Subscriber access provided by UNIV OF WESTERN ONTARIO
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 28
Environmental Science & Technology
1
Light absorption enhancement of black carbon
2
aerosol constrained by particle morphology
3
Yu Wu1, Tianhai Cheng1*, Dantong Liu2*, James D. Allan2, 3, Lijuan Zheng4, Hao Chen1
4 5
1
6
Earth, Chinese Academy of Sciences, China
7
2
8
Manchester, Manchester M13 9PL, UK
9
3
National Centre for Atmospheric Science, University of Manchester, Manchester M13 9PL, UK
10
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
12 13
*Correspondence to:
[email protected] and
[email protected] 14 15
ACS Paragon Plus Environment
1
Environmental Science & Technology
Page 2 of 28
16
Abstract
17
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
23
~3.5, suggesting that previous seemingly contrast results of E abs can be explicitly described by
24
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
27
overestimation is suggested to improve model predictions of BC climate impact.
28 29
ACS Paragon Plus Environment
2
Page 3 of 28
Environmental Science & Technology
30
Introduction
31
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
36
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
40
enhancements5-7. In current climate models, Eabs is either assumed to be a constant enhancement
41
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
43
enhancements can reach to ~3.5 in some experimental conditions10. In contrast, field
44
observations in urban environments indicated E abs is a quite small value of 1.06 after BC aging5.
45
For the estimations of BC direct radiative forcing, the ratio of the highest to the lowest is at least
46
twice due to the unclear parameterizations of Eabs 11, 12. Until now, a proper description of Eabs
47
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
50
in bare conditions are aggregated by small carbon spherules8, 13. The particles tend to be coated
51
with a thin layer of other aerosol components in the atmosphere through the coagulation and
ACS Paragon Plus Environment
3
Environmental Science & Technology
15
Page 4 of 28
52
condensation of secondary aerosol compounds14,
53
mixing with other larger aerosols, such as sulfate, organics, dust, and sea salt. Particle
54
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.
60
Recent microphysical studies indicate that BC aging causes not only growth of co-emitted
61
aerosols but also dramatic changes in particle morphology, which highly affects their optical
62
properties18, 19. The optical properties of individual BC-containing particles are validated by the
63
comprehensive laboratory and field ambient data reported by Liu et al.20. Our results indicate that
64
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
66
considering particle morphology if BC aging states are exactly obtained. It is also found that
67
Eabs may be overestimated by current climate models in a certain aging range with Eabs rapidly
68
rising, which widely appears in the ambient, leading to a misunderstanding on the estimations of
69
Eabs . Further, a correction coefficient is suggested for climate models on the parameterizations of
70
BC absorption properties.
71
ACS Paragon Plus Environment
4
Page 5 of 28
Environmental Science & Technology
72
Materials and Methods
73
Aging mechanism of black carbon particles
74
Recent studies suggested that the morphology of BC-containing particles can be quantified into
75
four categories by China et al.18, 21: bare, partly coated (thinly coated), partially encapsulated, and
76
heavily coated (see left column of Figure S1). According to microscopy images, freshly emitted
77
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
84
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-
87
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
ACS Paragon Plus Environment
5
Environmental Science & Technology
Page 6 of 28
92
Particle morphology of emitted BC particles changes rapidly after emission, and their absorption
93
tends to be enhanced during condensation and coagulation processes30, 31. The mass ratio of non-
94
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
96
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
ACS Paragon Plus Environment
6
Page 7 of 28
Environmental Science & Technology
114
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
ACS Paragon Plus Environment
7
Environmental Science & Technology
Page 8 of 28
136
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
155
ACS Paragon Plus Environment
8
Page 9 of 28
Environmental Science & Technology
156
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:
ACS Paragon Plus Environment
9
Environmental Science & Technology
M non-BC
173
ρnon-BC
4 4 3 + Fs,in Ns π a3 = π Rnon -BC 3 3
Page 10 of 28
(5)
174
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
176
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
186
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
ACS Paragon Plus Environment
10
Page 11 of 28
Environmental Science & Technology
193
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
ACS Paragon Plus Environment
11
Environmental Science & Technology
Page 12 of 28
213
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
223
particles, the thickness of the non-BC ( Rnon -BC ) is zero. The size of the BC core is constant, and
224
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.
229
ACS Paragon Plus Environment
12
Page 13 of 28
Environmental Science & Technology
230
Results and Discussion
231
Variations of BC particle morphologies during aging strongly constrain the modeling predictions
232
on optical properties of BC-containing aerosols. Ambient and laboratorial observations are given
233
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
236
absorption cross sections (MAC) of laboratory BC aerosols are measured by PASS-3 (DMT Inc.)
237
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.
244
ACS Paragon Plus Environment
13
Environmental Science & Technology
Page 14 of 28
245 246
Figure 1 Measured (yellow) optical properties of black carbon (BC) aerosol compared with
247
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 (
249
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).
255
ACS Paragon Plus Environment
14
Page 15 of 28
Environmental Science & Technology
256
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
261
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
ACS Paragon Plus Environment
15
Environmental Science & Technology
Page 16 of 28
272 273
Figure 2 Relative deviations of the measured optical properties and their simulations (
274
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
ACS Paragon Plus Environment
16
Page 17 of 28
Environmental Science & Technology
286
BC particles with different masses are simulated, from 0.25-10fg. The gray area indicates the
287
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.
302
ACS Paragon Plus Environment
17
Environmental Science & Technology
Page 18 of 28
303 304
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
ACS Paragon Plus Environment
18
Page 19 of 28
Environmental Science & Technology
315
~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
ACS Paragon Plus Environment
19
Environmental Science & Technology
Page 20 of 28
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.
ACS Paragon Plus Environment
20
Page 21 of 28
Environmental Science & Technology
358
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/).
ACS Paragon Plus Environment
21
Environmental Science & Technology
Page 22 of 28
380
Supporting Information
381
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
References
385
1. Stocker, T. F.; Qin, D.; Plattner, G. K.; Tignor, M.; Allen, S. K.; Boschung, J.; Midgley, P. M.
386
Climate change 2013: the physical science basis. Intergovernmental panel on climate change,
387
working group I contribution to the IPCC fifth assessment report (AR5). New York. 2013
388
2. Ramanathan, V.; Carmichael, G. Global and regional climate changes due to black carbon.
389
Nat. Geosci. 2008, 1(4), 335-358.
390
3. Boucher, O.; Balkanski, Y.; Hodnebrog, Ø.; Myhre, C. L.; Myhre, G.; Quaas, J.; Samset, B.
391
H.; Schutgens, N.; Stier, P.; Wang, R. Jury is still out on the radiative forcing by black carbon. P
392
Natl. Acad. Sci. USA 2016, 113(35), E5092-E5093.
393
4. Gustafsson, Ö.; Ramanathan, V. Convergence on climate warming by black carbon aerosols. P
394
Natl. Acad. Sci. USA 2016, 113(16), 4243-4245.
395
5. Cappa, C. D.; Onasch, T. B.; Massoli, P.; Worsnop, D. R.; Bates, T. S.; Cross, E. S.;
396
Davidovits, P.; Hakala, J.; Hayden, K. L.; Jobson, B. T.; Kolesar, K. R. Radiative absorption
397
enhancements due to the mixing state of atmospheric black carbon. Science 2012, 337(6098),
398
1078-1081.
399
6. Jacobson, M. Z. Comment on “Radiative Absorption Enhancements Due to the Mixing State
400
of Atmospheric Black Carbon”. Science 2013, 339, 1078.
401
7. Cappa, C. D.; Onasch, T. B.; Massoli, P.; Worsnop, D. R.; Bates, T. S.; Cross, E. S.;
402
Davidovits, P.; Hakala, J.; Hayden, K. L.; Jobson, B. T.; Kolesar, K. R. Response to comment
ACS Paragon Plus Environment
22
Page 23 of 28
Environmental Science & Technology
403
on" Radiative absorption enhancements due to the mixing state of atmospheric black carbon".
404
Science 2013, 339(6118): 393-393.
405
8. Bond, T. C.; Doherty, S. J.; Fahey, D. W.; Forster, P. M.; Berntsen, T.; DeAngelo, B. J.;
406
Flanner, M. G.; Ghan, S.; Kärcher, B.; Koch, D.; Kinne, S.; Kondo, Y.; Quinn, P. K.; Sarofim,
407
M. C.; Schultz, M. G.; Schulz, M.; Venkataraman, C.; Zhang, H.; Zhang, S.; Bellouin, N.;
408
Guttikunda, S. K.; Hopke, P. K.; Jacobson, M. Z.; Kaiser, J. W.; Klimont, Z.; Lohmann, U.;
409
Schwarz, J. P.; Shindell, D.; Storelvmo, T.; Warren, S. G.; Zender, C. S.Bounding the role of
410
black carbon in the climate system: A scientific assessment. J. Geophys. Res-Atmos. 2013,
411
118(11), 5380-5552.
412
9. Jacobson, M. Z. Short-term effects of controlling fossil-fuel soot, biofuel soot and gases, and
413
methane on climate, Arctic ice, and air pollution health. J. Geophys. Res-Atmos. 2010, 115(D14),
414
1307-1314.
415
10. Mikhailov, E. F.; Vlasenko, S. S.; Podgorny, I. A.; Ramanathan, V.; Corrigan, C. E. Optical
416
properties of soot-water drop agglomerates: An experimental study. J. Geophys. Res-Atmos.
417
2006, 111, 1576-1585.
418
11. Jacobson, M. Z. Strong radiative heating due to the mixing state of black carbon in
419
atmospheric aerosols. Nature 2001, 409, 695-697.
420
12. Peng, J.; Hu, M.; Guo, S.; Du, Z.; Zheng, J.; Shang, D.; Zamora, M. L.; Zeng, L.; Shao, M.;
421
Wu, Y. S.; Zheng, J.; Wang, Y.; Glen, C. R.; Collins, D. R.; Molina, M. J.; Zhang, R. Markedly
422
enhanced absorption and direct radiative forcing of black carbon under polluted urban
423
environments. P Natl. Acad. Sci. USA 2016, 113, 4266-4271.
ACS Paragon Plus Environment
23
Environmental Science & Technology
Page 24 of 28
424
13. Wang, Y.; Liu, F.; He, C.; Bi, L.; Cheng, T.; Wang, Z.; Zhang, H.; Zhang, X.; Shi, Z.; Li, W.
425
Fractal Dimensions and Mixing Structures of Soot Particles during Atmospheric Processing.
426
Environ. Sci. Technol. Lett. 2017, 4(11), 487-493.
427
14. Adachi, K.; Buseck, P. R. Internally mixed soot, sulfates, and organic matter in aerosol
428
particles from Mexico City. Atmos. Chem. Phys. 2008, 8, 6469-6481.
429
15. Castro, L. M.; Pio, C. A.; Harrison, R. M.; Smith, D. J. T. Carbonaceous aerosol in urban and
430
rural European atmospheres: estimation of secondary organic carbon concentrations. Atmos.
431
Environ. 1999, 33(17), 2771-2781.
432
16. Zhang, R.; Wang, G.; Guo, S.; Zamora, M. L.; Ying, Q.; Lin, Y.; Wang, W.; Hu, M.; Wang,
433
Y. Formation of urban fine particulate matter. Chem. Rev. 2015, 115(10), 3803-3855.
434
17. Pósfai, M.; Buseck, P. R. Nature and climate effects of individual tropospheric aerosol
435
particles. Annu. Rev. Earth pl. Sci. 2010, 38, 17-43.
436
18. China, S.; Mazzoleni, C.; Gorkowski, K.; Aiken, A. C.; Dubey, M. K. Morphology and
437
mixing state of individual freshly emitted wildfire carbonaceous particles. Nat. Commun. 2013,
438
4, 2122.
439
19. Wu, Y.; Cheng, T.; Zheng, L.; Chen, H. Effect of morphology on the optical properties of
440
soot aggregated with spheroidal monomers. J. Quant. Spectrosc. Radiat. Transfer. 2016, 168,
441
158-169.
442
20. Liu, D.; Whitehead, J.; Alfarra, M. R.; Reyes-Villegas, E.; Spracklen, D. V.; Reddington, C.
443
L.; Kong, S.; Williams, P. I.; Ting, Y.; Haslett, S.; Taylor, J. W.; Flynn, M. J.; Morgan, W. T.;
444
McFiggans, G.; Coe, H.; Allan J. D. Black-carbon absorption enhancement in the atmosphere
445
determined by particle mixing state. Nat. Geosci. 2017, 10, 184-188.
ACS Paragon Plus Environment
24
Page 25 of 28
Environmental Science & Technology
446
21. China, S.; Scarnato, B.; Owen, R. C.; Zhang, B.; Ampadu, M. T.; Kumar, S.; Dzepina, K.;
447
Dziobak, M. P.; Fialho, P.; Perlinger, J. A.; Hueber, J.; Helmig, D.; Mazzoleni, L. R.; Mazzoleni,
448
C. Morphology and mixing state of aged soot particles at a remote marine free troposphere site:
449
Implications for optical properties. Geophys. Res. Lett. 2015, 42(4): 1243-1250.
450
22. Adachi, K.; Chung, S. H.; Friedrich, H.; Buseck, P. R. Fractal parameters of individual soot
451
particles determined using electron tomography: Implications for optical properties. J. Geophys.
452
Res-Atmos. 2007, 112(D14), 161-167.
453
23. Chakrabarty, R. K.; Beres, N. D.;, Moosmüller, H.; China, S.; Mazzoleni, C.;, Dubey, M. K.;
454
Liu L.; Mishchenko, M. I. (2014). Soot superaggregates from flaming wildfires and their direct
455
radiative forcing. Sci. Rep. 2014, 4, 5508.
456
24. Cheng, T.; Wu, Y.; Chen, H. Effects of morphology on the radiative properties of internally
457
mixed light absorbing carbon aerosols with different aging status. Opt. Express 2014, 22, 15904-
458
15917.
459
25. Mishchenko, M. I.; Liu, L.; Cairns, B.; Mackowski, D. W. Optics of water cloud droplets
460
mixed with black-carbon aerosols. Opt. Lett. 2014, 39, 2607-2610.
461
26. Bond, T. C.; Bergstrom, R. W. Light absorption by carbonaceous particles: An investigative
462
review. Aerosol Sci. Technol. 2006, 40, 27-67.
463
27. Adler, G.; Riziq, A. A.; Erlick, C.; Rudich, Y. Effect of intrinsic organic carbon on the
464
optical properties of fresh diesel soot. P Natl. Acad. Sci. USA 2010, 107(15), 6699-6704.
465
28. He, C.; Liou, K. N.; Takano, Y.; Zhang, R.; Levy Zamora, M.; Yang, P.; Li, Q.; Leung, L. R.
466
(2015). Variation of the radiative properties during black carbon aging: theoretical and
467
experimental intercomparison. Atmos. Chem. Phys. 2015, 15(20), 11967-11980.
ACS Paragon Plus Environment
25
Environmental Science & Technology
Page 26 of 28
468
29. Wu, Y.; Cheng, T.; Zheng, L.; Chen, H. Black carbon radiative forcing at TOA decreased
469
during aging. Sci. Rep-UK 2016, 6, 38592.
470
30. Zhang, R.; Peng, J.; Wang, Y.; Hu, M. Reply to Boucher et al.: Rate and timescale of black
471
carbon aging regulate direct radiative forcing. Proc. Natl. Acad. Sci. USA. 2016, 113, E5094-
472
E5095.
473
31. Liu, S.; Aiken, A. C.; Gorkowski, K.; Dubey, M. K.; Cappa, C. D.; Williams, L. R.; Herndon,
474
S. C.; Massoli, P.; Fortner, E. C.; Chhabra, P. S.; Brooks, W. A.; Onasch, T. B.; Jayne, J. T.;
475
Worsnop, D. R.; China, S.; Sharma, N.; Mazzoleni, C.; Xu, L.; Ng, N. L.; Liu, D.; Allan, J. D.;
476
Lee, J. D.; Fleming, Z. L.; Mohr, C.; Zotter, P.; Szidat, S. Enhanced light absorption by mixed
477
source black and brown carbon particles in UK winter. Nat. Commun. 2015, 6, 8345.
478
32. Kahnert, M.; Nousiainen, T.; Lindqvist, H. Review: Model particles in atmospheric optics. J.
479
Quant. Spectrosc. Radiat. Transfer 2014, 146, 41-58.
480
33. Wu, Y.; Gu, X.; Cheng, T.; Xie, D.; Yu, T.; Chen, H.; Guo, J. The single scattering
481
properties of the aerosol particles as aggregated spheres. J. Quant. Spectrosc. Radiat. Transfer
482
2012, 113(12), 1454-1466.
483
34. Wu, Y.; Cheng, T.; Gu, X.; Zheng, L.; Chen, H.; Xu, H. The single scattering properties of
484
soot aggregates with concentric core–shell spherical monomers. J. Quant. Spectrosc. Radiat.
485
Transfer 2014, 135, 9-19.
486
35. Wu, Y.; Cheng, T.; Zheng, L.; Chen, H. Models for the optical simulations of fractal
487
aggregated soot particles thinly coated with non-absorbing aerosols. J. Quant. Spectrosc. Radiat.
488
Transfer 2016, 182, 1-11.
ACS Paragon Plus Environment
26
Page 27 of 28
Environmental Science & Technology
489
36. Kahnert, M. Optical properties of black carbon aerosols encapsulated in a shell of sulfate:
490
comparison of the closed cell model with a coated aggregate model. Opt. Express 2017, 25,
491
24579-24593.
492
37. Wu, Y.; Cheng, T.; Zheng, L.; Chen, H.; Xu, H. Single scattering properties of semi-
493
embedded soot morphologies with intersecting and non-intersecting surfaces of absorbing
494
spheres and non-absorbing host. J. Quant. Spectrosc. Radiat. Transfer 2015, 157, 1-13.
495
38. Adachi, K.; Chung, S. H.; Buseck, P. R. Shapes of soot aerosol particles and implications for
496
their effects on climate. J. Geophys. Res-Atmos. 2010, 115, 4447-4458.
497
39. Cheng, T.; Wu, Y.; Gu, X.; Chen, H. Effects of mixing states on the multiple-scattering
498
properties of soot aerosols. Opt. Express 2015, 23(8), 10808-10821.
499
40. Ramana, M. V.; Ramanathan, V.; Feng, Y.; Yoon, S. C.; Kim, S. W.; Carmichael, G. R.;
500
Schauer, J. J. Warming influenced by the ratio of black carbon to sulphate and the black-carbon
501
source. Nat Geosci. 2010, 3(8), 542-545.
502
41. Mackowski, D. W.; Mishchenko, M. I. A multiple sphere T-matrix Fortran code for use on
503
parallel computer clusters. J. Quant. Spectrosc. Radiat. Transfer 2011, 112, 2182-2192.
504
42. Mackowski, D. W. A general superposition solution for electromagnetic scattering by
505
multiple spherical domains of optically active media. J. Quant. Spectrosc. Radiat. Transfer 2014,
506
133, 264-270.
507
43. Liu, L.; Mishchenko, M. I.; Arnott, W. P. A study of radiative properties of fractal soot
508
aggregates using the superposition T-matrix method. J. Quant. Spectrosc. Radiat. Transfer 2008,
509
109, 2656-2663.
510
44. Kahnert, M. On the discrepancy between modeled and measured mass absorption cross
511
sections of light absorbing carbon aerosols. Aerosol Sci. Technol. 2010, 44, 453-460.
ACS Paragon Plus Environment
27
Environmental Science & Technology
Page 28 of 28
512
45. Yon, J.; Liu, F.; Bescond, A.; Caumont-Prim, C.; Rozé, C.; Ouf, F. X.; Coppalle, A. Effects
513
of multiple scattering on radiative properties of soot fractal aggregates. J. Quant. Spectrosc.
514
Radiat. Transfer 2014, 133, 374-381.
515
46. Wu, Y.; Cheng, T.; Zheng, L.; Chen, H. Sensitivity of mixing states on optical properties of
516
fresh secondary organic carbon aerosols. J. Quant. Spectrosc. Radiat. Transfer J. 2017, 195,
517
147-155.
518
47. Liu, F.; Yon, J.; Bescond, A. On the radiative properties of soot aggregates-Part 2: Effects of
519
coating. J. Quant. Spectrosc. Radiat. Transfer. 2016, 172, 134-145.
520
48. Cui, X.; Wang, X.; Yang, L.; Chen, B.; Chen, J.; Andersson, A.; Gustafsson, Ö. Radiative
521
absorption enhancement from coatings on black carbon aerosols. Sci. Total Environ. 2016, 551.
522
51-56.
523
49. Yao, X.; Lau, A. P.; Fang, M.; Chan, C. K.; Hu, M. Size distributions and formation of ionic
524
species in atmospheric particulate pollutants in Beijing, China: 1-inorganic ions. Atmos. Environ.
525
2003, 37, 2991-3000.
526
50. Liu, D.; Taylor, J. W.; Young, D. E.; Flynn, M. J.; Coe, H.; Allan, J. D. The effect of
527
complex black carbon microphysics on the determination of the optical properties of brown
528
carbon. Geophys. Res. Lett. 2015, 42, 613-619.
529
51. Chakrabarty, R. K.; Gyawali, M.; Yatavelli, R. L. N.; Pandey, A.; Watts, A. C.; Knue, J.;
530
Chen, L. A.; Pattison, R. R.; Tsibart, A.; Samburova, V.; Moosmüller, H. Brown carbon aerosols
531
from burning of boreal peatlands: microphysical properties, emission factors, and implications
532
for direct radiative forcing. Atmos. Chem. Phys. 2016, 16, 3033-3040.
533
ACS Paragon Plus Environment
28