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Berkeley ENE,RES C200 - CHAPTER 7 - Air Pollution

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CHAPTER7Air Pollution7.1 INTRODUCTION7.2 OVERVIEW OF EMISSIONS7.3 THE CLEAN AIR ACT7.4 THE POLLUTANT STANDARDS INDEX7.5 CRITERIA POLLUTANTS7.6 TOXIC AIR POLLUTANTS7.7 AIR POLLUTION IN THEWORLD'SMEGACITIES7.8MOTORVEHICLE EMISSIONS7.9 STATIONARY SOURCES7.10 AIR POLLUTIONANDMETEOROLOGY7.11 THE POINT-SOURCE GAUSSIAN PLUMEMODEL7.12INDOORAIR QUALITYPROBLEMSREFERENCES"Fromaparticulateexposurestandpoint,a2-percentdecrease inenvironmentaltobaccosmoke(passivesmoking)wouldbeeouive-lenttoeliminatingall the cost-firedpowerplantsin the country. "-KirkR.Smith, East-West Center,Programon Environment,Honolulu, 19937.1INTRODUCTIONAirpollutioniscertainlynota newphenomenon.Indeed.earlyreferencesto itdatetotheMiddleAges,whensmokefromburningcoalwasalreadyconsideredsuch aseriousproblemthatin 1307,KingEdwardIbannedits use in lime kilns inLondon.Inmorerecenttimes,thoughstilldecadesago,severalseriousepisodesfocusedattentionontheneedtocontrolthequalityoftheairwebreathe.TheworstoftheseoccurredinLondon,in 1952. Aweekofintensefogandsmokeresultedinover4000excessdeathsthatweredirectlyattributedtothepollution.IntheUnitedStatesthemostalarmingepisodeoccurredduringa four-dayperiodin ItJ4i-iinDonora,Pennsylvania.when2()deathsandalmost6000 illnesseswerelinkedto airpollution.Atthetime,Donorahauapopulationofonly14.0()().makingthis thehighestpercapitadeathrateeverrecordedfor an airpollutionepisode.Thoseairpollutionepisodesweretheresultsofexceptionallyhighconccntra-tionsofsulfuroxidesandparticulatematter.theprimaryconstituentsofindustrialsmogorsulfuroussm0!i'Sulfuroussmogiscausedalmostentirelybycombustion01327;jillc.napter IAllr'UIIUlIUIIFuel (H. C. S, N.Ph. ash) + air (N, +0,)....,Emissions(C02•H20.CO. NO,. SO,. Ph. particulates) + Ash (7.4)Of course, most combustion takes place in air, not in a pure oxygen environment.and air is roughly 7Rpercent nitrogen (N2)and 21 percent oxygen(OJ.When the tem-peratureof combustion is high enough, some of that nitrogen reacts with the oxygen inair to form various nitrogen oxides(NOJ.Since thisNO,is formed when combustiontemperaturesare high, it is referred to as thermalNO,.Nowlet'sadd a simple representation of the photochemical reactions that produceozone(03)andotherconstituents of photochemical smog. Hydrocarbons(He)andotherorganic compounds that readily vaporize are called volatile organicClJIIl/WI/IIi/S(VOCs). VOCs react withNO,in the presence of sunlight to produce photochcmicalsmog:plant stack gases arecreatedduringcombustion. Of these it is combustion thataccounts for the great majority of emissions, and it is the gases and particulatematterreleased when fuels areburnedthat havebeenthe focus of most of the technical andlegislative pollution control efforts.In its simplest form. we can imagine the complete combustion of a purl' hydro-carbon fuel such as methane (CH.):(71)(73)(HC)I(7.2)+ O2)+Heat~ThermalNO,CH.+ 20,~CO,+ 2H,OairCH4+ O2...., mostly(CO, + 2 H20)+ traces ofleoThe products of combustion are simple carbon dioxide(C02)and water(H,O).nei-therof which had been considered an air pollutant until we realized that the accumula-tion of CO2in thc atmosphere was enhancing theearth'snatural greenhouse effect (aswill be described in the next chapter).Ifthetemperatureof combustion is not high enough, or there is not enough oxy-gen available. or if the fuel is not given enough time to burn completely. then thefu~,1will not be completely oxidized and some of the carbon will be released as carbonmonoxide(CO)instead of CO2,Also, some of the fuel will not be completely burned.so there will be emissions of various partially cornbustcd hydrocarbons that we willrepresent by(HC).So we can write the following descriptive reaction to representincomplete combustion ofourpure hydrocarbon fuel. methane:So far we have assumed that the fuel beingburnedwas a pure hydrocarbon suchas methane. In reality. of course. most fuels have anumberofotherelements in them.such as nitrogen. sulfur. lead (in gasoline). andotherunburuablc materials called ash.Burning fuel with these "impurities" in them releases additionalNO,(called [uclNOx) , oxides of sulfur (SO,), lead(Ph).more particulate matter. and ash.Combining the effects of incomplete combustion. combustion in air. and cornbus-tion of fuelsthatare notpurehydrocarbons yields the following qualitative descriptionof combustion:fossil fuels, especially coal, instationarysources suchpowerplantsandsmelters. Incontrast,the airpollutionprobleminmanycities is caused by emissions ofcarbonmonoxide, oxides ofnitrogen,and various volatile organic compounds, which swirlaroundin theatmospherereacting witheachotherand with sunlight to formphoto-chemical smog.Althoughstationarysources alsocontributeto photochemical smog,theproblemismostclosely associated withmotorvehicles. Amajoreffect of effortsin theUnitedStatestocontrolbothsulfurous smog and photochemical smog hasbeenthe eliminationofthosedramatic,peakconcentrationsof pollutionthatwereresponsible for theairpollution episodes justmentioned.Intheirplace, however, isthemoreinsidiousproblemof morbidityandmortality increases associated withlong-termexposuretolowerconcentrationsof pollution. Thehumantoll is muchmoredifficult todocument,butestimatesplace thecurrentexcessdeathscausedbyair pollution (mostly small particles) at several tens ofthousandsperyearin theUnitedStatesalone.Much of the work on air pollution in the last few decades hascenteredon a smallset of six substances, called criteria pollutants,thathavebeenidentified as contributorstobothsulfurousandphotochemical smog problems. The sources, transport, effects,andmethodsof controlling these criteria pollutants will be a principal focus of thischapter.Morerecently,attentionhasbeenshifting toward the characterization and con-trol of a growing list of especially hazardous air pollutants, many of which we areexposed to inourhomes and workplaces, where we spend roughly 90percentofourtime. As thequoteat the begining of thischaptersuggests,modestimprovements inindoor air quality can improve public health as much as major reductions in the tradi-tionaloutdoorsources, which havebeenthe focus of most of the scientific and politicalefforts of the past 50 years.In the nextchapterwe will discuss the emissions and impacts of carbon dioxide,chlorofluorocarbons, andothertrace gases that are affecting global climate and caus-ing stratosphericozonedepletion. In anumberof ways these gasesareso differentfrom the usual air pollutantsthatthey deserve special treatment.Thereare many


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