Chapter 20
Acid Rain
Acid rain is a rain or any other form of precipitation that is unusually acidic, i.e. elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure through the process of wet deposition.Acid rain is caused by emissions of compounds of ammonium, carbon, nitrogen, and sulphur which react with the water molecules in the atmosphere to produce acids. The various gases like sulphur dioxide and nitrogen dioxide react with water vapours in presence of sunlight and form sulphuric acid and nitric acid mist. The reaction takes place in the following manner.
SO2 + H2O = H2SO3 (Sulphurous acid)
SO2 + O3 (Ozone) = SO3 + O2
SO3 + H2O = H2SO4 (Sulphuric acid)
2NO + O2 = 2NO2
2NO2 + H2O = HNO3 (Nitric acid) + HNO2(Nitrous acid)
The sulphuric acid and nitric acid remain in a vapour state when the temperature is high. The condensation begins with the fall of temperature and it mingles with the rain, fog or snow and this increases its acidity
Causes of Acid Rain:
The oxides of nitrogen, or NOx, and sulphur dioxide, or S02, are the two main sources of acid rain. Sulphur dioxide, which is a colourless gas, is given off as a by-product when fossil fuels that contain sulphur are burned.
This gas is produced due to various industrial processes, like the processing of crude oil, utility factories, and iron and steel factories. Natural means and disaster can also result in sulphur dioxide being released into the atmosphere, such as rotting vegetation, plankton, sea spray, and volcanoes, all of which emit about 10% sulphur dioxide.
On the whole, industrial combustion is responsible for 69.4% sulphur dioxide emissions into the atmosphere, and vehicular transportation is responsible for about 3.7%.Nitrogen oxide is the other chemical that acid rain is made up of. Any nitrogen compound that contains oxygen atoms of any amount is known as oxides of nitrogen. For example, nitrogen dioxide and nitrogen monoxide are oxides of nitrogen.
These gases are produced in firing processes which involve extremely high temperatures, e.g., utility plants and automobiles, as well as in chemical industries, such as in the production of fertilizers.
Five per cent of nitrogen oxide is emitted by natural processes like lightning, volcanic eruptions, forest fires, and action of bacteria in the soil. Industrial processes emit 32% and vehicular transportation is responsible for 43%.
Nitrogen oxide, which is a dangerous gas in itself, causes damage to the respiratory organs by attacking the membranes in them, thus increasing the chances of respiratory diseases. It also causes smog and is a contributory factor for the damage of the ozone layer in the atmosphere. When there is acid rain, the nitrogen oxide can be carried far away from the original location of the rain.
Effects of Acid Rain:
Acid rain has serious implications for continuation of life on this planet. The main acid rain problems are:
i. Effects on Water Bodies:
Most rivers and lakes have a pH that ranges between 6 and 8. Acid rain that may fall directly in these water bodies or may be washed into them as surface runoff, alter their chemical environment. The flora and fauna in these water bodies are adapted for life in the original pH value of the water.Altered acidity of their environment may be a threat to their survival. Although, some soils are capable of buffering increase in acidity, water bodies that have soil with poor buffering capacity may release aluminium ions from the soil which is toxic for aquatic life forms. Read more on water pollution.
ii. Effects on Soil and Forests:
Acid rain is being cited as one of the major causes of degradation of the forests at higher altitudes of the Appalachian Mountains from Maine to Georgia. Forests are affected directly as well as indirectly by acid rain. When leaves are frequently exposed to acid rain they are stripped off the essential nutrients present in them.
Acid rain falling on the soil, change soil acidity. In its bid to neutralize this change in pH, soil releases substances that are toxic for trees growing on it. Acidic water also dissolve nutrients in the soil and as it runs off the surface, it carries these essential minerals away with it, before they can be absorbed by flora growing on the forest floor.
iii. Acid Rain Effects on Aquatic Flora and Fauna:
Acid rain has a long term can directly effect on the aquatic life, as the high amount of sulphuric acid and nitric acid levels in acid rains are directly consumed by aquatic animals and plants. The harmful acids affect the ability of fish to take in nutrients, salt, and oxygen.
Aquatic animals intake oxygen from the water through their gills, but harmful acids leads to mucus formation in the gills, which hinders their ability to respire. Acid rains affect the pH level of the water which reduces the absorption capacity of essential nutrients of the aquatic life. This also hampers the reproduction process in fish leading to weak or brittle eggs.
iv. Effects of Acid Rain on Humans:
Most of all, acid rain affects human health adversely. It has the ability of harming us via the atmosphere as well as the soil where the food we eat is grown. Acid rain results in toxic metals breaking loose from the chemical compounds they occur in naturally.While toxic metals may be dangerous, but as long as they exist in combination with other elements, they are not harmful. Once acid rain causes these toxic metals to be released they can infiltrate into the drinking water, and the animals or crops that humans use as sources of food.
This contaminated food can damage the nerves in children, or result in severe brain damage, or even death. Scientists suspect that aluminum, one of the toxic metals affected by acid rain, is associated with Alzheimer’s disease.
Another adverse health effect of acid rain on humans is the respiratory problems it causes. The emissions of nitrogen oxide and sulfur dioxide cause respiratory problems like throat, nose and eye irritation; headache; asthma; and dry coughs. Acid rain is particularly harmful for those who have difficulty in breathing or suffer from asthma. In fact, even the lungs of healthy people can be damaged by the pollutants in acid air.
v. Acid Rain Effects on Historical Monuments:
Acid rain destroys stained glass windows, corrodes metal and also ruins the paint colour. Acid rain reacts with calcium to form calcium bicarbonate, which can be easily washed away. One drastic effect of acid rain can be seen on one of the Seven Wonders of the World, Taj Mahal in India.
The marble walls and pillars of this great man-made monument are found to be getting eroded by acid rains. Even St. Paul’s Cathedral in London, and the Statue of Liberty in New York are few such examples.
3. Control Measure:
The numbers of possible solutions for acid rain that are available to us are aplenty:
i. One of the most fundamental acid rain solutions is to utilize fuels that burn more cleanly, or to burn coal more efficiently. This will greatly reduce the possibilities of acid rain developing in the atmosphere.
ii. As fast as industrial power plants are concerned, the best solution is to attach devices known as ‘scrubbers’ in the chimneys of these plants. These scrubbers reduce the amount of sulfur produced in the smoke by 90 – 95%.
iii. Vehicles and cars must be mandatory required to comply with very tight and efficient emission standards. Fitting catalytic converters into the exhaust pipes of vehicles also reduces the amount of sulfur dioxide produced by the vehicles.
iv. For industrial power plants, there are many more acid rain solutions that must be enforced, as they are clearly the biggest contributors to the formation of acidified water droplets in the atmosphere. Industries must regularly inspect and clean all their emission equipment and chimneys and pipes.
v. All these acid rain solutions will be pointless unless people are informed and educated about the ill-effects and harms of acid rain. A widespread and nationwide effort must be made to make people aware. Only after that is done will all the acid rain solutions actually make a difference.
Acid rain is one of the biggest environmental hazards that we are facing today, and strong measure must be taken to prevent it, before it is too late. Governments need to sit up and take notice, and do much more than what they are already doing.
Acid rain adversely affects plants, animals and human beings, and as a result it is not something that we can afford to ignore. It is our duty towards ourselves and towards our fellow human beings to do all we can to prevent and reduce the presence and increase of acid rain in our environment
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