Chapter 1- Measures of Atmospheric Composition

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1、1CHAPTER1.MEASURESOFATMOSPHERICCOMPOSITIONTheobjectiveofatmosphericchemistryistounderstandthefactorsthatcontroltheconcentrationsofchemicalspeciesintheatmosphere.Inthisbookwewillusethreeprincipalmeasuresofatmosphericcomposition:mixingratio,numberdensity,andpartialpressure.Aswewillsee,eachmeas

2、urehasitsownapplications.1.1MIXINGRATIOThemixingratioCXofagasX(equivalentlycalledthemolefraction)isdefinedasthenumberofmolesofXpermoleofair.Itisgiveninunitsofmol/mol(abbreviationformolespermole),orequivalentlyinunitsofv/v(volumeofgaspervolumeofair)sincethevolumeoccupiedbyanidealgasisproporti

3、onaltothenumberofmolecules.Pressuresintheatmospherearesufficientlylowthattheidealgaslawisalwaysobeyedtowithin1%.Themixingratioofagashasthevirtueofremainingconstantwhentheairdensitychanges(ashappenswhenthetemperatureorthepressurechanges).Consideraballoonfilledwithroomairandallowedtoriseinthea

4、tmosphere.Astheballoonrisesitexpands,sothatthenumberofmoleculesperunitvolumeinsidetheballoondecreases;however,themixingratiosofthedifferentgasesintheballoonremainconstant.Themixingratioisthereforearobustmeasureofatmosphericcomposition.Table1-1liststhemixingratiosofsomemajoratmosphericgases.T

5、hemostabundantismolecularnitrogen(N2)withamixingratioCN2=0.78mol/mol;N2accountsfor78%ofallmoleculesintheatmosphere.Nextinabundancearemolecularoxygen(O2)withCO2=0.21mol/mol,andargon(Ar)withCAr=0.0093mol/mol.ThemixingratiosinTable1-1arefordryair,excludingwatervapor.Watervapormixingratiosinthea

6、tmospherearehighlyvariable(10-6-10-2mol/mol).Thisvariabilityinwatervaporispartofoureverydayexperienceasitaffectstheabilityofsweattoevaporateandthedryingrateofclothesonaline.GasesotherthanN2,O2,Ar,andH2Oarepresentintheatmosphereatextremelylowconcentrationsandarecalledtracegases.Despitetheirlo

7、wconcentrations,thesetracegasescanbeofcriticalimportanceforthegreenhouseeffect,theozonelayer,smog,2andotherenvironmentalissues.Mixingratiosoftracegasesarecommonlygiveninunitsofpartspermillionvolume(ppmvorsimplyppm),partsperbillionvolume(ppbvorppb),