acidic pollutants can be deposited from the atmosphere to the earth

8/14/2019 Acidic Pollutants Can Be Deposited From the Atmosphere to the Earth

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Acidic pollutants can be deposited from the atmosphere to theearths surface and wet and dry forms. The common term to describethis process is acid deposition. The term acid precipitation is used tospecifically describe wet forms of acid pollutant that can be found inrain snow, fog and cloud vapor. An acid can be defined as anysubstance that when dissolved in water dissociates to yield corrosivehydrogen ions. The acidity of substances dissolved in water iscommonly measured in terms of pH (defined as the negative logarithmof the concentration of hydrogen ions). According to this measurementscale solutions with pHs less than 7 are described as being acidic,while a pH greater than 7.0 is considered alkaline. Precipitation

normally has a pH between 5.0 to 5.6 because of natural atmosphericreactions involving carbon dioxide. For comparison, distilled water pureof any other substances, would have a pH of 7.0. Precipitation isconsidered to be acidic when its pH falls below 5.6 (which is 25 timesmore acidic than pure distilled water). The term acid rain was first usedtwo centuries later when Angus Smith published book called AcidRain in 1872. Some air pollutants do more than irritate your eyes andlungs. The nitrogen dioxide and sulfur dioxide in smog dissolve in waterto form nitric acid and sulfuric acid. These strong acids fall to earth asacid rain.

Acid deposition can form as a result of two processes. In somecases, hydrochloric acid can be expelled directly into the atmosphere.

More commonly it is due to secondary pollutants that form from theoxidation of nitrogen oxides (NOx) or sulfur dioxide (SO2) gases thatare released into the atmosphere. Acid precipitation formation can alsotake place at the Earth's surface when nitrogen oxides and sulfurdioxide settle on the landscape and interact with dew or frost.Emissions of sulfur dioxide are responsible for 60-70% of the aciddeposition that occurs globally.
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More than 90% of the sulfur in the atmosphere is of human origin. Themain sources of sulfur include:

Coal burning - coal typically contains 2-3% sulfur so when it isburned sulfur dioxide is liberated.

The smelting of metal sulfide ores to obtain the pure metals.Metals such as zinc, nickel, and copper are all commonlyobtained in this manner.

Volcanic eruptions - although this is not a widespread problem, avolcanic eruption can add a lot of sulfur to the atmosphere in aregional area.

Organic decay. Ocean spray.

After being released into the atmosphere, sulfur dioxide can either bedeposited on the Earth's surface in the form of dry deposition or it can

undergo the following reactions to produce acids that are incorporatedinto the products of wet deposition.

Some 95% of the elevated levels of nitrogen oxides in the atmosphereare the result of human activities. The remaining 5% comes fromseveral natural processes. The major sources of nitrogen oxidesinclude:


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Combustion of oil, coal, and gas. Bacterial action in soil. Forest fires. Volcanic action. Lightning.

Acids of nitrogen form as a result of the following atmosphericchemical reactions.

Finally, the concentrations of both nitrogen oxides and sulfur dioxidesare much lower than atmospheric carbon dioxide which is mainlyresponsible for making natural rainwater slightly acidic. However,these gases are much more soluble than carbon dioxide and thereforehave a much greater effect on the pH of the precipitation.


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Acid deposition influences the environment in several differentways. In aquatic systems, acid deposition can affect these ecosystemsby lowering their pH. However, not all aquatic systems are affectedequally. Streams, ponds, or lakes that exist on bedrock or sedimentsrich in calcium and/or magnesium are naturally buffered from theeffects of acid deposition. Aquatic systems on neutral or acidic bedrockare normally very sensitive to acid deposition because they lack basiccompounds that buffer acidification. The severity of the impact of aciddeposition on vegetation is greatly dependent on the type of soil the

plants grow in. Similar to surface water acidification, many soils have anatural buffering capacity and are able to neutralize acid inputs. Ingeneral, soils that have a lot of lime are better at neutralizing acidsthan those that are made up of siliceous sand or weathered acidicbedrock. In less buffered soils, vegetation is effected by acid depositionbecause:

Acid precipitation can cause direct damage to the foliage onplants especially when the precipitation is in the form of fog orcloud water which is up to ten times more acidic than rainfall.

Dry deposition of SO2 and NOx has been found to affect the

ability of leaves to retain water when they are under waterstress.

Acidic deposition can leach nutrients from the plant tissues

weakening their structure.

The combination of these effects can lead to plants that have reducedgrowth rates, flowering ability and yields. It also makes plants morevulnerable to diseases, insects, droughts and frosts. Acid depositionalso influences the economic livelihoods of some people. Forestry andagriculture are affected by the damage caused to vegetation.


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An important regional effect of human alteration of to majorbiogeochemical cycles is acid precipitation, rain or snow whose pH islowered by the presence of sulfuric acid and nitric acid, derived in largepart from the burning of fossil fuels. These acids enter the atmosphereand may travel hundreds of kilometers before the settle to earthprecipitation or as dry particle. Acid precipitation is now a phenomenonof all major industrial countries and is particularly wide spread.Ecologists in Canada studied the effects of acid precipitation on small

lakes by adding enough sulfuric acid to two lakes to reduce their pHfrom about 6.6 to 5.2. In both lakes, nitrifying bacteria failed to adaptto these moderately acidic conditions, with the result that the nitrogencycle was blocked and ammonium accumulated in the water. When theecologists stopped adding acid to one oh the lakes, its pH increase to5.4, and nitrification resumed after a lag of about 1 year. Thisexperiments show that lakes are very sensitive two acidification butcan recover quickly when pH returns to normal values. Acidprecipitation also changes soil chemistry. Since 1963, scientists havemonitored the chemistry of both precipitation and stream water atHubbard Brook experimental forest in New Hampshire. The pH of

stream water leaving the forest has changed relatively little asprecipitation has become more acid, but large amount of calcium andmagnesium have flowed out of the water shed. When pollutionoriginates in one area but causes problems in another, as acidprecipitation does, solving the problem is politically difficult. Acidprecipitation is caused by the generation of the energy upon whichmodern societies depend. The number of sources emitting these oxidesis now so great that almost complete removal will be necessary tocorrect the problem even if no sources are added.


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Strauss, E & Lisowski M. Life : The Science ofBiology. Singapore: Pearson Education Inc.,2000.

Lutgens, Frederick. Earth Science 10th ed.,Singapore : Pearson Education, 2003.

Thomson. 1997. Biology Concepts andApplication . 3rd Ed., Wadsworth: PublishingCompany.

Biological Science: An Ecological Approach. 1854.Chief Settle: Seuquamish Tribe Washington Territory.

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In 1974 two chemists at the University of California at Irvine usedcomputer model calculations to predict that an important part of theearths s atmosphere, the ozone layer, was in danger of being seriouslydamaged or even destroyed by chlorofluorocarbons, gaseouscompound widely used in aerosols in refrigerants. Ozone forms a layerin the stratosphere about 30 kilometers above the earth surface. Thislayer protects plant and animal life from ultra violet light by screeningout about 99 percent of the ultraviolet rays falling on the stratosphere.The U.S Government banned the used of chlorofluorocarbons as thepropellant gas in aerosol sprays in 1978. However, air conditioners,refrigerators, and insulating foam products also emitchlorofluorocarbons. If emissions continue at to this rate, 5 to 9 % ofthe present Ozone layer could disappear over the next 50 years, itmight be disappearing even faster. Since 1977, British meteorologistshave observed that an ozone Hole , in which the zone is reduced asmuch as 40%, opens up every October over the South Pole. Thethinning of the ozone has led to some predictions for the future. Overthe lifetime of people alive today in the U.S. there could be a dramaticincrease in skin cancer, cataracts and fatal malignant melanomas.Agricultural crops and aquatic plant communities could be damaged.

Ozone thinning could contribute to the green house effect of thegasses in earths atmosphere world wide. The ozone layer is thinning asthe total amount of ozone decreases. The main cause of the thinning ofthe ozone layer is a group of chlorine containing chemicals known uschlorofluorocarbons or CFCs. These odorless, non corrosive compoundswere once widely used as propellants in aerosol cans and in theproduction of plastic foam. In the stratosphere, they are bombarded byultraviolet rays and they break apart. This process releases chlorineatoms that reacts with ozone and deflate the ozone layer. Depletion ofthe ozone layer increases Earths exposure to UV radiation. CFCs havebeen banned in the U.S. and many other nations. Substitutes are being

developed for some types of CFCs. However, CFCs persist in theatmosphere for many years. So even though CFC use has droppeddramatically, Ozone destruction will continue for many years. Ozone isthen created when a single atom of oxygen and a molecule of oxygencollide.

This must happen in the presence of a third, neutral molecule thatacts as a catalyst by allowing the reaction to take place without itself


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In 1985, scientists discovered that the high altitude ozone layer,which shields organisms from harmful ultraviolet radiation, had thinned

greatly over Antarctica. Ozone is produced by the action of sunlight onatmospheric oxygen in tropical regions and is transported to highlatitudes, was it is destroyed. Chlorine compounds, produced mainly byhumans, appear to be the main cause of the unusually high rates ofozone destruction.

The Ozone layer, almost twice as high above sea level as the topof Mount Everest the Highest place on Earth. Each September throughmid- October the layer gets thinner at high latitudes. Ozone thinning isso pronounced, it was once known as an Ozone Hole. In 1995, thethinning above Antarctica spanned an area twice as big as Europe. Thethinning at high northern latitudes exceeded to percent. Sixty years

from today, the protective layer may be reduced by 30 percent or moreover heavily populated may be reduced by 30 percent or more heavilypopulated regions of North America, Europe and Asia.


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acidic pollutants can be deposited from the atmosphere to the earth

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