How did acid rain (酸雨)affect the plant

General Information

Acid rain (or more accurately acid precipitation)[1] occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.[2] The term "acid rain" is sometimes used more generally to include all forms of acid deposition - both wet deposition, where acidic gases and particles are removed by rain or other precipitation, and dry deposition removal of gases and particles to the Earth's surface in the absence of precipitation.[3] Acid rain is defined as any type of precipitation with a pH that is unusually low.[4] Dissolved carbon dioxide dissociates to form weak carbonic acid giving a pH of approximately 5.6 at typical atmospheric concentrations of CO2.[5] Therefore a pH of less than 5.6 has sometimes been used as a definition of acid rain.[6] However, natural sources of acidity mean that in remote areas, rain has a pH which is between 4.5 and 5.6 with an average value of 5.0 and so rain with a pH of less than 5 is a more appropriate definition.[7]

The US EPA says, "Acid rain is a serious environmental problem that affects large parts of the US and Canada" [8] Acid rain accelerates weathering in carbonate rocks and accelerates building weathering. It also contributes to acidification of rivers, streams, and forest damage at high elevations.

Adverse effects

Soils
Soil biology can be seriously damaged by acid rain. Some tropical microbes can quickly consume acids[11] but other types of microbe are unable to tolerate 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 and leach away essential nutrients and minerals [12].


[edit] Forests and other vegetation
Acid rain can slow the growth of forests, cause leaves and needles to turn brown and fall off and die. In extreme cases trees or whole areas of forest can die. The death of trees is not usually a direct result of acid rain, often it weakens trees and makes them more susceptible to other threats. Damage to soils (see above) can also cause problems. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain.[12]

Other plants can also be damaged by acid rain but the effect on food crops is minimised by the application of 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.[12] Acid Rain depletes minerals from the soil and then it stunts the growth of the plant.

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Acid rain can penetrate into the soil can mix with water,
when the plant absorb water from the soil,
the acid in water will affect the enzyme acticity,
like photosynthesis and respiration
since they are controlled by enzyme.

Also, the concentration of water molecules
in the water containing acid is lower than the water
inside the leave cells, and the water willmoves out through
the cell membrane by osmosis,
with these effect on the plant,
the plant will therefore die !

2006-12-03 19:32:22 補充：
The acid will affect the soil and the root of the plant cannot absorb minerals in the soil normally, and this will also decrease the nutrients supply to the plant.

Acid rain (or more accurately acid precipitation)[1] occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.[2] The term "acid rain" is sometimes used more generally to include all forms of acid deposition - both wet deposition, where acidic gases and particles are removed by rain or other precipitation, and dry deposition removal of gases and particles to the Earth's surface in the absence of precipitation.[3] Acid rain is defined as any type of precipitation with a pH that is unusually low.[4] Dissolved carbon dioxide dissociates to form weak carbonic acid giving a pH of approximately 5.6 at typical atmospheric concentrations of CO2.[5] Therefore a pH of less than 5.6 has sometimes been used as a definition of acid rain.[6] However, natural sources of acidity mean that in remote areas, rain has a pH which is between 4.5 and 5.6 with an average value of 5.0 and so rain with a pH of less than 5 is a more appropriate definition.[7]
The US EPA says, "Acid rain is a serious environmental problem that affects large parts of the US and Canada" [8] Acid rain accelerates weathering in carbonate rocks and accelerates building weathering. It also contributes to acidification of rivers, streams, and forest damage at high elevations.
Natural emissions
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.

[edit] Human emissions
The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation, factories and motor vehicles. The gases can be carried hundreds of miles in the atmosphere before they are converted to acids and deposited.

[edit] Gas phase chemistry
In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via a termolecular 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 + H2O → H2SO4
Nitric acid is formed by the reaction of OH with Nitrogen dioxide:

NO2 + OH· → HNO3
For more information see Seinfeld and Pandis (1998).[5]

[edit] 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

Hydrolysis
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-

Oxidation
There are a large number of aqueous reactions of sulfur which oxidise it 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 catalysed by Iron and Manganese in the cloud droplets).
For more information see Seinfeld and Pandis (1998).[5]
圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/Acid_rain_woods1.JPG/350px-Acid_rain_woods1.JPG



圖片參考：http://upload.wikimedia.org/wikipedia/en/4/42/Volcanic_injection.jpg



圖片參考：http://upload.wikimedia.org/wikipedia/en/thumb/9/9f/Origins.gif/350px-Origins.gif



圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Processes involved in acid deposition (note that only SO2 and NOx play a significant role in acid rain).