Light absorption enhancement of black carbon aerosol constrained by

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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

8

Manchester, Manchester M13 9PL, UK

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3

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

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4

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

11

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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ACS Paragon Plus Environment

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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

18

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

19

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

20

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

21

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

22

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

25

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

26

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

32

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

33

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

34

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

35

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,

37

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

38

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

39

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

42

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.

48 49

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,

55

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

56

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

. Further aging may lead to BC particles

57 58

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

59

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

65

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

78

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

79

Df  Rg  Ns = k 0    a 

80

Rg2 =

1 Ns

Ns

∑r

2

i

(1)

(2)

i =1

81

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

82

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

83

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

85

aggregate radius, and

86

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

88

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

89

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

90

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.

91


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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

95

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

97

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

98

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

99

(SP2, DMT)

20

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

100

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

101

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

102

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

103

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

104

Supporting Information.

105 106

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

107

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

108

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

109

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

110

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

111

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

112

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

113

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

115

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

116

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

117

(see Figure S1 of supplementary materials).

118 119

Aggregate model using the superposition T-matrix method

120

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

121

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

122

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

123

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

124

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

125

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

126

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

127

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

128

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

129

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

130

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

131

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

132

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

133

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

134

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

135

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.

137

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

138

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

139 140

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

141

and BC components ( MR )

142

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

143

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

144

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

145

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

146

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

147

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

148

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

149

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

150

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

151

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

152

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

153

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

154

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

40

. 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

157

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

158

M BC = ρ BC N s

4 π a3 3

(3)

159

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

160

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

161

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

162

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

163

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

164

investigated in Section S1.2.

165 166

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

167

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

168

169

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.

170 171

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

172

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


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M non-BC

173

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

175

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

177

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

178

the large non-BC host.

179 180

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

181

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

182

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

183

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

184

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

185

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

187

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

188

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

189

reflect the general single scattering properties of aerosol mixtures.

190 191

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

192

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

194

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

MAC＝Cabs

195

4 3 π RBC ρBC 3

(6)

196

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

197

respectively.

198 199

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

200

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

201

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

202

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

203

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

204

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

205

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

206

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

207

simultaneous measurements of their optical properties.

208 209

Single core-shell sphere model using the Mie method

210

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

211

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

212

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

214

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

215

their complex morphologies, components and multiple scattering43-45.

216 217

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

218

morphologies, according to the following equations:

219

220

221

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)

222

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

225

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

226

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

227

frequently found with the microscopy measurements may generate significant effect for

228

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.

234

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

235

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

238

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

239

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

240

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

241

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

242

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

243

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

248

(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,

250

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

251

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

252

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

253

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

254

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-

257

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

258

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

259

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

260

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

262

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

263

frequently found by microscopy36,

264

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

265

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

266

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

267

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

268

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

269

is suggested that this estimate by morphologically constrained theoretical calculations can

270

provide enough accuracy for the assessment of BC absorption enhancements.

46-47

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

271


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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

275

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

276

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

277

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

278

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

279

280

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

281

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

282

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

283

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

284

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

285

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

288

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

291

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

292

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

305

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

307

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

308

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.

310 311

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

313

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

314

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-

316

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

318

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

319

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

320

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-

323

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

335

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


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336

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

337

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

359

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

360

Author Contributions

361

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

365

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

377

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

382

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

383

indices; Comparison with previous observed BC absorption enhancements.

384

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Light absorption enhancement of black carbon aerosol constrained by

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