Acid Rain

ACID RAIN
Definition
Acid rain is rain or any other form of precipitation which is unusually acidic. It has harmful effects on plants, aquatic animals and buildings. Acid rain is mostly caused by human emissions of sulfur and nitrogen compounds which react in the atmosphere to produce acids. In recent years, many governments have introduced laws to reduce these emissions.
The term "acid rain" is commonly used to mean the deposition of acidic components in rain, snow, fog, dew, or dry particles. The more accurate term is "acid precipitation." Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. "Clean" or unpolluted rain has a slightly acidic pH of about 5.0, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid.

H2O (l) + CO2 (g) → H2CO3 (aq)

Carbonic acid then can ionize in water forming low concentrations of hydronium ions:

2H2O (l) + H2CO3 (aq) ⇌ CO32- (aq) + 2H3O+(aq)
The extra acidity in rain comes from the reaction of primary air pollutants, primarily sulfur oxides and nitrogen oxides, with water in the air to form strong acids (like sulfuric and nitric acid). The main sources of these pollutants are vehicles and industrial and power-generating plants

Emissions of chemicals leading to acidification
The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides which are oxidized to form nitric acid are of increasing importance due to stricter controls on emissions of sulfur containing compounds. 70 Tg(S) per year in the form of SO2 comes from fossil fuel combustion and industry, 2.8 Tg(S) from wildfires and 7-8 Tg(S) per year from volcanoes.

Natural Phenomena
The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and those from biological processes that occur on the land, in wetlands, and in the oceans. The major biological source of sulfur containing compounds is dimethyl sulfide.The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe.

Human activity
The coal-fired Gavin power plant in Cheshire, OhioThe principal cause of acid rain is sulphuric and nitrogen compounds from human sources, such as electricity generation, factories and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of kilometres in the atmosphere before they are converted to acids and deposited. In the past, factories had short funnels to let out smoke, but this caused many problems; thus, factories now have longer smoke funnels. However, this causes pollutants to be carried farther, causing greater ecological damage.

Chemical processes
Gas phase chemistryIn the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via a intermolecular reaction:

SO2 + OH• → HOSO2•

which is followed by:

HOSO2• + O2 → HO2• + SO3

In the presence of water sulphur trioxide (SO3) is converted rapidly to sulphuric acid:

SO3(g) + H2O(l) → H2SO4(l)

Nitric acid is formed by the reaction of OH with Nitrogen dioxide:

NO2 + OH• → HNO3

Acid deposition

Processes involved in acid deposition (note that only SO2 and NOx play a significant role in acid rain).

Wet deposition
Wet deposition of acids occurs when any form of precipitation (rain, snow, etc) removes acids from the atmosphere and delivers it to the Earth's surface. This can result from the deposition of acids produced in the raindrops (see aqueous phase chemistry above) or by the precipitation removing the acids either in clouds or below clouds. Wet removal of both gases and aerosol are both of importance for wet deposition.
Dry deposition
Acid deposition also occurs via dry deposition in the absence of precipitation. This can be responsible for as much as 20 to 60% of total acid deposition. This occurs when particles and gases stick to the ground, plants or other surfaces.