The Science Knowledge Pool: Chemistry of Acid Rain

Any form of precipitation which is acidic in nature is called acid rain. Acid rain is the result of excessive emissions of sulfur and nitrogen caused by human activity, which reacts with other compounds to form acids. Acid rain has detrimental effects on animals, plants and infrastructure.
In its purest state, rainwater is like distilled water. It does not have carbon dioxide dissolved in it. It is neutral, with a pH level of 7. pH is the concentration of hydrogen ions in an aqueous solution. If the pH level is above 7, it is said to be basic, and if it is below 7, it is said to be acidic in nature.

As rainwater falls through the atmosphere, particles suspended in the air are dissolved in it. These substances are generally dust, pollen grains and carbon dioxide (CO2). Emissions of volcanoes and lightning tend to decrease the pH level of acid rain, making it even more acidic. CO2 combines with water to form carbonic acid (H2CO3).

H2O(l) + CO2(g) = H2CO3(aq)

Carbonic acid ionizes in water to form low concentrations of carbonate and hydronium ions.

2H2O(l) + H2CO3(aq) = CO32-(aq) + 2H30+(aq)

Carbonic acid is a weak acid. It brings down the pH of the rainwater to 6.0-5.2. With pH levels ranging between 6.0-5.2, rainwater is acidic, but still not dangerous. This is a reversible reaction.

The problem occurs when rainwater combines with gaseous oxides of sulfur, nitrogen, and phosphoric and hydrochloric acid mists. The latter two and sulfur are released into the atmosphere from automobile exhausts, industries and electric power plants. Nitrogen forms a major part of atmospheric composition. These chemicals bring down the acid rain pH level to 5.6-3.5. Sometimes, the pH level can even become as low as 2. This phenomenon of acidic rainwater precipitation, is called acid rain. Rain, snow, sleet, freezing rain, hail, fog and dew are other forms of precipitation.

Acid Rain
Sulfuric acid and nitric acid are the main acids present in acid rain. Sulfuric acid is formed as follows:
  • Sulfur released into the atmosphere combines with atmospheric oxygen to form sulfur dioxide (SO2)
  • Sulfur dioxide reacts with atmospheric water to form sulfurous acid - SO2(g) + H20(l) = H2SO3(aq)
  • Sulfurous acid is also present in acid rain.
  • Sulfur dioxide gradually oxidizes to form sulfur trioxide (SO3) - 2SO2(g) = O2(g) = 2SO3(g)
  • Sulfur trioxide reacts with water to form Sulfuric acid (H2SO4) - SO3(g) + H20(l) = H2SO4(aq)
Nitrogen dioxide(NO2) is formed as follows:
  • Nitrogen combines with atmospheric oxygen to form nitrogen dioxide (NO2). Nitrogen dioxide reacts with water to form nitrous acid (HNO2) and nitric acid (HNO3) - 2NO2(l) + H2O(l) = HNO2(aq) + HNO3(aq)
Acid rain is a mild combination of mainly sulfuric and nitric acid. Sulfurous acid and nitrous acid are less stable and are present only in very low amounts. Following are the various adverse effects of acid rain on living organisms and infrastructure.

Buildings and Monuments: Acid rain causes severe damage to buildings and marble statues. Acid rain reacts with the calcium carbonate (CaCo3) to form soluble calcium hydrogen carbonate or calcium bicarbonate, Ca(HCO3)2 - CaCO3 + Acid rain = Ca(HCO3)2(aq)
Calcium bicarbonate is a powdery substance, which is easily washed away with water or more specifically, rainwater. This is the way acid rain has partly eroded many world-famous monuments and buildings like the Taj Mahal in India, St. Paul's Cathedral in London, and the Statue of Liberty in New York.

Acid rain can destroy stained glass windows in churches, bridges made of steel, and railway tracks. It corrodes metal, ruins the paint color, weakens leather and forms a crust on glass surfaces.

Trees and Plants: Acid rain washes away important minerals from leaves and soil. Acid rain also blocks the small pores on the leaves' surface, through which they take in carbon dioxide. With improper functioning of leaves, the growth of trees gets retarded. This results in a loss of leaves, stunted growth and damaged bark. Such trees are more prone to attacks by fungi and insects. This can even result in the death of trees.

Soil: Soil contains many harmful minerals such as mercury and aluminum. These elements can't be absorbed by plants and trees and are thus harmless. Upon contact with acid rain, these chemicals undergo chemical reactions with the acids. As a result, compounds of aluminum, lead and mercury are formed. Plants and trees can easily absorb these compounds. Such elements, which are extremely harmful to living forms, ultimately affect the entire food chain. These chemicals not only harm the flora, but also the animals that feed on them.

Water Bodies: Harmful substances like aluminum, lead and mercury, as discussed above, are washed away from the soil to neighboring water resources by the acid rain, thus resulting in water pollution. These chemicals and their acids do not allow the flora and fauna to grow and reproduce. This also results in depletion of oxygen levels in water, thereby making it difficult for fish to respire. They die due to suffocation and poisoning caused by the presence of these chemicals.

Birds that feed on fish that are affected by these harmful chemicals, accumulate these elements in their systems. These chemicals are then passed on to animals that feed on such birds. In this manner, chemicals get introduced to each trophic level of the food chain. In each trophic level, the concentration of chemicals gets multiplied. This phenomenon is known as biomagnification.

Human Health: Acid rain is harmful to human health. Drinking water contaminated by aluminum, mercury and lead is highly dangerous for human health.

Acids are very small and fine particles. They are normally in a liquid state. When they are present in the atmosphere, they easily enter the lungs while breathing. Research has proven that these particles can even lead to cancer.

Solutions to Acid Rain

Acid rain is potent enough to destroy life on Earth. It damages anything it comes in contact with. It ruins forests, water bodies, soil, infrastructure, and the health of living beings. It's high time we take all possible measures to control it. There is a solution to every problem, and acid rain is no exception. Take a look at the various measures we can take.
  • Human beings should reduce the use of fossil fuels. This would lessen the emission of nitrogen and sulfur in the environment.
  • Public transportation, car pools and walking can reduce nitrogen, sulfur and lead emissions into the atmosphere.
  • Sulfur and nitrogen are mostly released into the atmosphere from the burning of fossil fuels (e.g. Coal). Switching over to alternative forms of energy such as geothermal, water, wind, and solar power would help to a great extent.
All the above measures are simple steps that can easily be adopted on an individual level. Acid rain has become an international issue because of its serious and definite danger to life on earth. Many international treaties have been signed i.e. the Sulfur Emissions Reduction Protocol and the Convention on Long-Range Transboundary Air Pollution. Installing Flue Gas Desulfurization e.g. wet scrubber in coal-burning power plants to remove sulfur-containing gases - is one of the steps taken by the USA, and is followed by a number of developed and developing countries.

Let's take all the steps possible to give the next generation of the human race and all living organisms on this planet, a cleaner and safer environment to live in.