Optical properties of bare BC aggregates
The spectral dependence of mass extinction, absorption and scattering
coefficients (MEC, MAC, MSC) is presented in Fig. for an
ensemble of synthetic open-chain-like aggregates, as described in
Table (and with a size parameter X=2πaeff/λ<4.5). Large differences are found in optical
properties of BC aggregates compared to equivalent volume spherical
particles, biases in the numerical simulations and relevance for radiative
forcing estimates are discussed in and
.
It is well known that bare/uncoated fresh BC absorbs more radiation than it
scatters . Therefore, MAC represents the dominant contributor
to the MEC. report BC MAC values larger than
5 m2 g-1. Predicted values of MEC, MAC and MSC (see
Eqs. –) are shown in Fig. for a composite of BC
aggregates with similar porosity and monomers size but increasing monomer
numbers (see Table ). MAC values are strongly wavelength
dependent (see also , and ). At
550 nm MAC predicted values, using a BC density (ρ) of
1.8 g cm-3 , ranging between 5.32 and
5.65 m2 g-1 and that are not strongly sensitive to the aggregate
size. The latter finding is in agreement with the fractal theory by
, which maintains that the mass absorption coefficient
should not be a strong function of the size, but rather a strong function of
the refractive index and physical shape (as well as mixing)
e.g.,.
The range of predicted MAC values at 550 nm is in agreement with field
measurements by and modeled values by
and .
(a) SSA and (b) g spectral dependency for bare
BC aggregates (cases BL1–BL4). All the computed optical values are
averaged over 1000 particle orientations.
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However, several studies e.g.,
report larger values. Reasons might be related to different indices of
refraction or density values; for instance, the values predicted here are
lower than the published values at 550 nm by because of
differences in the adopted refractive indices. At a wavelength of 550 nm,
the refractive index by , adopted in these simulations, has
lower real and imaginary indices than the value of 1.95–0.79i recommended
by (see Table ), which was adopted in
simulations by . In this study, as in
, we used a BC ρ1 value of 1.8 g cm-3. If
we use a value of ρ1 equal to 1.4 g cm-3 and the
refractive index, we find for the cases BL1–BL4 MAC values
at 550 nm of about 7 m2 g-1. As a reminder, the OPAC (Optical
Properties of Aerosols and Clouds) code uses a
density value as low as 1 g cm-3 for BC. MSC and SSA values, as shown
in Fig. , are slightly more sensitive to the aggregates size
than to the MAC (see also , for the SSA dependence on
aggregate compactness). SSA values are lower than those predicted by
, due as well to the differences in the refractive
indices used in the simulations. The SSA magnitude and spectral variation
presented in this study are both in agreement with laboratory measurements by
. At 550 nm, SSA shows little variability in cases
BL1–BL4 with an average value of 0.19 ± 0.02. Just looking at MAC and
MEC, one could argue that the implementation of optical properties of bare BC
aggregates in chemical transport and radiative transfer models might be
greatly facilitated by the fact that some of the properties of BC aggregates
are little sensitive to aggregate size in the UV, Vis and NIR spectra. Such a property would reduce the need
for complex parametrizations of BC aggregates' optical properties to
accurately model the chain structure, and monomer size of the aggregate (see
, for sensitivity to monomer size). This assumption fails
when looking at the asymmetry parameter (g) spectral dependency for the
cases BL1–BL4 in Fig. b, where g presents a strong
sensitivity to the BC aggregate size in the entire spectral range under
study. In Fig. , DDSCAT predicts the lowest g values for
the BL1 case, intermediate values for the case BL4 and higher values for
cases BL2 and BL3. For wavelengths longer than 800 nm the differences in g
values between the cases BL2, BL3 and BL4 are minimized.
AAE, EAE and SAE values are wavelength dependent (see
Table and ). In the spectral range
between 340 and 1600 nm, AAE values are consistent with observations and
theoretical results with values of approximately 1, while in the spectral
range between 400 and 675 nm AAE values approach 1.2 (in agreement with
). The range of values of AAE, EAE and SAE is also fairly
consistent with . For example, we found a SAE average
value of 1.79 ± 0.37, which is in the range of values reported by
of 1.61 ± 0.05 and by
of 1.88.
Summary of optical properties predicted by DDSCAT for bare BC
aggregates at 550 nm. AAE and EAE have been calculated in different
wavelength ranges: (a) 340–1000 nm, (b) 400–675 nm and (c) 340–1600 nm
(spectral range not shown in Fig. ). MAC and MEC values
are provided at 550 nm. MSC values have not been included in the table, as
they can be calculated by the difference between MEC and MAC values.
Case
AAE
EAE
SAE
MAC(550)
MEC(550)
SSA(550)
g(550)
a (b) (c)
a (b) (c)
a (b) (c)
m2 g-1
m2 g-1
BL1
1.26 (1.24) (1.04)
1.36 (1.33) (1.15)
1.49 (1.32) (1.82)
5.60
6.80
0.17
0.42
BL2
1.24 (1.23) (1.11)
1.38 (1.41) (1.26)
1.58 (2.20) (2.32)
5.72
6.93
0.17
0.60
BL3
1.18 (1.21) (1.05)
1.29 (1.32) (1.18)
1.33 (1.73) (1.90)
5.38
6.94
0.22
0.73
BL4
1.18 (1.18) (1.04)
1.27 (1.35) (1.15)
1.60 (1.92) (1.70)
5.53
6.98
0.20
0.76
Optical properties of mineral dust
In all the field campaigns presented here, we have found mineral dust
particles with jagged surfaces and irregular shape (see Fig. ).
In particular, in Fig. a, c, and d dust particles were found to be
silica rich and with a plate-like morphology. We found that the DDSCAT-predicted optical properties have a large variability depending on the
modeled dust shape, despite having the same aspect ratio. In
Fig. , we present the residual of the Qabs,scat,ext=Cabs,scat,ext/πaaeff2 for an ensemble of spheroids
(E1, E2, E3, E4, E5) and rectangular prisms (S1, S2, S3, S4, S5) with
AR = 1.75. The difference in the Qabs,scat,ext is small for
cases E1 and S1, and it is larger (up to about 50 % in the
Qext,scat at 550 nm) for larger particle sizes (cases S4 and S5).
The sensitivity of Qabs,scat,ext to shape confirms the limited
range of applicability of spheroids over different types and sizes of mineral
dust aerosols, in agreement with previous modeling studies
. Extended
studies on the sensitivity to shape of mineral dust particle optical
properties in the UV–NIR range can provide useful constrains on the envelope of
values to be expected during measurements in ambient air
i.e.,. From Fig. ,
it is also evident that simplifications, in handling mineral dust particle
shape, can generate positive (and at times negative) biases in retrieved AOD
(aerosol optical depth)
and opacity, when ellipsoids are adopted in the retrieval and aerosol at the
site resemble more the synthetic rectangular prisms/modeled particles. The
magnitude of the biases are strictly dependent on the wavelength and size of
the particles. For example, if aerosols at the site resemble more the
rectangular prism than the ellipsoidal shape, large positive biases (up to
50 %) in retrieved AOD can be expected at 550 nm for particles with an
aeff between 700 and 1000 nm, as mineral dust particles (cases S4
and S5) modeled as rectangular prisms have a higher Qext than
ellipsoids (cases E4 and E5). No AOD biases should be expected at 550 nm
depending on the two shape assumptions for particles smaller than 700 nm.
Whereas an average AOD bias of 15 ± 7 % in the shorter wavelength
range (340–500 nm) and 10 ± 13 % for longer wavelength range
(550–1020 nm) should be expected.
Differences in percentage between extinction, absorption and
scattering efficiency for spheroids vs. rectangular prisms.
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Optical properties of BC aggregates<?xmltex \hack{\break}?> internally mixed
We have modeled binary internal mixtures of BC aggregates and mineral dust,
as visualized in Fig. . The BC aggregates are on the
surface of the mineral dust particles. Given the plate-like structure of
Fig. a, c and d, we opted to model mineral dust shape as
rectangular prisms. The chosen shape does not cover the whole range of
variability encountered in ambient air, but it
does for our cases (see
Fig. , it adds a degree of complexity in the description of
mineral dust shape compared to ellipsoids).
Visual representation of polluted dust, as an internal mixture of BC
and mineral dust. The shape of the particle is represented by an array of
coordinates (small dots or spheres) to which is associated a dipole moment.
Brown dots represent the dust particle dipoles, while grey, small spheres
represent the dipoles of the BC aggregate. The cases BL2S1, BL2S2, BL2S3,
and BL2S5 have BL2, respectively, on the surface of S1, S2, S3 and S5. Arrows
show that sides of the rectangular prism can vary keeping the aspect ratio
constant to a value of 1.75.
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In Fig. , we present the MAC, MSC, and MEC spectral dependency
for three different aerosol types: (1) an ensemble of bare mineral dust particles
with aspect ratio of 1.75 and increasing size (cases S1–S5), (2) one
bare BC aggregate (case BL2), and (3) internal mixtures of the two types
(cases BL2S1–BL2S5, where BL2 is mixed, respectively, with S1, S2, S3,
and S5).
Bare mineral dust aerosols (see cases S1–S5 in
Fig. ) have low MAC values compared to bare BC aggregates
(i.e., case BL2 in Fig. ) in the UV and NIR regions. The MAC
values of bare dust are wavelength dependent with larger values predicted in
the UV–Vis range. Smaller dust particles have higher MAC. DDSCAT predicts for
bare/unpolluted dust at 550 nm a MAC average value of
0.13 ± 0.03 m2 g-1 (± standard deviation).
The internally mixed particles (cases BL2S1–BL2S5, also referred to as
polluted dust) have higher MAC values for smaller particles (BL2 has the
highest MAC). As expected, DDSCAT predicts higher MAC values for polluted
dust than for unpolluted/bare dust, with an average MAC value of
0.26 ± 0.27 m2 g-1 at 550 nm.
Furthermore, MSC values of bare mineral dust aerosols have a strong variability
with size and wavelength. DDSCAT predicts an average MSC value at 550 nm of
2.1 ± 1.9 m2 g-1 for dust particles ranging in size from
0.18 to 1μm. When considering just the accumulation mode, with dust size
ranging between 0.5 and 1μm, DDSCAT predicts a smaller MSC average
value of 0.8 ± 0.2 m2 g-1. Fine-mode particles compared to
coarse-mode particles have larger MSC values because smaller particles
scatter light more efficiently at visible wavelengths. ,
after reviewing 60 studies of ground-based observations, report at 550 nm for
the fine-mode dust an average MSC value of
3.3 ± 0.6 m2 g-1, while they report in the accumulation
mode smaller MSC values of 0.9 ± 0.8 m2 g-1, in agreement
with our study. The MSC of larger dust particles (cases S3, S4, and S5) does not
show a strong spectral dependency, while the opposite is true for small
particles (cases S1 and S2); see Fig. c. It should be noted
that the spectral variability of AOD is used in remote sensing in
interpreting aerosol type. For example, mineral dust aerosol is assumed to
have a “spectrally flat” AOD, while biomass burning or polluted aerosol
usually exhibit a strong wavelength dependence. The spectral dependencies in
Fig. a demonstrate that small mineral dust aerosol
particles and polluted dust have also a strong AOD spectral dependence, those
characteristics might be a potential source of classifications of aerosol
type, size and amount.
(a) MEC, (b) MAC, and (c) MSC spectral
dependency for a BC aggregate (BL2) internally mixed with a mineral dust
particle represented as a rectangular prism (BL2S1–BL2S5; see
Fig. ). MEC, MAC, and MSC are normalized by total mass of the internally mixed particle (see Table ). All the
computed optical values are averaged over 1000 particle orientations.
![]()
(a) Cext, (b) Cabs and
(c) Cscat ratios of an external vs. internal mixture of BC
and mineral dust aerosols.
![]()
Furthermore, representation of the state of aerosol mixing, whether internal
(such cases BL2Si with i = (1, 2, 3, 5)) or external (such as cases
BL2 plus Si i = (1, 2, 3, 5)) might affect the overall optical
properties of the aerosols (see Fig. ). We found that for
smaller particles (cases S1, S2, BL2, BL2S1, and BL2S2) external and internal
mixtures predict similar values of Cabs,scat,ext in the entire
spectral range, with ratios, respectively, of 1.09 ± 0.06,
0.96 ± 0.05 and 1.02 ± 0.07.
The latter might be due to the combination of (1) small electromagnetic
interactions between the BC aggregate and the mineral dust particle, due to
the small size parameter; and (2) the small difference in size between BC and
mineral dust particles (with a mixture / core size ratio smaller than
2.8). However, we found for larger particles (with larger size parameters)
with larger differences in Cext, abs, scat values, depending on the
parametrization of the mixing configurations (such as external, cases
BL2 + S3, BL2 + S5, BL2S3, and internal BL2S3 and BL2S5). For those
cases, simulations using external mixture representations give smaller
Cabs values compared to internal mixtures (with average ratio of
0.87 ± 0.30) for wavelengths shorter than 550 nm, while larger values
(average ratio of 1.35 ± 0.49) for wavelengths larger than 550 nm.
Furthermore, Cscat values for external mixtures are smaller than
internal mixtures in most of the spectral range studied (and similarly for
Cext values) with average ratios of 0.59 ± 0.30 and
0.49 ± 0.27 for wavelengths shorter and larger than 550 nm. The internal
mixture might lead to larger Cscat (and similarly for
Cext) values because of larger scattering interactions and
electromagnetic coupling between mineral dust and BC, which might lead to an
increase in scattering compared to the external mixtures; similar results
were found in .
SSA for different particle sizes in the accumulation mode:
(a) finer particles and (b) larger particles.
![]()
g for different particle sizes in the accumulation mode:
(a) finer particles and (b) larger particles.
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The SSA spectral signatures of bare BC (BL2), an ensemble of mineral dust
(cases S1–S5), and internal mixtures of the two aerosol components
(BL2S1–BL2S5) are shown in Fig. . Bare mineral
dusts (cases S1–S5) show a typical decrease in the SSA magnitude for
wavelengths shorter than 500 nm, with SSA values ranging from 0.85 to 0.96
depending on the size of the dust particle, with smaller values attributed to
larger particles. The range of values predicted by DDSCAT, in this study, is
in agreement with values of 0.7–0.97 for Sahara dust reported by
, where the authors attributed variability in measured
values to the presence of a significant number of large particles. Furthermore,
analyses of the SSA values of Saharan dust from the AERONET reported averages of 0.95 at 0.67μm . SSA
values of 0.95–0.99 have been reported during the Saharan Dust Experiment
(SHADE) and the Dust Outflow and Deposition to the Ocean (DODO)
.
estimated the SSA for pure dust aerosol during the Dust and Biomass-burning
Experiment DABEX, to be consistently high (ranging
between 0.98 and 0.99).
For wavelengths shorter than 500 nm, small polluted dust particles
(BL2S1 and BL2S2) show a stronger decrease in the SSA magnitude compared to
unpolluted dust particles (S1 and S2); perturbation of dust optical properties
of the same order of magnitude was also found in the Aerosol Characterization
Experiment (ACE) field campaign . DDSCAT predicts for
internally mixed particles larger than 500 nm (BL2S3–BL2S5) an increase of
SSA at all wavelengths compared to bare dust particles (S3–S5). Such a
“cut off” in SSA values is due to the fact that simulations predict for
small internally mixed particles (cases BL2S1 and BL2S2), where dust
particles are small in size; a steep increase in the absorption and no
significant variation in the scattering properties compared to bare mineral
dust (S1 and S2). The latter leads to smaller SSA values of internal mixtures
compared to bare mineral dust particles. Furthermore, when mineral dust particles
are large (cases S3–S5) and therefore the BC mass (case BL2) results
comparatively much smaller than the mass of cases S3 and S5) DDSCAT
simulations predict a steep increase in the scattering but less in the
absorption; therefore, the prevailing scattering vs. absorption for those cases
is associated with larger SSA values compared to bare mineral dust.
In an attempt to synthesize the differences between the above-discussed
optical properties of bare BC and internal mixtures, we found that with the
increase in size of mineral dust, the absorption increases; however, also the
scattering of the internal mixture (cases BL2S1–BL2S5) increases, leading to
larger SSA values for internal mixtures compared to bare BC (case BL2) (not
shown here, as we provide MAC normalized by the total mass of the particle,
not just BC mass). The increase in the absorption, despite no embedding (no
“lens effect”; see also ) is due to absorption
properties of mineral dust.
DDSCAT predicts a wavelength-dependent asymmetry parameter g (see Fig. ), BC has higher
spectral dependency than dust, mostly due to the variation in real part of
the BC refractive index with wavelength. DDSCAT predicts at 550 nm higher
g values for internally mixed polluted dust than bare mineral dust; larger
g values are predicted when modeling an external mixture compared to an
internal mixture,
differences can amount up to about 37 % (see Table ).
Summary of simulated optical properties for mineral dust and
internal mixtures with BC aggregates. AAE and EAE have been calculated in two
different wavelength ranges: (a) 340–1020 nm and
(b) 400–675 nm.
Case
aeff
AAE
EAE
SEA
MAC(550)
MEC(550)
SSA(550 nm)
g(550 nm)
[nm]
a (b)
a (b)
a (b)
m2 g-1
m2 g-1
S1
180
3.92 (5.68)
2.8 (2.62)
2.75 (2.37)
0.14
3.70
0.96
0.75
S2
284
2.90 (4.16)
0.22 (0.95)
0.25 (1.56)
0.17
5.10
0.96
0.81
S3
500
2.15 (3.70)
0.65 (0.80)
1.17 (0.18)
0.12
0.82
0.85
0.60
S4
700
1.89 (3.13)
0.10 (0.47)
0.69 (1.5)
0.13
1.32
0.90
0.79
S5
1000
1.53 (2.60)
0.18 (0.26)
0.74 (0.60)
0.10
0.70
0.86
0.81
BL2S1
190
1.84 (2.39)
2.20 (2.23)
2.46 (2.19)
0.64
3.97
0.84
0.77
BL2S2
289
2.00 (2.48)
1.08 (0.14)
0.27 (1.63)
0.27
5.00
0.94
0.81
BL2S3
503
2.10 (3.17)
0.19 (0.93)
0.34 (2.68)
0.13
2.19
0.94
0.79
BL2S5
1010
2.68 (4.20)
0.45 (0.97)
0.92 (1.48)
0.09
2.82
0.96
0.88