ABSTRACT
A pollutant emitted into the atmosphere is subject to various dynamicandchemicalprocessessuchastransport,mixing, recirculation,trapping,photochemicalproduction,titration, coagulation,anddeposition.›eseprocessesarecloselyrelated tonaturalenvironmentsaswellasanthropogenicactivities. ›eLosAngelesairbasinisanexample;worstairqualityo¢en occursinanurbanareaovercomplexterrain,inwhichcomprehensivežowpatternsplayacriticalroleinthebuildupofpollutants.Atthesametime,climatologicalfactorssuchashighactinic žuxpromotetheformationofsecondarypollutants,whichare, likeozoneandŸneparticulatematters,notdirectlyemitted fromsources(primarypollutants)butchemicallyproducedin theairfromthoseprimarypollutants.Inhomogenoussurfaces, includingland/watercontrastandurban/nonurbandi§erences, addfurthercomplexitytothenatureoftheproblems.Despite multidecade-longe§ortstoimprovetheunderstandingofsuch processesandeventuallytoprotectpublichealthandwelfare fromadverseairquality,theattainmentofairqualitytothelevel deŸnedinNationalAmbientAirQualityStandards(NAAQS) remainselusive,especiallyforO3 and PM2.5. Many of the major urbancentersintheUnitedStateshavebeenclassiŸedasnonattainmentofO3 and PM standards with limited improvement tobeexpectedinthenearfuture.Still,betterunderstanding ofphysicalandchemicalprocessesinvolvedinairpollutionis criticaltoachievebetterairquality.Inthiscontext,wediscuss fundamentalsoftransport,dispersion,andmixingprocessesas wellaschemicalreactionsinvolvedinthemostcommonlycited pollutants(Section6.2),andtypicalmethodologiestoelucidate
such fundamentals (Section 6.3). ›e Phoenix and Los Angeles airbasins,twoofthemostpopulatedandpollutedurbancentersintheUnitedStates,weretakenasexamplestoillustrate suchtheories(Section6.4).Finally,regulatorye§ortstoreduce adversee§ectsofairpollutiononhumanhealthandwelfareare briežy described in Section 6.5.
