Acidity is measured using a pH scale, with the number 7 being neutral.
Consequently, a substance with a pH value of less than 7 is acidic,
while one of a value greater than 7 is basic. It is also worthwhile to note that the pH scale is logarithmic; that is, a substance of pH of 6 is
10 times more acidic than another with a pH of 7. Generally, the pH of 5.6 has been used as the Acidity is measured using a pH scale, with the number 7 being neutral.
WHAT CAUSES ACID RAIN?
One of the main causes of acid rain is sulphur dioxide. Natural sources which emit this gas are volcanoes, sea spray , rotting vegetation
and plankton. However, the burning of fossil fuels, such as coal and oil, are largely to be blamed for approximately half of the emissions
of this gas in the world. When sulphur dioxide reaches the atmosphere, it oxidizes to first form a sulphate ion. It then becomes sulphuric
acid as it joins with hydrogen atoms in the air and falls back down to earth. Oxidation occurs the most in clouds and especially in heavily
polluted air where other compounds such as ammonia and ozone help to catalyze the reaction, converting more sulphur dioxide to
sulphuric acid. However, not all of the sulphur dioxide is converted to sulphuric acid. In fact, a substantial amount can float up into the
atmosphere, move over to another area and return to earth unconverted. The following are the stoichiometric equations for the formation
of sulphuric acid:
S (in coal) + O2 yields SO2
2 SO2 + O2 yields 2 SO3
SO3 + H2O yields H2SO4
Nitric oxide and nitric dioxide are also components of acid rain. Its sources are mainly from power stations and exhaust fumes. Like
sulphur dioxide, these nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric acid. These reactions are also
catalysed in heavily polluted clouds where iron, manganese, ammonia and hydrogen peroxide are present.*
The following is a chart which summarizes the effect of the pH level of the lake on its lifeforms.
pH LEVEL EFFECTS
<6 *Basic forms of food die off. Eg. Mayflies and stoneflies are important food sources for fish. They can't survive at this pH level.
<5.5 *Fish cannot reproduce.
*Young have difficulty staying alive.
*More deformed adult fish due to lack of nutrients.
*Fish die of suffocation.
<5.0 *Fish population die off.
<4.0 *Very different lifeforms, if any, from before.
The "safe" level of mercury in food has been set at about 0.05 parts per million. Indians and Eskimos in parts of Canada and the United States eat fish and seal meat with mercury levels as high as 15.7 and even 32.7 parts per million.
Fish, being one of the primary members of the food chain, is food for many other lifefoms, including hunans. Because toxic materials such as mercury are deposited in the fish due to acid rain, it is dangerous for humans to consume the fish. Like the domino effect, fewer fish can be sold as food, fishermen lose their hobby and people selling fishing supplies are affected. Amphibians are also affected; like the fish, they cannot reproduce in an acidic environment.
Hence,the effects of acid rain on lakes and its aquatic ecosystem are numerous and overwhelmingly magnified as we move down the food web.
EFFECT ON TREES AND SOILS
One of the most serious impacts of acid precipitation is on forests and soils. Great damage is done when sulphuric acid falls onto the earth as rain. Nutrients present in the soils are washed away. Aluminium also present in the soil is freed and this toxic element can be absorbed by the roots of trees. Thus, the trees are starved to death as they are deprived of their vital nutrients such as calcium and magnesium. Not all of the sulphur dioxide is converted to sulphuric acid. In fact, a substantial amount can float into the atmosphere, move over to another area and return to the soils unconverted. As this gas returns back to earth, it clogs up the stomata in the leaves, thus hindering photosynthesis. Research has been made where red spruce seedlings were sprayed with different combinations of sulphuric and nitric acid of pH ranging from 2.5 to 4.5. The needles of these seedlings were observed to develop brown lesions. Eventually, the needles fall off. It was also found that new needles grew more slowly at higher concentrations of acid used. Because the rate at which the needles were falling was greater than the rate at which they were replenished, photosynthesis was greatly affected, The actual way in which these needles were killed is still not yet known. However, studies have shown that calcium and magnesium nutrients are washed away from their binding sites when sulphuric acid enters the system. They are replaced by useless hydrogen atoms and this inhibits photosynthesis.
EFFECT ON MATERIALS
Acid rain also damages materials such as fabrics. For example, flags that are put up are being "eaten away" by the acidic chemicals in the precipitation. Books and age-old art that are centuries old are also being affected; the ventilation systems of the libraries and musuems that hold them do not prevent the acidic particles from entering the builings and so, they get in and circulate within the building, affecting and deteriorating the materials.
EFFECT ON HUMANS
Among one of the serious side effects of acid pollution on humans is respiratory problems. The SO2 and NO2 emmisions give rise to respiratory problems such as asthma, dry coughs, headaches, eye, nose and throat irritations. An indirect effect of acid precipitation on humans is that the toxic metals dissolved in the water are absorbed in fruits, vegetables and in the tissues of animals. Although these toxic metals do not directly affect the animals, they have serious effects on humans when they are being consumed. For example, mercury that accumulate in the organs and tissues of the animals has been linked with brain damage in children as well as nerve disorders, brain damage and death. Similarly, another metal, Aluminium, present in the organs of the animals, has been associated with kidney problems and recently, was suspected to be related to Alzheimer's disease.
EFFECT ON THE ATMOSPHERE
Some of the constituents of acid pollution are sulphates, nitrates, hydrocarbons and ozone. These exists as dry particles in the air and contribute to haze, affecting visibility. This makes navigation especially hard for air pilots. Acid haze also interferes with the flow of sunlight from the sun to the earth and back. In the Arctic, this affects the growth of lichens which in turn, affect the caribou and reindeer which feed on it.
Labels: Acid rain, animals, carbon, enviromental change, fish, sulphur
