Acid rain is rain or any other form of precipitation that is unusually acidic. It has harmful effects on plants, aquatic animals, and infrastructure. 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 haveintroduced laws to reduce these emissions.
"Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog and cloudwater, dew) and dry (acidifying particles and gases) acidic components. A more accurate term is “acid deposition”. 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 pHgreater than 7 are bases. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals.
H2O (l) + CO2 (g) → H2CO3 (aq)
Carbonic acid then can ionize in water forming low concentrationsof hydronium and carbonate ions:
2 H2O (l) + H2CO3 (aq) [pic] CO32− (aq) + 2 H3O+ (aq)
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
The principalnatural 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 isdimethyl sulfide.
Acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe.
Human activityThe principal cause of acid rain is sulfur 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 manyproblems locally; thus, factories now have taller smoke funnels. However, dispersal from these taller stacks causes pollutants to be carried farther, causing widespread ecological damage.
Gas phase chemistry
In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction:
SO2 + OH· → HOSO2·
which is followed by:
HOSO2· + O2 →HO2· + SO3
In the presence of water, sulfur trioxide (SO3) is converted rapidly to sulfuric acid:
SO3 (g) + H2O (l) → H2SO4 (l)
Nitric acid is formed by the reaction of OH with nitrogen dioxide:
NO2 + OH· → HNO3
Chemistry in cloud droplets
When clouds are present, the loss rate of SO2 is faster than can be explained by gas phase chemistry alone. This isdue to reactions in the liquid water droplets.
Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:
SO2 (g) + H2O [pic] SO2·H2O
SO2·H2O [pic] H+ + HSO3−
HSO3- [pic] H+ + SO32−
There are a large number of aqueous reactions that oxidize sulfur from S(IV) to S(VI),leading to the formation of sulfuric acid. The most important oxidation reactions are with ozone, hydrogen peroxide and oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud droplets).
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...