Acid Rain

Acid Rain Broadly refers to acidic precipitations—in wet form such as acidic rain, fog, and snow; or in dry form such as acidic gases and particles. Acid rain was first noticed in the late 1960s, when declining fish stocks were observed in Scandinavian lakes, and precipitation was found to be more acidic. Acid rain has also destroyed forests and acidified lakes in Canada as well.

The term acid rain is a misnomer, because even uncontaminated rain has a pH level below 7 and is therefore acidic. The addition of sulphurous and nitrous gases to the atmosphere causes precipitation to become even more acidic as they combine with water. The natural pH of rainwater is about 5.6, but the pH of acidic rain is 4.0–5.0. A decrease of one pH unit represents about a tenfold increase in the acidity of rain.

Acid rain occurs when sulfur dioxide (SO2) and nitrogen oxides (NOx), released from fossil fuel burning and industrial processes, react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. Sunlight increases the rate of most of these reactions. The result is a mild solution of sulfuric acid and nitric acid. When this acidic precipitation reaches the ground, it affects plants, animals, fishes, and other living things to varying degrees.

About half of the acidity in the atmosphere is composed of dry depositions blown by the wind onto buildings, homes, trees, etc. When the other half arrives in the form of rain, all of these dry deposits are also washed out. This, in turn, makes the runoff even more acidic and harmful to the environment.

High-altitude vegetation communities are more at risk from acidification because they may be exposed to the cloud bases, which have low pH values due to the acids produced by the reaction of acidification gases with hydroxyl radicals and monatomic oxygen. Areas with “acid” bedrock are also more at risk because they lack buffering capacity; this means these soils are already acidic and hence lack cations such as calcium, which react with acid soil and groundwaters to raise the pH in drainage waters.

Acid rain causes corrosion of buildings in urban areas; reduced visibility; forest damage from occult precipitation; acidification of lakes, rivers and ground water; decline in fish population (or decline in aquatic ecology); and changes in soil flora and fauna (forest and terrestrial ecosystem damage). Acid rain also damages forests by damaging tree leaves, thus limiting available nutrients, or by exposing trees to toxic substances slowly released from the soil.

Acid rain can also cause extensive damage to buildings, marble sculptures (such as Taj Mahal), limestone, slate, and mortar. These materials become pitted and weakened mechanically as the soluble sulphates are leached out by rainwater. Acid rain also increases the process of weathering.