Mar 16, 2000 - of Ferrare et al. [1998a, b] clearly show the importance of ..... Finland, 1996. Ogren, J. A., S. Anthony, J. Barnes, M. H. Bergin, W. Huang, L. M..
BNL-66601-00/04-Rev JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 105, NO. D5, PAGES 6807-6816, MARCH 16, 2000
Comparison of aerosol optical depth inferred from surface measurements with that determined by Sun photometry for cloud-free
conditions
at a continental
U.S. site
M. H. Bergin,•,2,3 S. E. Schwartz,• R. N. Halthore,• J. A. Ogren,2 D. L. Hlavka 4
Abstract. Evaluationof the forcingof climateby aerosolscatteringof shortwaveradiationin cloud-freeconditions(directaerosolforcing)requiresknowledgeof aerosolopticalpropertieson relevantspatialandtemporalscales.It is convenient to measurethesepropertiesat the surface. However,beforethesemeasurements canbe usedto quantitatively estimatedirectclimateforcing, it is necessary to determinethe extentto whichthesepropertiesare representative of the entire atmospheric column.In thispaperwe compareaerosolopticaldepth(AOD) determinedby Sun photometryat the SouthernGreatPlains(SGP) Atmospheric RadiationMeasurement(ARM) site in northcentralOklahomafor severalcloud-freedayswith estimatesof AOD basedon two methods.First,the aerosolextinctionmeasuredat the surface(takenasthe sumof the aerosol scattering andabsorption coefficients at instrumental relativehumidityof-20%) is multipliedby the mixingheightdeterminedfrom temperatureprofilesfrom radiosondemeasurements. Even underconditions of vigorousmiddaymixingthisapproach underestimates AOD by asmuchas 70% usingdry aerosolmeasurements andby roughly40% whenhygroscopic growthof aerosol underambientrelativehumidityis takenintoaccount.Thisdiscrepancy is attributedprimarilyto underestimation of aerosolcolumnextinction,asconfirmedby examinationof normalizedaerosol backscatter profilesobtainedfrommicropulselidar (MPL), whichshowsubstantial contributions of aerosolloadingabovetheatmospheric boundarylayer.The secondapproachusesMPL profiles of normalizedaerosolbackscatter to estimatetheverticalprofileof aerosolextinctionusing surfacevalues.The resultingAOD's are on average30% lessthanmeasuredvalues.This discrepancy is attributedto hygroscopic growthof aerosolsin the atmospheric column.The results showthatat the SGP siteevenunderconditions of vigorousmixingin theatmospheric boundary layer the aerosolopticaldepthcannotbe estimatedwith surfacemeasurements of aerosol extinctionunlessinformationon theverticalprofile of aerosolextinctionis takeninto account. 1.
Introduction
The decreasein planetaryabsorptionof shortwaveradiation due to scatteringby anthropogenicaerosolsduring clear-sky conditions,termeddirect aerosolradiativeforcing, is estimatedto be roughly1 W m-2on a globalannualaverage[Charlsonet al., 1992; Kiehl and Briegleb, 1993; IntergovernmentalPanel on ClimateChange,1995] and may be as greatas 50 W m-2locally and instantaneously near sourceregions [Schwartz, 1996]. In largepartbecauseof the patchynatureof aerosolforcingon both temporal and spatial scales,as well as the general lack of knowledge of aerosol radiative properties,the uncertaintyin estimatesof globalmeandirectaerosolradiativeforcingis at least •EnvironmentalChemistry Division, BrookhavenNational Laboratory, Upton, New York. 2 Climate Monitoring and DiagnosticsLaboratory,National Oceanic and AtmosphericAdministration,Boulder,Colorado. • Now at Georgia Institute of Technology, School of Civil and Environmental Engineering and School of Earth and Atmospheric Sciences,Atlanta, Georgia. 4 Science Systemsand Applications,NASA Goddard Space Flight Center, Greenbelt,Maryland.
a factor of two [Penner et al., 1994; IPCC, 1995; Schwartzand Andreae, 1996; Quinn et al., 1996]. The key aerosolpropertygoverningdirect shortwaveradiative forcing is aerosoloptical depth (AOD) (integral of the aerosol extinctioncoefficientwith height; throughoutthe paper the term AOD refersto the vertical optical depth,i.e., air massequal to one) which is a measureof the aerosolloading, or "extensive" aerosol property [Ogren et al., 1996]. Pertinent "intensive" aerosolpropertiesare singlescatteringalbedo(fractionof aerosol extinction that is due to scattering versus absorption), and upscatterfraction (fraction of light scatteredinto the upward hemisphere)[Charlson et al. 1992; Haywood and Shine, 1995; Boucher and Anderson, 1995; Nemesure et al., 1995; Schwartz 1996]. Estimatesof direct radiativeforcinghave generallyrelied on surfacemeasurements of aerosolradiativeproperties[Charlson et al., 1992; IPCC, 1995] under the assumptionthat aerosol propertiesoverthe entiretropospheric columnare similarto those at the surface. However, it has yet to be shownon a systematic basisthat aerosoloptical propertiesat the surfaceare suitably representative of the integratedcolumnpropertiesto justify this assumption. Several
recent
studies
have
raised
concern
over
the
determinationof AOD by Sun photometry [Kato et al., 1997; Halthore et al., 1998]. Model estimatesof the broadbanddirect normal solar irradianceduring cloud-free conditionsbased on AOD values determined from Sun photometry agree with measuredvalues, but the same AOD's lead to an overestimation
Copyright2000by theAmericanGeophysical Union. Papernumber1999JD900454 0148-0227/00/1999JD900454509.00 6807
6808
BERGIN
ET AL.:
COMPARISON
OF AEROSOL
OPTICAL
DEPTHS
AT SGP ARM
SITE
and that the single scatteringalbedo was close to unity. The estimatedandmeasuredAOD's agreedwithin roughly20%. In this paper we report aerosolmeasurements made at the Southern Great Plains (SGP) Atmospheric Radiation Measurement(ARM) site in Oklahoma, where aerosoloptical propertiesrelated to climate forcing are continuouslybeing measuredat the surface. Aerosol properties measuredat the surfaceinclude aerosollight scatteringcoefficientmeasuredat aerosol extinction. threewavelengthsand absorptioncoefficient.The measurements In principlesucha closureexperimentmightbe carriedout by are madeat a low referencerelativehumidity(-20%) in orderto comparingAOD estimatedby Sun photometrywith in-situ measurethe propertiesthat are intrinsic to the aerosol and vertical integrated aerosol extinction measurementsfrom an independent of atmospheric relativehumidity. Measurements of aircraft. However, such ideal experimentspresent challenging aerosolcolumn propertiesinclude AOD at six wavelengthsby experimentaldifficulties[Remeret al., 1997] and are generally Sun photometryand the vertical profile of normalizedaerosol by a micropulselidar (MPL). quite limited becauseof the effort and expenses associated with backscattering aircraftoperations.Althoughthe aerosolscatteringcoefficientis readily measuredby meansof an integratingnephelometer, the measuredvalue is generallynot equal to the ambientscattering 2. Theory and Approach of the diffuse downwelling irradianceby as much as 40%. A possibleexplanationfor this discrepancy is an unaccounted for continuum-likeatmosphericabsorptance over the solarspectrum [Kato et al., 1997;Halthoreet al., 1998]. Halthoreet al. [1998] suggest that AOD inferred by Sun photometry may be overestimated by -0.02. For this reasonit also it is importantto ascertainwhether the aerosol optical depth measuredby Sun photometryis identicalto that given by the columnintegralof
coefficient because the ambient RH is not identical to that in the
The first objectiveis to ascertainthe accuracyto whichaerosol opticaldepthcan be estimatedbasedon surfacemeasurements of aerosol extinction and mixing height estimates.The second, relatedobjectiveis to ascertainthe accuracyof estimatingAOD size-resolved aerosol measurements for two aircraft descents using normalized lidar backscatterprofiles to scale extinction duringclear-skyconditionsover the Atlantic Ocean.They found measurements at the surfacewith heightin orderincludethe effect that in the caseof pollutedcontinentalair the estimatedAOD was of the vertical structureof aerosolextinction. In this section,the within a few percent of AOD measuredby Sun photometry, theoreticalframeworkfor estimatingaerosolopticaldepthusing whereas for a case impacted by Sahara dust the AOD was bothapproaches is presented. underestimatedby-50%. The discrepancywas attributed to The AOD r at a givenwavelengthis equalto the integralof the horizontal spatial variability in aerosol. Remer et al. [1997] particle extinction coefficient ocp,withheight fromthesurface to comparedAOD measurementsmade at the surfacewith AOD the top of the atmosphere, toa; estimatesbasedon the vertical integrationof humiditycorrected toa extinctionmeasurements madefor severalflightsoff the eastcoast of the United States.The estimatedAOD's were typically 50% nephelometerdue to heatingof the sampleairstreamwithin the nephelometercontrolvolume [Bergin et al., 1997]. Clarke et al. [1996] estimatedthe changein aerosoloptical depth basedon
less than measured values. The difference was considered most
likely dueto aerosolsaloft,whichwerenot accounted for because the flightswere confinedto the lower 2 km of the atmosphere. Hegget al. [1997] measured verticalprofilesof aerosolextinction as well as the humidity dependenceof aerosollight scattering from an aircraftfor severalclear-skydaysoff the eastcoastof the United States.Aerosolopticaldepthsestimatedfrom the vertical profileswere within -•15% of that estimatedby Sunphotometry. In the presentstudywe takean alternativeapproachof estimating AOD from surface-based measurements of aerosolscatteringand absorption,which, althoughsubjectto the concerns raisedabove regardingrelativehumidity(RH) influence,can in principlebe much more widespread and continuous than aircraft
z= loep(RH) dz sfc
becausesurfaceaerosolmeasurements are madeat a dry reference RH (typically-20%), relating AOD to the measuredextinction coefficientrequires that equation (1) be written in terms of particleextinctioncoefficientat a dry referencerelativehumidity,
Oep(RHref), as toa
x= Ioep(RHref) Fep(RH ) dz
(2)
sfc
measurements.
Severalpreviousstudieshave attemptedto integrateaerosol surfacemeasurements with height in order to estimateaerosol propertiespertinentto radiativeforcingof climate[Quinn et al., 1996]. Hoppel et al. [1990] estimatedAOD's in the marine boundarylayerfor severaldaysbasedon surfacemeasurements of aerosol size distributions and scattering coefficients. The estimated AOD's were highly correlated with surface
where Fep(RH ) isthelightextinction hygroscopic growth factor whichis equalto Oep(RH)/oep(RHref). Equation 2 canalsobe writtenin termsof the aerosolscattering, Osp(RHref), and absorption coefficients, Oap(RHref), at a dryreference RH with the hygroscopic growthfactorsfor light scattering andabsorption
of Fsp(RH ) andFap(RH ) as
measurements but were a factor of 2 to 4 lower than the direct
measurements. The discrepancywas attributedto the lack of knowledgeof the verticalprofilesof aerosolextinction.VeeJkind et al. [1996] estimatedAOD's for three clear-skydays in the Netherlandsbasedon surfacemeasurements of aerosolscattering coefficientand its dependence on RH, togetherwith estimatesof mixing height based on lidar measurements,under the assumptions that the aerosolwas entirelywithin the mixedlayer
toa
z= Iosp(RHref) Fsp(RH ) dz st2: toa
+ Ioap(RHref) Fap(RH ) dz sfc
(3)
BERGIN ET AL.: COMPARISON OF AEROSOL OPTICAL DEPTHS AT SGP ARM SITE
6809
The first approach to estimating aerosolopticaldepthis basedon 10m-highstackat 800 1min-'.A portionof the flow (150 1min-') A portion of the the assumption that the aerosolis entirelywithin a well-mixed is heated to maintain an RH of-40%. boundarylayer of heightHML,whereHMLis determinedas the conditionedairstream(30 1min-')thenpassesthroughan impactor transitionbetweenan unstableto stable atmosphericlapse rate. with a 10-[tm particlediametercut size. The airstreamis then The aerosol extinction coefficient at the reference RH is sampledby an integratingnephelometer (TSI model3653) anda determinedfrom measurements of the aerosol scatteringand ParticleSootAbsorptionPhotometer, PSAP (RadianceResearch). heatsthe air by an additionalseveraldegrees absorption coefficients at the surface.Under theseassumptionsThe nephelometer (5.0 "C + 0.5 "C basedon 1-min nephelometer data at the SGP theaerosol opticaldepthusingapproach I, 'cI, canbewrittenas site) resultingin a further decreasein RH. The nephelometer
,cI = [•sp(Sfc) Fsp(RH) + •ap(Sfc)Fap(RH)]HML
(4)
measures theaerosol scattering coefficient, C•sp, atwavelengths of 450, 550, and 700 nm. Noise levels for scatteringcoefficient measurements for the SGP nephelometeras determinedfrom the standarddeviations of mean values for filtered air are 0.33, 0.19,
The second approachinfers the vertical profile of aerosol extinctionfrom the magnitudeof the attenuation-andrangecorrectedprofile of aerosolbackscatter return,normalizedto the surfacevalue,asdeterminedby micropulselidar (MPL), scaledby
and 0.46 Mm-' for wavelengthsof 450, 550, and 750 nm, respectively. Scattering coefficient measurements are not corrected for nephelometernonidealities as described by Andersonand Ogren [1998] sincethe particlesarepredominantly the aerosol extinction coefficient measured at the surface. The in the submicronmode at the SGP site [Sheridanet al., 1998], aerosolopticaldepthusingapproach II, 'Cli, is estimated as where such correctionsare typically less than 10%. Scattering follows:
coefficient measurements are usedto determine the Angstr6m exponent,g (negative slope of the scatteringcoefficientversus
'Cli= [C•sp(Sfc ) rsp(RH) toa
,
(5)
+C•ap(Sfc ) Fap(RH)] l[3rc(z)/[3rc(sfc) dz sfc
wavelength curvewhenplottedona log-logscale),whichsupplies qualitative information on particlesize.The PSAPmeasures the
aerosol absorption coefficient, C•ap, at a wavelength of 565nm.
Both C•ap andC•sp (measured at550nm)areused toestimate the single scatteringalbedo co o (ratio of aerosolscatteringto
where[3rc(z) is the attenuation-and range-corrected 180øMPL extinction).The aerosolobservation system(AOS) is similarto returnsignalas a functionof height.Implicitin equation (5) is that at aerosolmonitoringstationsmaintainedby the National
thattheratioof aerosol extinction tobackscattering, C•ep/[3rc, is Oceanic and AtmosphericAdministration(NOAA) Climate constantwith height. In addition,MPL profilesof aerosol Monitoringand Diagnostics Laboratory (CMDL) [Ogrenet al., backscatter can be used to estimate the fraction of aerosol
1996, Bergin et al., 1997].
backscatter due to aerosolwithin the boundarylayer: 3.2. Aerosol Optical Depth
HML
l[3rc (z)dz FH= toasfc
.
(6)
l[3rc (z)dz
Atmospheric extinctionis determined at wavelengths of 414, 499, 608, 662, 859, and 938 nm from measurements of diffuse andtotaldownwe!ling solarirradiance usinga MultifilterRotating ShadowbandRadiometer (MFRSR) [Harrison et al., 1994' Michalskyet al., 1995]. At each wavelengththe direct beam
sfc
signalis evaluatedas total minusdiffusesignaland the direct normalsignalis determined by multiplying thedirectbeamsignal In thispaperwe usesurface measurements of aerosolscattering by the secantof the solarzenithangle.Extinctionopticaldepths and absorption coefficients madeat a low referencerelative are thendeterminedby Langleyanalyses[Harrisonet al., 1994; humidity(-20%) togetherwith mixingheightsderivedfrom Michalskyet al., 1995]. Apparentaerosoloptical depth is sondemeasurements to estimateAOD's basedon equation(4) for obtainedby subtracting extinctiondueto Rayleighscattering and severalcloud-freedaysat the SGP ARM site.The comparisons known gaseousabsorptionfrom measuredextinctionoptical are made at 1730 UTC (--, 1130 LT) sincethis time period denth corresponds roughlyto thedailymaximum in surfaceshortwave Theprecision of theAOD estimate is + 0.02 [Halthoreet al., irradiance. The time also coincides with the launching of
radiosondes whichsupplyverticalprofilesof temperature andRH. We also use normalized attenuation-andrange-corrected180ø MPL returns to scale surface measurements of the aerosol
extinction with heightto estimate aerosolopticaldepthbasedon
1997].As previously mentioned, it hasbeensuggested thatthe AOD is overestimatedby-0.02, due to the absorptionof shortwaveradiationby an unaccounted for species[Halthoreet al., 1998].The AOD's reportedin thispaperarethe "apparent" AOD's that may include a contributionof-0.02 from the
equation (5). Theestimated aerosol opticaldepths arecompared unaccounted forabsorbing species. TheAngstr6m exponent of the
with AOD's measuredby Sunphotometry.
3. Experimental Methods
apparentAOD is evaluatedas the negativeslopeof the AOD versuswavelengthcurvewhenplottedon a log-logscalewhichis
estimated using450-and700-nmwavelengths (i.e., Angstr6m exponent= - log (r70,,/r45,,) I log(700/450)).
3.1. Measurementof AerosolScatteringand Absorptionat the Surface
Severalaerosolproperties are measured continuously at the surfaceat the SGP ARM site. Ambientair is sampledthrougha
3.3. Vertical
Distribution
of Normalized
Aerosol Backscatter
A micropulse lidar(MPL) is usedto determine thenormalized attenuation-and range-corrected 180ø backscatterprofile with
6810
BERGIN ET AL.: COMPARISON
I
5000 4500 4000
-
3500
-
3000
-
2500
--
2000
--
1500
--
1000
--
.....
4)5:30:00
UTC
-11:30:00
UTC
.....
14:30:00
UTC
....
17:30:00
UTC
-20:30:00
I
4. Results
-
UTC
_
-'
nephelometer, RHneph; surface measurements of the aerosol scattering coefficient at 550nm,C•sp,dry; andsinglescattering
__
coefficient(estimatedusing450-and700-nmwavelengths), the AOD at 550 nm measured by the MFRSR, XMFRSR,and
50
450-and700-nmwavelengths) overthe entirecolumn,fiMFRSR. Missing data in Table 1 representperiodswhen measurements
--
;
and Discussion
In order to maximize the likelihood that the atmospheric boundarylayerwaswell mixed,thereforemeetingthe assumption of estimating AOD using method I, we have restricted comparisons to measurements nearmidday(1730 UTC) on cloudfree days.The datapertinentto the analysesare givenin Table 1, which includesboundarylayer mixing heights(estimatedfrom sondepotentialtemperatureprofiles),HML;the meanand standard deviation of relative humidity in the lower 4 km of the atmosphere,RHML; the RH in the sample volume of the
'_
'
albedo, tOo,dry; Angstr6m exponent of the lightscattering
500 l
]•ngstr6m exponent (estimated usingAOD valuesadjusted to
0
10
OF AEROSOL OPTICAL DEPTHS AT SGP ARM SITE
15
20
25
30
35
I
I
40
45
werenot beingmadeor datawere invalid.
Potential Temperature (C) 4.1. Relationship Between Aerosol Extinction Coefficient at Figure 1. Potentialtemperatureprofilesversusheightestimated the Surface and Aerosol Optical Depth from balloon sonde measurementsof temperaturefor several timesduringthe day of September4, 1996 The relationshipbetween apparentaerosol optical depths inferredfromMFRSR measurements (logarithmically interpolated to 550 nm from measurements at 499 and 608 nm), 'rMFRSR, heightat a wavelengthof 523 nm [Spinhirne,1993]. The vertical versusthe aerosolextinctioncoefficientmeasuredat a dry RH, samplingresolutionis 300 m, with a laser pulse frequencyof Oep,dry, isshown in Figure 2. Thecorrelation ofthedataisweak r•= 0.55), with a slopeof 2982 m (with a 95% 2500Hz. Retrievals of aerosol backscatter withheight,fix(z),are (linearregression averagedover 1-mintime intervals.The MPL is calibratedduring confidenceintervalsof + 593 m), which is a roughindicatorof a 60-90 min cloud-freeperiodwhen aerosolloadingsare low (i.e., the aerosolmixing heightfor the timesover which the datawas AOD lessthan 0.1). The retrievalsare calibratedby makingthe collected.The slopesof the linearregressions at wavelengths of assumption that the backscatter at 523 nm for layersin the upper 450 and 700 nm are not statisticallydifferent (at a 95% troposphere (-7-13 km) is entirelydue to molecularscattering. confidenceinterval) from the 550 nm value. The low r• value The attenuation in the signaldueto lowertropospheric aerosolis couldbe due to severalpossiblereasonsincludingday-to-day estimatedfrom the MPL signalprofile underthe assumption of an variabilityin the mixing heightsat the SGP ARM site, lack of aerosolextinctionto 180øbackscatter ratioof 23 [Spinhirneet al., mixing of surfaceair with air in and/orabovethe boundarylayer 1980].The integrated MPL signalin the uppertroposphere is then (i.e., aerosollayersexistaloft that are responsiblefor a significant fit to the integratedmolecular signal in order to obtain a fraction of aerosol extinction), and/or differencesin aerosol calibrationvalue.The calibrationvaluerepresents the adjustment physicalproperties(size distribution)and chemicalcomposition factor appliedto the MPL signalfor a known scatteringcross with height.It is alsopossiblethat the discrepancy arisesfrom the section(due to Rayleighscattering).Oncethe calibrationvalueis inabilityof the aerosolsamplingsystemto transportcoarsemode obtained it is applied to the backscattersignal to get the aerosolsto the aerosolinstrumentation. Although,Sheridanet al. backscatter crosssection(m-' st-'). [1998] reporta meanratio of submicron(diameter< 1.0 um) to total (diameter< 10.0 um) light scatteringcoefficientof 0.85 (standarddeviation= 0.08), suggesting that coarsemodeaerosol 3.4. Atmospheric State Parameters doesnot, in general,significantlyinfluencethe light scattering Vertical profiles of temperature, pressure, and relative budgetat the SGP site.The lack of a strongrelationshipbetween humidityare regularlymeasuredat the SGP ARM site by means the extinction coefficient measured at the surface and the AOD is of balloon-borneradiosondes.Here we use potentialtemperature not surprisingbecauseof the dependenceof AOD on the vertical profiles to estimatemixing heights at 1730 UTC (-1130 local distributionandpropertiesof aerosols. standardtime) for cloud-free days as the transitionfrom an Figure3 comparesmeasuredvaluesof 'rMFRSR with AOD's unstable to stable atmosphere. Cloud-free conditions are estimated by equation (4) ('ri) usingsurface measurements made determinedby first removingdayshaving significantbackscatter at a dry referencerelativehumidity(i.e., F,,,(RH) = F.,.,(RH)= 1). above 5 km as determinedby MPL profiles and then removing Thereis a somewhat stronger relationbetween •rI and'rMFRS R days having RH values correspondingto liquid water or ice (linear regression r'-= 0.78)thanfor thatbetween C•ep,dry and supersaturation.Figure 1 shows several potential temperature 'rMFRSR.This increasein correlationis attributedto the fact that profiles over the courseof a single day (September4, 1996). 'rI accounts forday-to-day variability in mixingheights. Estimated During the morningthe atmosphere is stablenearthe surface.As AOD's aresystematically lowerthan'rMFRSR. This is attributed the surfaceheats,an unstablelayer developedwhich extendsto in part to the fact that the RH valueswithin the nephelometer are 1200m + 200mat 1730UTC, and 1400m + 200mat2030 in generallessthan in the atmospheric boundarylayer (Table 1). UTC. Thereforeit is necessary to accountfor hygroscopic growthwhen
BERGIN ET AL.' COMPARISON OF AEROSOL OPTICAL DEPTHS AT SGP ARM SITE
i
6811
6812
BERGIN
ET AL.:
COMPARISON
OF AEROSOL
-
0.40
-
0.35
--
_ r2= 0.55 slope= 2982 m +/- 593 m
0.30--
ß
AT SGP ARM
SITE
ß
0.25
.--
4.2. Estimating AOD With Surface Extinction Coefficient Measurements and Micropulse Lidar Profiles of Aerosol Backscatter
0.20
0.15 ß ••/e ß 0.10• •,• ß 0'05 I•'
DEPTHS
atmosphericspecies[Halthore et al., 1997] resultsin 'cI values that are lower than 'CMFRS R by- 40%, thus insignificantly 'contributingto the discrepancybetweenoptical depths. These resultsbring into questionthe assumptionof constantaerosol extinctionwith heightwithin the mixed layer, andzero extinction abovethe mixed layer, that is implicit in the evaluationof 'cI by equation(4).
0.50 0.45
OPTICAL
'
_
To examine
the vertical
distribution
of aerosol we show in
Figure 4 the attenuation-andrange-correctedvertical profiles of aerosol 180 ø backscatter normalized
to backscatter in the lowest
MPL heightrange(surface to 270m), 13rc/13rc(sfc ). Alsoshownare
the cumulative aerosol backscatterwith height, normalized to unity, and the mixing heights as determinedfrom radiosonde temperatureprofiles, HML.The measurements are for cloud-free 0 20 40 60 80 100 120 140 daysat 1730 UTC (-1130 local standardtime), when, as noted above,conditionsarelikely to favora well mixedboundarylayer. As is evident in Figure 4, there is considerablevariationof Figure 2. AerosolOptical Depth at 550 nm estimatedfrom aerosolbackscatterwithin the mixed layer as well as substantial MFRSR measurements, 'CMFRS R, versusthe aerosolextinction contributionof aerosolbackscatteringfrom altitudesabove the boundary layer (i.e., above HM0. The fraction of aerosol coefficient, Yep(estimated asthesumofYapandVsp) measured at a dry referenceRH (-20%) backscatter withintheboundary layer,FH, asdetermined by the o.oo
I
I
I
I
I
I
O;ep,dry (Mm 4)
intersections of the H•L and normalized cumulative backscatter
curvesand estimatedfrom equation(6), rangesfrom 19% to 72% (mean = 44%, standarddeviation = 16%). The substantial using nephelometer measurementsin the estimation of contributions of aerosolabovethe boundarylayer,evenunderthe atmospheric properties. Becausesuchinformationis not available conditionsof vigorousmixing, suggeststhat it is insufficientto for the time periodof the presentmeasurements it is necessary to omit the contributionof aerosolfrom abovethe mixed layer to rely on measurements from other studies.Rood et al. [1987] AOD, at leastunderthe conditionsof the measurements reported measured Fsp(RH ) relative toa reference RH< 35%overseveralhere. That is, aerosoloptical depth must explicitly employthe daysin Los Angeles.The nephelometer measurements at the SGP verticalintegrationof aerosolextinction(methodII, equation(5)), site are made at a reference RH < 35%, similar to the reference
RHused byRoodetal. [1987].Roodetal. [1987]report Fsp(RH ) valuesrangingfrom roughly1.0 to 1.3 at an ambientRH of 30% and 1.3 to 1.7 at an ambient RH of 60%, and point out that
0.5
Fsp(RH ) depends onthethermodynamic state oftheaerosol (i.e. dry, subsaturated, or metastable)as well as on additionalfactors including the aerosol dry size distribution and chemical composition[Mclnneset al., 1998; Hegg et al., 1993] both of which likely vary between locations.Several previousstudies [Covert et al., 1980; Waggoneret al., 1983; Charlsonet al., 1984;Dougleet al., 1998;Mclnneset al., 1998]reportvaluesof
0.4
0.3
Fsp(RH=60% ) forcontinental aerosols thatfallwithintherange of valuesreportedby Roodet al. [1987]. In addition,preliminary datafrom continuousmeasurements madeby a dual-nephelometer humidographsystemduringJanuaryand Februaryof 1999 at the
0.2
SGPsiteshowthatFsp(RH=60% ) at550nmranges from1.1to 1.5 (P. Sheridan,personalcommunication,1998). Therefore,for
theatmospheric RHvalues reported in Table1 anFsp(RH ) of 1.7 likely representsan upper limit. Figure 3 also shows lines corresponding to 'CMFRS R / 'cI = 1.0, and 'CMFRS R / 'cI = 1.7. On the basis of previousmeasurements the envelopeof the two curvesshouldencompass the datapointsif hygroscopic growthis the solereasonfor the lack of agreementbetween'CMFRS R and
'cI.Evenwithan assumed upperlimitFep(RH ) valueof 1.7 (assuming equivalent values forbothFsp(RH ) andFap(RH)), 'cI
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
•MFRSR
values are still systematicallylower than 'CMFRS R by-50% Figure 3. AerosolOptical Depth at 550 nm estimatedfrom (standard deviation of 28%). Assuming that 'CMFRS R is extinction coeffecient, •ep, measured at thesurface at a dry overestimatedby 0.02 due to absorptionby an unknown referenceRH (-20%), 'ci, versus'CMFRSR
BERGIN ET AL.: COMPARISON
61.
ii
I
I
I
I
•
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OF AEROSOL OPTICAL
I
DEPTHS AT SGP ARM SITE
6813
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ß ß
I ''
eeeeeeeeeeeeeee•eß ee e
o
0.0
0.2
0.4
0.6
0.8
1.0
1.2
I•, / I•,(sfc),Norm.cun•
1.4
1.6
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
I•,•/ I•,(sfc),Norm.Cum.
Figure4. Attenuation-and range-corrected MPL1800backscatter at523nmnormalized tothebackscatter in the
lowest MPLrange (137c/137c(sfc)) (dashed line)andnormalized cumulative MPLbackscatter withheight (solid line)
6814
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ET AL.'
COMPARISON
OF AEROSOL
OPTICAL
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AT SGP ARM SITE
wavelengthsduringdifferenttime periods.Nonetheless,the work of Ferrare et al. [1998a, b] clearly show the importanceof
directlymeasuring theverticalprofileof Sa at thewavelength of 0.25
0.20
0.15-
......
.
interestin order to increasethe accuracyof retrievalsof aerosol backscatter profiles from lidar measurements.The overall uncertaintyin estimatingzII is greaterthan the 0.02 suggested overestimation of ZMFRSRdueto the atmospheric absorptance by an unknown species discussedby Halthore et al. [1998]. Thereforewe cannotuse our studyto reachany conclusions as to the existenceof an additionalabsorptance by an unknownspecies duringclear-skyconditions.
- -
.
Information
0.10
- -
on the differences
in aerosol
size distributions
betweenthenephelometer andthe atmosphere maybe obtainedby
comparing the Angstr6mexponents for the light scattering coefficientof the dry aerosolas measuredby the nephelometer,
o.ooF-' I 0.00
0.05
I
I
I
I
0.10
0.15
0.20
0.25
0.30
shownby the errorbarsin Figure6, is due to the 0.02 uncertainty
Figure 5. Aerosol optical depth at 550 nm estimatedfrom measurements
at
the
surface
less than•ne-'h dry(mean value of30%lower witha standard
deviation of2•). The uncertainty associated with fiMFRSR, as
'•MFRSR extinction
fine•'h that for the extinctionof the atmospheric 1• , dr', .Y_ versus aerosolcolumnas measured by the MFRSR,fiMFRSR-Higher valuesof the Angstr6mexponentgenerallyrepresent smaller particlesizes.As shownin Figure6, fiMFRSR is considerably
scaled
with
MPL
backscatter profiles,:II, versusthe AOD measured by the MFRSR, :MFRSR
in theestimation of ZMFRSR(notefor comparisons of fiMFRSR and fineph,dry we do not includefiMFRSR valueswith uncertainties greater thantheabsolute valueof fiMFRSR) ßThe theoretical relationship betweenAngstr6mexponent, •, and particlesizeis examined in Figure7. TheAngstr6m exponent is
estimated(using wavelengthsof 700 and 450 nm) for several lognormalsize distributionsof specifiedmassmedian diameter (MMD) and geometricstandarddeviation(GSD), for spherical AOD's estimated fromequation (5) (Xli) usingsurface-basedaerosolsof refractive index 1.53 using Mie scatteringtheory scatteringand absorptioncoefficientmeasurements madeat a dry calculationsas describedby Bergin et al. [1997]. For unimodal lognormal distributions, fi is rather sensitive to MMD and RH (-20%) and MPL backscatterprofiles for which both MPL rather than assuminga uniform profile of extinctionwith height withinthemixedlayer(methodI, equation(4)).
insensitive toGSD.Themean values offineph dr and andnephelometer measurements areavailableare shownin Figure relatively 5 for severalcloud-freedays at 1730 UTC. Also shownare lines dMFRSRas givenin Table 1 are 1.7, and 1.3, respectively. for XMFRSR/ XlI = 1.0 and XMFRSR / xII = 1.7; asnotedabove these represent the envelope of values ranging from no hygroscopicgrowth in the atmosphereto an upper limit 3.01:1:1:1:1:1:1:1: hygroscopicgrowth factor of 1.7, as previouslydiscussed.The ratiosof XMFRSR / XlI rangefrom 0.95 to 1.6 (meanof 1.27, 2.5 standarddeviationof 0.22). The valuesof XlI are thuswithin the range of values expectedwhen hygroscopicgrowth of aerosol 2.0 under atmosphericconditionsis taken into account,suggesting thatthereis no systematic discrepancy betweenXMFRSR andXlI when hygroscopicgrowthis considered.It is also importantto
pointout thatthereis additional uncertainty in Xli dueto the assumption of a constant extinction/backscatter ratio,Sa,usedin the MPL retrievals of 13x(z). Ferarreet al. [1998a,b] present profiles of theaerosol extinction/backscatter ratio,Sa,determined
1.5
1.0
0.5
from scanning raman lidar (SRL) measurementsfor several eveningsduringApril 1994. Ferrare et al. [1998a, b] showthat the majorityor the aerosolbackscatter duringthe night is below 2
0.0
km and that within this region Sa varies with height.
-0.5
Unfortunately, there is a significant (•+/- 30%) amount of
uncertainty in Sa profiles makingit difficulttojudgeif changes in -1.0 $a with height representactual atmospheric variabilityor -1.0-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 measurement uncertainty. A 30% uncertainty in the Sa usedin MPL retrievals of 13x(z) results in anuncertainty of-10% in the aneph,dry ZlI estimates shown in Figure5. Although, if Sa isdoubled values of zII increase onaverage by40%,whichwouldexplaintheentire Figure6. Nephelometer •ngstr6m exponent, fine*'h . P ,.dry' versus discrepancy between ZlI andXMFRS R. It is difficultto directly MFRSR/•ngstr6m exponent, fiMFRSR (estimated using 450nm o
relate SRL and MPL measurementssince they are at different
and700 nm wavelengths)
BERGIN
ET AL.:
COMPARISON
OF AEROSOL
4.0 3.5 3.0
GSD=I.4 .....
GSD=I.6
___
GSD=I.8
---
GSD=2.0
2.5
2.0
1.0
OPTICAL
DEPTHS
AT SGP ARM SITE
6815
accountday-to-dayvariationin mixing height. Still that approach underestimates AOD by-50% (standarddeviationof 28%) even when hygroscopicgrowth is taken into account. The AOD's estimated using MPL backscatterprofiles scaled to surface measurements of aerosolextinctionat a dry RH (-20%) are lower than measuredAOD's by-30% but in agreementwithin the range of representativehygroscopicgrowth factorsbetweenthe RH in the nephelometerand the atmosphere.The necessityof including the relative humidity growth factor to accountfor drying of the aerosolsin the samplingline and nephelometeris supportedby
theJtngstr6m exponents whichare-30% lowerfor extinction in
the total column as measured by MFRSR, than those 0.5 characterizinglight scatteringmeasuredat the surfacemeasured by a nephelometer.This provides strong evidencethat aerosol 0.0 particlesin the nephelometerare generally smaller than in the -0.5 atmosphere, consistent with the hypothesis that the underestimation of AOD by dry scattering coefficient -1.0 measurements made at the surfaceis linked, in part, with the 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 drying of aerosolswithin the nephelometer.The underestimation of aerosoloptical depthby methodI is due mainly to failure of Mass Median Diamter (Ixm) the assumptionthat the aerosol is present in a well mixed Figure 7. Theoretical estimation of Jtngstr6m exponentboundarylayer without contributionsto aerosolextinctionfrom (estimated using 450nmand700nmwavelengths), fitheory, for above. As shownin Figure4, micropulselidar (MPL) profilesof lognormalsize distributionsof specifiedmassmediandiameter normalized attenuationand range-correctedaerosol backscatter indicate that only-50% of the aerosol extinction is due to andgeometricstandard deviation,andrefractiveindexof 1.53 boundarylayer aerosol.Additional sourcesof error include the unknownhygroscopicgrowth coefficientof the aerosolas well as variationin this coefficient,and in relativehumidity,from day-toAccordingto Figure 7, this corresponds to an increasein MMD day, and with height. by a factorof 1.1 to 1.3. This rangeoverlapswith the reported The resultsof the presentcomparisonssuggestthat surface diametergrowthfactorrangeof 1.1 to 1.5 reportedfor polluted of the aerosolscatteringand absorptioncoefficient continentalaerosol[Svenningsson et al., 1992]. It is importantto measurements can be used along with informationof the vertical profile of point out that changesin fi as a functionof RH dependon the aerosol light scattering (from the MPL) and assumptions aboutthe hygroscopic natureof the aerosol(i.e., chemicalcomposition) and aerosol hygroscopicity to estimate the AOD to within roughly drysizedistribution of theaeros'ol, whichwerenotmeasured at 30% of measuredvalues at the SGP site. In contrast,mixing the SGP site. Therefore the above discussion must be viewed as qualitative.
heightsestimatedfrom temperature profilesdo not yield accurate estimates of AOD becauseof significantcontributionof aerosol extinctionabovethe boundarylayerto aerosolextinctionat least
5. Summary and Conclusions
possibleto usesurfacemeasurements to estimateaerosoloptical depthwithoutadditionalinformationon the verticalprofile of
for this location.Overall, resultsshowthat at the SGP site it is not
Two methodsof estimating the aerosolopticaldepthhavebeen aerosol extinction. examinedusingdataobtainedat the SGPARM sitefor cloud-free days near the daily solar maximum(---1730UTC, 1130 local Acknowledgments. MHB was supportedin part by the Global standardtime) at which time the boundarylayer is likely to be ChangeDistinguished Postdoctoral Fellowships programsponsored by the well mixed. First, aerosol extinction measurementsmade at the
surface(takenasthe sumof the aerosolscatteringand absorption coefficientsat instrumentalrelative humidity of--- 20%) are multipliedby the mixing height determinedfrom temperature
U.S. Departmentof Energy(DOE), Office of Healthand Environmental Research,and administeredby the Oak Ridge Institutefor Scienceand Education.Work at BrookhavenNational Laboratorywas supportedby the EnvironmentalSciencesDivision of the U.S. DOE, as part of the Atmospheric RadiationMeasurement Programandwasperformedunder
profiles fromradiosonde measurements 0:I). Second, micropulsethe auspicesof DOE contractDE-AC02-98CHI0886. lidar (MPL) attenuation-and range-corrected backscatter profiles are used to scale surface measurements of aerosol extinction with
height 0:II). The estimatedAOD's are comparedwith measurements obtainedby Sunphotometry0:MFRSR).
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