ACID RAIN

Background

     No liquid found on the earth is naturally neutral. Of the total water present on the earth, some may be acidic and others may be alkaline or basic.
     The unit used to measure acidity or alkalinity of any liquid is termed as pH. pH is a symbol indicating how acidic or basic a solution is in ratios of relative concentration of hydrogen ions in a solution. The liquid which has a pH of around 7 is termed neutral. The value of pH below 7 is an indicator of its acidity. Low the value of pH, greater the acidity. If value of pH exceeds 7, it indicates alkalinity. Higher the value of pH greater is the alkalinity. pH is usually measured on a scale of 0 to 14.
     Rainwater is always slightly acidic in nature. This is because carbon dioxide present in the atmosphere, reacts with rainwater to form carbonic acid, H2CO3. However, we never refer to normal rain as acid rain. The term acid rain was first used by Robert Angus in 1872. Acid rain literally means the presence of excessive acids in rainwater. The acids which are mainly identified as present in rainwater are sulfuric acid, H2SO4 and, HNO3.There is no fixed ratio of concentrations of these two acids but this depends on the presence of amount of sulfur and nitrogen in the atmosphere. Minor quantity of another acid called hydrochloric acid (HCI) is also present in acid rain.
     "Acid Rain," or more precisely acid precipitation, is the word used to describe rainfall that has a pH level of less than 5.6. This form of air pollution is currently a subject of great controversy because of it's worldwide environmental damages. A pH from 6.5 to 8, is considered the safe zone. Between these numbers, organisms are in very little or no harm.

What is Acid Rain?

     Acid rain is rain that has been made acidic by certain pollutants in the air. Rain is acidified by oxides of sulfur and nitrogen. Acid rain usually has a pH of less than 5.6. It is formed when pollutants called oxides of sulfur and nitrogen, contained in power plant smoke, factory smoke, and vehicular exhaust, react with the moisture in the atmosphere. It is a broad term referring to a mixture of wet and dry deposition (deposited material) from the atmosphere containing higher than normal amounts of nitric and sulfuric acids.

Wet Deposition

     Wet deposition refers to acidic rain, fog, and snow. If the acid chemicals in the air are blown into areas where the weather is wet, the acids can fall to the ground in the form of rain, snow, fog, or mist. As this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depends on several factors, including how acidic the water is; the chemistry and buffering capacity of the soils involved; and the types of fish, trees, and other living things that rely on the water.

Dry Deposition
     In areas where the weather is dry, the acid chemicals may become incorporated into dust or smoke and fall to the ground through dry deposition, sticking to the ground, buildings, homes, cars, and trees. Dry deposited gases and particles can be washed from these surfaces by rain storms, leading to increased runoff. This runoff water makes the resulting mixture more acidic. About half of the acidity in the atmosphere falls back to earth through dry deposition.

Causes of Acid Rain
     Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain. Sulfur dioxide and nitrogen oxides dissolve very easily in water and can be carried very far by the wind. As a result, the two compounds can travel long distances where they become part of the rain.
     There are two primary sources of acid rain are sulfur dioxide (SO₂), and oxides of nitrogen (NO_X). Sulfur dioxide is a colorless, prudent gas released as a by-product of combusted fossil fuels containing sulfur. A variety of industrial processes, such as the production of iron and steel, utility factories, and crude oil processing produce this gas. Sulfur is present in nearly all the sources of energy, i.e. coal, fuel, wood or petroleum products. Sulfur dioxide can also be emitted into the atmosphere by natural disasters or means.
     The other chemical that is also chiefly responsible for the make-up of acid rain is nitrogen oxide. Nitrogen oxide is a dangerous gas by itself. These gases are by-products of firing processes of extreme high temperatures (automobiles, utility plants), and in chemical industries (fertilizer production). Natural processes such as bacterial action in soil, forest fires, volcanic action, and lightning make up nitrogen oxide emission. It also contributes to ozone damage, and forms smog.
      These oxides are swept up into the atmosphere and can travel thousands of kilometers by wind.
     Thus, Increasing air pollution due to human activities, has led to a rise in the atmospheric concentrations of nitrogen oxides (NO_X) and sulfur oxides (SOS_X).During rainfall, these Oxides react with molecules of water and form nitric acid (HNO₃) and sulfuric acid (H₂SO₄) respectively. Both these acids are strong in nature and thus make rainfall acidic.

Acid Rain Formation
     Acid rain is formed when oxides of nitrogen and sulfur combine with moisture in the atmosphere to make nitric and sulfuric acids. These acids can be carried away far from its origin.

Sulfur oxides are formed as:
      S         +      O₂     →     SO₂
      2SO₂   +      O₂     →     2SO₃

Nitrogen oxides are formed as:
      N₂        +        O₂        →        2NO
      NO       +        O_{3           →}         NO₂       +       O₂
      NO₂     +        O₃         →         NO₃       +       O₂
      NO₃     +        NO₂      →          N₂O₅
During rainfall, these oxides of nitrogen and sulfur react to form nitric acid and sulfuric acid respectively, as :
     2SO₂      +       O₂      +       H₂O       →        2H₂SO₄
     N₂O₅      +       H₂O       →        2HNO₃
Nitric acid and Sulfuric acid, thus formed fall on earth with rainfall, thus causing acid rain.

Effects of Acid Rain

On Aquatic Ecosystem

On Aquatic Ecosystem

On Materials

On Human beings: Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct. Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or swimming in clean water. However, the pollutants that cause acid rain—sulfur dioxide (SO₂) and nitrogen oxides (NO_X)—do damage human health.
     Acid rain can adversely affect nervous, respiratory and digestive system through drinking water polluted by it. It will harm people when they breathe in smog (acid smog), acid rain in one of its many forms. Acid rain can harm people indirectly.
     Industrialization bringing acid rain to India there is growing evidence to believe that the incidence of acid rain is increasing in the country. Many experts link the phenomenon with increasing industrialization in the country. India Meteorological Department has found increasing acidity in rain samples from Pune and Nagpur also. The samples were acidic with pH values less than 5