Acid rain is the mixture of wet and dry deposition in the atmosphere which contains more than normal sulfuric and nitric acid. It can occur in many forms – fog, rain, snow et cetera. The primary cause of acid rain is the emission of gases like sulfur dioxide and nitrogen oxide which react with water and oxygen in the atmosphere to form acids. These emissions are due to both natural and man-made reasons e.g. volcanoes, decaying vegetation and burning of fossil fuels. It is worth noting that unpolluted rain is also somewhat acidic with a pH 5.7.
Chemical Reactions involved in formation of acid rain –
Analysis shows that the presence of sulfuric and nitric acid is the primary reason of acidity in rain.
Formation of nitric acid –
Firstly, oxygen and nitrogen combine to give nitric oxide.
N2(g) + O2(g) → 2NO(g)
Nitric oxide then reacts rapidly with excess oxygen to give nitrogen dioxide, the compound responsible for the brown color of smog
2NO(g) + O2(g) → 2NO2(g)
When nitrogen dioxide dissolves in water, it forms nitric and nitrous acid.
2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)
Oxygen further oxidizes nitrous acid to nitric acid, the overall reaction is
2N2(g) + 5O2(g) + 2H2O(l) → 4HNO3(aq)
Formation of sulfuric acid – Roasting sulfide ores to obtain metals such as zinc and copper produces large amounts of SO2 via reactions such
2ZnS(s) + 3O2(g) → 2ZnO(s) + 2SO2(g)
Regardless of the source, the SO2 dissolves in rainwater to give sulfurous acid.
SO2(g) + H2O(l) → H2SO3(aq)
which is eventually oxidized by oxygen to sulfuric acid
2H2SO3(aq) + O2(g) → 2H2SO4(aq)
Harmful effects of Acid Rain –
- Surface water and Aquatic animals – Acid rain harms sensitive water bodies whose soil does not have the ability to neutralize the acidic compound. In these bodies where neither the water body nor the soil are able to neutralize the acidity the acid causes the soil to release aluminium which is a toxic substance and harms aquatic life living in that water body. Acid rain may harm or kill individual fish, reduce fish population numbers, completely eliminate fish species from a water body, and decrease biodiversity
- Effects on forests –Acid rain causes injury, slower growth or even complete eradication of forests. Acid rain is not usually fatal for tree but it does cause the tree’s leaves, limits the nutrients available to trees. Likelihood of a tree which is exposed to acid to die is more than that of a tree which is not.
- Damage to automotive coating- Vehicles whose paint is exposed to acid rain are damaged and there have been innumerable cases which prove the same. The effect is most evident on dark colored vehicles. Millions of dollars are spent by automotive manufacturers in providing an anti-acid rain coating to the paint of its vehicles
- Damage to material – Acid rain and the dry deposition of acidic particles contribute to the corrosion of metals (such as bronze) and the deterioration of paint and stone (such as marble and limestone). These effects significantly reduce the societal value of buildings, bridges, cultural objects (such as statues, monuments, and tombstones), and cars. Taj Mahal is a live example of the harmful effect acid rain has on buildings, this drives up maintenance costs of buildings.
- Human health – The harm from acid rain is not from direct contact with acid rain but from the pollutants which cause acid rain i.e. sulfur dioxide and nitrogen oxide. These gases interact with elements in the atmosphere to form fine sulfate and nitrate particles which are inhaled into people’s lungs and cause lung and heart disorders such as asthma and bronchitis.
Prevention methods –
- Flue gas desulfurization- Coal fired power stations should use FGD to desulfurize the flue gases which are emitted into the atmosphere. This process removes 95% of the sulfur from flue gases and has significant potential in reducing sulfur emissions into the atmosphere.
- There are other measures taken in an attempt to reduce acid rain such as Vehicle Emission norms are put into place to reduce harmful emissions of which sulfur and nitrogen dioxide are also a component. Other international treaties such as Sulfur Emissions reduction protocol have been signed.
Currently there are other more serious environmental problems to be solved. Acid rain is a very small component of the environmental problems our industrial civilization has created. The major reason of all such problems is harmful gas emissions which result from burning of fossil fuels. If the gas emissions are controlled then problem of acid rain will automatically solve itself.