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Smoghangingovercitiesisthemostfamiliarandobviousformofairpollution.Buttherearedifferentkindsofpollution—somevisible,someinvisible—thatcontributetoglobalwarming.Generallyanysubstancethatpeopleintroduceintotheatmospherethathasdamagingeffectsonlivingthingsandtheenvironmentisconsideredairpollution.Anairpollutantisasubstanceintheairthatcanhaveadverseeffectsonhumansandtheecosystem.Thesubstancecanbesolidparticles,liquiddroplets,orgases.Apollutantcanbeofnaturaloriginorman-made.Pollutantsareclassifiedasprimaryorsecondary.Primarypollutantsareusuallyproducedfromaprocess,suchasashfromavolcaniceruption.Otherexamplesincludecarbonmonoxidegasfrommotorvehicleexhaust,orthesulfurdioxidereleasedfromfactories.Secondarypollutantsarenotemitteddirectly.Rather,theyformintheairwhenprimarypollutantsreactorinteract.Groundlevelozoneisaprominentexampleofasecondarypollutant.Somepollutantsmaybebothprimaryandsecondary:theyarebothemitteddirectlyandformedfromotherprimarypollutants.Carbondioxide,agreenhousegas,isthemainpollutantthatiswarmingEarth.Thoughlivingthingsemitcarbondioxidewhentheybreathe,carbondioxideiswidelyconsideredtobeapollutantwhenassociatedwithcars,planes,powerplants,andotherhumanactivitiesthatinvolvetheburningoffossilfuelssuchasgasolineandnaturalgas.Inthepast150years,suchactivitieshavepumpedenoughcarbondioxideintotheatmospheretoraiseitslevelshigherthantheyhavebeenforhundredsofthousandsofyears.Othergreenhousegasesincludemethane—whichcomesfromsuchsourcesasswampsandgasemittedbylivestock—andchlorofluorocarbons(CFCs),whichwereusedinrefrigerantsandaerosolpropellantsuntiltheywerebannedbecauseoftheirdeterioratingeffectonEarth'sozonelayer.Anotherpollutantassociatedwithclimatechangeissulfurdioxide,acomponentofsmog.Sulfurdioxideandcloselyrelatedchemicalsareknownprimarilyasacauseofacidrain.Buttheyalsoreflectlightwhenreleasedintheatmosphere,whichkeepssunlightoutandcausesEarthtocool.Volcaniceruptionscanspewmassiveamountsofsulfurdioxideintotheatmosphere,sometimescausingcoolingthatlastsforyears.Infact,volcanoesusedtobethemainsourceofatmosphericsulfurdioxide;todaypeopleare.Coalandpetroleumoftencontainsulfurcompounds,andtheircombustiongeneratessulfurdioxide.Industrializedcountrieshaveworkedtoreducelevelsofsulfurdioxide,smog,andsmokeinordertoimprovepeople'shealth.Butaresult,notpredicteduntilrecently,isthatthelowersulfurdioxidelevelsmayactuallymakeglobalwarmingworse.Justassulfurdioxidefromvolcanoescancooltheplanetbyblockingsunlight,cuttingtheamountofthecompoundintheatmosphereletsmoresunlightthrough,warmingtheEarth.Thiseffectisexaggeratedwhenelevatedlevelsofothergreenhousegasesintheatmospheretraptheadditionalheat.Mostpeopleagreethattocurbglobalwarming,avarietyofmeasuresneedtobetaken.Onapersonallevel,drivingandflyingless,recycling,andconservationreducesaperson’scarbonfootprint—theamountofcarbondioxideapersonisresponsibleforputtingintotheatmosphere.Onalargerscale,governmentsaretakingmeasurestolimitemissionsofcarbondioxideandothergreenhousegases.OnewayisthroughtheKyotoProtocol,anagreementbetweencountriesthattheywillcutbackoncarbondioxideemissions.Anothermethodistoputtaxesoncarbonemissionsorhighertaxesongasoline,sothatpeopleandcompanieswillhavegreaterincentivestoconserveenergyandpolluteless.