arise from combustion sources. The increasing demand for electricity and the rise in the number of motor vehicles in recent decades has meant that emissions of acidifying pollutants have increased dramatically from human sources particularly since (1950s).
Emissions of such pollutants are heavily concentrated in the northern hemisphere, especially in Europe and North America in 1970s. As a result precipitation is generally acidic in these countries. In the 1970s and 1980s Scandinavian countries began to notice the effect of acid deposition on trees and freshwater. Much of the pollution   causing this damage was identified as being transported from other more  polluting countries. Acid rain became an international concern.
Acid rain is determined by the hydrogen ion content (H+) of the rain water pH. This scale was invented  by a Danish Scientist in (1909) it is called Sorenson. The pH scale ranges from O, which is strongly acid, to 14 which is strongly alkaline the scale point 7 being neutral. The pH scale  is logarithmic rather than linear, so there is a ten fold increase in acidity  with each pH unit, such that rainfall with pH5  is ten times more acidic than pH6 rainfall with pH4 is 100 time more acidic than pH6 and rainfall  with pH3 is 1000 time more acidic than pH6 (Baso, 1996).
Acid rain became particularly prominent as a media issue during the (1980s). However during the (1970s) many countries started to notice changes in fish population in lakes and damage to certain trees. By the late 1970s concern led to international effort to identify the cause and effect of long range (transboundary). Transport of air pollutants, and thus during the (1980s) much research was conducted in Europe and Northern America.