Light Absorption Enhancement of Black Carbon Aerosol Constrained

May 22, 2018 - Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL , United ...
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Environmental Processes

Light absorption enhancement of black carbon aerosol constrained by particle morphology Yu Wu, Tianhai Cheng, dantong Liu, James Allan, Lijuan Zheng, and Hao Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00636 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Environmental Science & Technology

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Light absorption enhancement of black carbon

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aerosol constrained by particle morphology

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Yu Wu1, Tianhai Cheng1*, Dantong Liu2*, James D. Allan2, 3, Lijuan Zheng4, Hao Chen1

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1

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Earth, Chinese Academy of Sciences, China

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2

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Manchester, Manchester M13 9PL, UK

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National Centre for Atmospheric Science, University of Manchester, Manchester M13 9PL, UK

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China Aero Geophysical Survey and Remote Sensing Center for Land and Resources, Beijing,

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China

State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital

Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of

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*Correspondence to: [email protected] and [email protected]

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Abstract

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The radiative forcing of black carbon aerosol (BC) is one of the largest sources of uncertainty in

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climate change assessments. Contrasting results of BC absorption enhancement ( E abs ) after

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aging are estimated by field measurements and modeling studies, causing ambiguous

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parameterizations of BC solar absorption in climate models. Here we quantify E abs using a

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theoretical model parameterized by the complex particle morphology of BC in different aging

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scales. We show that E abs continuously increases with aging and stabilizes with a maximum of

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~3.5, suggesting that previous seemingly contrast results of E abs can be explicitly described by

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BC aging with corresponding particle morphology. We also report that current climate models

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using Mie Core-Shell model may overestimate E abs at a certain aging stage with a rapid rise of

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E abs , which is commonly observed in the ambient. A correction coefficient for this

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overestimation is suggested to improve model predictions of BC climate impact.

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Introduction

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Black carbon aerosol (BC) is the second most important anthropogenic components of global

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warming after CO2 in terms of instantaneous top-of-atmosphere forcing, and its poorly qualified

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climate impact is one of the grand challenges in atmospheric climate science1, 2. An important

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issue is the systematic discrepancy between model and observation estimates of BC light

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absorption enhancements ( Eabs ) after aging, which transfer directly into large uncertainties in

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model estimates of BC radiative forcing3, 4. The conflicting results of field observations,

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laboratory measurements and theoretical modeling on Eabs indicate that the light absorption of

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BC aerosols can be negligibly (~1) or significantly (~3.5) enhanced by mixing with co-emitted

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and secondary particulate matter, causing an intense debate about the issue of BC absorption

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enhancements5-7. In current climate models, Eabs is either assumed to be a constant enhancement

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factor in the low end of this wide range (such as 1.5)4, 8 or calculated by the simplified core-shell

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sphere structure, offering a range of 2-39. Laboratory measurements demonstrated that the

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enhancements can reach to ~3.5 in some experimental conditions10. In contrast, field

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observations in urban environments indicated E abs is a quite small value of 1.06 after BC aging5.

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For the estimations of BC direct radiative forcing, the ratio of the highest to the lowest is at least

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twice due to the unclear parameterizations of Eabs 11, 12. Until now, a proper description of Eabs

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varying with BC aging has not been validated, leading to a curial question of BC climate impact.

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Results of in situ measurements and laboratory studies indicate that freshly emitted BC particles

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in bare conditions are aggregated by small carbon spherules8, 13. The particles tend to be coated

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with a thin layer of other aerosol components in the atmosphere through the coagulation and

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condensation of secondary aerosol compounds14,

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mixing with other larger aerosols, such as sulfate, organics, dust, and sea salt. Particle

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morphology of these aged BC aerosols is complex, and depends highly on the degree of aging,

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ambient temperature, and relative humidity. Previous observations indicated that the aging may

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lead to more compact black carbon aggregates in the large non-BC particles16, 17.

. Further aging may lead to BC particles

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In this study, we report a detailed analysis of the qualified BC absorption enhancements using a

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theoretical model considering their realistic particle morphologies dependent on the aging scales.

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Recent microphysical studies indicate that BC aging causes not only growth of co-emitted

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aerosols but also dramatic changes in particle morphology, which highly affects their optical

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properties18, 19. The optical properties of individual BC-containing particles are validated by the

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comprehensive laboratory and field ambient data reported by Liu et al.20. Our results indicate that

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previous conflicting results of Eabs were possibly observed in different BC aging states, which

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lie in the range of the modeling descriptions. The observed Eabs can be simulated by the model

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considering particle morphology if BC aging states are exactly obtained. It is also found that

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Eabs may be overestimated by current climate models in a certain aging range with Eabs rapidly

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rising, which widely appears in the ambient, leading to a misunderstanding on the estimations of

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Eabs . Further, a correction coefficient is suggested for climate models on the parameterizations of

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BC absorption properties.

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Materials and Methods

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Aging mechanism of black carbon particles

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Recent studies suggested that the morphology of BC-containing particles can be quantified into

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four categories by China et al.18, 21: bare, partly coated (thinly coated), partially encapsulated, and

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heavily coated (see left column of Figure S1). According to microscopy images, freshly emitted

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BC particles consist of hundreds of small spherical primary particles combined into branched

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aggregates, described by the well-known fractal law22, 23:

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Df  Rg  Ns = k 0    a 

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Rg2 =

1 Ns

Ns

∑r

2

i

(1)

(2)

i =1

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where N s is the number of monomers in the cluster, a is the mean radius of the monomers, k 0

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is the fractal prefactor, and Df is the fractal dimension. For an aggregate, Df describes its space-

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filling characteristic, while k 0 is strongly influenced by shape anisotropy (stringiness) and

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monomer packing density. Rg , called the radius of gyration, is a measure of the overall

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aggregate radius, and

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Bond and Bergstrom reported the value of mean radii of BC monomer a in the range of 0.01-

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0.025µm26. In the field observations, the numbers of monomers N s has been observed in the

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range of 50-300, and it may vary up to ~80014. Previous measurements and simulations

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suggested the fractal dimensions ( D f ) of bare and heavily coated BC particles varied from

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approximately 1.8 to 3.0, with the fixed fractal prefactor of 1.227-29.

ri is the distance from the ith monomer to the center of the cluster24, 25.

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Particle morphology of emitted BC particles changes rapidly after emission, and their absorption

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tends to be enhanced during condensation and coagulation processes30, 31. The mass ratio of non-

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BC and BC components in individual BC-containing particle ( M R = ( M P − M BC ) M BC ), which

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is determined by a morphology-independent measurement of total particle mass ( M P ) and

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refractory BC mass ( MBC ), is considered to be a key indicator for BC aging20. The total particle

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mass ( M P ) and BC mass ( M BC ) in the same individual particle was measured by the coupling of

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a Centrifugal Particle Mass Analyser (CPMA, Cambustion) and a single-particle soot photometer

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(SP2, DMT)

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: selected by the known and quantifiable charge-to-mass ratios across a narrow

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and well-defined mass distribution, then the particles with fixed mass when through the CPMA

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and the BC mass and scattering cross sections at 1064nm were further measured by the SP2. The

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mass absorption coefficient of laboratory generated BC at green (532 nm) was measured by a

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photoacoustic soot spectrometer (PASS-3, DMT). Details are described in the Section 1.1 of

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Supporting Information.

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Realistic BC particle morphologies during aging are qualified for modeling BC optical properties

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dependent on MR . The non-BC/BC mass ratio is zero ( M R = 0 ) for indicating bare BC particles

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freshly emitted from incomplete combustions without mixing non-BC materials, and their

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particle morphology appears as a fractal aggregated chain-like structures consisting of hundreds

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or thousands of spherules. The augment of non-BC/BC mass ratio indicates the mixing of

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aggregated BC monomers with larger non-BC components in the individual particles, resulting

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in more compact BC structures and various mixing states. According to microphysical

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measurements by China et al.18, the single BC particles may be thinly coated or partially

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encapsulated, and further aging brings on compact BC aggregate heavily coated with the large

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non-BC materials. These more realistic morphologies of BC-containing aerosols, quantitatively

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related to the non-BC/BC mass ratios, are applied for the modeling of their optical properties

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(see Figure S1 of supplementary materials).

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Aggregate model using the superposition T-matrix method

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According to the morphological and chemical features in field-emission scanning electron

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microscope images, the morphologies of BC-containing aerosols with different mixing states are

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constructed and integrated by a novel aggregate model. As shown in the middle column of

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Figure S1, bare, partly coated (thinly coated), partially encapsulated (semi-embedded), and

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heavily coated (internally mixed) states of BC-containing particles are modeled. Freshly emitted

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BC in the bare condition consists of hundreds of small spherical primary particles combined into

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branched aggregates. According to the diffusion limited aggregation (DLA) method, the

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aggregations of BC monomers are constructed with the given fractal parameters32, 33. These bare

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BC particles tend to be partly coated or partially encapsulated with other aerosols. BC aggregates

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with concentric core-shell spherical monomers are simulated for their partly coated states34, 35.

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These monomers for aggregation are assumed to be core-shell structures with BC core and non-

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BC shell. Bare BC particles can be considered to be the aggregated BC monomers without a non-

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BC shell. The partially encapsulated morphologies of BC-containing particles are represented by

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the aggregated BC partially embedded in the host non-BC particle36, 37. The generation of BC

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aggregate is divided into two alternating processes: an inner aggregation and an outer

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aggregation of the larger non-BC host. The developed BC monomers of the inner aggregation are

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all inside of non-BC host, while those of the outer aggregation are all outside of the non-BC host.

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Further aging brings on compact aggregated BC heavily coated or internally mixed by large

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spherical non-BC particles, and all BC monomers are inside of the non-BC particle38, 39.

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In the aggregate model, the aging scale of BC aerosol is determined by the mass ratio of non-BC

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and BC components ( MR )

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states of BC-containing particles in this study: (1) without mixing with non-BC, the mass ratio is

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zero for bare BC ( MR =0 ); (2) larger mass ratios means more non-BC components in the

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individual BC-containing particles, and their partly coated states ranged from 0 to 5 ( 0 < MR ≤ 5);

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(3) BC particles with partially encapsulated states are in the range of 0.1 to 10 ( 0.1 ≤ M R ≤ 10 );

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and (4) the mass ratios of heavily coated BC are larger than 5 ( MR > 5 ). The realizations with

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smaller mass ratios can hardly be modeled for BC-containing particles with partially

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encapsulated and heavily coated states for MR < 0.1 and MR < 5, respectively, due to the limited

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space of the non-BC host particle. The fractal parameters of aggregated BC particles vary with

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their aging states. In this study, the fractal prefactor ( k 0 ) is assumed to be 1.2, and the fractal

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dimension ( Df ) is 1.8, 2.4, and 2.8 for partly coated (including bare), partially encapsulated, and

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heavily coated states, respectively. For BC particles with the same mixing states, the variations

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of fractal prefactor and fractal dimension are not considered. Sensitivities of the fractal

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parameters on BC optical properties are discussed in Section S1.2.

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. Typical ranges of this mass ratio are assumed for different aging

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The measured BC masses ( M BC ) are related to the mean radii of BC monomers ( a ), BC mass

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density ( ρ BC ) and monomer numbers of the individual BC particles ( N s ), as the following:

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M BC = ρ BC N s

4 π a3 3

(3)

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where the mass density of BC ( ρ BC ) is assumed to be 1.8 g/cm3 according to the suggestion of

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Bond and Bergstrom26. For the given BC mass ( M BC ), if the mean radii of BC monomers ( a )

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are fixed, the monomer numbers of the individual BC particles ( N s ) can be calculated for the

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modeling of BC aggregates. Note that the monomer numbers should be integers for the

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modeling. The effect of BC monomers radii and numbers on their optical properties is also

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investigated in Section S1.2.

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In the partly coated states, the thickness of the non-BC shell ( a s ) for each monomer can be

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further calculated by the non-BC mass ( Mnon-BC =MP − MBC ):

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M non -BC = ρ non -BC N s

4  3 π ( a + as ) − a 3    3

(4)

where ρnon -BC is the mass density of non-BC component, and it vary for different aerosol types.

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The single non-BC particle is assumed to be a single sphere for partially encapsulated and

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heavily coated states. Their radii ( Rnon -BC ) can be calculated by the following equations:

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M non-BC

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ρnon-BC

4 4 3 + Fs,in Ns π a3 = π Rnon -BC 3 3

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(5)

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where Fs,in is the ratio of BC monomers inside the large non-BC particle. In this study, this ratio

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of BC-containing particles is assumed to be 0.5 for the partially encapsulated states, indicating

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half BC monomers are embedded in the large non-BC host, while half are outside. This ratio

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grows to 1 for the heavily coated states, because all of the BC monomers internally mixed with

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the large non-BC host.

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Optical properties of the aggregate model for the BC-containing aerosols are calculated using the

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superposition T-matrix method41, 42. It uses the numerically exact solution methods to Maxwell’s

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equations, can be used to calculate the T-matrix descriptions of the light scattering from the

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cluster with an appropriate superposition technique and analytically obtain the random-

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orientation cross sections and scattering matrices of these clusters. The superposition T-matrix

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method is applicable to a wide range of particle sizes, and it generates all scattering and

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absorption characteristics of particles. The random orientation scattering properties are obtained

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analytically from the superposition T-matrix method. They need to be averaged by multiple

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calculations for different realizations with the same morphological and chemical parameters to

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reflect the general single scattering properties of aerosol mixtures.

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Cross sections of absorption and scattering ( Cabs and Csca ) were calculated, and corresponding

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mass cross sections (MAC and MSC, respectively) are further computed and normalized, which

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are defined as the cross sections per unit mass of aerosols. For the example of absorptions, the

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normalization of absorption cross sections is divided by the BC mass26, 27:

MAC=Cabs

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4 3 π RBC ρBC 3

(6)

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where RBC and ρBC are the radii of volume-equivalent sphere and the mass density for BC,

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respectively.

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In this aggregate model, the morphological and chemical parameters of BC-containing particles

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can be calculated by the effective radii or masses of BC and non-BC components, and thus the

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proportion of their mixing states is the only supplementary parameter for climate models.

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According to the microphysical measurements18, the proportions of bare, partly coated, partially

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encapsulated, and heavily coated BC can be qualified and considered important indicators for

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regional and seasonal BC emissions. For the applications of climate models, the ranges of non-

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BC/BC mass ratios for different mixing states would vary and be adjustable to reflect their

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proportions in the specific locations and conditions, and it can be determined by the

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simultaneous measurements of their optical properties.

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Single core-shell sphere model using the Mie method

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The radiative properties of soot aerosols in climate models are commonly obtained based on the

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morphological simplification of homogenous spheres for freshly emitted states and the single

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core-shell spheres for aged states. Their optical properties are generally calculated using the

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Lorenz-Mie-Debye theory and Mie Core-Shell model. However, large discrepancies have been

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measured and simulated between the aggregates and the equivalent sphere approximations due to

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their complex morphologies, components and multiple scattering43-45.

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The volume-equivalent radius of BC ( RBC ) is related to their masses and their aggregated

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morphologies, according to the following equations:

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RBC =

3

M BC = 3 Ns a 4 πρ BC 3

(7)

The thickness of the non-BC ( Rnon -BC ) shell is

Rnon−BC = 3

3  M BC M non−BC  +   − RBC 4π  ρBC ρnon−BC 

(8)

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BC aging states are qualified by the augment of non-BC thickness. For freshly emitted BC

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particles, the thickness of the non-BC ( Rnon -BC ) is zero. The size of the BC core is constant, and

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BC aging leads to a thicker non-BC shell. The multi-scattering of aggregated BC monomers in

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the individual BC-containing particles are not considered by the morphologically simplifications

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of the single core-shell sphere model, and BC particles with partially encapsulated states

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frequently found with the microscopy measurements may generate significant effect for

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qualifying BC absorption enhancements5, 32.

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Results and Discussion

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Variations of BC particle morphologies during aging strongly constrain the modeling predictions

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on optical properties of BC-containing aerosols. Ambient and laboratorial observations are given

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to validate these morphologically constrained theoretical calculations during BC aging20.

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Scattering cross sections ( Csca ) of individual BC-containing aerosols are measured by SP2 (DMT

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Inc.) at 1, 064nm, in the ambient (Figure 1A) and laboratory (Figure 1B) environments. Mass

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absorption cross sections (MAC) of laboratory BC aerosols are measured by PASS-3 (DMT Inc.)

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at 532nm (Figure 1C). Figure 1 demonstrates that current climate models using the classical Mie

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Core-Shell model largely overestimate BC scattering and absorption cross sections, and they fail

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to reproduce the variations of M R -dependent optical properties. The consideration of BC

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realistic particle morphologies dependent on aging leads to a substantial improvement of

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modeling estimations on BC optical properties for both bare and heavily coated conditions,

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suggesting a reliable approach to assess BC absorption enhancements by mixing non-BC

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aerosols.

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Figure 1 Measured (yellow) optical properties of black carbon (BC) aerosol compared with

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two simulated results using the Mie Core-Shell model (blue) and the Aggregate model

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(red). The scattering cross sections ( Csca ) dependent on the non-BC/BC mass ratio (

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M R = Massnon − BC MassBC ) of ambient (A) and laboratory (B) BC-containing particles at 1,

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064 nm, and the mass absorption cross sections (MAC) of laboratory BC-containing

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particle ensembles (C) at 532 nm are shown. The scheme of Aggregate model was

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integrated by BC morphologies dependent on the non-BC/BC mass ratio (bottom of A).

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This more realistic model was derived from the previous microphysical photographs by

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China et al.18 (see Figure S1).

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Figure 2 shows the qualified discrepancies between current climate models using the Mie Core-

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Shell model. The observations of BC optical properties varied with the non-BC/BC mass ratio,

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which likely indicates that the simulation errors may be caused by their morphologically

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simplifications. When the mass ratio M R is between 0.1 and 2, the Mie Core-Shell model tends

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to provide greater relative diversity, up to ~120% on Csca and ~30% on MAC. The possible

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reason is that the internal mixing in this model is not suitable for describing the optical properties

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of many BC-containing particles with partly coated and partially encapsulated states, which

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frequently found by microscopy36,

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measurements indicate that BC monomers tend to be aggregated compact and fully coated by

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larger non-BC particles, approaching the single core-shell sphere structure and leading to

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relatively smaller simulation errors on BC optical properties25, 38. The model predictions of M R -

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dependent optical properties are improved by integrating BC realistic morphologies during their

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aging, leading to the decline of simulation errors of MAC from ~20.6% to ~5.1% on average. It

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is suggested that this estimate by morphologically constrained theoretical calculations can

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provide enough accuracy for the assessment of BC absorption enhancements.

46-47

. For the larger mass ratio ( M R ), microphysical

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Figure 2 Relative deviations of the measured optical properties and their simulations (

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RD ( sim, mea ) = Csim − Cmea Cmea ×100% ). The Mie Core-Shell model (blue) and the

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Aggregate model (red) are applied for simulations. The M R -dependent Csca of both

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ambient and laboratory BC-containing particles at 1, 064 nm (A), and the MAC of

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laboratory BC-containing particle ensembles (B) at 532 nm, are compared. The averaged

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relative deviations of the two models are plotted by the dotted lines.

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Figure 3 demonstrates that Eabs continuously increases with aging and finally reaches a stable

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range, with a maximum of ~3.5, showing a multi-stage process similar to Peng et al.12. At the

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start stage of BC aging with the non-BC/BC mass ratio less than ~1, BC absorption enhancement

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is limited to less than 5%. When M R is between ~1 and ~200, E abs rapidly rises with the

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augment of the non-BC mass. The averaged E abs becomes stable from the estimations to be ~2.5

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for larger M R , but floating up to ~3.5, due to particle morphologies and sizes. In this study, the

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BC particles with different masses are simulated, from 0.25-10fg. The gray area indicates the

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simulated range between minimum and maximum values of Eabs . Qualified descriptions of Eabs

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during aging contribute to a better understanding of previous ambient and laboratorial

289

observations (Figure 3). Cappa et al. indicated a factor of 1.06 on Eabs by ambient

290

measurements and attribute it to the limited Eabs caused by the commonly found BC particles

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with partially encapsulated states4. This situation is reproduced by the aged BC particles, with

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M R in the range of 1 to 3, corresponding to the aging stage with the BC particles that partially

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encapsulated and thinly coated in modeling. This small magnitude of Eabs is likely observed in

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the ambient environments with partly aged BC aerosol, largely due to the continued fresh

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emissions in their observation sites. Recent measurements suggested a range of 2.3 to 2.7 for

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Eabs , and it is in agreement with the modeling results after fully aging12, 48. Although there are

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large spatial and seasonal variabilities in the ambient BC absorption, the dynamic evolutions of

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M R -dependent E abs constrained by the particle morphologies will benefit the parameterization

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of BC absorption in regional and global climate models. Future work is needed for the

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representations of aging time scales in different regions and seasons, as a function of

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morphologies dependent on M R during BC aging.

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Figure 3 Light absorption enhancement ( E abs = C abs , coated C abs , bare ) of the simulations using

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the Aggregate model. The results of E abs are affected by BC size and morphologies (gray

306

area), and their MR-dependent mean E abs are red points. The simulated M R -dependent

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E abs results are compared with the previous measurements (right), and the simulations of

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the Aggregate model (red) and Mie Core-Shell model (blue) during the Rise stage and

309

Stable stage are also compared in the left-top sub-figure.

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At the rapidly rise stage of E abs , current climate models using the Mie Core-Shell model may

312

overestimate E abs , leading to the systematic discrepancy between model and observation

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estimates of E abs . This stage is important for the assessments of BC climate impact, because

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major individual BC-containing particles may lie in this stage with M R in the range of ~1 to

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~200, in the ambient environments with low relative humidity. For example, the volume-

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equivalent radii of dry sulfate typically lie in the range between 0.2-1µm, according to previous

317

observations49, and the mass ratio of sulfate and BC is limited to ~20 correspondingly. For the

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partly coated and partially encapsulated states of BC aerosols, previous studies also indicated

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that the large systematic discrepancy of optical properties between Mie Core-Shell model and

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observations can be reduced by considering BC complex particle morphologies43-45. However,

321

the diversity would be eliminated for fully aged BC, possibly because the BC monomers had a

322

more compact aggregation and their contributions became limited with the augment of the non-

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BC components. It is also shown in the left-top sub-figure of Figure 3 that the overestimation of

324

the Mie Core-Shell model on Eabs varied with M R , at the rise stage of Eabs . During the aging

325

process, Eabs is overestimated to a maximum of ~100%, showing a considerable uncertainty in

326

the modeling description of BC aerosol. A correction coefficient is suggested to improve the Mie

327

Core-Shell

328

Eabs = 0.92 + 0.11× e

model

predictions MieCS −1.07 0.55

on

Eabs

by

an

exponential

fitting

function:

(see Figure S13).

329 330

A general description on the variations of Eabs with BC aging is provided for the

331

parameterization of BC absorption in climate models, suggesting a clear solution to settle

332

disputes on the aging-enhancement of BC absorption. The small observed values of Eabs

333

correspond to a partly aging stage with the BC particles that partially encapsulated and are thinly

334

coated. The large results of Eabs are produced by heavily coated BC particles in their fully aged

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stage. Cappa et al concluded that climate models using the Mie Core-Shell structure may

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overestimate the magnitudes of Eabs from ambient observations5. Our results confirm this

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overestimation, but indicating that it only happens in the certain aging stage with rapidly

338

increasing Eabs . Current climate models would benefit from the suggested exponential function

339

with a correction coefficient, because predictions of Eabs varying with BC aging are largely

340

improved by the morphologically constrained theoretical modeling.

341 342

Our analysis shows that change of BC particle morphologies depending on aging is an important

343

constraint of Eabs . The magnitude of Eabs is significant to be a maximum of ~3.5 after fully

344

aging, supporting the key role of BC aerosol in global warming. Our results indicate that the

345

underestimation of absorption enhancement of ambient BC is largely due to the fractal

346

aggregated structure and particle heterogeneity. The fractal aggregated structure of carbonaceous

347

aerosol is widely observed by microscopy, exhibiting strong correlations with their optical

348

properties. Moreover, the absorption of these heterogeneous particles could be affected by

349

variation of mixing state during aging. According to the qualified evolution of Eabs ,

350

understanding of the spatial and temporal variabilities of Eabs should be further studied by

351

investigating BC aging scales and their corresponding morphologies in different environments.

352

Moreover, the morphologies and heterogeneity of brown carbon (BrC) and BrC-containing

353

particles should also be the subject of future work to assess the radiative forcing of light-

354

absorbing carbonaceous aerosol. The attachment of organics on the carbonaceous particles may

355

generate variation of the microphysical properties, potentially influence the climate forcing50, 51.

356

The morphologically constrained parameterizations of light absorbing carbonaceous aerosol in

357

climate models would be helpful to assess net climate impact.

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Corresponding Author

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*Correspondence to: [email protected] and [email protected]

360

Author Contributions

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Y. Wu, T. Cheng and D. Liu designed the research; D. Liu and J. D. Allan prepared

362

observations; Y. Wu, and L. Zheng performed model simulations; Y. Wu, T. Cheng, and H.

363

Chen performed data analysis, Y. Wu and D. Liu wrote the paper.

364

Funding Sources

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This research was supported by the National Key Research and Development Program of China

366

(2017YFC0212302), National Natural Science Foundation of China (41401386, 41371015,

367

41001207 and 41501401), the Major Special Project-the China High-Resolution Earth

368

Observation System (30-Y20A21-9003-15/17), and the UK Natural Environment Research

369

Council project COM-PART (Grant ref: NE/K014838/1). The Manchester chamber has received

370

funding from the European Union's Framework 7 EUROCHAMP2 Network and currently from

371

the Horizon 2020 research and innovation programme through the EUROCHAMP-2020

372

Infrastructure Activity under grant agreement no. 730997.

373

Notes

374

The authors declare that they have no competing financial interests.

375

Acknowledgement

376

The authors appreciate Dr. Dantong Liu and his cooperators for the use of laboratory and field

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ambient data in ref. 20.

378

We thank Dr. Daniel Mackowski and Dr. Michael Mishchenko for the code of the superposition

379

T-Matrix method (MSTM) (http://www.eng.auburn.edu/users/dmckwski/scatcodes/).

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Supporting Information

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Comparisons with ambient and laboratorial measurements; MR-dependent scattering and

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absorption cross sections; Sensitivity of morphological parameters; Sensitivity of refractive

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indices; Comparison with previous observed BC absorption enhancements.

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