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JOURNAL

OF GEOPHYSICAL

RESEARCH,

VOL. 103, NO. D4, PAGES 32,041-32,050, DECEMBER

27, 1998

Effects of black carbon content, particle size, and mixing on light absorption by aerosols from biomass burning in Brazil J. Vanderlei Martins,•,2,3Paulo Artaxo,• Catherine Liousse,4 Jeffrey S. Reid,2 Peter V. Hobbs, 2 and Yoram J. Kaufman 3 Abstract. Black carbonmassabsorptionefficienciesof smokeparticleswere measured for varioustypesof biomassfires during the Smoke, Clouds,and Radiation-Brazil (SCAR-B) experimentusingthermal evolutionmeasurementsfor black carbonand optical

absorption methods. Theobtained results rangebetween 5.2and19.3m2 g-1 withan average valueof 12.1+ 4.0m2 g-1. Particlesizedistributions andopticalproperties were alsomeasuredto provide a full set of physicalparametersfor modelingcalculations.Mie theorywas used to model the optical propertiesof the particlesassumingboth internal and externalmixturescouplingthe modelingcalculationswith the experimentalresults obtainedduring the campaign.For internal mixing,a particle model with a layered structureconsistingof an absorbingblack carboncore, surroundedby a nonabsorbing shell,was assumed.Also, for internal mixing, a discretedipole approximationcode was used to simulatepacked soot clusterscommonlyfound in electron microscopy photographsof filters collectedduring the experiment.The modeledresultsfor layered spheresand packedclustersexplainblack carbonmassabsorptioncoefficientsup to values

of about25 m2 g-l, butmeasurements showevenhighervalueswhichwerecorrelated with the chemicalcompositionand characteristicsof the structureof the particles. Unrealistichigh valuesof black carbon absorptionefficiencieswere linked to high concentrations of K, which influencethe volatilizationof black carbon (BC) at lower temperaturesthan usual,possiblycausingartifactsin the determinationof BC by thermal technique.The modelingresultsare comparedwith nephelometerand light absorption measurements.

the sameasfor pure BC particles.However,for smokeaerosol, it is likely that BC particlesthat form at relatively high temLight absorptionby aerosolparticleshas a heatingeffect in peratureswill be coatedby a nonabsorbing shell (to form an the atmosphere,which contrastswith the cooling effect by internal mixture). The external shell is likely formed by gasnonabsorptive particles.The balancebetweenthe coolingand to-particle conversion and condensationof volatile comthe heating effectsdependson the absorptionand scattering pounds.Thus a reasonablemodel for biomassburningpartipropertiesof the particles.Smokeparticlesproducedby bio- cles is a layered sphere, with a highly absorbingBC core massburninghave a significantfraction of light-absorbingma- surroundedby a nonabsorbingshell. terial composedby blackcarbonparticles.Black carbonis the Ackerman and Toon [1981] explored the optical properties only importantabsorberto be taken into accountin radiative of internal and external mixtures using the layered-sphere transfer calculations of smoke aerosols. The same amount of model and volume-homogeneousmixtures,where the absorbBC for different typesof mixingmay result in fairly different ing material is homogeneouslymixedwith nonabsorbingcomabsorptionproperties. The efficiencywith which a certain pounds.Another type of internalmixturecommonlyfound in amount of BC will absorb light is expressedby a BC mass smoke aerosols are long-chain aggregatesof BC particles absorption efficiency (OZaBC).This coefficient relates the formed at high temperaturesclose to the fire. These chain amount of BC in the particle with the light absorptioncross aggregatescan alsobe coatedwith nonabsorbingmaterialsto sectionand dependson the sizeof particlesand on the type of form an internallymixedheterogeneousstructure.After aging mixing between BC and nonabsorbingcomponents(such as and interactionswith water vapor and clouds,these opened organicmatter and sulfates).In externalmixturesconsistingof clustersusuallycollapseto form closelypackedspherical-like individualpure BC particlesin parallelwith nonabsorbing parstructures[Hallet et al., 1989]. These packed structuresare ticles,the BC massabsorptionefficiencyis, in general, about likely to be externallycoated and alsohave nonabsorbingmaterial in its internal structure.Figure 1 showsexamplesof some •Institutode Ffsica,Universidadede S•o Paulo, S•o Paulo, Brazil. possible internal and externalmixturesof BC particlesand 2Department of Atmospheric Sciences, Universityof Washington, Seattle. nonabsorbingmaterialsin smokefrom biomassfires. 3NASAGoddardSpaceFlightCenter,Greenbelt,Maryland. In the presentstudy,we useMie theoryto estimatethe mass 4Centre de Faibles Radioactivites,CNRS-CEA, Gif sur Yvette, absorptionefficiencyof BC particlesinternallyand externally France. mixed with nonabsorbingmaterials.Calculationsfor particles Copyright1998 by the American GeophysicalUnion. with a layered structurewere performed using a Mie code developedby/lckermanand Toon[1981],asfurther developed Paper number 98JD02593. 0148-0227/98/98 JD-02593509.00 and made availableby W. Wiscombe.CloselypackedBC clus1.

Introduction

32,041

32,042

MARTINS

Internal mixing with layered structure:

ET AL.: LIGHT

ABSORPTION

BY BC PARTICLES

tain amount of BC absorbslight for different typesof particle mixing and sizes.Figure 2 showsMie calculationsof the BC massabsorptionefficiencyfor pure BC spheres(at ,k = 0.55 tzm) as a functionof the particleradius.The BC massabsorp-

External mixing: Black carbon

shell )•//U • Non-absorbing (organics, sulfates, etc.) O OO

tionefficiency forpureBCparticles (9 = 1.85g cm-3) remains below10m2g- ], regardless of particlesizeandfor threevalues

Black carbon core

Non-absorbing particles

Internal mixing in soot aggregates

of the complexrefractive index coveringthe range commonly foundin the literature [Horvath,1993a].Calculationsof the BC mass absorption efficiencies over a realistic accumulation mode biomassburning particle size distribution[e.g., Reid et

al., thisissue(b)] produces valuesmuchlowerthan10m2g-•

Internally mixed particlescomposedof an absorbingcore surroundedby a nonabsorbingshellhave,in general,a greaterBC massabsorptionefficiencythan pure BC particles.This is becausethe nonabsorbingshellincreasesthe total cross-sectional Open sootcluster Closedsootcluster area of the particle and focuseslight toward the absorbing Figure 1. Examplesof somepossiblemixturesbetweenblack core, causingthe sameamount of BC in the mixed structureto carbon(BC) and nonabsorbingmaterialsin smokeparticles. absorbmore than pure BC particles. Figure 3 showscalculatedBC massabsorptionefficiencyfor particleswith a BC core surroundedby a nonabsorbingshell. ters were simulated by a three-dimensional mathematical Along the text, shell radius is defined as equal to the particle structurewith thousandsof dipoles, the optical propertiesof radius.It canbe seenthat for particleswith a layeredstructure whichwere calculatedusingthe discretedipole approximation the value of aaBc depends on the fraction of BC in each [Draine and Flatau, 1994;Purcelland Pennypacker,1973]. Di- particle and on the size of the particles,but it can be much larger than that for pure BC particles.For the studiedparampole calculationswere performed using the code DDSCAT, made availableby B. Draine and P. Flatau. Resultsfrom closed eters, the larger the particle radius the larger the effect of the clusters are compared with those from the layered-sphere nonabsorbingcoatingin aaBo up to a maximum aaBc value model and with the Maxwell-Garnett averagefield theory for for a particle radius of about 0.25 /zm. For a BC fraction of homogeneousinternally mixed materials [Bohrenand Huff- 0.5%, and monodispersedparticleswith an externalradiusof man, 1983].The modelingresultsare comparedwith measure- 0.25 /zm, aaBc can reachvaluesas highas 30 m2 g-•. The ments of BC mass absorptionefficiencyobtained during the different behavior as a function of the shell radius can be explainedby the relative size of the BC core, which is deterSCAR-B field project in Brazil. minedby the fractionBC/TPM (blackcarbondividedby total particle mass),the externalparticle radius,and the densityof 2. Theoretical Modeling each component.The shell radius is also important in deterThe massabsorptionefficiencyof an aerosolparticle(aa) is mining the value of the mass absorption coefficient. For a definedas the absorption coefficient of the particles(m-•) particle radius about 0.05 /zm, the BC mass absorotion effi-

dividedby theparticlemassconcentration (g m-3). Similarly, the BC massabsorptionefficiency(aaBc) is defined as the

absorption coefficient of the particles(m-l) dividedby the massconcentration ofblackcarbon(gm--•)in theaerosol. The BC massabsorptionefficiencyindicateshow efficientlyacer-

Percentage

25

of BC .....

10

•15 10

0.5%



1%

........

2_%

--

5%



20% 75%

Refractive indices: 1.5

-0.5i

1.5

-1.0•

2.0

-10i

0

0.1

0.2

0.3

0.4

05

Shellradius (•m)

0

0.1

0.2

0.3

0.4

0.5

radius (um)

Figure 3. Calculatedvaluesof black carbonmassabsorption efficiency(c•a]•c) at ,k = 0.55/•m for an internalmixtureof BC and nonabsorbingmaterial in a layeredstructure.The particle structureconsistsof an absorbingBC core with a surrounding nonabsorbingshell. The refractive index of the BC was as-

Figure 2. Calculatedvaluesof black carbonmassabsorption sumedto be 2.0-1.0i, the density1.85g cm-3;for the nonefficiency(aaBc) for pure BC sphericalparticles(density absorbingshell, the refractive index was assumedto be 1.5-

1.85 g cm-3) at a wavelength of 0.55 /•m. Three complex 10-6i andthe density1.5g cm-3. The amountof BC andthe refractiveindices(n), spanningvaluescommonlyfoundin the literature,were usedin thesecalculations(n = 1.5-0.5i, n 1.5-1.0i, and n = 2.0-1.0i).

radius

of the shell determine

the radius

of the BC core. The

radiusof the shellis definedin thiswork as equalto the radius of the particle.

MARTINS

30

ET AL.: LIGHT

ABSORPTION

Shell radius

-

(tam)

•E 20

.....

0.5O

--

0.35

--

-- - O.25 0.15 0.10

10

0.05

-

i 0.2

0.6

32,043

dependson the sizesof the absorbingparticles.Figure 4 shows an exampleof the spectraldependenceof the BC massabsorption efficiencyfor severalparticle sizesbasedon the layeredspheremodelwith an absorbingcore and a nonabsorbingshell with 5% BC in mass.The resultsshow that smaller particles (0.05 to 0.15 /•m) have strongerwavelengthdependencefor light absorptionthan larger particles.This suggeststhat spectral measurementsof light absorptionby aerosolparticlescan provide information on the type of mixing and on the size of the absorbers.

, 0.4

BY BC PARTICLES

0.8

wavelength(p rn)

Figure 4. Calculatedvaluesof black carbonmassabsorption efficiency(aa Be) as a function of the wavelengthfor several particle radii basedon Mie calculationsfor the layered-sphere model.Particlestructureis an absorbingBC core (5% in mass) and a nonabsorbingexternal shell. Optical parametersare the same as for Figure 3.

ciencyis around10 m2 g-1 andis almostindependent of the ratio BC/TPM. Severalauthorsobtainedvalues around 10 m2

g-• for •a [•c of aerosolparticles[Clarkeet al., 1987;Japaret al., 1986;Roesslera•d Faxvog,1980].The value of •a [•c is also dependenton the real part of the refractiveindex of the nonabsorbingshell. For a range of real refractive indicesaround values reported in the literature for organic materials and sulfatc,higher refractiveindiceswill produce higher valuesof •a m-(at/X = 0.55 /•m). Spectral measurementsof light absorptionassociatedwith measurementsof the particlesizedistributionscan be usefulin determiningthe type of mixingbetweenBC and nonabsorbing particlcs.The wavelengthdependenceof the light absorption

Other factors,such as the shapeand structureof the particles, can affect the BC massabsorptionefficiency. Black carbon is commonlyfound in large chain aggregatesclose to the fires. The optical properties of these fresh aggregateshave been estimatedto be equivalentto individual pure BC particles. These aggregatescan also have a nonabsorbingexternal layer, and after interacting with water vapor and clouds,they can collapseto form more closelypackedstructures[Hallet et al., 1989]. Scanningelectron microscopy(SEM) photographs showthat in somecases,most of the particlesin regional hazes in Brazil are composedof compact closed clusters,with the particle radiusvaryingover the whole sizedistribution[Martins et al., this issue].These closedstructureshave different optical properties from the initial fresh chain aggregates.Depending on size ranges and structures,the BC massabsorption efficiencyfor thesepacked clusterswith a nonabsorbinglayer will be larger than those of pure BC particles. Figure 5a showsa transmissionelectronmicroscope(TEM) photographof a smoke particle composedof a closedcluster aggregateand, apparently,a coatingof lower densitymaterial. Figure 5b illustrates the mathematical model we used to calculate the optical properties of suchclosedclustersusing the discretedipole approximationcode (DDSCAT). Each dot in Figure 5b representsa singledipole,with different symbolsfor

x non-absorbing * BlackCarbon

(a)

(b)

Figure 5. (a) Transmissionelectronmicroscopyphotographof a closelypackedBC clusterfrom biomass burning.]'he structureshowsevidencesof a particlecoatingwith a different material from that of the isolated spheres.Photographcourtesyof F. Echalar, Institute of Physics,University of S•o Paulo. (b) Threedimensionalmathematicalstructureusedfor simulatinginternal mixturesof a closelypackedclusterusingthe discretedipole approximation.Each dot in the figure representsa singledipole;the large circlescomposedof many dots correspondto singleBC particles.

32,044

MARTINS

ET AL.: LIGHT ABSORPTION

BC and nonabsorbingmaterials.The black dots representdipoleswith the dielectricpropertiesof BC, the x symbolsare dipoleswith dielectricpropertiesof nonabsorbing materials.A complexrefractiveindexof 2.0-1.0i was assumedfor BC and

BY BC PARTICLES 12

10

1.5-1.0 x 10-6i for nonabsorbing particles, withdensities of 1.85and 1.5 g cm-3, respectively. The modeledstructureis three dimensionalwith BC particles in touch with each other and all the free spacesfilled with nonabsorbingmaterial. This mixture resultsin a volume ratio ([7BC/[7tot)of 0.52, which meansthat BC correspondsto 64% of the particle mass.It is important to recall that even if the bulk BC/TPM fraction in aerosolsare of the order of 5-15 %, isolatedparticlesmay have a higher BC fraction dependingon the particularmixture for eachcase(e.g., Figure 5a: particleis likely composedpredom-

inantlyby BC). The BC massabsorptionefficiency(aa BC)for this structurewas calculatedusingthe DDSCAT code. Figure 6 comparesresultsfrom the packedclusterstructure with aaBc for pure BC particles and for the layered-sphere model.For all particleradius,aaBc for the closedclusteris

largerthanfor pureBC particles, witha peakof 10m: g-• at a particleradiusabout0.1 rim for the givenBC/TPM ratio. The closedclusteralsoshowedaa BCvaluesup to 35% higher than resultsfor the layered spheremodel for particle radius larger than 0.1 rim. Moreover, also in Figure 6 these results are comparedwith resultsfrom the Maxwell-Garnett mean field theory. In this theory, one determinesaveragedielectricfunctionsfor the composedmaterial,which are then usedin a Mie codefor estimatingopticalproperties.The mixingrule usedfor this calculation considersa two component mixture with a nonabsorbingmatrix containingsphericalabsorbinginclusions ponderedby volume. The Maxwell-Garnet theoryand the mixing rule usedin thiswork is describedin detailsby Bohrenand Huffman [1983]. Despite problemsin determiningaverageeffective refractive indices for mixed materials,we present an intercomparisonof the DDSCAT resultsand the mean field theory as an example of a simple model for a complexstructure. Good agreementwas found between the DDSCAT results and the Maxwell-Garnet theory combine_d with Mie calculations in estimating aaBc over a large range of particle sizes.There was also good agreement for the particle mass scatteringcoefficient, although this does not assurethat the mean field theory will reproduceall the optical propertiesof closedclustersat any wavelength,mixing ratio, or particle size. By analogywith the layered-spheremodel, a smaller mixing ratio will provide a large aa BCvalue. Extrapolationsusingthe mean field theorysuggestthat thesepackedclustersshouldnot

d

4 2

0

0.1

0.2

0.3

0 4

0.5

0 6

ParticleRadius(gm)

l

ß DDSCAT 52.3% BC pure BC

....

Maxwell-Garnet 50% BC ayered sphere 523%BC

Figure 6. Calculatedvaluesof black carbonmassabsorption efficiencyfor a BC cluster(at ,k = 0.55 rim) with a nonabsorbing coatingaround individualparticlesversusthe particle radiususingthe discretedipole approximation(DDSCAT). Results from DDSSCAT are comparedwith pure BC particles (solidcurve),layered-sphere model,and calculationsusingthe Maxwell-Garnett theory consideringa volume ratio of about 52 or 64% in mass.

1982;Delumyeaet al., 1980]. Most of the methodsapplied on filtersproduceartifactsthat dependon particleloading,typeof filter, and type of particlesbut, on the other hand, have the advantageof concentratingthe particlesin a smallvolume for further analysis.The aethalometertechniquecombinesautomated optical attenuation measurementsin filters with good time resolutiondependingon massconcentrations[Hansenet al., 1982;Ruosset al., 1993]. Light absorptiontechniquesrequire empirical calibrationsand intercomparisonswith different methodsfor quality assurance.During SCAR-B, light absorptionwas measuredsimultaneouslyby several techniques (optical extinctioncell, integratingplate techniqueon Teflon filters, opticalreflectanceon Nucleporefilters, and absorption photometryor aethalometertechnique)aboardthe University of Washington(UW) C131-A aircraft;Reidetal. [thisissue(a)] present an intercomparison of these techniques for the SCAR-B data set, consideringa combinationof the UW extinction cell plus a nephelometeras a referencemethodology [Weissand Hobbs,1992].Reid et al. [thisissue(a)] concluded that for the various techniquesused on the C-131A, optical reflectance(OR) providedthe bestcorrelation(r = 0.9) with reachaa BCvalueslargerthan20 m2 g-l for realisticparticle light absorptionmeasurementsfrom the Weiss-Hobbsoptical sizesand mixing ratios. Therefore the obtained resultsare in extinctioncell (OEC) over severalorders of magnitude.Rethe same range as those obtained with the layered-sphere flectance techniqueshave been applied by other authors to model. obtain optical properties of aerosol particles [Lindbergand Laude, 1977;Pattersonand Marshall, 1982].Hanel [1988] discussesa photometricmethodologyfor measuringlight absorp3. Measurements tion by aerosol particles and concludesthat its results are 3.1. Techniques equivalentto a diffusereflectancetechniqueand to calorimetBlack carbonmassabsorptionefficienciesfor aerosolparti- ric measurements[Hanel and Hillenbrand, 1989]. Particulate carbon concentrations in the aerosol were detercleswere obtainedfor all samplescollectedsimultaneously in Nuclepore and Quartz filters during the Smoke, Clouds,and minedby thermal analysis[Cachieret al., 1989]of quartzfilters Radiation-Brazil (SCAR-B) experiment.The determination at the Centre de Faibles Radioactivites, CNRS-CEA, Gif sur of ozaBcdependson measurementsof the absorptioncoeffi- Ivette, France. The exposedfilters were subjectedto HC1 vacient and BC massconcentration.Measurementsof light ab- pors for 24 hours in order to remove carbonates.The carbon sorptionby aerosolshavebeen discussed by manyauthors[e.g., remaining on the filters after this treatment is referred to as Horvath, 1993a, b; Clarke, 1982a, b; Clarke et al., 1987, 1996; total atmosphericparticulate carbon (TC). Both total and Campbellet al., 1995; Campbelland Cahill, 1996;Hanel et al., black carbonmeasurements were performedon filter aliquots

MARTINS

ET AL.: LIGHT ABSORPTION

by coulometrictitration with a Str6hleinCoulomat©702C.The black carbon fraction (BCTh) is analyzed after the thermal removal of the organiccompoundsat 340øCduring 120 min under pure oxygen.OC is assumedto be the difference between TC and BC. It must be recalled, however, that the BC

BY BC PARTICLES

32,045

sorption efficienciesshowsc•aBcvaluesbetween 5.2 and 58.2

m2 g-• separated in twogroups withdistinctchemical signatures.The first group has a K/BCTh ratio smallerthan 0.78 and

c•ancvaluesbetween5.2 and 19.3m2 g-J, with an average valueof 12.1 _+4.0 m2 g-•. The secondgroupfor K/BCTh

and OC separationis method dependent[Reidet al., this issue larger than 0.78 showsvery scattered results for c•anc with valueof (a); Petzoldand Niessner,1995].The publisheduncertaintyin valuesbetween18.0and58.2m2 g-• andan average the BC content is of the order of 10% when BC/TC ratio is in

37 _+14 m2 g-•. This separation is stronglyobserved in the

the 10-35% range,which correspondsto 70% of samplesfor the SCAR-B campaign.It mustbe notedthat followingReid et al. [this issue(a)] and for the SCAR-B data set, BC concentrations could be increasedby 25%, due to thermal method sensitivityto catalyticreactionswith K and Na [Novakovand Corrigan, 1995]. The Nuclepore, Quartz, and Teflon filters were collectedin parallelin a grab-bagsamplingsystemaboard the UW C131-A aircraft using the same inlet. Therefore becauseof samplingsimilaritywith the quartz filters and good correlationwith the OEC results(r = 0.9), absorptionmeasurementsfrom Nucleporefilterswere consideredin this work to calculatethe absorptioncoefficients. Several other aerosol properties were measured simultaneously,includingparticle size distributions,light scattering, and humidificationfactors[Hobbs,1996;Reid et al., this issue (b); Reid and Hobbs, this issue].Sampleswere collectedin different conditions and environments, including smoke plumes,regionalhazes,backgroundair, and clouds.Nuclepore filters were also subjectedto PIXE (proton-inducedX-ray emission),SEM (scanningelectronmicroscopy),and gravimetric analysis.The Teflon filters were analyzed gravimetrically and by ion chromatography.The elemental and ionic compositionsprovided information on the dependenceof qaBc on the chemical compositionof the particle, and the electron microscopyresults provided information on the micromorphologyand structureof the particles.

relationshipof c•auc and the concentrationsof K, Ca, and Mg for eachsample,asit canbe seenin Figures8a-8c. High values of c•anc correlatewell with high K, Ca, and Mg concentrations. Accordingto Novakovand Corrigan[1995], K has a catalytic effect on BC, loweringits volatilization temperature and causing artifactson BC measurementsobtained by thermal evolution techniques.This effect can explainthe separationof c•anc in two different groupsand suggestsan upper limit value of K/BCTh -- 0.78, under which BCTh presentedreasonablevalues. The authors also suggesta similar effect associatedwith Na, but we did not find any associationbetween the ratio Na/BCTh and c•aBc. Thus sampleswith K/BCTh < 0.78 were selectedfor the measurement of c•anc in this data set. This procedureneedsto be studied and generalizedfor other data sets, suggestinga quality assuranceprotocol for thermal BC

3.2.

Faxvog,1980]. However, Chyleket al. [1995] discussthe possibility of BC massabsorptionefficiencyreachingvalues up to

data.

The K/BCTh = 0.78 upper limit is also in accordancewith the optical modeling calculationspresentedin section2. As showedin Figure 3, the maximumvalue for a monodispersed particle size distribution using the layered-spheremodel is

about30 m2 g-• Integratingover a realisticaccumulationmode sizedistributionfor biomassburningparticles[e.g.,Reid et al., thisissue(b)], thisvalue reachesa maximumof about25

m2 g i. Similarvalueswerefoundfor a packedclusterof BC particlesembeddedin a nonabsorbingmatrix. Also, c•auc valuescommonlyreported in the literature are generallybelow 25

m2 g-l [e.g.,Liousse et al., 1993;Horvath,1993a;Roessler and

Results

Figure 7 showsa histogramof the measuredBC mass absorption efficienciesfor all the Nuclepore filters collected in parallel with quartz filters. The distribution of BC mass ab-

13

t7

2t

25

29

33

37

4t

45

49

53

57

61

q,a•c(m=g"•)

Figure 7. Histogram of measuredBC massabsorptionefficienciesobtained during SCAR-B. Results are separated in two groupswith K/BCTh < 0.78 and K/BCTh > 0.78. As discussedin the text, estimatesof c•auc were obtainedbasedon the groupwith K/BCTh < 0.78 producingan averagec•auc =

12.1_+4.0 m2 g-i.

100m2 g-• in specialcircumstances (e.g.,waterdroplets with a small BC core out of center). Using the layered-spheremodel describedin section2, the optical properties of the aerosol particleswere estimated on the basis of the parameters measured on the UW C131-A aircraft, namely, simultaneousmeasurementsof light absorption and massconcentrations(Nucleporeand Teflon filters), BCTh (quartz filters), particle size distributionwith a passive cavityaerosolspectrometerprobe (PCASP) and a differential mobilityparticle sizer (DMPS), and total light scatteringand backscatteringat three wavelengths.The PCASP measures particleswith diameter between 0.1 and 3.0 /•m, while the DMPS measuresparticles diameter in the range of 0.01-0.6 /•m. Five caseswere modeled assuminga constantfraction of BC for all the particles in the size distribution. For the BC core, a complexrefractive index of 2.0-1.0i at 0.55 /•m was

assumed, with a densityof 1.85gcm-3. Particlesizedistributions were consideredto be lognormal. Figure 9 showsa comparisonbetweenthe measuredvaluesof c•auc versusthe relative amount of thermal BC in each sample separatingcases with K/BCTh < 0.78. Calculated results using the layeredspheremodel with three measuredparticle size distributions are superimposedon the experimentalvalues, as well as the asymptoticc•auc value for pure BC particleswith similar particle size distributions.In this case,the c•auc values for pure BC particlesrepresent external mixing between BC and non-

32,046

MARTINS

0

ET AL.: LIGHT

10

20

ABSORPTION

30

40

BY BC PARTICLES

50

60

70

1.2

0.8 0.6 0.4

0.2

0.7 0.6 ,-



0.5

0.4

o 0.3

..............

'

it: 0.2 o.1

..........

m--:-

ß ........

: 0

10

20

30

mI 40

50

60

70

(xasc(m2g-l) Figure 8. Ratios(a) K/BGrh, (b) Ca/BCTh,and (c) Mg/BCThversusozaBcfor all the analyzedsamples. The upper limit K/BCrh = 0.78 is pointedin the figure showingthe separationfor reliable aaBc values.

absorbingparticles.The modeledresultscanaccountfor aa Bc

evaporate;for example, organics,ammonium sulfate, etc.). The comparisonbetweenthe "hot" and the "cold" size distrithe data set and all aanc valuesfor K/BCTh < 0.78. Table 1 butionssuggesteda predominanceof internal mixingbetween showsgeneral characteristicsof the studiedsamples.There is volatile and nonvolatileparticles.Despite the large difference no clear correlation between aanc and location, type of com- in the sizedistributionsfor the "hot" and "cold" samples,the bustion,or type of sample.On the other hand,SEM picturesof total numberof particleswasreducedby only 15% by heating, the samplesalsosuggesta separationin two groupsof particles showingthat mostof the particleswere not completelyvolatile with distinctmicromorphology.The first group,which haslow and suggestinginternal mixing. Figure 11 showsgood agreeaa Bc values,has spherical-likeshapeand relativelyhomoge- ment between particle count distribution measuredwith the neousparticles(Figure 10a).The secondgroup,whichhashigh DMPS and PCASP, despitethe fact that the DMPS measures aa nc values,showsa large concentrationof nonsphericalpar- particle sizes on the basis of the electrical mobility of the ticles and compactclustersof small particles(Figure 10b), particles,andthe PCASP measurements are derivedfrom light similar to the one shownin Figure 5. scattering.Volume size distributionscalculatedfrom the meaFigure 11 showsan exampleof the particlesizedistribution, sured number size distributions, in combination with massconand the DMPS "hot" size distribution,which correspondsto centrationmeasurements,were used to estimateparticle denthe size distribution of the same sample after heating the sity. Backscatteringmeasurementsfrom the nephelometer aerosolto 340øC(which causesmost volatile compoundsto were usedto estimatethe asymmetryfactor (#) as a function

valuesbetween5 and25 m: g-l, whichexplains about70%of

MARTINS ET AL.: LIGHT ABSORPTIONBY BC PARTICLES

32,047

Table1. Characteristics of SCAR-BSamples Listedin Orderof BC MassAbsorption Efficiency BC Mass

Absorption

Sample

Location of Sampling

1 2 3 4

Brasilia Cuiabfi Cuiabfi Marabfi

5 6 7

Porto Velho Brasfiia Marabfi

8

Cuiabfi

9 10

Porto Velho Porto Velho

11 12

Cuiabfi Marabfi Brasfiia Brasfiia Brasfiia Marabfi

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Porto Velho Porto Velho Cuiabfi Marabfi Marabfi Marabfi Marabfi Marabfi Porto Velho Cuiabfi Marabfi Porto Velho Brasfiia Marabfi Marabfi Cuiabfi Marabfi Marabfi Marabfi Marabfi

Typeof Sample

MCE

emission factor

5.2 7.2 7.9

regional haze

Cu processed (fire top) smoldering(EF) background

0.881

emission factor in cloud

0.949

regional haze background regional haze regional haze regional haze emission emission emission emission

factor factor factor factor

0.936 0.958 0.896

regional haze emission factor

0.894

regional haze emission factor emission factor emission factor emission factor in cloud emission factor smoke column emission factor

0.883 0.828 0.946

0.975

emission factor emission factor

0.949

regional haze regional haze factor

0.966

emission factor emission factor

0.855

5.85 12.94 4.32

8.3

7.10 5.56

9.1

5.18

9.2 10.3 10.7 11.4 11.4 11.4 12.3

14.07 5.98 2.25 4.60 4.59 5.76 9.73

12.5 13.3 13.4

5.78 10.74 7.00

14.1

5.03

15.4

4.19

17.1 17.4

1.85 4.41 5.05 5.82 6.51 7.98 11.82 3.77 5.41

18.1 19.3

25.2 25.7

0.920

BCTh/TPM , %

8.9

21.3 25.1

regional haze

emission in cloud

Efficiency, m2 g-1

25.9 27.8

1.87

29.9 37.0 37.5 41.9 55.0 56.2 57.3 58.3

4.00 2.20 2.33 0.57 1.89 3.15 1.93 2.23

Samplenumbers are the sameasin Table2. Note that MCE corresponds to modifiedcombustion efficiency.

of the particlegeometricstandarddeviation,usingthe expressionproposedby Marshallet al. [1995].The real refractive

7O

6O

indicesof the nonabsorbing shellswere estimatedin an itera5O tiveprocess to fit the scattering coefficient (o-,) measured by the nephelometerat a wavelengthof 0.55/xm. The amountof 'm40 BC wasallowedto varyuntilit fittedthe absorption coefficient (o-,) estimated fromthe Nucleporefilters.After convergence, •ø30 o-,, o¾,the BC content,g, andc•aBCwere comparedwith the 2O

measured

values.

Table2 showscomparisons betweensomeof the modeling resultsandthe measurements. The averagereal refractivein-

10

dexobtained fromthemodelwas1.48+ 0.08(varying between

5

1.37 and 1.59). A real refractiveindex as small as 1.37 can be

explained by possible underestimates of o-x by the nephelometer, uncertaintiesin the particlesizedistribution,or inconsis-

10

AmountofthermalBC ....

external mixing

o

15

(%) K/BCth > 0.78

• internal mixing ß K/BCth < 0.78 tencies between themodeled particlestructure andshape(layered sphere)and the actualparticles.However,the singlescattering albedo(tOo)andthe asymmetry factorare relatively Figure 9. Measuredand modeledBC massabsorptioneffi-

well producedby the model. The BC contentestimatedfrom

ciencies(aa Bc) at A -- 0.55/amversusthe amountof BC in the

sample.Solidcurvesrepresentthe modeledaauc for three the modelshowedpoor agreement with the measuredBCTh. particlesizedistributionsgivenin Table 1. The dashedline is Large uncertaintiesin the BC determination from the model theasymptotic limitfor aa uc assuming externalmixingandfor are associated to someextentwith largevaluesof aaBc de- the measuredparticle size distributions.

32,048

MARTINS

ET AL.: LIGHT ABSORPTION

BY BC PARTICLES

,

!'%.•.•;•.,.½•. --,.%?•?....,,•-.,-,,..,

i

,

'F.....

•-

,

6.E+05 t '-'" .... •1 i

,..i.::.;.:(..'.'.;':: ...... •5• '•;i:..:.,.,'. -.:;t½':;':*'x•!•;::

-'

5.E+05• -•-- '& ....

..., :,... ,....:;,;. .... 7%; •?:.:;...:.:,-....%

-'C.......

;•

-

-

_

2. E+05

"• ....

"-.**?'" .:'•"

":':'"'•:½5:-5 ....,.• ....... . ......

1.E+05---

""'" .,..

0.E+00

:•-•':;..•,.-;•' .... •'•;-'.". .•., :.'3: '½ ?-" .•.....; ' '"'

0

ß ;,;;•;•..•;, ?•-'.. ...... .:.'•,.•:e.-.': ...:f";•:".•,' ?%?' .'.•

,

'-•.',,,_ ,A ___•X-- I - -

0.1

0.2

0.3

0.4

0.5

0.6

diameter (pro)

I I Cold--•Hot o Pcasp -- fitting 1 Figure 11. Measured particle size distributions from the DMPS (differential mobility particle sizer) and from the PCASP (passivecavity aerosol spectrometerprobe). DMPS cold correspondsto measurementsat ambient temperature and hot to measurementsat 340øC.The solid cu•e represents a lognormal fit to the combined DMPS "cold" plus PCASP data. The difference between DMPS resultscold and hot suggestspredominanceof internal mixing.

rived from the combinationof the thermal techniqueand optical absorption measurements(e.g., sample 27). Recall that

themodelcannotexplainvaluesof o•aBclargerthan25m2 for the particle size distributionsconsidered.Even for monodisperseparticles an optimized particle size would provide a maximumvalue of c•aBcaround30 m2 g , as shown in Figure 3. Figure 10. (a) Typical scanningelectron rnicroscopy(SEM) photographof aerosolparticleswith low c•a•(, w•lucs.Most of 4. Conclusions the particles have spherical-likeshape. (b) Typical SEM phoBlack carbon mass absorption efficiency (-auc-) depends tographof smokeparticleswith high •a Be-valucs.The photograph showsa large concentrationof nonsphericalparticles stronglyon particle mixing, size distribution,and morphology and compact clusters of small particles similar to the one of the particles.in agreementwith the literature, resultsshow shownin Figure 5.

Table 2. Optical and PhysicalPropertiesof Aerosol ParticlesMeasuredand CalculatedUsing Mie Theory in Actual SamplesCollccted During SCAR-B Aboard the UW C13 I-A Aircraft Sample 4 Results

Calculated

smoldering,Marabfi

Type D# erg

DgV ergV p, gcm Rind shell 1

ers, m 1

era,

Measured

m

600

%BCTh g

oza eo m2g- •

Sample 27 Measured

Calculated

flaming, Marabfi

Sample 10 Measured

Calculated

haze, Porto Velho

Sample 7 Measured

Calculated

in cloud, Marabfi

Sample 20 Measured

smoldering,Marabfi

0.12

0.087

0.19

0.12

0.11

1.89 0.27 1.63 1.21

1.87 0.25 1.62 1.00

1.78 0.52 1.78 1.20

1.79 0.25 1.62 1.28

1.89 0.27 1.67 1.04

6.4E-4 1.30E-4 0.83 7.1 0.533 8.3

1.440 6.4E-4 1.30E-4 0.83 4.1 0.550 15.3

1.11E-3 3.79E-4 0.74 5.41 0.485 25.9

1.525 1.11E-3 3.80E-4 0.74 9.6 0.490 15.2

4.72E-4 6.75E-5 0.87 4.60 0.617 11.4

1.370 4.72E-4 7.05E-5 0.87 4.3 0.689 12.5

4.9E-4 1.54E-4 0.76 14.07 0.52 9.2

1.587 4.9E-4 1.54E-4 0.76 7.8 0.50 16.6

Calculated

8.70E-4 1.84E-4 0.83 4.41 0.534 17.4

1.460 8.66E-4 1.84E-4 0.83 5.3 0.55 15.3

The Mie calculationswere performed for layered-sphereparticlesusingthe code developedby Ackerman and Toon, further improved and made availableby W. Wiscombe.NASA GSFC. Size distributionswere measuredby the PCASP and DMPS instruments,ersand g was obtained by a onboardnephelometer(• = 0.55/xm), and era wasobtainedby a reflectancetechniqueapplied to Nuclepore filters intercalibratedwith an optical extinctioncell. All measurementswere taken in parallel. Sample numbersare the sameas presentedin Table 1. Read 6.4E-4 as 6.4 x

10-4. Dg and erg,DgV, and ergV,correspond to the geometricmeandiameterand standarddeviationof the numberand volumesize distribution,respectively."Rind shell" representsestimatesfor refractive index of the nonabsorbingshell

MARTINS

ET AL.' LIGHT

ABSORPTION

that the mass of BC was underestimated for samples with high-K contentproducingunrealistichigh valuesfor BC mass absorptionefficiencies.An empirical upper limit K/BCTh = 0.78 hasbeen definedin this paper to selectreliable resultsfor mass absorptionefficiencies.Below this value the measured massof BCThprovidedgood resultsfor aa Bc. Similar behaviors were found for concentrationsof Ca and Mg. The BC massabsorptionefficiency(aa Bc) for particlesfrom

biomass burningin Brazilrangedfrom5.2to 19.3m2 g-• with an averagevalueof 12.1 _+4.0 m2 g-•. In additionto the

BY BC PARTICLES

32,049

Chylek, P., G. Videen, D. Ngo, R. G. Pinnick, and J. D. Klett, Effect of black carbon on the optical properties and climate forcing of sulfate aerosols,J. Geophys.Res., 100, 16,325-16,332, 1995. Clarke, A.D., Integratingsandwich'A new methodof measurementof the light absorptioncoefficientof atmosphericparticles,Appl. Opt., 21, 3011-3021, 1982a.

Clarke, A.D., Effects of filter internal reflection coefficient on light absorptionmeasurementsmade usingthe integratingplate method, Appl. Opt., 21, 3021-3031, 1982b. Clarke, A.D., K. J. Noone, J. Heintzenberg, S. G. Warren, and D. S. Covert,Aerosol light absorptionmeasurementtechniques:Analysis and intercomparisons, Atmos.Environ.,21, 1455-1465, 1987. Clarke, A.D., J. Ogren, and R. Charlson,Comment on "Measurement of aerosolabsorptioncoefficientfrom Teflon filters usingthe integratingplate and integratingspheretechniques"by D. Campbell,S. Copeland,and T. Cahill, AerosolSci. Technol.,24, 221-224, 1996. Delumyea, R. G., L. C. Chu, and E. Macias, Determination of elemental carboncomponentof soot in ambient aerosolsamples,At-

layered spheremodel for internal mixtures,a packed cluster model was proposedfor biomassburning particlesfrom regional haze. For a particular casewith BC/TPM -- 0.64, the packedcluster structurepresentedozaBcvalues up to 35% higher than resultsobtainedfor the layered spheremodel for mos. Environ., 14, 647-652, 1980. particleswith radius above 0.1 /•m. Modeled results using clusterand layeredspheremodelsplus Mie theory accounted Draine, B. T., and P. J. Flatau, Discrete dipole approximation for

for aaBcvaluesbetween5 and25 m2 g-• depending on the

scatteringcalculations,J. Opt. Soc.Am. A., 11, 1491-1499, 1994. Hallet, J., J. G. Hudson, and C. F. Rogers, Characterizationof combustion aerosolsfor haze and cloud formation, Aerosol Sci. Technol.,

size distribution and BC fraction. The modeled results pro10, 70-83, 1989. vided goodestimatesfor the opticalpropertiesof the biomass burning aerosolcollectedin Brazil. The large range of aaBc Hanel, G., Single scatteringalbedo, asymmetryparameter, apparent refractiveindex, and apparentsootcontentof dry atmosphericparmeasuredfor biomassburning particlesmust be taken into ticles,Appl. Opt., 27, 2287-2294, 1988. accountwhen using optical measurementsto determine the Hanel, G., and C. Hillenbrand, Calorimetric measurementsof optical mass of BC. An

accurate

determination

of the mass of BC

through optical methods dependson knowing the value of c•aBc very well for eachparticulartype of particles.Estimates of aa nc are alsoimportantto connectBC emissionswith light absorptionby particlesin the atmosphere. There was no clear correlationbetween aanc and location, typeof combustion,or type of sample.Sampleswith low aa nc valuescontainedmostlyhomogeneousparticleswith sphericallike shape.Samplescontainingasymmetricalparticlesand cluster-like structuresshowedhigher aa nc values.This characteristic was attributed either to the particular internal mixing contributingto enhancethe light absorptionpropertiesor to earlier volatilization of BC particlesduring the thermal measurementsalso enhancedby the particular internal mixing. Acknowledgments. J. V. Martins thanks the Brazilian agencies

absorption,Appl. Opt., 28, 510-516, 1989. Hanel, G., R. Busen,C. Hillenbrand, and R. Schloss,Light absorption measurements:New techniques,Appl. Opt., 21,382-386, 1982. Hansen, A.D. A., H. Rosen, and T. Novakov, Real time measurement

of the aerosolparticles,Appl. Opt., 21, 3060-3062, 1982. Hobbs,P. V., Summaryof typesof data collectedon the Universityof Washington'sConvair C-131A aircraft in the Smoke, Clouds and Radiation-Brazil(SCAR-B) field studyfrom 17 August-20 September 1995,Cloud and Aerosol Res. Group, Dep. of Atmos. Sci.,Univ. of Washington, Seattle, March 1996. (Also available on http:// cargsun2.atmos.washington.edu/.) Horvath, H., Atmospheric light absorption--A review,Atmos. Environ., Set.A, 27(3), 293-317, 1993a. Horvath, H., Comparisonof measurementsof aerosoloptical absorption by filter collectionand a transmissometricmethod,Atmos. Environ., Ser. A, 27, 319-325, 1993b.

Japar, S. M., W. W. Brachaczek,R. A. Gorse, J. M. Norbeck, and W. R. Pierson,The contributionof elemental carboninto the optical propertiesof rural atmosphericaerosols, Atmos.Environ.,20, 12811289, 1986.

FAPESP (project93/5017-3and 96/2672-9)and CAPES (project437/ Lindberg, J. D., and L. S. Laude, Measurement of the absorption 95) for financialsupport.The Universityof Washington'sparticipation coefficientof atmosphericdust,Appl. Opt., 13, 1923-1927, 1977. in SCAR-B was supportedby the following grants: NASA NAGWLiousse, C., H. Cachier, and S. G. Jennings,Optical and thermal

3750 and NAG 11709; NSF ATM-9400760, ATM-9412082, and ATM9408941; NOAA NA37RJ0198AM09; and EPA CR822077. We thank Alcides C. Ribeiro, Ana L. Loureiro, and Tarsis Germano for assis-

tance during samplingand analysis.We are very grateful to H61ane Cachierfor her carefulreviewandimportantdiscussions improvingthe contentsof the paper.

measurements

of black carbon

aerosols content

in different

envi-

ronments:Variationsof the specificattenuationcrosssection,sigma (o-),Atmos.Environ.,Ser.A, 27, 1203-1211,1993. Marshall, S. F., D. S. Covert, and R. J. Charlson,Relationshipbetween asymmetryparameter and hemisphericbackscatterratio' Implications for climate forcing by aerosols.Appl. Opt.. 34. 6306-6311, 1995.

References Ackerman,P. T., and O. B. Toon, Absorptionof visibleradiationin the atmosphere containingmixturesof absorbingandnonabsorbing particles,Appl. Opt., 20, 3661-3667, 1981. Bohren, C. F., and D. R. Huffman, Absorptionand Scatteringof Light by SmallParticles,pp. 213-219, Wiley Intersci.,New York, 1983. Cachier, H., M.P. Brdmond, and P. Buat-Mdnard, Determination of atmosphericsootcarbonwith a simplethermal method,Tellus,Ser. B, 41, 379-390, 1989.

Campbell,D., and T. Cahill, Responseto "Comment on 'Measurement of aerosolabsorptioncoefficientfrom Teflon filtersusingthe integratingplate andintegratingspheretechniques'by D. Campbell, S. Copeland,andT. Cahill"byA. Clarke,J. Ogren,and R. Charlson, Aerosol Sci. Technol., 24, 225-229, 1996.

Campbell,D., S. Copeland,and T. Cahill, Measurementsof aerosol coefficientfrom Teflon filtersusingintegratingplate and integrating spheretechniques, AerosolSci. Technol.,22, 287-292, 1995.

Martins, J. V., P. V. Hobbs,R. E. Weiss,and P. Artaxo, Sphericityand morphologyof smokeparticlesfrom biomassburningin Brazil, J. Geophys.Res., this issue. Novakov,T., and C. E. Corrigan,Thermal characterizationof biomass smokeparticles,Mikrochim.Acta, 119, 157-166, 1995. Patterson, E. M., and B. T. Marshall, Diffuse reflectance and transmissionmeasurementsof aerosolabsorption,in LightAbsorptionby AerosolParticles,edited by H. E. Gerger and E. E. Hindman, Spectrum, Hampton, Va., 1982. Petzold, A., and R. Niessner, Method comparison study on sootselectivetechniques,Mikrochim.Acta, 117, 215-237, 1995. Purcell,E. M., and C. R. Pennypacker,Scatteringand absorptionof light by nonsphericaldielectricgrains,Astrophys.J., 186, 705-714, 1973.

Reid, J. S., and P. V. Hobbs,Physicaland opticalpropertiesof young smokefrom individualbiomassfires in Brazil, J. Geophys.Res., this issue.

Reid, J. S., P. V. Hobbs, C. Liousse, J. V. Martins, R. E. Weiss, and

32,050

MARTINS

ET AL.: LIGHT

ABSORPTION

T. F. Eck, Comparisonsof techniquesfor measuring shortwave absorptionand the black carboncontent of aerosolsfrom biomass burning in Brazil, J. Geol)hys.Res.,this issue(a). Reid, J. S., P. V. Hobbs, R. J. Ferek, D. R. Blake, J. V. Martins, M. R. Dunlap, and C. Liousse,Physical,chemical,and opticalpropertiesof regionalhazesdominatedby smokein Brazil, J. Geophys.Res.,this issue(b). Roessler,D. M., and F. R. Faxvog,Optical propertiesof agglomerated acetylenesmokeparticlesat 0.5145/xm and 10.6/xmwavelengths, J. Opt. Soc.Am., 70, 230-235, 1980. Ruoss, K., R. Dlugi, C. Weigl, and G. H/finel, Intercomparisonof different Aethalometerswith an absorptiontechnique:Laboratory calibrationsand field measurements, Atmos.Environ.,27A(8), 1221-

BY BC PARTICLES

P. Artaxo and J. V. Martins, Instituto de Ffsica, Universidade de S•o

Paulo, C.P. 66318, CEP 05315-970, S•o Paulo, Brazil. (e-mail: [email protected]) P. V. Hobbs and J. S. Reid, Department of AtmosphericSciences, Universityof Washington,Seattle, WA 98195-1640. Y. Kaufman, NASA Goddard Space Flight Center, Code 913, Greenbelt, Maryland 20771. C. Liousse, Centre de Faibles Radioactivites, CNRS-CEA, Av de la Terrasse, 91198 Gif sur Yvette, France.

1228, 1993.

Weiss,R. E., and P. V. Hobbs, Optical extinctionpropertiesof smoke from the Kuwait oil fires,J. Geophys.Res., 97, 14,537-14,540,1992.

(ReceivedOctober 6, 1997;revisedJuly 1, 1998; acceptedAugust3, 1998.)