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AerosolsandClimate
AerosolsandClimate
Editedby
KenS.Carslaw
SchoolofEarthandEnvironment,UniversityofLeeds,Leeds,UnitedKingdom
Elsevier
Radarweg29,POBox211,1000AEAmsterdam,Netherlands TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UnitedKingdom 50HampshireStreet,5thFloor,Cambridge,MA02139,UnitedStates
Copyright©2022ElsevierInc.Allrightsreserved.
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Notices
Knowledgeandbestpracticeinthisfieldareconstantlychanging.Asnewresearchandexperiencebroadenour understanding,changesinresearchmethods,professionalpractices,ormedicaltreatmentmaybecome necessary.
Practitionersandresearchersmustalwaysrelyontheirownexperienceandknowledgeinevaluatingandusing anyinformation,methods,compounds,orexperimentsdescribedherein.Inusingsuchinformationormethods theyshouldbemindfuloftheirownsafetyandthesafetyofothers,includingpartiesforwhomtheyhavea professionalresponsibility.
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ISBN:978-0-12-819766-0
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Publisher: CandiceJanco
AcquisitionsEditor: AmyShapiro
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CoverDesigner: VictoriaPearsonEsser
TypesetbySTRAIVE,India
Symbols
c
topoftheatmosphere
Albedooftheclear(cloud-free)partofthe skyatthetopoftheatmosphere
cloud)atthetopoftheatmosphere
e
Aerodynamic,mobility,optical,volumeequivalentdiameterofaparticle
Saturationvaporpressureofwaterfora particleofdiameter
Cloudyfractionofamodelgridcell
Fractionofachemicalspeciesinthe particlephase
Netradiativeflux(incomingminus outgoing)
Radiativeeffect(netradiativeflux, incomingminusoutgoing)ofaerosol–radiationinteractionsatthetopofthe atmosphere
sky
definedas1750or1850)
Instantaneousradiativeforcingdueto aerosol–cloudinteraction,dueto aerosol–radiationinteractionatthetopof theatmosphere
Instantaneousaerosolradiativeforcingat thetopoftheatmosphere
Effectiveradiativeforcingdueto aerosol–cloudinteraction,dueto aerosol–radiationinteractionatthetopof theatmosphere
Exponentinsupersaturationdependencyof CCNconcentration
k
ExponentinempiricalformofStefanBoltzmannequation
m Massconcentrationofparticles
Refractiveindex
N>3nm, N>50nm, etc. Particlenumberconcentrationlargerthan 3nmdiameter,largerthan50nmdiameter (usuallydrydiameter)
Ni Iceparticlenumberconcentration
Nmode Integralparticlenumberconcentrationina lognormaldistribution“mode”
p PressurehPa P(Θ)Phasefunction(probabilitytobescattered atascatteringangle Θ)
ql Clouddropletmassmixingratioinair (l ¼ liquid)
q
qt Totalwater(vaporplusliquid)massmixing ratioinair
qv Watervapormassmixingratio(specific humidity)
(saturationspecifichumidity)
(scatteringplusabsorption)
r
r
r
Dropleteffectiveradius(area-weighted meanradiusofapopulationofcloud droplets)
Aerosoleffectiveradius(area-weighted meanradiusofapopulationofaerosol particles)
Medianradiusofalognormaldistribution function
Medianradiusofalognormalaerosol particlemode
r
r
fluxatthetopoftheatmosphere
Downwardlongwave(terrestrial)radiative fluxatthetopoftheatmosphere
RLW,cloudy " Upwardlongwave(terrestrial)radiative fluxofthecloudysky
RLW,cloud-free " Upwardlongwave(terrestrial)radiative fluxatthetopoftheatmosphereforthe clear(cloud-free)partoftheskyatthetop oftheatmosphere
Θ Scatteringangle(betweenincidentand scatteredradiation)
θ s Solarzenithangle(withrespecttovertical)Degreeorradian 10,11
θ v Scatteredzenithangle Degreeorradian 11 κ Hygroscopicityparameter – 5,12 λ Climatesensitivityparameter K(Wm 2) 1 2 λ Wavelengthofradiationm
λ Slopeoftheexponentialinthegamma
ρa Densityofairmaybeaffectedby condensedwater-Eq.(12.5)
ρl Massconcentrationofcloudliquidwater dropletsinair(liquidwatercontent)
ρp Densityofaparticle kgm 3
ρw Densityofliquidwater 1000kgm 3 12
σ , σ ∗ Stefan-Boltzmann(Planck)constant, empiricalvalueofStefan-Boltzmann constantforplanetaryemission
σ Geometricstandarddeviationofa lognormaldistribution – 4,5,6
^ σ Surfacetension Nm 2 12
σ abs Particlelightabsorptioncrosssectionm2 11
σ ext Particlelightextinctioncrosssection (scatteringplusabsorption) m 2 11
σ sca Particlelightscatteringcrosssectionm2 11
τa Aerosolopticaldepth
τa,anth Aerosolopticaldepthattributedto anthropogenicaerosol
τc Cloudopticaldepth
11
12
ϕs Azimuthangle DegreeorRadian 11
ϕv Scatteredazimuthangle DegreeorRadian 11
χ ViewzenithangleofasatelliteinstrumentDegreeorRadian 10
ω Verticalvelocity(inpressurecoordinates)Pas 1 12
ϖ 0 Single-scatteringalbedo – 11
aThesearetheunitstypicallyusedtoreportquantitiesinatmosphericaerosolscience.
Contributors
JamesAllan
DepartmentofEarthandEnvironmentalSciencesandNationalCentreforAtmosphericScience, UniversityofManchester,Manchester,UnitedKingdom
NicolasBellouin
DepartmentofMeteorology,UniversityofReading,Reading,UnitedKingdom
MassimoA.Bollasina
UniversityofEdinburgh,Edinburgh,UnitedKingdom
TamiC.Bond
DepartmentofMechanicalEngineering,ColoradoStateUniversity,FortCollins,CO,UnitedStates
SimonCarn
MichiganTechnologicalUniversity,Houghton,MI,UnitedStates
KenS.Carslaw
SchoolofEarthandEnvironment,UniversityofLeeds,Leeds,UnitedKingdom
WilliamCollins UniversityofReading,Reading,UnitedKingdom
AnnicaM.L.Ekman
DepartmentofMeteorologyandBolinCenterforClimateResearch,StockholmUniversity, Stockholm,Sweden
JiwenFan
PacificNorthwestNationalLaboratory,Richland,WA,UnitedStates
EdwardGryspeerdt
ImperialCollegeLondon,London,UnitedKingdom
RalphKahn
NASAGoddardSpaceFlightCenter,Greenbelt,MD,UnitedStates
ZbigniewKlimont
InternationalInstituteforAppliedSystemsAnalysis(IIASA),Laxenburg,Austria
HanneleKorhonen
FinnishMeteorologicalInstitute,Helsinki,Finland
BenKravitz
IndianaUniversity,Bloomington,IN,UnitedStates
ZhanqingLi UniversityofMaryland,CollegePark,MD,UnitedStates
XiaohongLiu
DepartmentofAtmosphericSciences,TexasA&MUniversity,CollegeStation,TX,UntedStates
NatalieMahowald
CornellUniversity,Ithaca,NY,UnitedStates
JosephR.McConnell
DivisionofHydrologicSciences,DesertResearchInstitute,Reno,NV,UnitedStates
BenjaminJ.Murray
SchoolofEarthandEnvironment,UniversityofLeeds,Leeds,UnitedKingdom
KirstyPringle
SchoolofEarthandEnvironment,UniversityofLeeds,Leeds,UnitedKingdom
JohannesQuaas
LeipzigUniversity,Leipzig,Germany
PhilipJ.Rasch
PacificNorthwestNationalLaboratory/UniversityofWashington,Seattle,WA,UnitedStates
BjørnHallvardSamset
CICEROCenterforInternationalClimateResearch,Oslo,Norway
JuliaSchmale
SchoolofArchitecture,CivilandEnvironmentalEngineering, EcolePolytechniqueFederalede Lausanne,Lausanne,Switzerland
AnjaSchmidt
InstituteofAtmosphericPhysics(IPA),GermanAerospaceCenter(DLR),Oberpfaffenhofen; MeteorologicalInstitute,LudwigMaximilianUniversityofMunich,Munich,Germany;Yusuf HamiedDepartmentofChemistry,UniversityofCambridge,Cambridge,UnitedKingdom
MichaelSchulz
ResearchDepartment,NorwegianMeteorologicalInstituteandDepartmentofGeosciences, UniversityofOslo,Oslo,Norway
CatherineE.Scott
SchoolofEarthandEnvironment,UniversityofLeeds,Leeds,UnitedKingdom
DuncanWatson-Parris
Atmospheric,OceanicandPlanetaryPhysics,UniversityofOxford,Oxford,UnitedKingdom
LauraJ.Wilcox
NationalCentreforAtmosphericScience,UniversityofReading,Reading,UnitedKingdom
HongbinYu
EarthSciencesDivision,NASAGoddardSpaceFlightCenter,Greenbelt,MD,UnitedStates
Acknowledgments
Wearegratefultomanycolleagueswhoreviewedseveralofthechaptersofthisbook:
JonathanAbbatt,DepartmentofChemistry,UniversityofToronto,Canada; TimAndrews,Met Office,UnitedKingdom; RobertAllen,EarthandPlanetarySciences,UniversityofCalifornia,Riverside,UnitedStates; MianChin,AtmosphericChemistryandDynamicsLaboratory,NASAGoddard SpaceFlightCenter,UnitedStates; PaulDeMott,DepartmentofAtmosphericScience,ColoradoState University,UnitedStates; OlegDubovik,UniversityofLille,Lille,France; DonGrainger,Atmospheric,Oceanic&PlanetaryPhysics,UniversityofOxford,UnitedKingdom; RuthPrice,School ofEarthandEnvironment,UniversityofLeeds,UnitedKingdom; JeffreyPierce,DepartmentofAtmosphericScience,ColoradoStateUniversity,UnitedStates; ZacharyLebo,DepartmentofAtmosphericScience,UniversityofWyoming,UnitedStates; Po-lunMa,PacificNorthwestNational Laboratory,UnitedStates; ThomasPopp,GermanAerospaceCenter(DLR),GermanRemoteSensing DataCenterAtmosphere,Germany; AlanRobock,SchoolofEnvironmentalandBiologicalSciences, RutgersUniversity,UnitedStates; AndrewSayer,NASAGoddardSpaceFlightCenter,UnitedStates; StevenSmith,JointGlobalChangeResearchInstitute,PacificNorthwestNationalLaboratory,United States; IvyTan,DepartmentofAtmosphericandOceanicSciences,McGillUniversity,Canada; MatthewToohey,InstituteofSpaceandAtmosphericStudies,UniversityofSaskatchewan,Canada; HuiWan,PacificNorthwestNationalLaboratory,UnitedStates; HailongWang,PacificNorthwest NationalLaboratory,UnitedStates; AlfredWiedensohler,DepartmentofExperimentalAerosoland CloudMicrophysics,LeibnizInstituteforTroposphericResearch,Germany; RobertWood, AtmosphericSciences,UniversityofWashington,UnitedStates; SabineUndorf,Departmentof Meteorology,StockholmUniversity,Sweden; DanieleVisioni,SibleySchoolforMechanicaland AerospaceEngineering,CornellUniversity,UnitedStates; KaiZhang,PacificNorthwestNational Laboratory,UnitedStates.
Acronymsandabbreviations
ACI Aerosol–cloudinteraction
ACSM AerosolChemicalSpeciationMonitor
ACTRIS Aerosols,CloudsandTracegasesResearchInfraStructureNetwork
AerChemMIP AerosolChemistryModelIntercomparisonProject
AeroCom Anopeninternationalinitiativeofscientistsinterestedintheadvancementofthe understandingoftheglobalaerosolanditsimpactonclimate
AERONET AerosolRoboticNetwork
AGCM Atmosphericgeneralcirculationmodel
AI Aerosolindex
AIRS AtmosphericInfraRedSounder
AMIP AtmosphericModelIntercomparisonProject
AMOC Atlanticmeridionaloverturningcirculation
AMS AerosolMassSpectrometer
AMV Atlanticmultidecadalvariability
AOD Aerosolopticaldepth
AOGCM Atmosphere-oceangeneralcirculationmodel
AR4,AR5,AR6 AssessmentreportsoftheIntergovernmentalPanelonClimateChange
ARI Aerosol–radiationinteraction
ATOFMS AerosolTime-of-FlightMassSpectrometer
ATSR AlongTrackScanningRadiometer
AVHRR AdvancedVeryHighResolutionRadiometer
BB Biomassburning
BC Blackcarbon
BVOC Biogenicvolatileorganiccompound
CALIOP Cloud-AerosolLidarwithOrthogonalPolarization
CALIPSO Cloud-AerosolLidarandInfraredPathfinderSatelliteObservations
CAMS CopernicusAtmosphereMonitoringService
CAPE Convectiveavailablepotentialenergy
CATS Cloud-AerosolTransportSystem
CCN Cloudcondensationnucleus/nuclei
CEDS CommunityEmissionsDataSystem
CFMIP CloudFeedbackModelIntercomparisonProject
CLAW Charlson,LovelockAndreae,Warren(authorsofapublication)
CLE Currentlegislation
CLRTAP ConventiononLong-RangeTransboundaryAirPollution
CMIP CoupledModelIntercomparisonProject
CPC(CNC) CondensationParticleCounter(CondensationNucleusCounter)
CPM Cloud-permittingmodelorconvection-permittingmodel
CPU Centralprocessingunit
CRM Cloud-resolvingmodel
CS Condensationsink
CTM Chemicaltransportmodel
DCC Deepconvectivecloud
DJF DecemberJanuaryFebruary
DMA DifferentialMobilityAnalyzer
DMPS DifferentialMobilityParticleSizer
DMS Dimethylsulfide
DNS Directnumericalsimulation(atmosphericmodel)
DSCOVR DeepSpaceClimateObservatory
EARLINet EuropeanAerosolResearchLidarNetwork
EBAS Adatabasehostingobservationdataofatmosphericchemicalcompositionand physicalproperties
EBC Equivalentblackcarbon(basedonopticalabsorption)
ECS Equilibriumclimatesensitivity
ELVOC Extremelylowvolatilityorganiccompound
EMIC Earthsystemmodelofintermediatecomplexity
ENSO ElNin ˜ oSouthernOscillation
ENVISAT EnvironmentalsatelliteoperatedbytheEuropeanSpaceAgency
EOF Empiricalorthogonalfunction
EOS EarthObservingSystem(NASA)
ERF Effectiveradiativeforcing
ESA EuropeanSpaceAgency
ESM Earthsystemmodel
EUMETSAT EuropeanOrganisationfortheExploitationofMeteorologicalSatellites
FAR FirstAssessmentReport(oftheIntergovernmentalPanelonClimateChange)
FAIR FiniteAmplitudeImpulseResponsesimpleclimatemodel/emulator
FF Fossilfuel
FT Freetroposphere
GAINS GreenhousegasAirpollutionINteractionsandSynergies
GAW GlobalAtmosphereWatch
GCCN Giantcloudcondensationnucleus/nuclei
GCM Generalcirculationmodel(alsoglobalclimatemodel)
GDE Generaldynamicequationofaerosol
GEOS GoddardEarthObservingSystem
GOES GeostationaryEnvironmentalSatellites
GOME GlobalOzoneMonitoringExperiment
GOMOS GlobalOzoneMonitoringbyOccultationofStars
GTP Globaltemperaturepotential
GWP Globalwarmingpotential
HNLC Highnutrient-lowchlorophyll
HS Apparenthydrologicalsensitivity
HSRL HighSpectralResolutionLidar
IAM Integratedassessmentmodel
IASI InfraredAtmosphericSoundingInterferometer
IMPROVE InteragencyMonitoringofProtectedVisualEnvironments
INP Ice-nucleatingparticle
IPCC IntergovernmentalPanelonClimateChange
IR Infrared
ITCZ IntertropicalConvergenceZone
IUPAC InternationalUnionofPureandAppliedChemistry
JJA JuneJulyAugust
LES Large-eddysimulation(atmosphericmodel)
LVOC Lowvolatilityorganiccompound
LW Longwave
M/R Maximum/random(overlap)
MACC MonitoringAtmosphericCompositionandClimate
MAGICC ModelfortheAssessmentofGreenhouseGasInducedClimateChange
MAM MarchAprilMay
MCB Marinecloudbrightening
MERRA Modern-EraRetrospectiveAnalysisforResearchandApplications
MIP ModelIntercomparisonProject
MISR Multi-angleImagingSpectroRadiometer
MME Multi-modelensemble
MOA Marineorganicaerosol
MODIS MODerateResolutionImagingSpectroradiometer
MS Massspectrometer
MXD Maximumlatewooddensity
NDC Nationallydeterminedcontribution
N:P Nitrogen:Phosphorus(nutrientratio)
NIR Near-infrared
NIST NationalInstituteofStandards
NMVOC Non-methanevolatileorganiccompound
NOAA NationalOceanicandAtmosphericAdministration
NOx Nitrogenoxides
NWP Numericalweatherprediction
OA Organicaerosol
OMI OzoneMappingInstrument
OPC(OPS,OPSS) Opticalparticlecounter(spectrometer,sizespectrometer)
OSIRIS OpticalSpectrographandInfraRedImagingSystem
PD Presentday
PDF Probabilitydensityfunction
PDO Pacificdecadaloscillation
PI Preindustrial
POA Primaryorganicaerosol
POLDER POLarizationandDirectionalityoftheEarth’sReflectances
POM Particulateorganicmatter
PPE Perturbedparameterensemble
RCP Representativeconcentrationpathway
RF Radiativeforcing(usuallydefinedasinstantaneousradiativeforcing)
RFMIP RadiativeForcingModelIntercomparisonProject
RGCM Regionalgeneralcirculationmodel
RH Relativehumidity
SAGE StratosphericAerosolandGasExperiment
SAR SecondAssessmentReport(oftheIntergovernmentalPanelonClimateChange)
SCIAMACHY SCanningImagingAbsorptionSpectroMeterforAtmosphericCHartographY
SDG Sustainabledevelopmentgoal
SEMS ScanningElectricalMobilitySpectrometer
SEVIRI SpinningEnhancedVisibleandInfraredImager
SIP Secondaryiceproduction
SLCF Short-livedclimateforcer
SLSTR SeaandLandSurfaceTemperatureRadiometer
SMPS ScanningMobilityParticleSizer
SOA Secondaryorganicaerosol
SON SeptemberOctoberNovember
SP2 SootParticlePhotometer
SRES SpecialReportonEmissionsScenarios
SSA Single-scatteringalbedo
SSP Sharedsocio-economicpathway
SST Seasurfacetemperature
STP Standardtemperatureandpressure
SVOC Semi-volatileorganiccompound
SW Shortwave
TAR ThirdAssessmentReport(oftheIntergovernmentalPanelonClimateChange)
TIR Thermalinfrared
TOA Topoftheatmosphere
TOMS TotalOzoneMappingSpectrometer
TRW Treeringwidth
UAP Ultrafineaerosolparticle
UT Uppertroposphere
UV Ultraviolet
UVAI UltravioletAerosolindex
VEI Volcanicexplosivityindex
VIIRS VisibleInfraredImagingRadiometerSuite
VNIR Visible-near-infrared
VOC Volatileorganiccompound
WBF Wegener-Bergeron-Findeisen
WMGHG Well-mixedgreenhousegas
WHO WorldHealthOrganization
WMO WorldMeteorologicalOrganization
Introduction
KenS.Carslaw
SchoolofEarthandEnvironment,UniversityofLeeds,Leeds,UnitedKingdom
1.1 Whatisaerosolandwhyisitimportantforclimate?
Earth’satmospherefromthesurfacetothetopofthestratosphereatabout40kmaltitudecontainsparticlesranginginsizefrommolecularclustersofnanometersindiameteruptoparticlesofseveralmicrometersnearthesurface. Aerosol isthetermusedtodescribethissuspensionofliquidandsolid particlesintheair.ThisbookiscalledAerosolsandClimate(plural)becausetherearemanydifferent typesofaerosolwithvaryingeffectsonclimate.Aerosolisimportantforclimatebecausetheparticles scatterandabsorbsolarandterrestrialradiationandbecausetheyarethenucleiuponwhichclouddropletsandiceparticlesform,whichdominateEarth’salbedo.
Particlesentertheatmospheremainlyatthesurfacebydirectemissionofmateriallikewind-blown seasprayandmineraldust,andfromnaturalandhuman(anthropogenic)combustionsourcesthatproduceparticlescomposedofcomplexmixturesoforganiccarbon,sootandothermaterial.Thereisalsoa tinyinputofparticlesatthetopoftheatmospherefromcosmicdust.Particlesalsoformdirectlywithin theatmospherefromgas-to-particleconversion(nucleation)ofawiderangeofinorganicandorganic gas-phasecompoundsderivedfromnaturalandanthropogenicsources.Theseparticlesstartlifeasmolecularclustersthatcaneventuallygrowuptoseveraltensofnanometersindiameterthroughcoagulationandpartitioningofawiderangeofgas-phasecompoundsintotheparticles.Particles subsequentlyundergomanyphysicalandchemicalchangesduringtransportthroughtheatmosphere, involvinginteractionofparticleswitheachother,furthercondensationofmateriallikesulfateandorganicmaterial,andinteractionwithclouds.Theseprocessesmixparticlestogether,creatingahugely diversearrayofparticlesizes,chemicalcompositions,andnumberconcentrations.
Inalmostanypartofthetropospheresubmicron-sizedparticlesareacomplexmixtureofvarying amountsofinorganicsaltandacidicspecies(sodium,ammonium,sulfate,nitrate,chloride),organic compoundsinwidelyvaryingstatesofoxidation,aswellasinsolublematerialsuchassoot(blackcarbon),mineralmaterial,andotherinclusions.Almostallparticleshaveatleastsomewaterassociated withthem,inamountsthatdependonthechemicalcompositionoftheparticleandambientrelative humidity.Someofthechemicalcomponentswerepresentwhentheparticleswereemittedandsome entertheparticlessometimelaterbycondensationofgas-phasecompoundspotentiallythousandsof kilometersfromwheretheparticlesorgaseswereemitted.Thisspatialvariabilityofsourcescombined withtherelativelyshortlifetimeofparticlesandgasesduetowetanddrydepositionontheEarth’s
https://doi.org/10.1016/B978-0-12-819766-0.00013-4 Copyright # 2022ElsevierInc.Allrightsreserved.
surfacecreatesahighlyheterogeneousdistributionofparticlesinthetropospherethatisfarmorechallengingtosimulateinaglobalmodelthanlong-livedgreenhousegases.
Inthecleanestpartsoftheatmosphereremotefromsourcesofparticlesorgasesthatcannucleateto formnewparticles,theparticlenumberconcentrationcanbelowerthan1cm 3 andthemassconcentrationlessthanafractionofamicrogrampercubicmeter.Inthemostpollutedregionswithstrong sourcesorslowremovalprocesses,numberconcentrationscanexceed105 cm 3 andmassconcentrationscanreachseveralhundredmicrogramspercubicmeter.Inthemajorityofenvironments,mostof theparticlemassresidesinthelargestsuper-micronparticlesandmostofthenumberliesinparticles smallerthanabout100nmdiameter.Theparticlesmostrelevantforclimatearethoseatintermediate sizesofseveraltensofnanometersuptoaboutamicrometer,whichreadilyformclouddropletsand scatterandabsorbsolarradiationefficiently.
Humanactivitieshaveprofoundlyalteredaerosolemissionstotheatmosphereprimarilythrough combustionoffossilfuels,butalsothroughagriculture,changesinlanduse,andalterationofnatural processeslikewildfires.Anthropogenicemissionsofsomeimportantaerosolspecieslikeblackcarbon andsulfurdioxide(whichformssulfateaerosol)exceednaturalemissions,resultinginsubstantialincreasesinaerosolparticlemassandnumberconcentrationsoverlargepartsoftheplanetwithimplicationsforEarth’sradiativeenergybalanceandclimate.Asweshowin Chapter2 andinalaterchapter onsatelliteobservations,continental-scaleplumesofaerosolfromhumanactivitiesarevisibleinsatelliteimagesthousandsofkilometersdownwindofsources.
TheeffectsofaerosolonEarth’sclimate arediverseandcomplexmainlybecausetheydependon manymorepropertiesoftheparticlesthanjustthenumberormassconcentration.Thescatteringand absorptionofradiation,whichalterEarth’sradiativeenergybalance(Fig.1.1),dependonthesizeofthe particles,theirchemicalcomposition,therefractiveindexofthematerial,andtheparticleshape,as wellasseveralenvironmentalfactorssuchasthehumidityoftheair(whichcontrolstheamountof waterabsorbedbytheparticles),thenatureofthesurfacesthattheyoverly,andtheirlocationrelative toclouds.Whenwereferto‘changesinaerosol’fromhumanactivitiesasadriverofclimatechangewe arereferringtochangesinanyoralloftheseparticlepropertiesaswellaschangesintheirhorizontal andverticaldistributionintheatmosphere.
Aerosolparticlesarealsothenucleiuponwhichclouddropletsandiceparticlesform.Clouddropletsaretypicallyseveraltensofmicrometersindiameter – 2–3ordersofmagnitudelargerthanthe aerosolparticlesuponwhichtheyform,sogloballytheyreflectconsiderablymoresolarradiationthan aerosolparticles.Changesinaerosolalterthenumberconcentrationandsurfaceareaofclouddroplets aswellasseveralotherclimaticallyimportantcloudpropertieslikewatercontent,thickness,andareal coverage.Theseaerosol-inducedchangestocloudpropertiesalterthereflectionandabsorptionofsolar radiationandtheabsorptionandemissionofterrestrialradiationbyclouds,withconsequenteffectson Earth’senergybalanceandclimate.Atinyfractionofaerosolparticleswithconcentrationsaslowas 10 4 cm 3 possessspecialpropertiesthatenablethemtoinitiatetheformationoficeinclouds,which canprofoundlyalterthebehaviorofcloudsbelow0°C.
Changesinaerosolcausedbyhumanactivitiesand naturalvariabilityaffectclimatebyaltering energyflowswithintheatmosphereandbetweenthesurface,atmosphere,andspace.Scatteringof solarradiationbacktospaceandtheabsorptionof radiationwithintheat mospherealterthenet amountofradiativeenergyintheclimatesystem(Fig.1.1).Thefirst-orderresponsetothischange inradiativeenergyisachangeinEarth’sglobalaveragetemperature,althoughthethermalinertiaof theoceanmeansthatittakesmanydecadestocenturiestore-establishanewequilibriumglobal
FIG.1.1
Themainwaysinwhichanincreaseinaerosolfromanthropogenicactivitiesaffectstheclimate.Scatteringand absorptionofsolarradiationbyaerosolandmodificationofcloudpropertiesresultsinanetlossofradiativeenergy fromtheplanet,andhenceacoolingeffectonclimate.Othereffects(notshown)includechangesinthe temperaturestructureandstabilityoftheatmosphere,changesintheabsorptionandemissionofradiationinthe terrestrial(infrared)spectrum,darkeningofsnowandicesurfaces,changesiniceparticleformationinclouds, andeffectsonthecarboncyclefromdepositionofnutrientsrequiredbybiota. 3 1.1
temperatureafteraerosolhasbeenperturbed.Regional-andhemispheric-scalechangesinatmosphericcirculationandlandtemperaturescanocc uronmuchshortertimescales,withregionally importantimpactsonclimate.
Changesinaerosolalsoaffectprecipitation.Thedistributionofprecipitationcanbeaffectedlocally andregionallyonthetimescaleofhourstodaysbychangesincloudmicrophysicalprocessestriggered bychangesindropletandiceparticleconcentrations.Thetotalamountofprecipitationcanalsobe affectedonregionalandglobalscalesonthetimescaleofdaystomonthsbychangesintheradiative energydepositedintheatmosphere,whichdeterminestheamountofwaterthatcancondense.Ultimately,onthetimescaleofdecadestocenturiesaerosol-inducedchangesinsurfacetemperaturefurther alterthehydrologicalcycleandprecipitation.
Ithasprovenextremelychallengingtoaccuratelyquantifythemagnitudeofaerosoleffectsonclimatedrivenbyanthropogenicemissions.Itisknownthattheneteffectofanthropogenicaerosolisan enhancementofreflectionofsolarradiationfromtheatmosphereandcloudsandthereforeanincrease inplanetaryalbedo.This radiativeforcing hascausedacoolingeffectonclimateovertheindustrial periodthatiscommensuratewiththewarmingeffectofanthropogenicgreenhousegases.Thefactthat
Earthhaswarmedovertheindustrialperiodindicatesthattheaerosolcoolingeffectissmallerthanthe greenhousegaswarming(unlessothermajorforcingshavenotbeenaccountedfor).Theradiativeforcingrelativetopreindustrialconditionshasbeenpersistentlyuncertaininclimatemodelsimulationsand observations,andthereisevenlessconfidenceinhowtheforcinghasaffectedglobalandregional temperaturesandweatherpatterns.Thedifficultystemsfromthemanywaysinwhichaerosolaffects climate,thehugediversityofaerosolpropertiesthatmatter,andtheenormousvariationsinaerosol propertiesaroundtheplanet.Subsequenteffectsonclouds,weatherpatterns,andregionalclimate drivenbychangesinaerosol,radiation,andtemperatureaddfurthercomplicationsthatchallenge ourunderstandingofmeteorologyandclimatedynamics.
Asidefromaerosoleffectsonthephysicalclimate(radiation,clouds,precipitation,temperature), therearealsonumerouseffectsinvolvingchangesinEarth’sbiotaintheoceanandonlandcausedby altereddepositionofnutrientspecieslikeiron,phosphorus,andnitrogen.Changesinbiotahavethe potentialtoaffectthecarboncycleandhencecarbondioxideconcentrations.
1.2 Aimsandscopeofthebook
Thisbookhasbeenwrittenforscientistsenteringwhathasbecomeahugelydiversefieldofscience. WhenSeanTwomey’sseminalbook AtmosphericAerosols (Twomey,1977)waspublished,theroleof aerosolinclimatechangedeservedoneshortchapterinabookfocusedonaerosoldynamics,optics, andelectricalproperties.Atthattime,whenthepotentialroleofanthropogenicaerosolinclimate changewasbeginningtobestudied(see Chapter2),aknowledgeofphysicsandsomebasicchemistry wassufficienttoinvestigatetheprocessesthatwereconsideredtobeimportantatthetime.Intheintervening40yearsorso,aerosol-climatesciencehasgrowntoencompassmanyaspectsofmeteorology,climatedynamics,cloudandradiationphysics,biogeochemistry,andotherdisciplines.Italso stimulatedandnowreliesheavilyonavastarrayofmeasurementsfromsatellitesandinsituinstrumentationaswellasnumericalmodelingfromthescaleofindividualcloudstoglobalEarthsystem models.Thechallengefornewentrantstothefieldofaerosol-climatescienceisthatmanyofthe bigquestionsneedtobetackledincollaborationsthatspanmanyorallofthesedisciplines.Asaconsequence,mostatmosphericaerosolscientistsnowfindthemselvesneedingtounderstandatleastthe basicsofawiderangeoftopicsoutsidetheirimmediatespecialism.
Afurtherchallengefornewentrantsisthatdiversificationofaerosolsciencehasalsoledtothe emergenceofseveralsubdisciplines,whichhasinevitablyledtothedevelopmentofspecializedterminology,concepts,definitions,andnomenclature.Suchspecializationmakesaerosol-climatescience increasinglyinaccessibletonewentrantsaswellastoscientistswhoseresearchdiversifies.Atthevery leastitmakesitchallengingtoswitchbetweenparallelsessionsatamajoraerosolconferenceorto participateactivelyinacollaborativeproject.Theaimofthisbookistoconnectthesediverseareas ofaerosolscienceandpresentthestateofknowledgeinaconsistentway.
Oneofthemainmotivationsforpursuingaresearchcareerinaerosol-climatescienceisthatthe climaticeffectsofaerosolarehighlyuncertain,asituationthathaspersistedsincethefirstattempts inthe1990stoevaluatetheseeffectsinaconsistentway.However,newentrantstothisfieldsoon appreciatethatanunderstandingofaerosolfundamentalsaloneisinsufficienttounderstandandtackle theuncertainty.Thisisbecausetheuncertaintystemsfromhowthefundamentalprocessesinteract, howtheyareappliedinmodels,andwhatassumptionsaremadebecauseofalackofprocess-level
understandingorlimitedcomputationalpowertorunlarge-scalemodels.Indeed,perhapsthegreatest separationofsubdisciplinesinaerosolscienceisbetweeninvestigationsofaerosolandcloudprocesses inever-increasingdetailandthedevelopmentoflarge-scalemodelsthatareultimatelyusedtodefine ouroveralllevelofunderstandingandtoinformclimatepolicy.Toaddressthisdisconnection,thebook includesadedicatedchapteronmodelingandeachofthesubsequenttopicchaptersincludesasection onmodelswiththeaimto‘liftthelid’onhowvariousprocessesarehandledinmodelsandwhatis neglectedortreatedinadequately.
Thisisnotabookaboutaerosolphysicalandchemicalfundamentals.Forthatthereareseveralother excellenttextbooksaimedatscientistsworkingwithinsubdisciplinesofaerosol-climatescience.Some fundamentalconceptsareintroducedineachchapter,especiallywherewethinktheconceptsmaybe difficulttoassembleinacoherentwayfromothersourcesorwheretheterminologyhasbeguntodeviatefromwhathasbeenusedinthesubdisciplines.Rather,thebookisabouthowfundamentalconceptsinaerosolsciencearebeingappliedinclimatescienceandhowtheconceptsareultimately translatedintobetterclimatemodels.
Thetopicscoveredinthebookreflectthemajorchallengesinthefieldofaerosol-climatescience. Indesigningthechapters,wewerealsoawareoftheneedtoprovideaprimerontopicsthathavebeen vitaltothedevelopmentofourunderstandingofglobalaerosol,includingthedevelopmentofemission inventories,ambientandremoteaerosolmeasurements,andmodeling.Wehavetriedtoavoidstructuringthechaptersaroundaerosolchemicalspeciesthatareimportantresearchfoci,suchasdust,black carbon,andorganicaerosols.Instead,theseareincorporatedintherelevantchaptersonprocesses, emissions,radiation,andobservations.ChaptersonvolcanicandArcticaerosolmightappeartobe theexceptions,butthesearealsoassociatedwithparticularregionsoftheatmosphere,sowedescribe theprocessesinthecontextoftheenvironmentsinwhichtheyoccur.
Followingthisintroduction,thebookisorganizedasfollows:
Chapter2 providesanoverviewofthe effectsofaerosolonclimate,includingchangesinradiative energyfluxes,temperature,andprecipitationonaglobalscale.Itintroducestheimportantconceptof aerosolradiativeforcing(theradiativeenergyimbalancedrivingclimatechange),atmosphericandradiative“adjustments”totheforcingthathaveproveddifficulttoquantify,long-termclimateresponse, andtheimportanceofaerosolinclimatesensitivity.Thischapteralsoprovidesabriefhistoryofour understandingofaerosoleffectsonclimate.
Chapter3 extendsthediscussionofaerosolclimaticeffectstothe roleofaerosolintheEarthsystem,inparticularthewaysinwhichaerosolaltersbiogeochemicalprocessesonlandandintheocean leadingtochangesinthecarboncyclethatalterclimate.Wealsodescribehowbiologicalprocesseson landandintheoceangenerateaerosolsandprecursorgasesthataffectclimateandare,inturn,affected byclimatechange.
Chapter4 describesthe propertiesanddistributionofaerosol.Thechapterdefinesthefundamental propertiesofaerosolparticlesthatdeterminetheeffectsonclimate – theparticlesizedistributionand chemicalcomposition – andhowthesepropertiesvaryspatiallyonthescaleofmeterstothousandsof kilometersandtemporallyonthetimescaleofhourstoyears.
Chapter5 describesthe aerosolphysicalprocesses thatshapetheaerosolphysicalandchemical properties,coveringparticleformationfromgas-phaseprecursors,particlegrowth,coagulation,exchangeofchemicalspecieswiththegasphase,wateruptake,anddryandwetdeposition.Weemphasizethetimescalesoftheseprocessesandhowtheyshapeparticlepropertiesduringtheirshort residencetimeintheatmosphere.
Chapter6 summarizestherepresentationof aerosolinclimatemodeling. Ouraiminthischapteris to‘liftthelid’onhowaerosols,clouds,andradiationarehandledinthewiderangeofmodelsusedin climatescience.Thisisavasttopic,sowefocusonhighlightingthemanyassumptionsthataremade, whytheyaremade,andhowtheyaffecttherealismandreliabilityofmodels.Modelingisoftenseenas adownstreamactivity,whichoughttoplaceitattheendofthebook.However,the“anatomy”ofa modeldescribedheresetsthesceneforthemore-detaileddescriptionsofmodelprocessesineach ofthesubsequentchapters.
Chapter7 describesthe historicalvariationsinaerosol asrecordedinicecoresandfromdirectand indirectmeasurementssincethe1960s.Itisthesechangesinaerosolthathavecausedchangesinclimate.Themeasurementrecordsarepatchyandoftendifficulttointerpretdirectlyintermsofatmosphericaerosolabundance,buttheypaintaconsistentpictureoflong-termclimaticallyimportant variations.
Chapter8 followsdirectlyfrom Chapters6and7 bydescribing aerosolandprecursorgasemissions,whichareavitalinputtoclimatemodelsandkeytounderstandinglong-termaerosoltrends.We explainhowemissionratesaredefinedandhowtheyaredevelopedforinputintomodels.Emissions derivefromhumanactivities(oftenreferredtoasanthropogenic)andfromnaturalsourceslikesea spray,fires,andthebiosphere.Weaddresshowemissionsareestimatedfromdataonhumanactivity andfrommeasurementsandmodelsofnaturalprocesses.
Chapter9 describesthe measurementofambientparticleproperties. Aerosolsciencemakesextensiveuseofambient(insitu)measurementstounderstandprocessesandtoconstrainthestateandbehavior ofmodels.Inthischapterwedescribemeasurementsofparticlenumberandmassconcentration,size distribution,wateruptake,opticalproperties,chemicalcomposition,andinteractionwithclouds.
Chapter10 describes satellitemeasurementsofaerosol. Satellitesprovidetheonlywaytoviewthe distributionofaerosolonaglobalscaleandtheyhavebeenacriticalpartofassessmentsoftheglobal radiativeeffectsofaerosolaswellastheeffectsonclouds.Inthischapterwedescribethekeyinstruments,measurementtechniques,andmethodsofretrievingaerosolpropertiesfromradiative measurements.
Chapter11 focuseson aerosol-radiationinteractions,includingthescatteringandabsorptionof solarandterrestrialradiationbyparticles,theneteffectaerosolonEarth’sradiativeenergybudget, andtheeffectthatanthropogenicemissionshavehadontheseprocesses.Afterdefiningsomefundamentalprocesses,estimatesofthemagnitudeofradiativeforcingfromsatellitemeasurementsand modelsarepresented,withfurtherdetailonhowsatellitemeasurementsaremadein Chapter10
Chapter12 focuseson aerosol-cloudinteractionsinshallowliquidclouds.Thechapterintroduces somefundamentalaspectsofcloudphysicsrequiredtounderstandhowcloudprocessesandproperties areperturbedbychangesinaerosol.Thechapterfocusesonshallowliquidcloudslikestratusandstratocumulusthatdominatethecloudradiativeeffectonclimate.Weoutlinehowperturbationofcloud propertiesbychangesinaerosolisquantifiedandaddresssomeofthereasonswhythemagnitudeof radiativeeffectsremainsverychallengingtoquantify.
Chapter13 addresses large-scaledynamicalresponsestoaerosol. Thischapterexplainshownonuniformcoolingandheatingintheatmosphereandatthesurfacecausedbyaerosolaffectregional-scale energyflows,resultinginchangesintheatmosphericcirculationonscalesrangingfromsubcontinental tohemispheric-widepatterns,withsubsequentimpactontemperatureandprecipitation.
Chapter14 describesourunderstandingof aerosoleffectsondeepconvectiveclouds.Theprocesses andclimaticeffectsindeepcloudsreaching10–15kminthetropospherearemorevariedandcomplex
thaninshallowcloudsowingtothecascadeofwaterandice-phaseprocessesthatoccurwithinacomplexdynamicalenvironment.Theeffectofchangesinaerosoloncloudvigorandthecreationofextensive,radiativelyimportantanvilcloudsisconsequentlyincompletelyunderstoodandregionally variable.
Chapter15 addresses aerosoleffectsoniceformationinclouds,whichisanotherimportantwaythat aerosolcanaffectcloudsandclimate.Thechaptercoversthepropertiesofice-nucleatingparticles,how iceisformedinclouds,andourcurrentunderstandingfrommodelsandobservationsoftheeffectsof theseparticlesonshallowmixed-phasecloudsinthelowertroposphereandcirruscloudsintheupper troposphere.
Chapter16 describesthe aerosolprocessesinpolarandhigh-latituderegions.Aerosoleffectsat highlatitudesdeservespecialtreatmentbecauseoftheirpotentialroleinthestronglyamplifiedrateof climatechangeintheArcticinparticular.Wedescribesomeoftheuniqueaerosolpropertiesandprocessesinthisregion,includingtheinteractionswithcloudsandthewaysinwhichthesedifferfrom otherregions.
Chapter17 exploresthe effectsofvolcanicaerosolonclimate.Volcaniceruptionsstandoutinthe climaterecordassubstantialperturbationstosurfacesolarradiation,globaltemperature,andprecipitationpatternscausedbysubstantialincreasesinmainlysulfuricacidaerosolinthestratosphere. Degassingvolcanoesandeffusiveeruptionsarealsoanimportantsourceofaerosoltotheglobaltroposphere.Wedescribehowthesedifferenteruptionstylesaffectaerosolinthetroposphereand stratosphere.
Chapter18 addressestheroleof aerosolinclimateengineering. Twoofthemostprominentproposalstodeliberatelymodifytheclimateinvolveaerosol – injectionofparticlesintothestratosphereto mimictheeffectsofvolcaniceruptions,andinjectionofseasprayparticlesintoshallowmarineclouds toincreasetheirreflectionofsolarradiation.Thischapterdescribestheprinciplesofthesetwomethods ofclimateengineering,includingpotentialinadvertenteffectsonaspectsofregionalclimate.
Chapter19 outlinestheroleof aerosolinclimateandairqualitypolicy. Manyofthechangesin aerosolabundanceovertheindustrialperiodhaveoccurredasaresultofpoliciesrelatedtoimproving airqualityandpreventingenvironmentaldegradation.Wedescribethesepoliciesaswellaswaysin whichfutureclimatechangepoliciessuchastheParis1.5degreetargetaccountforaerosoleffects.We alsooutlinehowairqualityandclimatearecoupledproblemsinwhichchangesinclimatecanalso affectaerosolandairquality.
1.3 Terminology,symbols,andunits
Aerosolscienceisaverybroaddisciplinewithrootsinfundamentalscienceslikephysicsandchemistry,establishedscienceslikemeteorologyandinstrumenttechnology(metrology),andsciencesthat haveexistedforonlyafewdecades,suchasclimatemodeling,airpollution,andpolicy.Thisbreadth bringswithitavastrangeofterminologies,acronyms,andsymbolstodescribephysicalquantities,as wellasadiverserangeofunits.Wehaveaimedforconsistencythroughthebook,butthereareafew areasofdivergence.
“Aerosol” isitselfinconsistentlyusedinaerosolscience.Inthisbookwerefertoaerosolasasuspensionofparticlesinair,sowemostlyrefertoitinthesingular.Thepluralisusedwhenreferring specificallytomultipletypesofaerosol,sodustandseasprayareaerosols.Thewordaerosolis
increasinglyusedasasynonymofparticlesuchas“aerosolgrowth”or“theaerosols,”meaningthe particles.Weavoidthisusage.
Diametersandradii areusedofteninterchangeablyinaerosolscience.Particlemeasurementsare almostalwaysreportedintermsofdiameter(althoughlesscommoninthestratosphere),whilethe equationsofparticlemicrophysics(growth,coagulation,etc.)almostalwaysuseradius:nobody learnedinschoolthatthevolumeofasphereis 1 6 π d 3 .Wehavenotattemptedtounifytheuseofradius anddiameterthroughthebook.
Acronymsandsymbols. Aerosolscienceisrifewithacronyms.Wehavetriedtokeeptheirusetoa minimum,butmanyarenowsodeeplyingrainedthattheyareoftennotdefinedinpublications.Alist ofacronymsisprovided.
Ithasbecomecommontouseacronymsandsymbolsforphysicalquantities,oftenwithasymbol usedinanequationbutanacronymforthesamequantityinthetext.ProminentexamplesareAOD (aerosolopticaldepth)andradiativeforcing(RF)inphrasessuchasRF ¼ 2Wm 2.Weavoidthisusage,whichisinconsistentwithIUPACguidelines(Cohenetal.,2006).InkeepingwithIUPAC,weuse singlelettersforphysicalquantities,suchas τa foraerosolopticaldepthand △ F forradiativeforcing, where △ indicatesthatitisachangeinradiativeflux F.Likewise,althoughCCNisanappropriate acronymforcloudcondensationnuclei,weuse NCCN fortheassociatedphysicalquantityofCCNnumberconcentration.TheonlycasewherewedivergefromthisusageiswithPM(particulatematterconcentration),whichisdeeplyembeddedinourfieldinrelationtoairqualityregulations.Werecognize thatourapproachmaymakesomeofthechaptersalittleunfamiliartospecialistswhoareusedtotheir acronyms,butwefeltthatconsistency(orsomethingapproachingit)wasthemoreimportant consideration.
References
Cohen,E.R.,Cvitas ˇ ,T.,Frey,J.G.,Holmstr€ om,B.,Kuchitsu,K.,Marquardt,R.,Mills,I.,Pavese,F.,Quack,M., Stohner,J.,Strauss,H.L.,Takami,M.,Thor,A.J.,2006.Quantities,UnitsandSymbolsinPhysicalChemistry. In:InternationalUnionofPureandAppliedChemistry,thirded. Twomey,S.,1977.AtmosphericAerosols,DevelopmentsinAtmosphericScience.ElsevierScientificPublishing Company,Amsterdam.
