section 3 acid · pdf filechapter 12 highlights 1 what causes air pollution? 2 air, noise, and...

314 Chapter 12 Air

Imagine that you are hiking through the forests of the AdirondackMountains in New York. You come to a lake and sit down to rest.You are amazed at how clear the water is; it is so clear that youcan see the bottom of the lake. But after a few minutes you feeluneasy. Something is wrong. What is it? Suddenly, you realize thatthe lake has no fish.

What Causes Acid Precipitation?This lake and thousands of lakes throughout the world are vic-tims of acid precipitation, which is also known as acid rain.

is precipitation such as rain, sleet, or snowthat contains a high concentration of acids. When fossil fuelsare burned, they release oxides of sulfur and nitrogen. Whenthe oxides combine with water in the atmosphere, they formsulfuric acid and nitric acid, which fall as acid precipitation.This acidic water flows over and through the ground, and intolakes, rivers, and streams. Acid precipitation can kill livingthings, and can result in the decline or loss of some local ani-mal and plant populations.

A (power of hydrogen) number is a measure of how acidicor basic a substance is. A pH scale is shown in Figure 12. As youcan see from the scale, the lower the pH number is, the moreacidic a substance is; the higher a pH number is, the more basic a substance is. Each whole number on the pH scale indicates a tenfold change in acidity.

pH

Acid precipitation

Objectives� Explain the causes of acid precipi-

tation.� Explain how acid precipitation

affects plants, soils, and aquaticecosystems.

� Describe three ways that acid pre-cipitation affects humans.

� Describe ways that countries areworking together to solve theproblem of acid precipitation.

Key Termsacid precipitationpHacid shock

S E C T I O N 3

Acid Precipitation

Increasing acidity Increasing basicity

1 2 3 4

4.0

5 6

6.0

8 10 11 129

3.5 5.0

Acid rain(extreme)

Acid rain(average) Rain

Human saliva

Tap water

Householdammonia

Seawater

Humanblood

Tomatojuice

Carbonatedsoftdrink DetergentsMilk

Lemonjuice Vinegar

7

Figure 12 � The pH scale measureshow basic or how acidic a substanceis. Below are the pH measurementsof some common substances.

Copyright© by Holt, Rinehart and Winston. All rights reserved.

Pure water has a pH of 7.0. Normal precipitation is slightlyacidic, because atmospheric carbon dioxide dissolves into the pre-cipitation and forms carbonic acid. Normal precipitation has apH of about 5.6. Precipitation is considered acid precipitation ifit has a pH of less than 5.0. Figure 13 shows how acid precipita-tion forms.

The pH of precipitation varies between different geographicareas. For example, Eastern Europe and parts of Scandinavia haveprecipitation with a pH of 4.3 to 4.5, whereas the remainder ofEurope has precipitation with pH values of 4.5 to 5.1. The pH ofprecipitation in the eastern United States and Canada ranges from4.2 to 4.8. The most acidic precipitation in North America occursaround Lake Erie and Lake Ontario. It has a pH of 4.2.

How Acid Precipitation Affects Soils and PlantsPlant communities have adapted over long periods of time to theacidity of the soil in which they grow. Acid precipitation cancause a drop in the pH of soil and water. This increase in theconcentration of acid is called acidification. Acidification changesthe balance of a soil’s chemistry in several ways. When the acidityof soil increases, some nutrients are dissolved and washed awayby rainwater. Increased acidity causes aluminum and other toxicmetals to be released and possibly absorbed by the roots of plants.Aluminum also causes root damage. Sulfur dioxide in water vaporclogs the openings on the surfaces of plants. Figure 14 shows theharmful effects of acid precipitation on trees.

Section 3 Acid Precipitation 315

OzoneO3

Oxygen atomO

OH

Acid precipitation

combines withSO2 + O2

combines withNO2

Sulfuric acidH2SO4

Nitric acidHNO3

Figure 13 � Sulfur oxides and nitro-gen oxides combine with water in theatmosphere to form sulfuric and nitricacids. Rainfall that contains theseacids is called acid precipitation.

Figure 14 � The trees in this forestin Poland show the dramatic effectthat acid precipitation can have onplants. Damage to more than 16 mil-lion acres in nine European countrieshas been linked to acid precipitation.


QuickLABNeutralizing Acid PrecipitationProcedure1. Pour 1/2 Tbsp of vinegar into

one cup of distilled water,and stir the mixture well.Check the pH of the mixtureby using pH paper. The pHshould be about 4.

2. Crush one stick of blackboardchalk into a powder. Pour thepowder into the vinegar andwater mixture. Check the pH ofthe mixture.

Analysis1. Did the vinegar and water mix-

ture become more or less acidicafter the powdered chalk waspoured in?

Acid Precipitation and Aquatic EcosystemsAquatic animals are adapted to live in an environment with aparticular pH range. If acid precipitation falls on a lake andchanges the water’s pH, acid can kill aquatic plants, fish, andother aquatic animals. The change in pH is not the only thingthat kills fish. Acid precipitation causes aluminum to leach out ofthe soil surrounding a lake. The aluminum accumulates in thegills of fish and interferes with oxygen and salt exchange. As aresult, fish are slowly suffocated.

The effects of acid precipitation are worst in the spring, whenacidic snow that accumulated in the winter melts and rushes intolakes and other bodies of water. This sudden influx of acidic waterthat causes a rapid change in the water’s pH is calledThis phenomenon causes large numbers of fish in a population todie, as shown in Figure 15. Acid shock also affects the reproduc-tion of fish and amphibians. They produce fewer eggs, and theseeggs often do not hatch. The offspring that do survive often havebirth defects and cannot reproduce.

To counteract the effects of acid precipitation on aquaticecosystems, some states in the United States and some countriesspray powdered lime (calcium carbonate) on acidified lakes in thespring to help restore the natural pH of the lakes. Because limehas a pH that is basic, the lime raises the pH of the water.Unfortunately, enough lime cannot be spread to offset all aciddamage to lakes.

Acid Precipitation and HumansAcid precipitation can affect humans in a variety of ways. Toxicmetals such as aluminum and mercury can be released into theenvironment when soil acidity increases. These toxic metals canfind their way into crops, water, and fish. The toxins then poisonthe human body.

Acid precipitation can lead to other human health prob-lems. Research has indicated that there may be a correlationbetween large amounts of acid precipitation received by a com-munity and an increase in respiratory problems in the commu-nity’s children.

The standard of living of some people is affected by acidprecipitation. Decreases in numbers of fish caused by the acidifi-cation of lakes and streams can influence the livelihood of com-mercial fisherman and people involved in the sport-fishingindustry. Forestry is also affected when trees are damaged byacid precipitation.

Acid precipitation can dissolve the calcium carbonate incommon building materials, such as concrete and limestone.Some of the world’s most important and historic monuments,including those made of marble, are being affected by acid pre-cipitation. For example, sulfur dioxide has caused black cruststo form on the carbonate stones of historic Greek monuments.

acid shock.

316 Chapter 12 Air

Figure 15 � Fish are vulnerable toacid shock, a sudden influx of acidicwater into a lake or stream thatcauses a rapid change in pH.


International Conflict and CooperationOne problem in controlling acid precipitation is that pollutantsmay be released in one geographical area and fall to the groundhundreds of kilometers away. For example, almost half of theacid precipitation that falls in southeastern Canada results frompollution produced in Ohio, Indiana, Pennsylvania, Illinois,Missouri, West Virginia, and Tennessee. Figure 16 shows areas ofthe world that produce pollutants and areas which are thenaffected by acid precipitation.

Because acid precipitation falls downwind, the problem of solv-ing acid precipitation has been difficult, especially on the interna-tional level. In the spirit of cooperation, Canada and the UnitedStates signed the Canada–U.S. Air Quality Agreement in 1991.Both countries agreed to reduce acidic emissions that flowed acrossthe Canada–U.S. boundary. More international agreements such asthis may be necessary to control the acid-precipitation problem.

Section 3 Acid Precipitation 317

1. Explain how acid precipitation forms.

2. Describe the harmful effects that acid precipitationcan have on plants, soils, and aquatic ecosystems.

3. Describe three ways in which acid precipitation canaffect humans.

4. Describe a way in which countries are workingtogether to solve the problem of acid precipitation.

CRITICAL THINKING5. Inferring Relationships In addition to negatively

affecting forestry and the fishing industry, howmight acid precipitation affect local economies?

6. Analyzing Viewpoints Write a short essay inwhich you discuss whether or not a country thatreleases significant amounts of pollutants into the airthat fall as acid precipitation in another countryshould be expected to pay some of the costs ofcleanup. WRITING SKILLS

S E C T I O N 3 Review

www.scilinks.orgTopic: Acid RainSciLinks code: HE4001

Figure 16 � Acid precipitation is aglobal problem.


HighlightsC H A P T E R 12

1 What Causes Air Pollution?

2 Air, Noise, and Light Pollution

3 Acid Precipitation

318 Chapter 12 Highlights

Key Termsair pollution, 303primary pollutant,

303secondary pollutant,

303smog, 308temperature

inversion, 308

Main Ideas� Primary pollutants are pollutants put directlyin the air by human activity.

� Secondary pollutants are formed when a pri-mary pollutant comes into contact with otherprimary pollutants or with naturally occurringsubstances and a chemical reaction takes place.

� Most air pollution comes from vehicles andindustry.

� Air pollution that hangs over cities andreduces visibility is called smog.

� Pollution can be trapped near the surface ofthe Earth by a condition known as temperatureinversion.

sick-buildingsyndrome, 310

asbestos, 312decibel (dB), 312

� Air pollution may have both long- and short-term effects on human health.

� The air indoors may be more polluted thanthe air outside. Plastics, cleaning chemicals,and building materials are major sources ofindoor air pollution.

� Noise is a pollutant that affects humanhealth and the quality of life.

� Inefficient lighting diminishes our view of thenight sky and wastes energy.

acid precipitation,314

pH, 314acid shock, 316

� Acid precipitation is precipitation such asrain, sleet, or snow that contains a high con-centration of acids.

� Acid shock occurs when a sudden influx ofacidic water enters a lake or stream and causesa rapid change in pH that harms aquatic life.

� Pollutants released in one geographical areamay fall to the ground hundreds of kilometersaway as acid precipitation—sometimes inanother country.


Using Key TermsUse each of the following terms in a sentence.

1. air pollution2. smog3. temperature inversion4. sick-building syndrome5. pH

For each pair of terms, explain how the meaningsof the terms differ.

6. primary pollutant and secondary pollutant7. asbestos and radon8. pH and acid precipitation9. acidification and acid shock

Understanding Key Ideas10. Which of the following air pollutants is not

a primary pollutant?a. particulate matterb. ozonec. sulfur dioxided. volatile organic compounds

11. A device used to clean exhaust gases beforethey exit an automobile’s tailpipe is called a(n)a. electrostatic precipitator.b. catalytic converter.c. scrubber.d. None of the above

12. The majority of sulfur dioxide produced byindustry comes froma. oil refineries.b. dry cleaners.c. chemical plants.d. coal-burning power plants.

Review

Chapter 12 Review 319

13. Which of the following substances is notinvolved in the chemical reaction that pro-duces smog?a. sunlightb. particulate matterc. automotive exhaustd. ozone

14. Which of the following respiratory diseasesis considered a long-term effect of air pollu-tion on human health?a. emphysemab. bronchitisc. pneumoniad. all of the above

15. Which of the following substances is a color-less, tasteless, and odorless radioactive gas?a. asbestosb. carbon monoxidec. radond. ozone

16. A sound measuring 40 dB has how manytimes the intensity of a sound that measures10 dB?a. 4 timesb. 30 timesc. 400 timesd. 1,000 times

17. Which of the following choices is not aneffective solution to the energy waste relatedto inefficient lighting?a. using low-pressure sodium lighting sourcesb. pointing lights on billboards and street

signs upwardc. placing light sources on time controlsd. shielding light to direct it downward

18. Which of the following numbers on the pHscale would indicate that a substance isacidic?a. 5.0b. 7.0c. 9.0d. none of the above

19. Normal precipitation has a pH ofa. 7.0.b. 5.6.c. 5.1.d. 4.5.

C H A P T E R 12

Predicting Exam Questions Before you take atest, do you ever attempt to predict what the ques-tions will be? For example, of the 10 multiple-choicequestions that appear on this page, how many wouldyou have predicted to be asked in a review of thischapter? Before your next test, predict and answerpossible exam questions.

STUDY TIP


ReviewShort Answer20. Define the term zero-emission vehicle. What

types of vehicles qualify as zero-emissionvehicles?

21. Identify five indoor air pollutants and exam-ples of sources of each pollutant.

22. Explain the health hazards that radon gasposes for humans.

23. Identify a chemical that is used to counteractthe effects of acid precipitation on aquaticecosystems.

24. Explain why acid precipitation is a source ofinternational conflict and why internationalcooperation is necessary to resolve the problem.

Interpreting GraphicsThe map below shows the pH of precipitationthat has been measured at field stations in thenortheastern United States. Use the map and legend to answer questions 25–26.25. Which area(s) of the northeastern United

States have the most-acid precipitation?26. Are the areas that have the highest pH

located close to or far from major cities?

Concept Mapping27. Use the following terms to create a concept

map: air pollution, primary pollutant, volatileorganic compound, scrubber, secondary pol-lutant, smog, and temperature inversion.

Critical Thinking28. Making Decisions Five states now have zero-

emission vehicle programs in place that willhelp decrease some primary pollutants. Whatwould be the advantages or disadvantages ofa federal program that required automobilemakers to produce a set number of ZEVsnationwide?

29. Making Decisions In some cities, noise-pollu-tion laws, such as restrictions placed on theuse of leaf blowers, have been put in place.Do you think the benefits of noise reductionoutweigh the costs of enforcing the law?

30. Inferring Relationships As you read underthe head “International Conflict andCooperation,” about half of the acid precipi-tation that falls in southeastern Canada is pro-duced by pollutants from the United States.How do the acid pollutants get from theirsources to southeastern Canada?

Cross-Disciplinary Connection31. Health Asbestos, lead paint, tobacco, and

many other products have been linked toadverse effects on human health. Researchone such case that has been brought into thecourts. Describe the allegations and the out-come of the trial and write a paragraph thatexplains whether you agree or disagree withthe decision.

Portfolio Project32. Make a Poster Create a poster similar to the

diagram that appears in Figure 9. This dia-gram may be of your home, your garage, aportion of your school, or a particular class-room in your school. Use the diagram toidentify and label potential sources of indoorair pollutants. Photographs may be used todocument these sources.

WRITING SKILLS

READING SKILLS

C H A P T E R 12

320 Chapter 12 Review

?

? ?


Chapter 12 Review 321

Read the passage below, and then answerthe questions that follow.

Lichens are unique organisms that consist of afungus and microscopic alga that live togetherand function as a single organism. The alga isthe photosynthetic partner, whereas the fungusabsorbs water and minerals and anchors theplant. Lichens form crusts or leafy growths onrocks, trees, and bare ground. Lichens do nothave roots. Instead, they absorb the nutrientsthey need directly from rain. Lichens growvery slowly and can live for centuries. Speciesof lichens have adapted to almost every envi-ronment in the world.

Lichens are sensitive to air pollution, par-ticularly sulfur dioxide. When lichens areexposed to high levels of sulfur dioxide, theyabsorb the sulfur that is contained in rain. Thesulfur destroys chlorophyll and inhibits photo-synthesis. So, lichens are good indicators of airpollution. Lichens usually disappear from areaswhere sulfur dioxide levels are high. Where theair is free of pollutants, a greater number oflichens will usually be present. In areas wheresulfur dioxide pollution is decreasing, lichenswill slowly return and colonize the area.

1. Which of the following statements aboutlichens is true?a. Lichens are present when sulfur diox-

ide levels are high.b. Lichens absorb nutrients through their

root systems. c. Lichens photosynthesize.d. Lichens grow only where the climate is

moderate.

2. Where would you be most likely to seethe greatest number of lichens?a. in areas where sulfur dioxide levels are

highb. in areas where sulfur dioxide levels

are lowc. in areas where sulfur dioxide levels are

decreasingd. in areas where sulfur dioxide is absent

MATH SKILLS

Use the graph below to answer questions 33and 34. 33. Analyzing Data The graph below shows the

change in air-pollution emissions in theUnited States between 1970 and 1997.Excluding NOx, which emissions categoryexperienced the greatest decrease over thisperiod of time?

34. Interpreting Graphics Why is lead, Pb, shownseparately from the other air pollutants?

WRITING SKILLS

35. Outlining Topics Outline the major sourcesof air pollution in the United States. Includeinformation about pollution sources andpollution types.

36. Writing Persuasively Write a letter to a leg-islator that expresses your concern about aparticular aspect of air, noise, or light pollu-tion that is important to you.

CO NOX VOC PM-10 SO2 Pb

Air Pollutants

0

50

100

150

200

250

Emis

sio

ns

(in

th

ous

and

s o

f to

ns)

0

30

60

90

120

150

Emis

sio

ns

(in

mill

ion

s o

f to

ns)

1970

1997

Now that you have read the chapter, take amoment to review your answers to the ReadingWarm-Up questions in your EcoLog. If necessary,revise your answers.

READING FOLLOW-UP


322 Chapter 12 Exploration Lab

The Acid TestAcid precipitation is one of the effects of air pollution. Whenpollutants that contain nitrogen or sulfur react with water vaporin clouds, dilute acid forms. These acids fall to Earth as acidprecipitation.

Often, acid precipitation does not occur in the same placewhere the pollutants are released. The acid precipitation usu-ally falls some distance downwind—sometimes hundreds ofkilometers away. Thus, the sites where pollutants that causeacid precipitation are released may not suffer the effects of acidprecipitation.

Coal-burning power plants are one source of air pollution.These power plants release sulfur dioxide into the air. Sulfurdioxide reacts with the water vapor in air to produce acid thatcontains sulfur. This acid later falls to Earth as acid precipitation.

In this investigation, you will create a chemical reaction thatproduces sulfur dioxide. The same acids that result from coal-burning power plants will form. You will see the effects of acidprecipitation on living things—in this case, plants.

Objectives� Perform a chemical reaction that

produces sulfur dioxide, a compo-nent of acid precipitation.

�Hypothesize what the effects ofacids that contain sulfur on plantswill be.

Materialsbeaker, 50 mLclear plastic bags, large (2)houseplants of the same type,

potted (2)sodium nitrite (2 g)sulfuric acid, 1 M (2 mL)twist tie or tape

USING SCIENTIFIC METHODS

Exploration Lab: MODELINGC H A P T E R 12

Procedure1. Place 2 g of sodium nitrite in a

beaker. Place a plant and thebeaker inside a plastic bag. Donot seal the bag.CAUTION: Steps 2–4 should becarried out only under a fumehood or outdoors.

2. Carefully add 2 mL of a 1 Msolution of sulfuric acid to thebeaker. Immediately seal thebag tightly, and secure the bagwith a twist tie or tape. CAU-TION: Because this reactionproduces sulfur dioxide, a toxicgas, the bag should have noleaks. If a leak occurs, moveaway from the bag until thereaction is complete and the gashas dissipated.

3. Seal the same type of plant inan identical bag that does notcontain sodium nitrite or sulfu-ric acid.

� Step 1 Place a plant and a beakerthat contains sodium nitrite into a plastic bag. Do not seal the bag.


4. After 10 minutes, cut both bags open. Stay at least 5 m fromthe bags as the sulfur dioxide gas dissipates. Keep the plantsand bags under the fume hood.

5. Predict the effects of the experiment on each plant over thenext few days. Record your predictions.

6. Observe both plants over the next three days. Record yourobservations below.

Analysis1. Examining Data How closely did your predictions about

the effects of the experiment on each plant match yourobservations?

2. Explaining Events What does this experiment suggest aboutthe effects of acid precipitation on plants?

Conclusions3. Drawing Conclusions In what ways is this a realistic model

of acid precipitation?4. Drawing Conclusions In what ways is this experiment not a

realistic simulation of acid precipitation?

Chapter 12 Exploration Lab 323

Day Control Plant Experimental Plant

1

2

3 DO NOT WRITE IN THIS BOOK

1. Analyzing Models Would you expect to see similar effectsoccur as rapidly, more rapidly, or less rapidly in the environment? Explain your answer.

2. Building Models Acid precipitation is damaging to plantsbecause the sulfur dioxide contained in the water vaporclogs the openings on the surfaces of plants and interfereswith photosynthesis. What kind of a safe model woulddemonstrate the damaging effects of acid precipitation inthe form of water vapor on plant photosynthesis? Wouldthis model be a realistic simulation of acid precipitation?

Extension


324 Chapter 12 Maps in Action

LIGHT SOURCES

M A P S K I L L S

This map of what the Earth looks like from spaceat night shows light sources that are human in ori-gin. The map is a composite image made fromhundreds of images taken by orbiting satellites.Use the map of light sources on Earth to answerthe questions below.

1. Inferring Relationships Using the brightness ofthe light sources on the map as a key, can youestimate the locations of some of the most denselypopulated areas on Earth? Where are some ofthese areas?

2. Inferring Relationships Some climatic condi-tions on Earth, such as extreme cold, heat, wetness,or a thin atmosphere, make parts of our planet less

habitable than others. Examples of areas on ourplanet that do not support large populationsinclude deserts, high mountains, polar regions, andtropical rain forests. From the map, can you identifyregions of the Earth where climatic conditions maynot be able to support large human populations.What are some of these places?

3. Finding Locations Many large cities are seaportsthat are located along the coastlines of the world’soceans. From the map, can you pick out lightsources along coastlines that might indicate the sitesof large ports? Identify some of these cities by name.

4. Inferring Relationships From the differences inthe density of the light sources, can you pick out anyinternational borders?


Chapter 12 Society and the Environment 325

THE DONORA, PENNSYLVANIA, KILLER SMOG

What Do You Think?Who do you think should beheld responsible for theDonora, Pennsylvania, disas-ter? Explain your answer.Given what you know aboutthe regulation of industrial pol-lutants under the Clean AirAct, do you think another inci-dent such as the Donora killersmog could happen in theUnited States today?

For the residents of the smallMonongahela Valley town ofDonora, Pennsylvania, living withthe smoke that billowed from thelocal zinc smelter was an everydayoccurrence—until October 26,1948. On that night, a temperatureinversion and an absence of windbegan to trap a deadly mixture ofsulfur dioxide, carbon monoxide,and metal dust that would hang inthe valley air for five days. Over thatperiod of time, 20 residents losttheir lives and 7,000 other resi-dents—about half of the town’s pop-ulation—suffered some form ofrespiratory problems.

The Weekend of the Killer SmogBy Saturday afternoon, October 29,1948, the yellowish smog hadbecome so thick that spectators inthe stands at a local high-schoolfootball game could not see theplayers on the field. Only the whis-tles of the referees could be heard.By nightfall, driving was unsafe.This proved to be catastrophicbecause doctors recommended thatany residents who suffered from

respiratory ailments be evacuatedfrom town. In an attempt to allevi-ate the suffering of people who werestruggling to breathe, several localfiremen carried oxygen tanksthrough the streets to differenthomes. Because of the low visibility,the firemen had to feel their wayalong buildings and fences. Becausethe supply of oxygen was limited,only a few breaths of oxygen couldbe given to each person. Eleven peo-ple died that night. A makeshiftmorgue was set up in the local com-munity center.

Even as the killer smog chokedthe valley, the zinc smelter continuedproduction throughout the night.The smelter continued sending moregases and dust into the air overDonora. The smelter was shut downonly when the magnitude of theproblem became apparent—6:00A.M. on Sunday, October 30, 1948.

Later that day, a drizzling rainbegan to fall and washed the pollu-tants from the sky. By the time therain fell, 20 people ages 52 to 85,who suffered from respiratory ail-ments, were dead. Thousands of

other people were at home in bed orwere filling the corridors and exam-ining rooms of the two area hospi-tals. People who were less affectedby the smog suffered from nauseaand vomiting, headaches, andabdominal cramps. Some victimswere choking or coughing up blood.The zinc smelter resumed operationon Monday morning, October 31.

The AftermathThe smog of Donora was one of theUnited States’ most serious environ-mental disasters. Shortly after theincident, investigations were under-taken by the PennsylvaniaDepartment of Health, the U.S.Public Health Service, and otheragencies. This was the first time anorganized attempt was made todocument the effects of air pollutionon health in the United States. Theknowledge that air pollution couldbe linked directly to the deaths ofindividuals resulted in legislation atthe local, regional, state, and federallevels. These laws were set to limitemissions of sulfur dioxide, carbonmonoxide, particulate matter, andother pollutants. The greatest legacyof the Donora tragedy was passageof the Clean Air Act of 1970.

� This historical photo from the Pittsburgh Gazette captures the town of Donora,Pennsylvania, as it is enveloped in smog at noon on Saturday, October 28, 1948.


section 3 acid · pdf filechapter 12 highlights 1 what causes air pollution? 2 air, noise, and...

Documents