The morphology and effective density of externally mixed
black carbon (extBC) aerosols, important factors affecting the radiative
forcing of black carbon, were studied using a tandem technique coupling a
differential mobility analyzer (DMA) with a single-particle soot photometer
(SP2). The study extended the mass–mobility relationship to large extBC
particles with a mobility diameter (dmob) larger than 350 nm, a size
range seldom included in previous tandem measurements of BC aggregates in
the atmosphere. The experiment was conducted at an urban site in Beijing
during a 19 d winter period from 23 January to 10 February 2018. Ambient
dry particles were selected by the DMA, and the size-resolved extBC particles
were distinguished from particles with a thick coating (internally mixed)
according to the time delay between the incandescence signal peak and the
scattering peak detected by the SP2. The masses of the extBC particles were
then quantified. The time differences between the DMA size selection and the
SP2 measurement were processed previously. The normalized number size
distributions were investigated at the prescribed dmob sizes in the
range of 140–750 nm to provide the typical mass of extBC at each dmob. On
this basis, the mass–mobility relationship of the ambient extBC was
established, inferring a mass–mobility scaling exponent (Dfm) (an
important quantity for characterizing the morphology of fractal-like BC
aggregates) with a value of 2.34±0.03 in the mobility range
investigated in this study. This value is comparable with those of diesel
exhaust particles, implying a predominant contribution of vehicle emissions
to the ambient extBC in urban Beijing. Compared to the clean period, a higher
Dfm value was observed in the polluted episode, indicating a more
compact BC aggregate structure than that in the clean period. The effective
densities (ρeff) of the extBC in the same dmob range were also
derived, with values gradually decreasing from 0.46 g cm-3 at 140 nm
mobility to 0.14 g cm-3 at 750 nm mobility. The ρeff values
were slightly lower than those measured using the DMA–aerosol particle mass
analyzer (APM) system. The difference in ρeff values was likely
due to the lower BC masses determined by the SP2 compared to those measured
by the APM at the same mobility, since the SP2 measured the refractory BC
(rBC) mass instead of the total mass of the BC aggregate, which consists of
both rBC and a possible fraction of nonrefractory components measured by the
APM. The ρeff values in the 280–350 nm dmob range were much
closer to the values for soot aggregates reported in the literature. It
might be related to the more compact structure of BC aggregates in this
range, resulting from the reconstruction effect by volatile and/or
semivolatile components in the atmosphere. The reconstruction effect might
also result in a hiatus in the increased dynamic shape factor in the range
of 200–350 nm, which presented an overall increase from 2.16 to 2.93 in the
140–750 nm dmob range.
Introduction
Black carbon (BC), a by-product of incomplete combustion, is the main
light-absorbing component in atmospheric aerosols. BC can lead to positive
radiative forcing second only to CO2 in magnitude and thus warming of
the earth's atmosphere (IPCC, 2013). However, there remains a large amount
of uncertainty regarding the radiative forcing induced by BC due to its
complexity and variability in morphology, mixing state and hygroscopicity.
Freshly emitted BC particles usually exhibit fractal-like aggregates
composed of a number of primary carbon spherules (Park et al., 2004;
Sorensen, 2011), which are generally hydrophobic. The condensation of
organic and/or inorganic components leads to the collapse of fractal-like
aggregates and, in turn, a compact structure of BC particles (Slowik et al.,
2007; Zhang et al., 2008). Changes in the morphology of BC particles affect
their optical properties. Encasement by organic and/or inorganic coatings
also increases the absorption of BC particles through the lensing effect
(Shiraiwa et al., 2010; Peng et al., 2016). In addition, water-soluble
coatings increase the hydrophilic ability of BC particles (Zhang et al.,
2008; McMeeking et al., 2011), indirectly affecting radiative forcing by
affecting cloud processes.
Laboratory studies indicate that freshly emitted BC particles can become
thickly coated within a few hours in the atmosphere (Pagels et al., 2009;
Peng et al., 2016). Thus, many studies have focused on the optical
properties and radiative forcing of thickly coated BC particles (Jacobson,
2001; Khalizov et al., 2009; Liu et al., 2017). However, in situ measurements have
shown that a great number of uncoated and/or thinly coated BC particles
exist in the ambient atmosphere, with a fraction even higher than that of
aged BC particles (Schwarz et al., 2008). In general, thickly coated BC
particles account for < 50 % of the BC-containing particles in
urban areas based on single-particle soot photometer (SP2) measurements
(Huang et al., 2012; Wang et al., 2014; Wu et al., 2016). The existence of a
large fraction of uncoated and/or thinly coated BC particles is likely due
to continuous emission from combustion processes such as vehicle exhaust
(Wang et al., 2017). Therefore, studies on the radiative forcing of BC
particles without thick coatings are also essential, especially in urban
areas. First, the morphologies and sizes of these quasi-bare BC particles,
which are the essential quantities for calculating the optical properties of
BC particles in numerical models, should be investigated (Scarnato et al.,
2013; Bi et al., 2013).
The morphology of fractal-like BC aggregates is generally characterized by a
quantity called the fractal dimension (Df), which has been well
documented in the review literature (Sorensen, 2011). The ideal
diffusion-limited cluster aggregation (DLCA), to which BC aggregates belong,
has a Df value of 1.78±0.1. Recent studies have also reported a
similar Df value of ∼1.82 for bare soot particles using
transmission electron microscopy (TEM) analysis of aerosol samples collected
in four different environments (Wang et al., 2017). A significant increase
in the Df was observed when the soot particles were partly coated or
embedded. In the past 2 decades, the morphologies of BC aggregates have
also been widely studied using tandem mobility techniques (Park et al.,
2008). Measurements obtained using an impactor (e.g., an electrical
low-pressure impactor, ELPI) or a particle mass analyzer (e.g., an aerosol
particle mass analyzer, APM, or a centrifugal particle mass analyzer, CPMA)
connected in tandem with a differential mobility analyzer (DMA) have
revealed the relationship between particle mass and mobility (Park et al.,
2003; Maricq and Xu, 2004; Olfert et al., 2007; Rissler et al., 2014;
Sorensen, 2011; and associated references therein). The derived
mass–mobility scaling exponents (Dfm), which have also been called
fractal dimensions in some of these references, varied over a wide range of
2.2–2.8 for diesel exhaust particles. These values were inherently higher
than the virtual Df, which is defined as the scaling exponents between
the radius of gyration of an aggregate and the radius of primary spherules
composing the aggregate, due to the improper interpretation of mobility
measurements, as demonstrated in detail in Sorensen (2011). The Df of
diesel particles obtained using TEM is ∼1.75, corresponding
to a large Dfm value of ∼2.35 based on the mass–mobility
relationship (Park et al., 2004). The mobility size-dependent effective
densities (ρeff) of BC aggregates were also determined from the
DMA–ELPI or DMA–APM (or CPMA) measurements, which were difficult to
characterize using TEM techniques.
The previous tandem measurements generally provided the mass–mobility
relationship of particles with a mobility diameter (dmob) not exceeding
350 nm due to the system detection limit (Park et al., 2003; Maricq and Xu,
2004; Olfert et al., 2007; Rissler et al., 2014). A condensation particle
counter (CPC) is connected next to the DMA–APM system to measure the number
concentrations of mobility size-selected particles at various APM voltages.
The voltage is proportional to the particle mass, and the voltage resulting
in the maximum concentration is in turn considered the typical voltage of
the mass of particles with a prescribed mobility size. Because large
particles (e.g., dmob > 350 nm) are less abundant in the
atmosphere than smaller particles, larger uncertainties exist in the
DMA–APM–CPC measurements for the larger particles (Geller et al., 2006).
Hence, the extrapolation of the mass–mobility relationship established on
the basis of tandem measurements of small mobility diameters (e.g.,
dmob < 350 nm) to large particles (e.g., dmob > 350 nm) is insufficient.
The SP2 was developed on the basis of the laser-induced incandescence
technique and provides advantages in the study of individual BC particle
properties, including mass, size and mixing state. The SP2 determines the
refractory BC (rBC) mass from particle to particle, thus providing the
masses of BC aggregates throughout a wide size range (70–500 nm
mass-equivalent diameter according to the manufacturer) with high
sensitivity and accuracy (Schwarz et al., 2006). Recently, a tandem system
consisting of an SP2 connected to a DMA was developed to study the
properties of size-resolved BC aerosols in the atmosphere. The mass
distributions and mixing states of the size-selected BC were investigated in
northern India using a DMA–SP2 tandem system (Raatikainen et al., 2017).
Coupling an SP2 with a volatility tandem DMA (VTDMA), the rBC core size
distributions of internally mixed BC and those measured by the VTDMA were
compared at the prescribed mobility size ranges. Subsequently, the
morphology and effective density of the internally mixed BC particles were
studied (Zhang et al., 2016). The hygroscopic properties of BC particles
were studied using a hygroscopicity tandem DMA (HTDMA)–SP2 coupling system
(McMeeking et al., 2011; Liu et al., 2013). Few studies have been performed
on the morphology and effective density of fractal-like BC aggregates that
are not coated with other components, especially those in the ambient
atmosphere, using DMA–SP2 measurements.
Using the DMA–SP2/CPC system, Gysel et al. (2012) revealed that the SP2 was
unable to reliably detect BC particles from a PALAS spark discharge soot
generator due to the lower detection limit of the SP2 for loosely packed
agglomerates made up of small primary spherules (∼5–10 nm in
diameter). However, they also claimed that a well-aligned SP2 was expected
to have a detection efficiency adequate to measure BC aggregates (e.g.,
diesel exhaust soot) in the atmosphere because these BC aggregates have
larger primary spherules and substantially higher effective densities than
the agglomerates made up of small primary spherules. Therefore, in this
study, a DMA–SP2 tandem system was built to examine the mass–mobility
relationship (from which the morphology and effective density were further
derived) of uncoated BC aggregates, especially in the large particle size
range (e.g., dmob > 350 nm), which has seldom been included in
previous tandem measurements. Moreover, the uncoated BC aggregates were
distinguished from the thickly coated BC particles using SP2, thus allowing
the study of the mass–mobility relationship of ambient BC aggregates in
different atmospheric environments. Previous DMA–ELPI or APM tandem
measurements were mainly conducted in the laboratory or in the source
environments (e.g., in a tunnel) where fresh BC aggregates were predominant.
Beijing, the capital of China, has suffered from severe air pollution issues
in recent years. Studies have revealed that emissions from coal combustion
and/or biomass burning for industry activities and residential heating have
played a predominant role in particulate pollution in Beijing, especially
during the polluted episodes (Zhang et al., 2013; Huang et al., 2014; Wu et
al., 2017; Ma et al., 2017a, b). Thus, the variation in the mass–mobility
relationship of uncoated BC aggregates was also compared for a polluted
episode and a clean episode to examine the possible influence of a source
change on the morphology of these BC aggregates. In addition, a better
mobility size resolution (33 logarithmic size bins from 20 to 750 nm) was
set for our DMA-SP2 system than was used in previous similar studies, in
which only a few mobility diameters in the range of ∼150–350 nm were selected (Zhang et al., 2016; Liu et al., 2013; McMeeking et al.,
2011). Similar to the study presented by Raatikainen et al. (2017), the high
size resolution is advantageous for calculating the BC mass and number size
distribution in the polluted region in our future studies.
MeasurementsExperimental setup
A tandem system comprising a size selection unit and a measurement section
was built and deployed in an ambient experiment that was conducted on the
roof of a building (approximately 8 m above the ground) on the campus of the
Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP, CAS)
during the winter from 23 January to 10 February 2018 (19 d in total).
Located in an urban area of Beijing, the site is a few hundred meters from
two main roads and thus may be significantly affected by vehicle emissions.
More information on the measurement site is described in previous studies
(e.g., Wu et al., 2016, 2017).
Schematic of the experimental setup for size-resolved measurements
of black carbon.
As shown in Fig. 1, polydisperse aerosols in the sample air were drawn
through the size selection unit (a model 3087 neutralizer, a model 3080
classifier and a model 3081 DMA, TSI Inc., Shoreview, MN, USA) to generate
quasi-monodisperse particles with a certain electrical dmob. Before
entering the system, the ambient air was dried by passing through a
30 cm Nafion dryer (model MD-700-12F-3, Perma Pure LLC, Toms River,
NJ, USA). A vacuum pump was used to draw the dry sheath air (e.g.,
particle-free indoor air) opposite to the flow direction of the sample air
to provide the appropriate vacuum degree required for the dryer. The
size-selected particles were delivered to the measurement section for
analysis by various methods, including an SP2 (Droplet Measurement
Technologies, Boulder, CO, USA), a CPC (model 3776, TSI Inc., Shoreview, MN,
USA) and two microaethalometers (model AE51, AethLabs, San Francisco, CA,
USA). The operational flow rates were set to 0.1, 0.3 and 0.15 L min-1 (STP) for
the SP2, CPC and two AE51s, respectively. The sheath flow rate was set to 3 L min-1, resulting in a ratio of sheath-to-sample flow rate of 4.3:1 for the
DMA. Particles in the range of 15–750 nm in mobility diameter could be
selected. The flow rate for each instrument was calibrated using a soap film
flowmeter (model Gilian Gilibrator-2, Sensidyne, Petersburg, FL, USA) before
the experiment to ensure the accuracy of the selected particle sizes and
measurements. The scientific purpose of this experimental setup was to study
the mixing states of size-selected BC particles, the mass and number size
distribution of BC, and the morphology and effective density of the
uncoated BC aggregates that are discussed in the current study. Because only
the DMA and SP2 were involved in the measurements presented in this study,
the setting and operation of the two instruments are described and
discussed in detail.
Particle size selection
The DMA was connected to an external computer on which a program was run to
control the voltage of the DMA, i.e., the particle mobility diameter
(dmob). A total of 33 dmob values were set in the program to
cyclically control the particles selected by the DMA and gradually increase
from 20 to 750 nm on the logarithmic scale. Stepwise size selection was
repeated until the operator stopped the program. A short cycle lasting for
18 s for each of the 33 diameters and a long cycle lasting for 36 s for each
size were set to alternately operate in this experiment (Fig. S1 in the
Supplement). The purpose of these settings was to identify the time
difference between the size selection and the subsequent measurement, as
described in the following sections.
Black carbon measurement
The individual particulate rBC mass was measured by the SP2 according to the
laser-induced incandescence signal when the particle passed through the
intense Nd:YAG intracavity continuous laser beam (Schwardz et al., 2006)
with a Gaussian distribution. The rBC mass in the SP2 detection range
(∼0.3–250 fg in this study, dependent on the laser intensity
of a specific instrument) is proportional to the peak of the incandescence
signal independent of the mixing state of the BC particles. If a BC particle
is coated with nonrefractory components, the coating will evaporate before
the rBC core incandesces, leading to a time lag between the peaks of
incandescence and scattering signals that are synchronously detected by the
SP2 (Moteki and Kondo, 2007). According to the frequency distribution of the
time lag, there was a significant distinction between thickly coated (i.e.,
internally mixed) BC particles (intBC) and thinly coated or uncoated (i.e.,
externally mixed) BC particles (extBC) (Fig. S2) with a minimum frequency at
∼2µs. BC-containing particles with delay times shorter
than 2 µs were identified as extBC. The delay time threshold might vary
slightly from one SP2 to another; for example, Zhang et al. (2016) reported
a short time lag of 1.6 µs. However, the delay time threshold should be
constant for a given instrument. In previous measurements using the same SP2
employed in this study, the critical delay time was maintained at 2 µs
regardless of the ambient conditions, such as the pollution level (Wu et
al., 2016, 2017). A fraction of BC-containing particles with thin or even
moderate coatings might also be recognized as extBC using the time delay
approach (Laborde et al., 2012). The effects of these thinly or even
moderately coated BC particles are discussed in Sect. 3.2 by reducing the
delay time threshold from 2 to 1.2 and 0.4 µs,
respectively.
The scattering signal of a single particle synchronously detected by the SP2
can be used to estimate the optical size of the particle. The mixing state
of a BC-containing particle can be deduced by comparing the optical size of
the particle and the mass-equivalent size of the rBC core. Because the
nonrefractory coating of a BC-containing particle is evaporated due to the
light absorption and heating of the rBC core when it passes through the
laser beam, the scattering cross section of this particle, which is
proportional to the scattering intensity at a given incident light
intensity, is gradually decreased. To estimate the initial optical size of
this particle, an approach called leading-edge-only (LEO) fitting was
developed (Gao et al., 2007). A small fraction of the measured scattering
signal in the initial stage before the particle is perturbed by the laser is
employed in the LEO fitting to reconstruct the expected scattering
distribution of the initial particle. In this method, the location of the
leading edge in the beam is also required, which is determined from a
two-element avalanche photodiode (APD) signal. Unfortunately, the notch in
the two-element APD of our SP2 failed to fix in an adequate position (e.g.,
before the peak location of the laser beam) in this experiment. Thus, the
optical size and the consequent coating thickness of the BC-containing
particle cannot be estimated. However, the coating thickness is not a
crucial quantity in our current study on the morphology and density of
uncoated BC aggregates. The coating thickness can provide a validation of
our discrimination of extBC but should have little influence on our final
analysis and the discussion presented in the following sections.
Before the experiment, the incandescence signal was calibrated using
DMA-selected monodisperse Aquadag particles. The effective densities of the
mobility size-selected Aquadag particles were determined based on the
polynomial equation as a function of the dmob reported in Gysel et al. (2011). The incandescence signal is more sensitive to the Aquadag particles
than to ambient BC particles because the Aquadag particle induces a higher
incandescence signal peak (by a factor of ∼25 %) than
fullerene soot or an ambient BC particle with the same mass (Laborde et al.,
2012). Thus, the peak intensity of the incandescence signal was reduced by a
factor of 25 % when calculating the calibration coefficient. The
calculated calibration factor, determined as the slope of the linear
regression of rBC masses against the scaled peak heights of SP2's broadband
incandescence signal, is consistent with the factor estimated using a
single-point scaling procedure (Baumgardner et al., 2012). The same
calibration was performed again after the experiment. The calibration
factors varied little (< 3 %), indicating the stability of the SP2
measurement during the entire experiment (Fig. S3). The uncertainty in the
individual rBC mass determination is estimated to be ∼10 %
due to the uncertainties in the rBC mass calibration and the effective
density of the calibration material. An additional uncertainty may also
arise in the determination of extBC masses when the time delay approach is used
to distinguish the mixing state of BC particles. The uncertainty will be
further discussed in Sect. 3.2.
Data processingIdentifying the time difference between the size selection and the SP2
measurement
There exists a considerable difference between the time recorded by the size
selection program and that recorded by the SP2, due to the time cost of the
particles transmitting from the DMA to the SP2, as well as the system clock
difference between the computer on which the size selection program runs and
that for the SP2 data acquisition. As shown in Fig. S1, the SP2 measurement
occurs significantly later than the size selection.
We have developed two methods to identify the time difference. The first
method involves finding the time difference between the local peak in the
particle number concentration (including both scattering and incandescence)
detected by the SP2 and the beginning of the corresponding size selection
cycle. During the experiment, stepwise size selection was cyclically
performed to produce quasi-monodisperse particles with sizes gradually
increasing from 20 to 750 nm. Thus, at the beginning of each new cycle,
the voltage of the DMA should first drop drastically from a high value to a
low one to make the particle size decrease from 750 to 20 nm. As a
result, some particles with sizes in an efficiently detectable range of the SP2
(∼100–500 nm) are measured during the descent period,
producing a local peak in the number concentration. Because it takes only a
few seconds for the descent, identifying the occurrence time of the local
peak position based on the SP2 clock and the beginning time of the size
selection based on the external computer clock provides the time difference
for each cycle.
The other method involves checking the consistency of the number and/or mass
size distributions between the short-duration cycle and long-duration cycle.
Although the durations of each size in the short cycle and long cycle are
different (18 s vs. 36 s), the time difference between the size selection
and the measurement should be uniform for adjacent short and long cycles.
Setting an initial time difference and calculating the mean number and/or
mass concentration of each particle size, the number and/or mass size
distributions are obtained. Then, the correlation coefficients between the
size distributions during short and long cycles are calculated. Changing the
time difference gradually, we can obtain a set of correlation coefficients
as functions of the time differences. The time difference resulting in the
maximum correlation coefficient is considered the difference between the
size selection and the measurement.
Since the detection efficiency of the SP2 decreases dramatically in the
small particle range (Fig. S4), the size distributions of the SP2-detected
particles are inadequate for further calculation of the correlation
coefficients. Therefore, the former method was employed in the current study
to identify the time difference between the size selection and the SP2
measurement. The latter method will be used to examine the time difference
between the size selection and the AE51 and CPC measurements in our future study
on the number and mass size distributions of BC.
Determination of the typical masses of extBC at prescribed mobility sizes
Particles in a certain size range are selected by the DMA instead of
absolutely monodisperse particles in a given mobility size due to the effect
of the transfer function. In addition, larger particles with multiple
charges are also selected. The frequency and number size distributions of
extBC as a function of the mass-equivalent diameter of rBC (dme) at
different mobility sizes are presented in Figs. S5 and S6, respectively.
Note that the number size distribution has been normalized by the peak value
of the corresponding distribution. Since the frequency and number size
distributions of extBC are quite insufficient at small particle sizes
(dme < 70 nm) due to the low detection efficiency of the SP2
(Fig. S4), only the distributions with a dmob larger than 140 nm are
presented. In the following study, we mainly address the morphology and
effective density of extBC in the 140–750 nm dmob range. The normalized
number size distributions at five representative dmob values (i.e., 140,
225, 350, 500 and 750 nm) are also shown in Fig. 2. The extBC particles with a
considerable dme range were observed for a certain dmob, indicating
a wide transfer function of the DMA due to the relatively low ratio of
sheath-to-sample flow (4.3:1). Multicharged particles also affected the size
distribution, especially in the dmob range of 100–400 nm (Ning et al.,
2013). As shown in Figs. S6 and 2, a minor peak is obviously observed in
the right tail of the major peak at each size distribution for dmob
values of < 350 nm.
Campaign average number size distribution of the mass-equivalent
diameter of the rBC core of extBC normalized by the peak value at five
representative mobility diameters (140, 225, 350, 500 and 750 nm) selected
by the DMA. Lognormal fitting is performed for the major peak of each
distribution.
As mentioned above, a fraction of thinly and/or moderately coated BC
particles might also be recognized as extBC according to the time delay between
the SP2 incandescence and scattering signal peaks. These particles also have
impacts on the size distribution of extBC for a given mobility size. A
thinly coated BC particle can be expected to have a larger mass than a bare
BC with the same mobility due to the restructuring of the thinly coated BC
particle by coating materials. These thinly coated BC particles will
increase the size distribution in the right tail when mixed with
multicharged particles. It is currently difficult or even impossible to
separate the effects of the thinly coated and multicharged particles based
on the size distribution of extBC. To examine the possible effect of these
thinly coated particles, we tightened the criterion of the delay time for
the discrimination of extBC, gradually decreasing from < 2.0 to
< 1.2 and < 0.4 µs. As shown in Figs. S5 and S6, a decrease in the delay time threshold results in a significant
reduction in the data volume used in the analysis but has few effects on the
major peak location of the distribution, which is used as the typical
dme of extBC for a given mobility size. The typical dme values,
determined as the mode values of the lognormal function that are employed to
fit the major peak of the size distribution at a certain mobility size, vary
little with the delay time thresholds (Table S1 in the Supplement). The maximum discrepancy in
the dme is < 3 % throughout the prescribed mobility size range
in this study (140–750 nm). The change caused by the delay time threshold mainly
appears in the right tail of the normalized number size distribution.
Reducing the delay time threshold to 0.4 µs results in a significant
decrease in the fraction of particles with a large dme compared to the
2.0 and 1.2 µs thresholds (Fig. S6). These large particles are
likely attributed to thinly and/or even moderately coated BC particles whose
structures are relatively more compact than the absolutely bare BC
particles. Therefore, we propose that thinly and/or even moderately coated
BC and multicharged particles should both have effects on the size
distribution of extBC, mainly in its right tail, but have little influence on
the typical dme, which is considered the peak dme of the
distribution for a given mobility size. The uncertainty in the typical
dme due to the time delay approach that was utilized to distinguish the
extBC is approximately 3 % at a given dmob, which is in turn
∼10 % of the corresponding mass of extBC. Combining the
uncertainty in the rBC mass determined by the SP2 (∼10 %),
the total uncertainty in the determined mass of extBC should be ∼20 % in the studied mobility range of 140–700 nm. To achieve an adequate
data volume for the analysis, the results and discussion presented in the
following sections are based on the database of extBC discriminated as
BC-containing particles with delay times of less than 2.0 µs, unless
otherwise specified.
Theoretical calculation of the morphology and effective density
The structure of extBC, agglomerated by primary spherules with diameters of
20–60 nm (Alexander et al., 2008), can be characterized by its mass–mobility
relationship, which is approximately expressed as a power-law relationship
between the mass of the agglomerate particle (m) and its mobility diameter
(dmob), expressed as
m=k⋅dmobDfm,
where the prefactor k is a constant and Dfm is the mass–mobility scaling
exponent, which was sometimes erroneously called the fractal dimension in
previous studies (e.g., Park et al., 2003). This quantity corresponds well
to the virtual Df and represents the morphology of the BC aggregates
(Sorensen, 2011). The Dfm value of a sphere is 3. Thus, the morphology
of a particle becomes increasingly closer to that of a sphere as the
Dfm increases gradually to 3.
The effective density (ρeff) of the extBC particles is calculated as
the ratio of the BC mass (m) measured using the SP2 and the BC volume, which
is based on the dmob selected by the DMA, expressed as
ρeff=6mπdmob3.
Combining Eqs. (1) and (2), ρeff can also be expressed as a function
of dmob,
ρeff=K⋅dmobDfm-3,
where K is a constant, corresponding to the prefactor k in the mass–mobility
relationship.
The dynamic shape factor is also calculated to indicate the morphology of
the extBC particles. It is derived from the ratio of the slip-corrected
mass-equivalent diameter (dme) and dmob, expressed as
χ=dmob⋅Cc(dme)dme⋅Cc(dmod),
where dme is calculated from the BC mass (m) by assuming the BC particle
to be a compact sphere with a density of 1.8 g cm-3 (Taylor et al.,
2015), and Cc is the Cunningham slip correction factor parameterized by
particle diameter (d)
Ccd=1+2λdα+βexp-γ⋅d2λ,
where λ is the mean free path of the gas molecules, which is set to
65 nm in this study according to Zhang et al. (2016). The values of the
three empirical parameters α, β and γ are 1.257, 0.4
and 1.1, respectively (Eq. 9.34 on p. 407 in Seinfeld and Pandis, 2006).
Results and discussionMass–mobility relationship of the ambient extBC
A power-law relationship was applied to the dmob-determined extBC mass
values, delivering a campaign average mass–mobility scaling exponent
(Dfm) of the ambient extBC (Fig. 3). In the dmob range of 140–750 nm,
the fitted Dfm is 2.34, with 1 standard deviation of 0.03. The fitted
Dfm is close to the lower limit of the Dfm values of diesel exhaust
particles presented in the literature, indicating the dominant contribution
of diesel exhaust to the extBC in our measurement site in urban Beijing.
Depending on the fuel type, engine type and load, the Dfm of diesel
exhaust particles measured by the DMA–APM or DMA–ELPI systems ranged between
2.22 and 2.84 (Olfert et al., 2007; Maricq and Xu, 2004; Park et al., 2003,
and references therein). The higher Dfm values in the literature are
likely attributed to the higher fraction of volatile and/or semivolatile
components (e.g., sulfate) in the diesel exhaust (Park et al., 2003; Olfert
et al., 2007). The presence of these volatile and/or semivolatile components
would result in a more compact structure of the particle, leading to a
higher Dfm value for coated particles than for bare BC aggregate.
Because the rBC mass instead of the whole particle mass of extBC was measured
by the SP2, a relatively low Dfm value was expected and reasonable in
this study. In addition, the relatively low Dfm value observed in urban
Beijing also likely implies high fuel quality (e.g., low sulfur content) and
efficient combustion in vehicle engines, which decrease the organic and/or
inorganic fractions in diesel exhaust particles. The Dfm value for the
ambient soot agglomerates measured with a DMA–APM system near a diesel-truck-dominated highway was 2.41 (Geller et al., 2006), slightly higher than
the value in our study.
The mass of extBC particles as a function of the mobility diameter in
the range of 140–750 nm (black circles), fitted by a power-law relationship
(red line). The power-law functions piecewise fitted in the 140–350 nm
mobility range (green line) and in the 350–750 nm mobility range
(blue line) are overlaid. The dashed lines represent the uncertainties in
the determined extBC masses.
According to Sorensen (2011), the ideal fractal-like DLCA with a virtual
Df of approximately 1.78 should have an expected Dfm≈2.2 in
the slip flow regime in which the BC aggregates are generally observed. The
slightly larger Dfm value of ambient extBC (2.34) in the current study might
indicate a more compact structure than the ideal fractal-like DLCA due to
the reconstruction effect by other components in the atmosphere. The
reconstruction effect appears to be more significant in the smaller particle
range than in the larger particle range. The smaller BC particles are more
likely to be coated by volatile and/or semivolatile materials, which will be
discussed in detail in the next section. We piecewise fitted the
mass–mobility relationship using the power-law function in the mobility
ranges of 140–350 and 350–750 nm. A Dfm of 2.51±0.04 that
was obtained in the smaller mobility range (140–350 nm) was obviously
larger than the fitted value in the whole size range (140–750 nm). In
contrast, a much lower Dfm with a value of 2.07±0.02 was observed
in the larger mobility range (350–750 nm). These results indicate that the
ambient extBC particles with larger mobility diameters were likely less
influenced by the reconstruction effect than those with smaller mobility
diameters.
The typical mass-equivalent diameters
(dme) and corresponding masses of
extBC for different mobility sizes
(dmob) selected by the DMA in the
whole campaign, in a polluted episode and in a clean period. The effective
densities (ρeff) and dynamic
shape factors (χ) at the
dmob selected by the DMA throughout
the whole campaign are also presented.
dmob (nm)dme (nm) Mass (fg) ρeff (g cm-3)χTotalPollutedCleanTotalPollutedClean14088.887.288.50.660.630.650.462.1616097.596.998.10.870.860.890.412.27180106.2106.1107.01.131.131.150.372.35200115.6116.1115.51.461.481.450.352.40225127.9128.6128.41.972.011.990.332.41250140.5142.2141.02.622.712.640.322.41280155.8158.0154.43.563.723.470.312.41315172.6174.8170.64.855.044.680.302.40350188.2191.8185.96.286.656.050.282.41400207.4213.7207.48.419.208.410.252.43450226.4232.3225.910.9411.8110.870.232.50500243.8251.4242.213.6514.9813.390.212.62560262.6271.1260.117.0618.7716.580.192.71630283.2293.5282.521.4223.8321.250.162.81700305.1312.7305.026.7628.8326.730.152.89750319.6328.8323.530.7633.4931.920.142.93
The variation in the morphology of extBC was further examined by comparing the
mass–mobility relationship in a polluted episode with that in a subsequent
clean period. As shown in Fig. S7, a polluted episode rapidly formed at
14:00 (local time, if not specified) on 26 January and lasted 1.5 d to 00:00 on 28 January 2018. The mean PM2.5 mass concentration was
72.1±23.1µg m-3 in this polluted episode, 3 times the
campaign average value (23.0±26.7µg m-3). The Dfm
value was 2.42±0.09 in the polluted episode, higher than that
(2.33±0.06) observed in the subsequent clean period from 01:00 on 28 January to 18:00 on 31 January 2018, during which the average PM2.5
concentration was merely 8.9±2.7µg m-3 (Fig. S8). The
higher Dfm in the polluted episode is mainly due to the increase in the
masses of extBC at large mobility sizes (e.g., dmob > 250). As
shown in Table 1, the typical masses of extBC in the 280–700 nm dmob range
in the polluted episode are ∼7 %–13 % larger than those in
the clean period. Although the differences might result from the uncertainty
(∼20 %) in the mass determination of extBC, the commonly
larger extBC masses (in the 280–700 nm dmob range) to some degree still
imply a possibly more compact structure of extBC aggregates in the polluted
episode, which might relate to changes in the dominant sources and the
ambient environment. Previous studies have revealed that regionally
transported pollutants emitted from coal combustion and/or biomass burning
played an important or even predominant role in polluted episodes in Beijing
(Wu et al., 2017; Ma et al., 2017a). Thus, a considerable fraction of
extBC aggregates from these sources is likely to coexist with the local
vehicle-emitted BC aggregates in the polluted episode, even though the
proportion of extBC in the total BC-containing particles decreased (Fig. S9).
These transported BC aggregates originating from coal combustion and/or
biomass burning might have a more compact structure than those from vehicle
exhaust due to the differences in the combustion environments and
efficiencies. In addition, the BC aggregates might also become more compact
due to the reconstruction effect by the volatile and/or semivolatile
components, which are generally abundant in polluted episodes. Both possible
factors are likely to result in the larger Dfm values in the polluted
episode.
Size-resolved effective densities of the ambient extBC
In contrast to the mass of extBC (m), the effective density of the extBC particles
(ρeff) showed a significant decreasing trend as the dmob
increased from 140 to 750 nm (Fig. 4 and Table 1). The highest ρeff of 0.46 g cm-3 was observed in the 140 nm dmob, likely
because the BC aggregates at the smallest size are made up of the fewest
primary spherules. When the dmob increased to 750 nm, ρeff
decreased to as low as 0.14 g cm-3, approximately one-third of that at
140 nm. The very low ρeff values agree well with the fractal-like
nature of the extBC particles.
The effective density (ρeff) of the extBC
particles as a function of the mobility diameter
(dmob) (black circles). The red
line represents the power-law fitting of ρeff versus
dmob. The variations in
ρeff with
dmob measured for the soot
agglomerates from diesel exhaust (Park et al., 2003) and near-traffic urban
environments (Rissler et al., 2014) are also presented as blue triangles and
red squares, respectively. The dashed lines represent the uncertainties in
the determined ρeff.
The ρeff values obtained by the DMA–SP2 measurements are close to
those of the lower limits of diesel exhaust particles measured by the
DMA–APM (or CPMA) or DMA–ELPI systems. Park et al. (2003) reported a
decrease in the ρeff of diesel exhaust particles under a moderate
(50 %) engine load from 0.95 to 0.32 g cm-3 as the
mobility diameter increased from 50 to 300 nm (Fig. 4). The ρeff values presented in Park et al. (2003) are approximately 1.25,
1.18 and 1.05 times those in our study at ∼150, 220 and
300 nm in mobility diameter, respectively. The differences in ρeff values between our study and the literature are generally within
the uncertainty (∼20 %) in the mass determination of
extBC at prescribed mobility sizes. However, the commonly lower ρeff
values are also likely due to the techniques used to determine the mass of
BC aggregates. Some previous studies on the ρeff of diesel
exhaust particles using the DMA–APM or DMA–ELPI tandem measurements also
showed a slightly larger ρeff throughout the comparable mobility
ranges (e.g., ∼150–350 nm) than that measured in this study
(Maricq and Xu, 2004; Olfert et al., 2007). The masses of the bare BC
particles were determined by the laser-induced incandescence technique of
the SP2. In a previous tandem system, the APM (or CPMA) or ELPI was utilized
to determine the typical mass of BC aggregates at a given mobility, and the
BC aggregates are likely composed of a fraction of volatile and/or
semivolatile components in addition to the bare primary particles. These
volatile and/or semivolatile components increase the mass of the whole
particle, resulting in a larger ρeff value for a certain mobility
causing a compact structure of the BC aggregate. For example, Olfert et al. (2007) found that the ρeff of diesel exhaust particles coated
with minor sulfate and water contents (∼2 % of the total
particle mass) was ∼0.4 g cm-3 at 299 nm, only slightly
larger than the value of diesel exhaust particles (0.32 g cm-3)
measured in Park et al. (2003) and that of extBC in the urban atmosphere (0.31 g cm-3) in our study at the same mobility size. However, the ρeff value increased significantly to ∼0.71 g cm-3
at a relatively high engine load of 40 % due to the high sulfate levels
(∼30 % of the total particle mass) in the diesel exhaust
particles (Olfert et al., 2007).
The ρeff values of ambient soot aggregates also showed a similar
decreasing trend with increasing dmob based on the DMA–APM system
(Geller et al., 2006; Rissler et al., 2014). Rissler et al. (2014) showed a
decrease in the average ρeff of BC aggregates from 0.94 to 0.31 g cm-3 in the near-traffic urban environment as the
dmob increased from 50 to 350 nm (Fig. 4), similar to that of the
freshly emitted diesel exhaust particles presented in Park et al. (2003).
However, based on the same method, the ρeff values of the ambient
BC aggregates that mostly originated from diesel exhaust (Geller et al.,
2006) are substantially different from those presented in Rissler et al. (2014), especially in the large particle size range. The ρeff at
∼350 nm was 0.17 g cm-3 in Geller et al. (2006),
approximately half of the value presented in Rissler et al. (2014). The
reason for the discrepancy might be related to the large measurement
uncertainties of the DMA–APM system for large particles, e.g., with
dmob sizes greater than 300 nm, since these large particles are less
abundant in the ambient atmosphere (Geller et al., 2006). Compared to the
results presented in Rissler et al. (2014), the ρeff values of
ambient extBC aggregates in our study are slightly lower, e.g., by
∼17 %, ∼18 % and ∼6 % for
dmob values of 150, 250 and 350 nm, respectively. The relatively
higher ρeff values are also likely attributed to the effects of
volatile and/or semivolatile components in the soot aggregates. Rissler et
al. (2014) found that the residual mass fraction of volatile and/or
semivolatile materials in the soot aggregates was ∼10 %,
even when the sample air was heated to 300 ∘C before entering the
system for measurement.
It is interesting to note that the ρeff values appear to be
closer to the values presented in the literature using the DMA–APM
measurements in the 280–350 nm dmob range (Fig. 4). As shown in Fig. 3,
larger typical masses of extBC in this range are also observed beyond the
logarithmic scaled linear curve that is fitted to the mass–mobility
relationship. The relatively larger masses and ρeff values might
imply a more compact structure of extBC aggregates in this range, which likely
results from the reconstruction effect by the ambient volatile and/or
semivolatile components. As shown in Fig. S9, the size-resolved number
fractions of extBC exhibit a minimum in the 280–350 nm dmob range,
regardless of whether they are associated with the polluted episode or the
clean period. This finding indicates that particles in this mobility range
are more likely to be thickly coated by other components than are particles
in the smaller or larger mobility ranges. Zhang et al. (2016) also observed
an increased coating thickness of the BC-containing particles in the
mobility range of 200–350 nm (Table 1 in the literature) using the
VTDMA–SP2 measurement at a suburban site ∼70 km away from our
observation site, although the variation in the coating thickness in the
larger mobility range was not investigated. Notably, the number fraction of
extBC at each mobility size presented in Fig. S9 is roughly calculated as the
ratio of the extBC number concentration to the sum of extBC and intBC, in which the
multiply charged effects were not corrected. Although the extBC particles
without coatings and/or with thin coatings are the focus of the current
study, the higher fraction of thickly coated BC particles in the 280–350 nm
dmob range implies a higher possibility that these extBC particles in the
same range were affected by volatile and/or semivolatile materials in the
atmosphere, in turn resulting in a more compact structure of these BC
aggregates. Further detailed studies of the size distribution of BC
(including extBC, intBC and both) and non-BC particles based on the combined
measurements of SP2 and CPC are needed in our further work to reveal the
potential mechanism for this phenomenon.
The dynamic shape factor of the
extBC particles as a function of the mobility
diameter in the range of 140–750 nm.
Although the ρeff of extBC at small sizes (dmob < 140 nm)
cannot be determined due to the lower limit of the DMA–SP2 system, we
extended the ρeff of extBC to a large size range (350 < dmob < 750 nm), which was barely investigated in previous studies
using tandem measurements. A continuous decrease in ρeff with
increasing dmob was observed even in the large size range between 350
and 750 nm (Fig. 4). It is reasonable to infer that the structure of the
extBC particles becomes looser when the fractal-like aggregates built up by the
primary spherules increase.
Dynamic shape factors of the ambient extBC
Due to their fractal-like structures, the extBC particles generally have large
dynamic shape factors (χ) with values in the range of 2.16 to 2.93
(Table 1), much larger than those of intBC with an average value of
∼1.2 (Zhang et al., 2016). The χ value declined
exponentially as a function of coating thickness of BC-containing particles
(Zhang et al., 2016). In contrast to the decrease in ρeff, the
χ values of extBC generally increase as dmob increases from 140 to
750 nm (Fig. 5). The extBC particles 750 nm in mobility diameter have a mean
χ value of 2.93, approximately 1.36 times that for 140 nm dmob
particles (Table 1). The larger particles have looser structures, resulting
in higher χ values. However, the χ values appear to vary
slightly in a narrow range between 2.40 and 2.41 in the size range of 200
to 350 nm (Fig. 5). The hiatus in the gradual increase in χ is also
likely related to the more compact structure of extBC particles in the 280–350 nm mobility range, which has been discussed in detail in the previous
sections.
Conclusions
The DMA–SP2 system was established to study the morphology and effective
density of the ambient extBC particles, especially in the larger mobility size
range, i.e., 350 < dmob < 750 nm, which was seldom
investigated in previous tandem measurements. Quasi-monodisperse particles
in the dmob range of 20–750 nm were stepwise selected using the DMA and
then delivered to the SP2 for rBC mass measurement and mixing state
discrimination. The time difference between the size selection and the SP2
measurement was previously processed using the local peak approach. The
normalized number size distribution of extBC, distinguished as having a delay
time between the incandescence signal peak and the scattering peak detected
by the SP2 of less than 2 µs, as a function of dme was investigated
at each prescribed mobility size in the range of 140–750 nm. The size
distributions at smaller mobility sizes were not presented due to the lower
limit of the rBC mass determined using the SP2. The peak dme, calculated
as the mode value of a lognormal function fitted to the major peak of the
size distribution, was determined as the typical dme value at each
mobility size. Consequently, the typical mass of extBC at each mobility size
was identified. Reducing the time delay threshold employed to discriminate
the extBC had few effects on the determined masses of extBC, implying the
reliability of our study for extBC particles. The uncertainty in the determined
extBC masses was ∼20 %, based on a combination of the
uncertainty in the SP2-measured rBC mass and the uncertainty related to the
time delay approach. On this basis, the mass–mobility relationship of
ambient extBC in urban Beijing was investigated. The campaign-average
Dfm value was estimated to be 2.34±0.03 by fitting the derived
extBC masses as a power-law function of dmob in the range of 140–750 nm,
close to the lower-limit Dfm value of diesel exhaust particles. A
relatively larger Df value was observed in the polluted episode than in
the clean period (2.42±0.09 vs. 2.33±0.06), implying a more
compact structure of BC aggregates in the polluted episode.
A decrease in ρeff with increasing dmob was observed, with
the ρeff value decreasing from 0.46 g cm-3 at a dmob
value of 140 nm to 0.14 g cm-3 at 700 nm. The ρeff values
derived using the DMA–SP2 measurement were slightly lower than those based
on the DMA–APM measurement. This difference was most likely due to the bias
in the extBC mass determination using the SP2 and APM techniques. The pure rBC
mass determined using the SP2 in this study was generally lower than the
total mass of the BC aggregate, which comprises both rBC and a possible
fraction of nonrefractory components. The ρeff values in the
280–350 nm mobility range appeared to be much closer to the values for soot
aggregates reported in the literature by using the DMA–APM tandem
measurement. This finding might be related to the more compact structure of
BC aggregates in this range, which was likely influenced by the
reconstruction effect of volatile and/or semivolatile components in the
atmosphere. The reconstruction effect might also result in a hiatus in the
gradually increased χ value in the range of 200–350 nm. Large χ
values generally increased from 2.16 to 2.93 with increasing dmob,
further implying the high fractal structure of extBC particles.
Data availability
Data used in this study are available from Yunfei Wu
(wuyf@mail.iap.ac.cn).
The supplement related to this article is available online at: https://doi.org/10.5194/amt-12-4347-2019-supplement.
Author contributions
RZ led and designed the study; YW designed the study, set up
the experiment, analyzed the data, and wrote and drafted the paper. YX and PT
collected the field data and contributed to data analysis; ZD provided the
size selection procedure and contributed to data analysis; RH and XX
finalized the paper. All coauthors provided comments on the paper.
Competing interests
The authors declare that they have no conflict of interest.
Financial support
This research has been supported by the National Key Research and Development Program of China (grant nos. 2017YFC0209601 and 2017YFC0212701) and the National Natural Science Foundation of China (grant nos. 41575150, 41775155, 91644217, and 91644219).
Review statement
This paper was edited by Paolo Laj and reviewed by two anonymous referees.
ReferencesAlexander, D. T., Crozier, P. A., and Anderson, J. R.: Brown carbon spheres
in East Asian outflow and their optical properties, Science, 321, 833–836,
10.1126/science.1155296, 2008.Baumgardner, D., Popovicheva, O., Allan, J., Bernardoni, V., Cao, J., Cavalli, F., Cozic, J., Diapouli, E., Eleftheriadis, K., Genberg, P. J., Gonzalez, C., Gysel, M., John, A., Kirchstetter, T. W., Kuhlbusch, T. A. J., Laborde, M., Lack, D., Müller, T., Niessner, R., Petzold, A., Piazzalunga, A., Putaud, J. P., Schwarz, J., Sheridan, P., Subramanian, R., Swietlicki, E., Valli, G., Vecchi, R., and Viana, M.: Soot reference materials for instrument calibration and intercomparisons: a workshop summary with recommendations, Atmos. Meas. Tech., 5, 1869–1887, 10.5194/amt-5-1869-2012, 2012.Bi, L., Yang, P., Kattawar, G. W., and Mishchenko, M. I.: Efficient
implementation of the invariant imbedding T-matrix method and the separation
of variables method applied to large nonspherical inhomogeneous particles,
J. Quant. Spectrosc. Ra., 116, 169–183,
10.1016/j.jqsrt.2012.11.014, 2013.Huang, R. J., Zhang, Y. L., Bozzetti, C., Ho, K.-F., Cao, J. J., Han, Y. M.,
Daellenbach, K. R., Slowik, J. G., Platt, S. M., Canonaco, F., Zotter, P., Wolf, R., Pieber, S. M., Bruns, E. A., Crippa, M., Ciarelli, G., Piazzalunga, A., Schwikowski, M., Abbaszade, G., Schnelle-Kreis, J., Zimmermann, R., An, Z. S., Szidat, S., Baltensperger, U., El Haddad, I., and Prévôt, A. S. H.: High secondary aerosol
contribution to particulate pollution during haze events in China, Nature,
514, 218–222, 10.1038/nature13774, 2014.Huang, X. F., Sun, T. L., Zeng, L. W., Yu, G. H., and Luan, S. J.: Black
carbon aerosol characterization in a coastal city in South China using a
single particle soot photometer, Atmos. Environ., 51, 21–28,
10.1016/j.atmosenv.2012.01.056, 2012.Gao, R. S., Schwarz, J. P., Kelly, K. K., Fahey, D. W., Watts, L. A.,
Thompson, T. L., Spackman, J. R., Slowik, J. G., Cross, E. S., Han, J.-H.,
Davidovits, P., Onasch, T. B., and Worsnop, D. R.: A novel method for
estimating light-scattering properties of soot aerosols using a modified
single-particle soot photometer, Aerosol Sci. Technol., 41, 125–135,
10.1080/02786820601118398, 2007.
Geller, M., Biswas, S., and Sioutas, C.: Determination of particle effective
density in urban environments with a differential mobility analyzer and
aerosol particle mass analyzer, Aerosol Sci. Technol., 40, 709–723,
10.1080/02786820600803925, 2006.Gysel, M., Laborde, M., Olfert, J. S., Subramanian, R., and Gröhn, A. J.: Effective density of Aquadag and fullerene soot black carbon reference materials used for SP2 calibration, Atmos. Meas. Tech., 4, 2851–2858, 10.5194/amt-4-2851-2011, 2011.Gysel, M., Laborde, M., Mensah, A. A., Corbin, J. C., Keller, A., Kim, J., Petzold, A., and Sierau, B.: Technical Note: The single particle soot photometer fails to reliably detect PALAS soot nanoparticles, Atmos. Meas. Tech., 5, 3099–3107, 10.5194/amt-5-3099-2012, 2012.
IPCC: Summary for policymakers, in: Climate Change 2013: The Physical
Science Basis. Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change, edited by: Stocker,
T. F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S. K., Boschung, J.,
Nauels, A., Xia, Y., Bex, V., and Midgley, P. M., Cambridge University
Press, Cambridge, United Kingdom and New York, NY, USA, 2013.Jacobson, M. Z.: Strong radiative heating due to the mixing state of black
carbon in atmospheric aerosols, Nature, 409, 695–697, 10.1038/35055518,
2001.
Khalizov, A. F., Xue, H. X., Wang, L., Zheng, J., and Zhang, R. Y.: Enhanced
light absorption and scattering by carbon soot aerosol internally mixed with
sulfuric acid, J. Phys. Chem. A, 113, 1066–1074, 2009.Laborde, M., Mertes, P., Zieger, P., Dommen, J., Baltensperger, U., and Gysel, M.: Sensitivity of the Single Particle Soot Photometer to different black carbon types, Atmos. Meas. Tech., 5, 1031–1043, 10.5194/amt-5-1031-2012, 2012.Liu, D., Allan, J., Whitehead, J., Young, D., Flynn, M., Coe, H., McFiggans, G., Fleming, Z. L., and Bandy, B.: Ambient black carbon particle hygroscopic properties controlled by mixing state and composition, Atmos. Chem. Phys., 13, 2015–2029, 10.5194/acp-13-2015-2013, 2013.Liu, D. T., Whitehead, J., Alfarra, M. R., Reyes-Villegas, E., Spracklen, D.
V., Reddington, C. L., Kong, S. F., Williams. P. I., Ting, Y.-C., Haslett,
S., Taylor, J. W., Flynn, M. J., Morgan, W. T., McFiggans, G., Coe, H., and
Allan, J. D.: Black-carbon absorption enhancement in the atmosphere
determined by particle mixing state, Nat. Geosci., 10, 184–188,
10.1038/NGEO2901, 2017.Ma, Q. X., Wu, Y. F., Zhang, D. Z., Wang, X. J., Xia, Y. J., Liu, X. Y.,
Tian, P., Han, Z. W., Xia, X. A., Wang, Y., and Zhang, R. J.: Roles of
regional transport and heterogeneous reactions in the PM2.5 increase during
winter haze episodes in Beijing, Sci. Total Environ., 599–600, 246–253,
10.1016/j.scitotenv.2017.04.193, 2017a.Ma, Q. X., Wu, Y. F., Tao, J., Xia, Y. J., Liu, X. Y., Zhang, D. Z., Han, Z.
W., Zhang, X. L., and Zhang, R. J.: Variations of chemical composition and
source apportionment of PM2.5 during winter haze episodes in Beijing,
Aerosol Air Qual. Res., 17, 2791–2803, 10.4209/aaqr.2017.10.0366,
2017b.Maricq, M. M. and Xu, N.: The effective density and fractal dimension of
soot particles from premixed flames and motor vehicle exhaust, J. Aerosol
Sci., 35, 1251–1274, 10.1016/j.jaerosci.2004.05.002, 2004.McMeeking, G. R., Good, N., Petters, M. D., McFiggans, G., and Coe, H.: Influences on the fraction of hydrophobic and hydrophilic black carbon in the atmosphere, Atmos. Chem. Phys., 11, 5099–5112, 10.5194/acp-11-5099-2011, 2011.Moteki, N. and Kondo, Y.: Effects of mixing state on black carbon
measurements by laser-induced incandescence, Aerosol Sci. Technol., 41,
398–417, 10.1080/02786820701199728, 2007.Ning, Z., Chan, K. L., Wong, K. C., Westerdahl, D., Močnik, G., Zhou, J.
H., and Cheung, C. S.: Black carbon mass size distributions of diesel exhaust
and urban aerosols measured using differential mobility analyzer in tandem
with Aethalometer, Atmos. Environ., 80, 31–40,
10.1016/j.atmosenv.2013.07.037, 2013.Olfert, J. S., Symonds, J. P. R., and Collings, N.: The effective density
and fractal dimension of particles emitted from a light-duty diesel vehicle
with a diesel oxidation catalyst, J. Aerosol Sci., 38, 69–82,
10.1016/j.jaerosci.2006.10.002, 2007.Pagels, J., Khalizov, A. F., McMurry, P. H., and Zhang, R. Y.: Processing of
soot by controlled sulphuric acid and water condensation – mass and mobility
relationship, Aerosol Sci. Technol., 43, 629–640,
10.1080/02786820902810685, 2009.Park, K., Cao, F., Kittelson, D. B., and McMurry, P. H.: Relationship
between particle mass and mobility for diesel exhaust particles, Environ.
Sci. Technol., 37, 577–583, 10.1021/es025960v, 2003.Park, K., Kittelson, D. B., and McMurry, P. H.: Structural properties of
diesel exhaust particles measured by transmission electron microscopy (TEM):
relationships to particle mass and mobility, Aerosol Sci. Technol., 38,
881–889, 10.1080/027868290505189, 2004.Park, K., Dutcher, D., Emery, M., Pagels, J., Sakurai, H., Scheckman, J.,
Qian, S., Stolzenburg, M. R., Wang, X., Yang, J., and McMurry, P. H.: Tandem
measurements of aerosol properties – a review of mobility techniques with
extensions, Aerosol Sci. Technol., 42, 801–816,
10.1080/02786820802339561, 2008.Peng, J. F., Hu, M., Guo, S., Du, Z. F., Zheng, J., Shang, D. J., Zamora, M.
L., Zeng, L. M., Shao, M., Wu, Y.-S., Zheng, J., Wang, Y., Glen, C. R.,
Collins, D. R., Molina, M. J., and Zhang, R. Y.: Markedly enhanced
absorption and direct radiative forcing of black carbon under polluted urban
environments, P. Natl. Acad. Sci. USA, 113, 4266–4271,
10.1073/pnas.1602310113, 2016.Raatikainen, T., Brus, D., Hooda, R. K., Hyvärinen, A.-P., Asmi, E., Sharma, V. P., Arola, A., and Lihavainen, H.: Size-selected black carbon mass distributions and mixing state in polluted and clean environments of northern India, Atmos. Chem. Phys., 17, 371–383, 10.5194/acp-17-371-2017, 2017.Rissler, J., Nordin, E. Z., Eriksson, A. C., Nilsson, P. T., Frosch, M.,
Sporre, M. K., Wierzbicka, A., Svenningsson, B., Löndahl, J., Messing,
M. E., Sjogren, S., Hemmingsen, J. G., Loft, S., Pagels, J. H., and
Swietlicki, E.: Effective density and mixing state of aerosol particles in a
near-traffic urban environment, Environ. Sci. Technol., 48, 6300–6308,
10.1021/es5000353, 2014.
Seinfeld, J. H. and Pandis, S. N.: Atmospheric chemistry and physics,
Chapter 9–Dynamics of single aerosol particles, John Wiley & Sons, Inc.,
Hoboken, New Jersey, 2006.Scarnato, B. V., Vahidinia, S., Richard, D. T., and Kirchstetter, T. W.: Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model, Atmos. Chem. Phys., 13, 5089–5101, 10.5194/acp-13-5089-2013, 2013.Schwarz, J. P., Gao, R. S., Fahey, D. W., Thomson, D. S., Watts, L. A.,
Wilson, J. C., Reeves, J. M., Darbeheshti, M., Baumgardner, D. G., Kok, G.
L., Chung, S. H., Schulz, M., Hendricks, J., Lauer, A., Kärcher, B.,
Slowik, J. G., Rosenlof, K. H., Thompson, T. L., Langford, A. Q.,
Loewenstein, M., and Aikin, K. C.: Single-particle measurements of
midlatitude black carbon and light-scattering aerosols from the boundary
layer to the lower stratosphere, J. Geophys. Res., 111, D16207,
10.1029/2006JD007076, 2006.Schwarz, J. P., Gao, R. S., Spackman, J. R., Watts, L. A., Thomson, D. S.,
Fahey, D. W., Ryerson, T. B., Peischl, J., Holloway, J. S., Trainer, M.,
Frost, G. J., Baynard, T., Lack, D. A., de Gouw, J. A., Warneke, C., and Del
Negro, L. A.: Measurement of the mixing state, mass, and optical size of
individual black carbon particles in urban and biomass burning emissions,
Geophys. Res. Lett., 35, L13810, 10.1029/2008GL033968, 2008.Shiraiwa, M., Kondo, Y., Iwamoto, T., and Kita, K.: Amplification of light
absorption of black carbon by organic coating, Aerosol Sci. Technol., 44,
46–54, 10.1080/02786820903357686, 2010.Slowik, J. G., Cross, E. S., Han, J. H., Kolucki, J., Davidovits, P.,
Williams, L. R., Onasch, T. B., Jayne, J. T., Kolb, C. E., and Worsnop, D.
R.: Measurements of morphology changes of fractal soot particles using
coating and denuding experiments: implications for optical absorption and
atmospheric lifetime, Aerosol Sci. Technol., 41, 734–750,
10.1080/02786820701432632, 2007.Sorensen, C. M.: The mobility of fractal agregates: A review, Aerosol Sci.
Technol., 45, 765–779, 10.1080/02786826.2011.560909, 2011.Taylor, J. W., Allan, J. D., Liu, D., Flynn, M., Weber, R., Zhang, X., Lefer, B. L., Grossberg, N., Flynn, J., and Coe, H.: Assessment of the sensitivity of core/shell parameters derived using the single-particle soot photometer to density and refractive index, Atmos. Meas. Tech., 8, 1701–1718, 10.5194/amt-8-1701-2015, 2015.
Wang, Q. Y., Huang, R. J., Cao, J. J., Han, Y. M., Wang, G. H., Li, G. H.,
Wang, Y. C., Dai, W. T., Zhang, R. J., and Zhou, Y. Q.: Mixing state of
black carbon aerosol in a heavily polluted urban area of China: implications
for light absorption enhancement, Aerosol Sci. Technol., 48, 689–697,
10.1080/02786826.2014.917758, 2014.Wang, Y. Y., Liu, F. S., He, C. L., Bi, L., Cheng, T. H., Wang, Z. L.,
Zhang, H., Zhang, X. Y., Shi, Z. B., and Li, W. J.: Fractal dimensions and
mixing structrures of soot particles during atmospheric processing, Environ.
Sci. Tech. Lett., 4, 487–493, 10.1021/acs.estlett.7b00418, 2017.Wu, Y., Wang, X., Tao, J., Huang, R., Tian, P., Cao, J., Zhang, L., Ho, K.-F., Han, Z., and Zhang, R.: Size distribution and source of black carbon aerosol in urban Beijing during winter haze episodes, Atmos. Chem. Phys., 17, 7965–7975, 10.5194/acp-17-7965-2017, 2017.Wu, Y. F., Zhang, R. J., Tian, P., Tao, J., Hsu, S.-C., Yan, P., Wang, Q.
Y., Cao, J. J., Zhang, X. L., and Xia, X. A.: Effect of ambient humidity on
the light absorption amplification of black carbon in Beijing during January
2013, Atmos. Environ., 124, 217–223, 10.1016/j.atmosenv.2015.04.041,
2016.Zhang, R., Jing, J., Tao, J., Hsu, S.-C., Wang, G., Cao, J., Lee, C. S. L., Zhu, L., Chen, Z., Zhao, Y., and Shen, Z.: Chemical characterization and source apportionment of PM2.5 in Beijing: seasonal perspective, Atmos. Chem. Phys., 13, 7053–7074, 10.5194/acp-13-7053-2013, 2013.Zhang, R. Y., Khalizov, A. F., Pagels, J., Zhang, D., Xue, H., Chen, J., and
McMurry, P. H.: Variability in morphology, hygroscopic and optical
properties of soot aerosols during internal mixing in the atmosphere, P.
Natl. Acad. Sci. USA, 105, 10291–10296, 10.1073/pnas.0804860105, 2008.Zhang, Y., Zhang, Q., Cheng, Y., Su, H., Kecorius, S., Wang, Z., Wu, Z., Hu, M., Zhu, T., Wiedensohler, A., and He, K.: Measuring the morphology and density of internally mixed black carbon with SP2 and VTDMA: new insight into the absorption enhancement of black carbon in the atmosphere, Atmos. Meas. Tech., 9, 1833–1843, 10.5194/amt-9-1833-2016, 2016.