EnvkomentalScience

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Acid Deposit ion.The Threat from Above

A n V A N C E D P I A C E M E N T

STUDENT INSTRUCTIONS

Objectives

DiscussionWhat Is Acid Deposition?How Does Acid Deposition Form?The Hydrologic CycleWhat Are the Effects of Acid Deposirion?Where Does Acid Deposition Occur?How Is Acid Deposition Neutralized?Solutions

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NotesPre,Lab QuestionsProcedure

Part I _Part II

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Laboratory Questions 1 0

Maps12

Cover Photo: Mt. Mitchell, NC, by Carolina staffphotographer William R. West, FBPA

@1998 Carolina Biological Supply Company printed in USA

Acid D.position: The Threat from Above

In this exercise, you determine the buffering effects of three different rypes ofbedrock, track the direction of wind patterns, and locate major pollutantsources to determine where acid deposition forms, where it falls, and where itmay affect aquatic habitats. You may also test warer samples you collect locallyto determine their sensitiviry to acid deposition.

Upon completion of this exercise, you should be able to' Explain how acid deposition forms and write the chemical equations that

describe its formation.

' Predict where acid deposition will occur in the United States and Canada,where it will be neutralized, and where it may acidift surface warer.

' T"st the pH of solutions using the appropriate rype of pH paper.. Interpret data displayed on maps.

Despite significant reductions in air pollutant emissions, acid depositionremains a threat to human-made sffuctures, aquatic organisms, fores$, andhuman health. In this activity, you examine the causes and effects of aciddeposition.

What Is Acid Deposition?Acid deposition is acidi. (pH < 5.0) and acid-forming subsrances that fal toearth. These materials may be wet, such as rain, snoq and fog, or dry such asparticles of sulfate and nitrate salts.

How Does Acid Deposition FormlAcid deposition forms when sulfur dioxide and nitrogen oxides (nitrogenmonoxide, nitrogen dioxide, and dinitrogen monoxide) gases are released intothe atmosphere. These gases react with oxygen ".rd *ut"r in the atmosphereto form sulfuric and nitric acids and sulfate and nitrate salts.

2SO2(d + O7k) -+ 2SO3(s)

SO3(d + H2O0) -+ H2SO +(aq)

H2SOa(u) + zHzo(t) -+ SO42-@q) + ZH3O + (aq)

NO, NOz,NzO(d + Oz(d + HzO(t)+ HNO3(aa)

HNO3 fuq) + HzO(l) + NO3- @q) + H3O+ (aq)

Many natural events, such as volcanic eruptions, forest fires, hot springs, andnatural geysers, produce sulfur dioxide and nitrogen oxide gases. Human

Objectives

Discussion

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activiry makes a significant contribution. In fact, anthropogenic (human-made) levels of sulfur in the atmosphere almost equal thl amount formednaturally, and-anthropogenic levels of nitrogen o*id., in the atmosphere doequal natural levels. The major sources of anthropogenic sulfur dioxide arecoal-burning electric utilities and industrial plants, ,ih.r.", the major sourcesof anthropogenic nitrogen oxides are coal-burning'el..iri. uriliries and motorvehicles.

The Hydrologic CycleAs snow or rain falls, carbon dioxide presenr in the atmosphere dissolves inthe water and reacts to form carbonic acid, H2co3, " *"rr. acid.

Coz(d + Hzo(I) -+ H2Co {aq)For this reason, unpolluted rainwater is acidic, with a pH value of 5.0 to 5.6.\rhen precipitation reaches the ground, it runs off into surface warer orinfiltrates the soil. The water th"t infiltrares the soil percolate, thro,rghpermeable rock and into groundwater. Both surface water and groundwaterproceed to the sea.'Water returns to the atmosphere when surface waterevaporates and plants ffanspire.

what Are the Effects of Acid DepositionlAcid deposition affects trees, human-made structures, and surface water. Aciddamages tree leaves, impairing a tree's abiliry to photosynthesize, and damagesbark, leaving the tree vulnerable to insects and Jis"ase. Tiees at highelevations are especially susceptible because cloud vapor can be 10 to 100times more acidic than acid rain and can bathe ffees in acid for days at a time.As water evaporates from the acid drops on the plant, the acidity increases toconosive levels' The acid removes the leaves' coating and burns the leaves,leaving brown spots. Acid also leaches nutrients, such as calcium andmagnesium, from the soil, depriving already weakened trees of essentialminerals' Acid also releases toxic aluminum ions from the soil which candamage plant roots.

Many ancient statues and buitdings are made of rwo different forms of calciumcarbonate(CaCo3)-marble and limestone. Marble and limestone reacr withacid deposition to form calcium sulfate, carbon dioxide, and water.

CaCO3(s) + H2SOa(U) -+CaSO+(s) + COz(d + HzOQ)Calcium sulfate binds soot and dust so that objects affected by acid depositionturn black and must be cleaned to retain their original appearance. Inaddition, calcium sulfate dissolves readily in warer. If the affe.t"d object isexposed to wet acid depositr_on_ or even to pure wate! it simply dissolves. TheParthenon, Tai Mahal, and Colosseum all show signs of ".orio.r. It is estimatedthat20o/o of this rype of erosion is due to acid dep"osition.Acid deposition also affects manufactured materials. Acid deposition corrodesmetals and damages paints and coatings meant to protect items from rust. Forexample, some automobile manufa.t.ri"., now use acid-resistant paints. Thesepaints add $5 to each vehicle for a total cost of $61 million per year for all newcars and trucks sold in the United States. Black spots upp"", occasionally on

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the Statue of Liberry, presumably due to acid deposition that dissolves thegreen patina to reveal weathered copper. Although many such items may berestored, maintenance costs money.

Perhaps the best-known effect of acid deposition is the acidification of surfacewater. In 1980, the US Environmenral Protection Agency conducted aNational Surface'Water Survey to determine how many lakes and srreamssuffered from chronic acidiry and what percentage of these waters werechronically acidic due to acid deposition. The survey included over 1000 lakeslarger than 10 acres and miles of streams thought to be affected by aciddeposition. The survey revealed that acid deposition was responsible forapproximately 75o/" of the acidified lakes and 507o of the acidified srreams.The affected areas included the Adirondacks, mid-Appalachian highlands, rheupper Midwest, and the high-elevation'S7est. The Adirondacks were the worsthit. According to the EPlt's t984 Eastern Lake Survey-Phase I, 10Zo of thelakes in the Adirondack mountains have pH values of 5.0 or less. This pH istoo acidic to support aquatic organisms, such as trout, bass, salamanders,crayfish, snails, and mayflies. The environmental impact quickly rises up tohigher trophic levels. For example, the Common Loon relies on fish u, ir, mainfood source and seldom breeds on acidic (pH < 5.5) rakes.

Canada's surface water also suffers from acid deposition. It is estimated that12,000 lakes in canada are acidic and 10,000 to 40,000 lakes may becomeacidic if present deposition rates continue. Interestingly, at least half of thesulfate deposited in Canada originates in the United States.

Acid deposition affects more than just the pH of surface water. The influx ofnitrogen from acid deposition into surface waters promotes plant growth.\fhen plants die, decomposers consume oxygen, lowering dirsol red oxygenlevels. Aquatic organisms suffocate and, soon, the water is devoid of life. TheEPA estimates that 30 to 40 percent of the nitrogen in Chesapeake Bay, forexample, is due to acid deposition.

The pollutants that cause acid deposition also threaten human health moredirectly. Lung cancer, asthma, bronchitis, and emphysema can be caused andaggravated by air pollutants. For example, cases of respiratory ailmenrc are50o/o higher in the most polluted parrs of Poland, Hungary and the czechRepublic than in the cleaner areas of those countries. The United States andCanada are not immune to this problem. Senior citizens, children, and peoplewith weakened immune systems are advised to stay inside during times of peakair pollution in many metropolitan cities.

Air pollutants also reduce visibiliry. Sulfate particles, for example, produce ahaze that is particularly noticeable in areas whose attraction is the view. Theseareas range from Shenandoah National Park in Vrginia to Grand CanyonNational Park in Arizona.

Where Does Acid Deposition Occur?The major sources of the sulfur dioxide emitted in the United States are coal-burning electric utilities (70y") in the midwest (see map) whereas the majorsources in Canada are industrial (60010). The gases rise into the atmospherewhere they are carried east and northeast by the prevailing westerly *i.rd,

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A d v a n c e d P l a c e m e n t E n v i r o n m e n t a l S c i e n c e

(Note 1r PB.E). These winds can disperse the air pollutanrs hundreds of milesfrom their source. In fact, most of the acid deposition that affects thenortheast United States and eastern Canada originates in the United States'midwest. As these gases ride the winds, they react to form sulfuric and nitricacids as well as sulfate and nitrate particles. Acid deposition is a regionalrather than a global problem because of the weakness of rhe wind currents.Even so, wind currents can carry air pollutants over political borders andcreate tension between neighboring countries. For example, tensions ran highin the late 1980s when it was found that air pollurants from the upperMidwest blow into Canada.

Motor vehicles account for 45o/o of nitrogen oxides emitted in the UnitedStates, approximately the same amount due to coal-burning electric utilities.In Canada, vehicles are responsible for 600/o of the total nitrogen oxide level.Thus, areas that have a large concentration of automobiles, such asmetropolitan cities and their surrounding areas, h?y also experience aciddeposition.

How Is Acid Deposition Neutralized?Surface waters and soils neutralize acid deposition in the same way rheyneutralize unpolluted precipitation. In water studies, this property is referredto as acid-neutralizing capacity (ANC) or alkaliniry and, in soil science, asbuffering capacity. The major contributors to alkaliniry in surface warer arehydrogen carbonates (HCo3-), carbonates (Corz-;, and hydroxides (oH-)that originate from minerals and rocks that the water encounters as it flows.Soils also affect ANC. Soils that have a large buffering capaciry such as soilsderived from limestone bedrock, produce lakes and streams with large ANCvalues. Soils with low buffering capaciry such as soils derived from inertbedrock (granite) and thin soils (common at high elevations) afford warerswith small ANC values. For example, surface waters in the high-elevationwest' such as the Cascades, Sierra Nevadas, and Rocky Mountains, have lowANC values. Ironically, the Midwest, where major air polluters are located, iswell-equipped to neutralize acid deposition because its soils are derived fromlimestone bedrock and have a large buffering capaciry. Some of the commonneutralization reactions are the following:

2HCO3-(q) + H2SOa(u) -> 7CO7k) + zHzo(t) + So4z-fu4)co32-(q) + H2so a@) + co2(d + H.zo(I) + so4z-@q)

zOH-(aq) + H2SO +(aq) -+ ZH2O(I) + SO42-(q)

Alkalinity is measured in either peqL (see Note 2, pE.B) or ppm CaCO3 (seeNotes 3 and 4). In both units of measure, the greater the number, the gieaterthe water's abiliry to neutralize acid.

'Water whose ANC < 0 peqlLis acidic by

definition. 'Water

whose ANC < 50 peqlLmay experience episodicacidification that results from snowmelt or heavy rainfalls.'Water whose ANC< 200 peqLis sensitive to acid deposition. For example, according to theEPAs 1984 Eastem Lake Survey-Phase l, IIo/o of the lakes in the Adirondacksare acidic, 360/o of the lakes may experience episodic acidification (this numberincludes acidic lakes), and 7lo/o are sensirive (includes acidic lakes and lakes

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that may experience episodic acidification). Use the following reladon tointerconveft ppm CaCO3 and p,eq,lL.

r ppm CaCO3 - Z}x peqlL

Solutions

Title IV of the 1990 Clean Air Act Amendmenrs conrains provisions tocontrol acid deposition and sets as its primary goal the reduction of annualsulfur dioxide emissions 10 million tons belo. rh" 1980 level (23 million tons).To this end, the EPA established the Allowance Ti"ading System in 1995.Under the auspices of this program, regulated companies are allocated permitscalled allowances which allow them toemit one ron of sulfur dioxide perallowance. The allowance may be used in the year allocated or saved for thefuture. companies may even buy, sell, or trade allowances.

The results of the program were immediate. In 1990, the 445 regulated unitsat 110 coal-buming electric utilities emitted 10 million tons of sulfur dioxide.In 1995, the first year of the program, these same utilities emitted onty 5.3million tons. Phase II of the program begins in the year 2000 and will expandto include 2000 units.

Another goal of the 1990 Clean Air Act Amendments is ro reduce annualnitrogen oxide emissions 2 million tons below the 1980 level (21 million tons).Phase I of this program began in 1996 and affects many of the same utilities asPhase I of the sulfur dioxide program.

The environment shows the results of these pollution control efform. TheUnited States Geological Survey reports thai wet deposition sulfateconcentration decreased 10 to 25o/o in 1995 compared ro the previous 12years' In addition, sulfate concentrations of surface waters in the northeasrernUnited States have fallen. Nitrate concenrrarions in Adirondack lakes havestarted to decline, in sharp contrast to the previous decade in which theseconcenffations increased.

Despite these reductions, the acid deposition control program encompassesonly half the emission mass produced by the United Si"t"r. Overall, 19 milliontons of sulfur dioxide and 21 million tons of nitrogen oxides were emitted in1994.

In Canada, the Eastem Canada Acid Rain Control Program was esrablished in1985 to limit wet sulfate deposition in the seven provinces east of Saskatchewanto no more than 20 kilograms per hectare p.r y""q, a deposition rare thought toprotect moderately sensitive aquatic systems. The program's initial sulfur Jioxideemission cap of 2.3 million tonnes (1 ton : 0.9 tonnes) was met in 1994, theprogram's first full year of implemenradon. For example, in 19g0 theIntemational Nickel company (INCo) in sudbury ontario emitted g65kilotonnes of sulfur dioxide. After the plant modemized its nickel and coppersmelte4 it emitted only 165 kilotonnes in 1994 ,35o/obelow its regulated limit of265 kllotonnes. Currently, Canada has a 2.3 million ronne .up ,h", applies tothe eastem provinces effective until 2000 and a national ."p lf 3.2 milliontonnes effective from 2000 and beyond. In lgg4,total s,rlf.,i dioxide emissions ineastem canada were 1.7 million tonnes, well below the cap.

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Notes

Pre-Lab Ouestions

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Phase I of Canada's NOxA/oC Managemenr Plan calls for reductions innitrogen oxide emissions of 125 kilotonnes by 2000. Additional regularions onvehicle emissions are expected to keep total nitrogen..ia. emissions inCanada consranr through ZOl0.

Sulfur dioxide emissions in the North America are declining. From 1980 to1994, emissions decreased 23o/o withadditional reductions predicted. Despitethis progress, nitrogen oxide emissions are forecasted to hold steady as thenumber of vehicles increases.

1' lvinds are often named for the direction from which they flow; forexample, the prevailing westerlies blow from the west to the east.2' One equivalent (eq) of a substance is the quantiry in moles of that

substance that neutralizes one mole of hydiog"" io"r, H+. ror-"*"*pt",only one-half mole of caco3 is required ,o *.rrrrlize one mole ofhydrogen ions because o.r. t iol" of calcium carbonate neutralizes twomoles of hydrogen ions.

co32-(u) + zH+ @d _+H7ce(a)The calcium ion does not react with the hydrogen ion. One equivalent ofcalcium carbonate equals one-half mole of .alc]rrm carbonate.

3' Parts per millioh, PPh, refers to the number of parts of a substance permillion parts of another substance. For example, 1.8 mg of calciumcarbonate in 1.0 x 106 mg of water is 1.8 ppm. Because the densiry ofwater is 1.0 g/mL, the mass of I L of water is 1000 g or 1.0 x 106 -*.Therefore, 1.8 ppm calcium carbonate is 1.8 mg calcium carbonate"per I Lf

ot water.

4' The results of alkaliniry tests are recorded as ppm CaCO3. Althoughwater may contain chemicals other than calcium carbonite, rhe identiryof these chemicals is unimportant; the test results are rreared as if thewater contained only calcium carbonate. In this way, the test resul6 arestandardi"ed and may be compared directly "rd ""riiy. Calcium carbonateserves as the reference because it is ubiquitous in natural waters.

1' A solution saturated with carbon dioxide at Ll"Chas a pH of 3.8. \7hydoes unpolluted rain not h-ave this pH? \7hat may happen to the pH ofunpolluted rain if carbon dioxide levels in the "r*orplr"re conrinue toincreasel

2' Write the chemical equadon that describes how calcium carbonareneutralizes acid.

3. Conven 32 ppm CaCO3 rc p,eqL.

4- \7hat step of the hydrologic cycle affords almost pure warerl5' When the temperature falls and solutions freeze, pure solvent freezes firsr,

and then' at a lower temperature, the solution freezes. \7hen thetemperature rises, the solid solution melts first, followed by the solidsolvent' Given this information, explain why mountain lakes and strearns

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Procedure

A c i d D e p o s i t i o n : T h e T h r e a t f r o m A b o v e

may experience episodes of increased acidity in the spring.

Part I. Laboratory Activity. Work in pairs.1. pH of unpollutedrain, Simulate the reaction of atmospheric carbon dioxide

(breath) with water that occurs in rain. Obtain a tesi tube, straw, andpipet. Pipet 1 mL of tap warer in a test tube. Add a drop of BogenUniversal Indicator, determine the pH from the color.hurr, and record.E*pry and rinse the test tube with tap water. Pipet 1 mL of tap warer inthe test tube and exhale gently through a straw into the water for 30seconds. Add a drop of Bogen Universal Indicator, determine the pH fromthe color chart, and record. Rinse the test tube with water in preparationfor Step 2.

2. pH of acid rain. Aburning match simulates the buming of sulfur in coal toform sulfur dioxide gas. Shake out water in the test rube. Place 3 drops ofBogen Universal Indicator in the test tube and tilt and rotate the tub. ,othat liquid droplets coat the inside surface. Inverr the test tube and clampto a ring stand or hold with test tube tongs. Determine the pH of thedrops from the color chart and record. Holding a match u.i". the tube,light the match and let it burn until you can no longer hold the match.Determine the pH of the drops from the color chari anC record. Rinse thetest tube with water and saye the tube for Step 3.

3. Effects of acid rain onhuman-mad.e structures. Place half a piece of chalk ina small (40 mL or larger) beaker. Measure 25 mLof 0.05 M sulfuric acid(concentrated acid rain) in a graduated cylinde! pour into the beakeq andrecord your observations. Place a zinc shot in your test rube. Pipet I mL0.05 M sulfuric acid into the tube and record your observatiols. Let thechalk and zinc shot sit in the acid overnight. Observe the next dav andrecord your observations.

4. Effect of bedrock on acid rain. Measure and record the pH of prepared ,AcidRain" with wide-range pH paper. Obtain 3 small beakers. Place one basaltspecimen in the first beaker, one granite specimen in the second beake4,and 10 marble chips in the third beaker. Add 20 mL of acid rain ro eachbeaker. Swirl each beaker once and let ir rest for five minures. Thenmeasure the pH of the liquid in each beaker. Let the samples sit in theacid rain ovemight. Measure and record the pH of the *"r., in thebeakers the next day.

5. Measure the pH and alkaliniry of surface water in your area.Although normal pH paper can be used ro measure the pH of mosrsolutions, use Lo Ion pH paper for samples that conrain few dissolvedions, such as distilled water and rain water.

a. How ro measure pH with Lo Ion pH paper.

i. Place a 3-in strip of Lo Ion pH paper in the sample tube.

ii. Fill the tube with sample warer.

iii. Cap the sample tube and shake.

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iv. \(/ait 1 minute.

v Hold the sample tube vertically against the black bar on the colorcnart,

vi. Compare the strit's color as seen through the liquid with the colorchart and record pH. If the pH from the color chart is 3.0, record theresult as 3 3,0. If the pH is 6.0, tecord as 2 6.0.

b. How to measure alkalini*

i. Fill a tination tube to the 5-mL line with sample water.

ii. Add one Bromcresol Green-Methyl Red (BCG-MR) IndicatorGblet. Cap and shake until the tablet dissolves. The solution willturn blue-green. If the solution tums pink, the sample has noalkalinity.

iii. Fill the direct reading titrator with Alkalinity Titration Reagent B,Fill the titrator until the plunger tip aligns with the zero graduationmark. Insert the tinator into the center hole ofthe titration tubecap.

iv. \i0hile gently stirring the tube, slowly depress the plunger to dtratethe sample until the blue-green color changes to pink, Read the testresult where the plunger tip meets the tirator scale. Record as fbtalAlkalinity in ppm CaCO3.

v, If the plunger tip reaches the bonom line on the titrator scale (200ppm) before the color change occurs, refill the titrator and conrinuethe titration. When recording the test result, be sure to include thevolume of the original amount of reagent disper$ed (200 ppm),

Part tr. Map Activity. Work individually.1. Using the Major Ninogen Dioxide and Sulfur Dioxide Generators mao

and the direction of the prevailing westerlies, predict where aciddeposition may occur. Outline these areas on the map with a solid line.

2. Using the United States and Canada Bedrock map and your results in partI, Step 5, predict where acid deposition may acidifi surface water. Outlinethese areas and lightly shade them in,

3. \frite the name of each state and province on the blank United Statesand Canada map.

Laboratory Ouestions 1. \Vhat is the pH of your rap waterl what is the pH of the simulatedunpolluted rain you created? Write the chemical equation that explainsthe acidity of unpolluted rain.

2. In Part I, Step 2, the buming match simulates the buming of sulfur in coalto form sulfur dioxide gas. What was the pH of the indicator solurionbefore you lit the match? $7hat was the pH of the indicaror solution afterthe match burned outl \fhy did the pH of the indicator solurion changeas the match bumed! Write the chemical equations (3) that describe howsulfur dioxide produces wet and dry acid deposition.

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3. \Uhat did you observe when you treated the chalk with concentrated"acid rain" (0.05 M sulfuric acid)? Vhat happened when you treated thezinc shot with concentrated acid rainl Name an item made out of calciumcarbonate and another object made out of metal and describe how acidrain affects them.

4. On the basis of your h?p, would you expect acid deposition to fall inOhiol New Yorkl Southem Saskatchewan? Vhy or why not? On thebasis of your h?p, would you expect acid deposition to fall where youlive? Why or why not?

5. From Part I, Step 4, rank the abilities of the different bedrocks toneutralize acid, from worst to best. Did the pH of the acid rain/bedrockmixtures change ovemightl

6. On the basis of your maps, would you expect acid deposition to acidifiisurface water in Ohio? New York? Southern Saskatchewanl Why or whynot? On the basis of your maps, would you expect acid deposition to falland acidifii surface water where you live? Vhy or why not?

7. Convert your surface water alkaliniry level from ppm CaCO3 to peqlLand classify the water as acidic (ANC < 0 p"qlL), ma1 experience episodicacidification (ANC S 50 pLeqlL), sensitive (ANC < 200 meq/L) , or buffered(ANC> 200 p,eqlL).

8. Mount Mitchell in North Carolina is the highest point east of theMississippi in North America. Suppose you and a friend go to Mt.Mitchell to hike one weekend. You both notice that the trees on the westside of the mountain have no leaves. Given that acid deposition affectsMt. Mitchell, list three rypes of wet acid deposition that may be involvedand describe how wet acid deposition affects tree leaves and roors.

9. You and your friend nodce that rees thrive on the easr face of Mt.Mitchell. Your friend argues that because fog occurs on both sides ofmountains and Mt. Mitchell experiences fog, acid fog does not harmtrees. Counter her argument with a possible explanation.

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