• EMISSIONS OF CHEMICALS
o NATURAL PHENOMENA
o HUMAN ACTIVITY
• GAS PHASE CHEMISTRY
o CHEMISTRY IN CLOUD DROPLETS
• ACID DEPOSITION
• DRY DEPOSITION
o SURFACE WATERS AND AQUATIC ANIMALS
o FORESTS AND OTHER VEGETATION
o HUMAN HEALTH
Acid rain is a rain or any other form of precipitation that is unusually acidic, elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure through the process of wet deposition. Acid rain is caused by emissions of compounds of ammonium, carbon, nitrogen, and sulfur which react with the water molecules in the atmosphere to produce acids. Governments have made efforts since the 1970s to reduce the production of sulfur dioxide into the atmosphere with positive results. However, it can also be caused naturally by the splitting of nitrogen compounds by the energy produced by lightning strikes, or the release of sulfur dioxide into the atmosphere by volcano eruptions.
Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog, cloudwater, and dew) and dry (acidifying particles and gases) acidic components. A more accurate term is “acid deposition”. Distilled water, once carbon dioxide is removed, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are Alkaline. “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, but unpolluted rain also contains other chemicals.
H2O (l) + CO2 (g) → H2CO3 (aq)
Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions:
2 H2O (l) + H2CO3 (aq) CO32− (aq) + 2 H3O+ (aq)
Acid deposition as an environmental issue would include additional acids to H2CO3.
EMISSIONS OF CHEMICALS
The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and those frombiological processes that occur on the land, in wetlands, and in the oceans. The major biological source of sulfur containing compounds isdimethyl sulfide.
Nitric acid in rainwater is an important source of fixed nitrogen for plant life, and is also produced by electrical activity in the atmosphere such as lightning. Acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe.
The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity, factories, and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of kilometers 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 locally thus, factories now have taller smoke funnels. However, dispersal from these taller stacks causes pollutants to be carried farther, causing widespread ecological damage. Also, livestock production plays a major role. It is responsible for almost two-thirds of all anthropogenic sources of ammonia produced through human activities, which contributes significantly to acid rain.
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)
Nitrogen dioxide reacts with OH to form nitric acid:
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 is due 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 SO2•H2O
SO2•H2O H+ + HSO3−
HSO3− 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 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 aerosols are both of importance for wet 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.
SURFACE WATERS AND AQUATIC ANIMALS
Both the lower pH and higher aluminium concentrations in surface water that occur as a result of acid rain can cause damage to fish and other aquatic animals. At pHs lower than 5 most fish eggs will not hatch and lower pHs can kill adult fish. As lakes and rivers become more acidic biodiversity is reduced. Acid rain has eliminated insect life and some fish species, including thebrook trout in some lakes, streams, and creeks in geographically sensitive areas, such as the Adirondack Mountains of the United States. However, the extent to which acid rain contributes directly or indirectly via runoff from the catchment to lake and river acidity (i.e., depending on characteristics of the surrounding watershed) is variable. The United States Environmental Protection Agency's (EPA) website states: "Of the lakes and streams surveyed, acid rain caused acidity in 75 percent of the acidic lakes and about 50 percent of the acidic streams".
Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to tolerate changes to low pHs and are killed. The enzymes of these microbes are denatured (changed in shape so they no longer function) by the acid. The hydronium ions of acid rain also mobilize toxins such as aluminium, and leach away essential nutrients and minerals such as magnesium.
2 H+ (aq) + Mg2+ (clay) 2 H+ (clay) + Mg2+ (aq)
Soil chemistry can be dramatically changed when base cations, such as calcium and magnesium, are leached by acid rain thereby affecting sensitive species, such as sugar maple (Acer saccharum).
FORESTS AND OTHER VEGETATION
Adverse effects may be indirectly related to acid rain, like the acid's effects on soil (see above) or high concentration of gaseous precursors to acid rain. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain.
Other plants can also be damaged by acid rain, but the effect on food crops is minimized by the application of lime and fertilizers to replace lost nutrients. In cultivated areas, limestone may also be added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in the case of wilderness lands. When calcium is leached from the needles of red spruce, these trees become less cold tolerant and exhibit winter injury and even death.
Scientists have suggested direct links to human health. Fine particles, a large fraction of which are formed from the same gases as acid rain (sulfur dioxide and nitrogen dioxide), have been shown to cause premature deaths and illnesses such as cancer and other diseases. For more information on the health effects of aerosols see particulate health effects.
Scrubbers remove 80-95 percent of the sulfur oxides. Certain types do not remove nitrogen oxides. They are costly to retrofit to existing power plants. They increase the electric generating costs by 10-15 %.
Fluidized Bed Combustion, This process also burns the coal more efficiently by about 5 %. This process removes 90 % of the sulfur oxides and 15-35 % of the nitrogen oxides.
Limestone Injection - Multistage Burner, This process removes 70 % of both sulfur and nitrogen oxides.
This thing can make prevent form acid rain, if this pollution continues; sure there will big danger to Mother Nature. So let prevent earth form acid rain and pollution.
acid rain.doc (Size: 68.5 KB / Downloads: 122)