Black carbon aerosols play an important role in climate
change because they directly absorb solar radiation. In this study, the
mixing state of refractory black carbon (rBC) at an urban site in Beijing in
the early summer of 2018 was studied with a single-particle soot photometer
(SP2) as well as a tandem observation system with a centrifugal particle
mass analyzer (CPMA) and a differential mobility analyzer (DMA). The results
demonstrated that the mass-equivalent size distribution of rBC exhibited an
approximately lognormal distribution with a mass median diameter (MMD) of
171 nm. When the site experienced prevailing southerly winds, the MMD of rBC
increased notably, by 19 %. During the observational period, the ratio of
the diameter of rBC-containing particles (Dp) to the rBC core
(Dc) was 1.20 on average for Dc=180 nm, indicating that the
majority of rBC particles were thinly coated. The Dp/Dc value
exhibited a clear diurnal pattern, with a maximum at 14:00 LST and a
Dp growth rate of 2.3 nm h-1; higher Ox conditions increased the
coating growth rate.
The microphysical properties of rBC were also studied. Bare rBC particles
were mostly found in fractal structures with a mass fractal dimensions
(Dfm) of 2.35, with limited variation during both clean and polluted
periods. The morphology of rBC changed with coating thickness increasing.
When the mass ratio of nonrefractory matter to rBC (MR) was <1.5,
rBC-containing particles were primarily found in external fractal structures,
and they changed to a core–shell structure when MR>6, at
which point the measured scattering cross section of rBC-containing
particles was consistent with that based on the Mie-scattering simulation.
We found that only 28 % of the rBC-containing particles were in core–shell
structures with a particle mass of 10 fg in the clean period but that proportion
increased considerably, to 45 %, in the polluted period. Due to the morphology
change, the absorption enhancement (Eabs) was 12 % lower than that
predicted for core–shell structures.
Introduction
Black carbon (BC) aerosol is one of the principal light-absorbing aerosols
in the atmosphere. BC is regarded as one of the most important components
contributing to global warming (Bond et al., 2013). BC has a much
shorter lifetime than CO2. Thus, BC's radiative perturbation on a
regional scale may be different from globally averaged estimates. It has
been reported that BC's direct radiative forcing can reach an order of ≥ 10 W m-2 over eastern and southern Asia (Bond et al., 2013). BC aerosols can
also influence the climate by altering cloud properties, such as the
evaporation of cloud droplets, cloud lifetime and albedo (Ramanathan et
al., 2001; Ramanathan and Carmichael, 2008). Ding et al. (2016) determined
that the existence of BC in the upper mixing layer could absorb downward
solar radiation, impeding the development of the boundary layer, which
aggravates air pollution. Moreover, BC aerosols have detrimental health
effects. Black carbon and organic carbon are regarded as the most toxic
pollutants in PM2.5 and lead to as many as ∼3 million
premature deaths worldwide (Adler et al., 2010; Apte et al., 2015).
Black carbon is typically emitted from the incomplete combustion of fossil
fuels and biomass. After being emitted into the atmosphere, BC particles
tend to mix with other substances through coagulation, condensation and
other photochemical processes that significantly change BC's cloud
condensation nuclei activity as well as its light absorption ability
(Bond et al., 2013; Liu et al., 2013). The model results suggest that
after BC's core is surrounded by a well-mixed shell, its direct-absorption
radiative forcing could be 50 % higher than that of BC in an external
mixing structure (Jacobson, 2001). Such an absorption enhancement
phenomenon is interpreted as exhibiting a “lensing effect”, in which a
non-absorbing coating causes more radiation to interact with the BC core and
thus more light is absorbed. This absorption enhancement effect has been
proven in laboratory studies (Schnaiter et al., 2005).
Shiraiwa et al. (2010) reported that the absorption enhancement of
BC in a core–shell structure increased with coating thickness and reached a
factor as high as 2. Nevertheless, field observation results demonstrated
large discrepancies (6 % to 40 %) in the absorption enhancement of aged BC
particles (Cappa et al., 2012; Lack et al., 2012). The discrepancies could
be attributed to the complex mixing state of BC in the real atmosphere,
which depends on the coating composition, the coating amount, and the size of
the BC core and structure. Bond et al. (2013) regarded the mixing state
of BC as one of the most important uncertainties in evaluating BC direct
radiative forcing. Furthermore, freshly emitted BC is initially hydrophobic.
Mixing BC with other soluble materials will significantly increase
BC-containing particles' hygroscopicity and thus their ability to become
cloud condensation nuclei (Bond et al., 2013; Popovicheva et al., 2011).
This ability is associated with the wet deposition rate and consequently
influences the lifetime and spatial distribution of BC particles in the
atmosphere. For these reasons, more observations are needed to determine the
specific spatial and temporal distribution of BC's mixing state, which would
be helpful for minimizing the uncertainty in evaluating BC's climatic and
environmental effects.
China's economy has grown rapidly in recent decades, accompanied by the
substantial emission of pollutant precursors. Annual emissions of BC in
China are reported to have increased from 0.87 Tg in 1980 to 1.88 Tg in 2009, comprising half of the total emissions in Asia and an average of
18.97 % of the global BC emissions during this period (Qin and Xie,
2012). Such substantial BC emissions greatly influence the regional climate
and environment (Ding et al., 2016; Menon et al., 2002). Although
spatio-temporal variations in BC and the corresponding optical properties
of aged BC have been recently reported (Cao et al., 2004, 2007; Zhang et al., 2009), the number of observational studies on BC's mixing
state remains insufficient. Recently, the single-particle soot photometer (SP2)
has been used as a reliable instrument for estimating the mixing state of BC
due to its single-particle resolution and high accuracy. Several studies
have used SP2 to investigate BC's mixing state in China (Gong et al.,
2016; Huang et al., 2012; Wang et al., 2016; Wu et al., 2017). Most studies
have primarily focused on the variability in BC's mixing state on severe
haze days during winter because of the extremely high concentrations of
particle matter and low visibility. In summer, higher radiation and high
hydroxyl radical concentrations favor photochemical reactions and thus
contribute to the condensation aging of BC. By using a smog chamber, Peng
et al. (2016) found that the amount of BC-containing particles increased
rapidly, owing to the photochemical aging of the BC-coating materials from
Beijing's urban environment, even in relatively clean conditions. Cheng
et al. (2012) noted that the changing rate of BC from an external to
internal mixing state can reach up to 20 % h-1 in summer. Thus, the mixing
state of BC should also be carefully considered on relatively clean days
during summer.
In this study, we used an SP2 to investigate BC in the urban areas of
Beijing, China, during early summer, focusing on the size distribution and
coating thickness of BC-containing particles. Field experiments using a
tandem system consisting of a centrifugal particle mass analyzer (CPMA;
Cambustion Ltd.) and a differential mobility analyzer (DMA; model 3085A, TSI
Inc., USA) with an SP2 were performed during two during clean and polluted
cases, focusing on BC-containing particles' microphysical properties,
including the effective density, morphology and light absorption
enhancement. The results of this paper were exhibited in the following
sequence: (1) the size distribution of BC core and its influence factors
such as seasons, weather conditions, etc.; (2) the coating thickness of BC
and its influence factors such as diurnal variation, Ox condition,
etc.; (3) the morphology of BC and its relationship with coating thickness;
and (4) the relationship of morphology of BC with its light absorption. Various
techniques have been developed to quantify the mass concentration of BC
aerosols, including optical, thermal, thermal–optical and photoacoustic
methods. For the SP2, the mass concentration of BC was measured on the basis
of incandescent signal emissions; therefore, refractory black carbon (rBC)
was used. The abbreviations and symbols used in this paper are listed in
Table S1 in the Supplement.
Observation and methodologySite description
The measurement of rBC particles was performed from 30 May to 13 June 2018
in an air-conditioned container located on the tower campus of the State Key
Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC),
Institute of Atmospheric Physics (39.97∘ N, 116.37∘ E). The sampling site is located between the
northern 3rd and 4th Ring Road of Beijing, approximately 50 m from the
closest road and 380 m away from the nearest highway (the Jingzang highway)
(Fig. 1b). Anthropogenic emissions from the experimental campus were
negligible. Thus, this site can represent the urban conditions in
Beijing well.
A single-particle soot photometer (SP2, Droplet Measurement Technologies,
Inc., Boulder, CO, USA) was used to determine the size distribution and
mixing state of rBC particles in the atmosphere. In the SP2 measuring
chamber, an intense continuous intracavity Nd : YAG laser beam is generated
(1064 nm, TEM00 mode). After an rBC-containing particle crosses the beam, it
is heated to incandesce by sequentially absorbing the laser power. The
maximum incandescence intensity (or the peak height of the incandescence
signal) is approximately linearly correlated with rBC mass, irrespective of
the presence of non-BC material or the rBC's morphology. The SP2 was
calibrated to determine the relationship between the incandescence peak
height and the mass of rBC particles using Aquadag aerosols (Acheson Inc.,
USA). Figure 2b illustrates the schematic diagram of the calibration system.
During calibration, monodisperse Aquadag aerosols were generated with an
atomizer (model 3072, TSI Inc., USA) and dried using a diffusion dryer.
Then, Aquadag aerosols with known mass (MrBC) were selected with a CPMA
and injected into the SP2 to obtain the corresponding laser-induced
incandescence (LII) signal. A recent study (Laborde et al., 2012)
demonstrated that the mass of rBC particles could be underestimated when
using Aquadag aerosol as the calibration material. We performed a correction
by multiplying by a factor of 0.75 for LII peak height during the
calibration, as described in Zhang et al. (2018) and Liu et al. (2014). The
LII peak–MrBC relationship was thus obtained (Fig. S1 in the Supplement). The uncertainty
of the derived rBC mass was estimated to be 20 %, which corresponds to an
uncertainty of ∼6 % of the mass equivalent size
(Dc),
Dc=6×MrBCπ×ρrBC3,
by using a 1.8 g cm-3 density for rBC material density (Bond et
al., 2013).
In addition to the incandescence channel, SP2 also has scattering channels
to directly measure the scattering cross section (σmeasured) of
every single particle. However, for rBC-containing particles, the particles
will evaporate during the measurement, since rBC can absorb the laser energy,
which results in a decrease in the rBC-containing particles' sizes and thus
a decrease in the σmeasured. The leading-edge-only (LEO)
fitting method was invented to obtain the scattering cross section of the
initial rBC-containing particles before evaporation
(Gao et al., 2007). With σmeasured and Dc, the diameter (Dp) of the rBC-containing
particle can be obtained using Mie theory with refractive indices of
2.26–1.26i for the rBC (Moteki et al., 2010) and 1.48–0i for the coatings
(Taylor et al., 2015) by assuming a core–shell structure. Thus, the
coating thickness of rBC can be directly determined by SP2, as denoted by
the shell–core ratio (Dp/Dc). The Dp derivation method based on
LEO fitting has been widely used (Taylor et al., 2015; Shiraiwa et al.,
2008; Liu et al., 2014; Laborde et al., 2013), and Liu et
al. (2015) estimated that the core–shell assumption will cause <6 % uncertainty in the derived Dp/Dc. The scattering signal of SP2
was calibrated using polystyrene latex spheres (PSL, Nanosphere Size
Standards, Duke Scientific Corp., USA) with known sizes (203±3 nm:
lot no. 185856; 303±3 nm: lot no. 189903; 400±3 nm:
lot no. 189904), as shown in Fig. S2. The calibration of the scattering channel
and the incandescence channel was also conducted after the observation. The
calibration coefficient varied little (<3 %), and the YAG power
(laser intensity index recorded by the SP2) fluctuated by 4.8±0.1,
indicating the stable condition of the SP2 during the observation period.
The detection efficiency of the SP2 was determined by comparing the number
concentrations of Aquadag as simultaneously measured by the SP2 and a
condensation particle counter (CPC; model 3775, TSI Inc., USA). For large
particles, the SP2 detection efficiency was approximately unity and
decreased gradually for smaller rBC particles (Fig. S3). For rBC with
Dc<70 nm, the detection efficiency of the SP2 fell
significantly below 60 %. The mass concentrations of rBC may be
underestimated because of the low detection efficiency of for smaller rBC
particles. By extrapolating a lognormal function fit to the observed mass
distribution, we found that rBC particles outside the detection range caused
an ∼15 % underestimation of the rBC mass concentration. To
compensate, the mass concentration of rBC was corrected by dividing by a
factor of 0.85 during the measurement.
In general, the SP2 can directly measure the mass of the rBC core
(MrBC) and thus the mass equivalent diameter (Dc). Additionally, the
scattering cross section (σmeasured) can be directly obtained
by the SP2, and the diameter of the rBC-containing particle (Dp) can be
derived using Mie theory.
Experiment
Two kinds of measurements were conducted in this study: a regular single-SP2
observation to provide the number and
mass size distribution and coating
thickness of the rBC-containing particles and a tandem CPMA-SP2–DMA-SP2
experiment to study the microphysical properties of the rBC-containing
particles.
Single-SP2 measurement
The regular single-SP2 observations were conducted from 30 May to 7 June and
9 to 12 June. An aerosol sampling inlet was placed at 4 m a.g.l. (meters above
ground level). A PM2.5 cyclone (URG-2000-30ENS-1) was used to selectively
measure particles with an aerodynamic diameter smaller than 2.5 µm
because rBC particles are typically present in the submicron mode. The
systematic configuration of the rBC measurements is presented in Fig. 2a. A
supporting pump with a flow rate of 9.6 L min-1 was used to guarantee a total
inlet flow rate of 10 L min-1 (the demanding flow rate of a PM2.5
cyclone) and to minimize particle loss in the tube. The residence time of
the sampling flow was estimated to be ∼17 s. Then, the sample
air was dried by passing through a Nafion dryer (MD-700-24S, TSI) at a flow
rate of 0.4 L min-1. The dried sample was measured with the SP2 and CPC.
Schematic diagram of (a) the measurement system, with the orange
dashed line denoting the tandem CPMA-SP2–DMA-SP2 measurement system, during the
periods 9 and 13 June, and (b) the calibration system.
Tandem CPMA-SP2–DMA-SP2 measurement
The tandem CPMA-SP2–DMA-SP2 experiments were conducted on 8 and 13 June: 8 June
is representative of a clean period, when the concentrations of PM2.5
and O3 averaged 20 µg m-3 and 60 ppbv, respectively, and Beijing
was mainly affected by a clean northern air mass (Fig. S5), while 13 June
is representative of a polluted period when the hourly mass concentration
of PM2.5 exceeded 110 µg m-3 – the air mass was from the
southern area of Beijing, which is one of the most polluted areas in China.
Thus, the tandem CPMA-SP2–DMA-SP2 experiment was conducted on 8 and 13 June to
study the detailed physical characteristics of rBC under different pollution
conditions.
As shown in Fig. 2a, the tandem system was similar to the regular single-SP2 observation system. The difference is the neutralizer, and a DMA or CPMA
was added in front of the SP2, as denoted by the orange dashed line.
Specifically, in the DMA-SP2 system, particles were first selected by DMA to
obtain particles with known mobility diameters (Dmob). Then, the
monodispersed particles were injected into the SP2 to obtain the
corresponding information. In practice, we set three Dmob points
(Dmob=200, 250, 300 nm). The duration of one set point is
∼20 min, and we recorded data 2 min after we changed the
set point to allow the system to stabilize. The purpose of the DMA-SP2 system
is to obtain the effective density of bare rBC. Bare rBC is defined as rBC
with Dp/Dc≈1.0, and the effective density of bare rBC
was calculated according to the following equation:
ρeff=6MrBCπDmob3.
In principle, the measured effective density is the same as the material
density if the particle has an ideal spherical shape with no void space.
Thus, the effective density is an indicator of particle compactness, as it
compares the effective density and the material density. Several studies
that include the coupling of DMA with an aerosol particle mass analyzer (APM) or CPMA have been conducted to
determine the ρeff-Dmob relationship of Aquadag rBC samples
in the laboratory (Moteki and Kondo, 2010; Gysel et al., 2011). The
relationship between the ρeff and Dmob of Aquadag is
presented in Fig. S4. The ρeff obtained using the DMA-SP2 system
in this study agreed well with previous research.
In the CPMA-SP2 system, particles with known mass (Mp) selected by CPMA
were injected into the SP2, and the Mp set points were 1, 2, 5
and 10 fg. The duration of one set point was ∼20 min, and we
waited 2 min after we changed the set points to record a measurement. The
purpose of the CPMA-SP2 system is to obtain morphological information about
rBC-containing particles with different coating degrees. Using a tandem
CPMA-SP2 system, the mass of an rBC-containing particle (Mp) and of the
rBC core (MrBC) can be simultaneously obtained. The coating thickness
can be represented by the mass ratio of the coating to the rBC core
(MR= (Mp-MrBC)/MrBC) without any assumptions. Knowing
Mp and MrBC, the scattering cross section of rBC-containing
particles can be calculated through Mie theory with refractive indices of
2.26–1.26i for the rBC and 1.48i for the coatings by assuming a core–shell
structure and a coating density of 1.5 g cm-3. The calculated scattering
cross section (σmodel) can be compared to the σmeasured by SP2, which can reflect the morphological characteristic
of rBC-containing particles; this comparison will be further discussed in
Sect. 4.1.2.
ResultsConcentrations of PM2.5, rBC and pollutant gases
The temporal variations in the concentrations of PM2.5, rBC and gaseous
pollutants (O3, NO2) during the project are presented in Fig. 3.
The regular pollutant concentrations, including PM2.5 (1405-F,
Thermo Fisher Scientific), NO2 (42c, Thermo Fisher Scientific) and
O3 (49i, Thermo Fisher Scientific), were obtained from a state-controlled
air quality site (2.5 km from LAPC), provided by the China National
Environmental Monitoring Center. The mass concentration of PM2.5 ranged
between 5 and 120 µg m-3 on a daily basis during the observation
period. The mixing ratios of both NO2 and O3 exhibited obvious
opposite diurnal variations. The maximum O3 concentration appeared at
14:00 LST on 2 June, with a value of 145 ppbv, reflecting high atmospheric
oxidant levels and strong photochemistry during the observation period. The
mass concentration of rBC was 1.2±0.7µg m-3 on average,
accounting for 3.5±2.4 % of PM2.5 on an hourly basis, which
was comparable to the previous filter-based measurement in Beijing, with an
average fraction of 3.2 % in the summer of 2010 (Zhang et al., 2013).
The mass concentration of rBC also exhibited a clear diurnal variation, with
a maximum at night and a minimum at noon.
Time series of aerosol/gaseous pollutants and meteorological
conditions during the observation period.
During the period from 1 to 6 June, the meteorological conditions were
characterized by low relative humidity (RH <40 %) and strong
solar radiation and were favorable for ozone formation. The mixing ratio of
ozone was relatively high from 1 to 6 June. On 7 June, a heavy rainfall
event occurred, and most of the major pollutants decreased due to
significant wet scavenging. The mass concentration of PM2.5 decreased
from 65 to 10 µg m-3, and the mass concentration of rBC decreased
from 2.6 to 0.2 µg m-3 from 03:00 to 07:00 LST on 7 June. The pollutant
concentration remained at a low level from 7 to 8 June. After 9 June, the
ambient RH increased to 80 %. Under high-humidity conditions, the mass
concentration of PM2.5 experienced steady growth, increasing from 10 to
120 µg m-3 and staying at a high level from 12 to 13 June. Thus,
the tandem CPMA-SP2–DMA-SP2 observations were conducted separately on 8 and
13 June, which separately represented the different PM2.5 pollution
conditions.
Size distribution of rBC
The number and mass distribution as a function of the Dc are illustrated
in Fig. 4. As presented, the number size distribution follows the lognormal
distribution and peaks at 70 nm; the mass size distribution is also fitted
by the lognormal distribution, and the mass median diameter (MMD) was 171 nm
during the project. A brief summary of the SP2 observations in China is
presented in Table 1. Most previous studies focused on the rBC
characteristics in winter, when a larger MMD (∼200–230 nm)
was obtained (Zhang et al., 2013; Wu et al., 2017; Wang et al., 2016; Huang
et al., 2012; Gong et al., 2016) than in this study. A similar MMD (180 nm)
was reported in urban Shenzhen during a summer observation period (Lan et
al., 2013), and a higher MMD (210–222 nm) was reported in winter. Liu et
al. (2014) also found a winter-high–summer-low trend for rBC sizes in
London, with Dc=149±22 nm in winter and 120±6 nm in
summer. Laboratory studies have proven that MMD is highly dependent on
combustion conditions (Pan et al., 2017) and material. Thus, MMD is a
suitable indicator of the sources of rBC. Several studies have suggested
that the MMD of rBC from biomass burning and coal is much larger than that
from traffic emissions (Wang et al., 2016; Schwarz et al., 2008). Huang
et al. (2012) found the MMD observed at rural sites to be much larger than
that observed at urban sites because urban sites are primarily affected by
rBC emitted from traffic sources and rural sites are more influenced by rBC
from coal combustion. The seasonal trends in MMD may be partially explained
by the different rBC sources in summer and winter.
Number and mass size distribution (dN/ dlogDc and
dM/ dlogDc) during the observation period.
Brief summary of some of the observations on the mixing state of
rBC-containing particles.
rBC typeSitePeriodMMD (nm)Dp/DcDescriptionReferenceUrbanemission(UE)Shenzhen, ChinaAug–Sep(summer)180*The measurement station wason a university campus locatedin the urban area of Shenzhen.Lan et al. (2013)Shenzhen, ChinaJan–Feb(winter)210*The measurement station wasthe same as the above site.Huang et al. (2012)Shanghai, ChinaDec (winter)2002–8The maximum PM2.5 massloading reached 636 µg m-3.Gong et al. (2016)Beijing, ChinaFeb–Mar(winter)213The same measurementsite as this study.Wu et al. (2017)This studyJun (summer)1711.20 (Dc=180 nm)London, UKJan–Feb(winter)1491.2–2 (Dc=110–150 nm)During the Clean Air forLondon (ClearfLo) project.Liu et al. (2014)Jul–Aug(summer)120Biomassburning(BB)AirbornemeasurementsSep (autumn)2101.33 (Dc=190–210 nm)The MMD and Dp/Dc wereboth higher for BB than UE.Schwarz et al. (2008)1891.2–1.4 (Dc=200 nm)Fresh, laboratory-producedbiomass-burning rBC.Pan et al. (2017)AirbornemeasurementsJul–Aug (summer)1952.35 (Dc=130–230 nm)During the second phaseof the BORTAS project.Taylor et al. (2014)
* Assuming that the density of rBC is 2 g cm-3.
Figure 5 provides the temporal variations in the mass size distribution of rBC
during the entire investigation period. Most rBC particles were within the
size range of 70–300 nm, with a clear diurnal pattern of the value of
dM/ dlogDc. The diurnal cycle reached a peak plateau between 03:00 and 07:00 LST and decreased gradually in the afternoon. The cycle was controlled by
the combined effects of the development of planetary boundary layer (PBL)
variation and on-road rBC emissions.
After the two rain events (4 and 7 June), the MMD decreased
significantly from 186 to 170 nm and from 183 to 159 nm, respectively,
as shown in Figs. 3 and S7. Taylor et al. (2014) observed that the
rBC core size distribution shifted to smaller sizes after a biomass-burning
plume passed through a precipitating cloud, attributing this shift to the
preferential nucleation scavenging of larger rBC cores. By counting the MMD
on non-rainy days and rainy days, Wang et al. (2018) also found that the
MMD decreased from 164±21 to 145±25 nm. The decrease in MMD
after rain events can be explained by the preferential wet scavenging of the
larger rBC-containing particles.
Time series of the mass size distribution of rBC. A southerly wind
period is selected when the wind direction is 135–225∘, and a
northerly wind period is the time when the wind direction is
325–45∘. The gray dashed line denotes the rainy period.
(a) Dependence of rBC's MMD on wind speed and wind direction
during the observation period. (b) MMD versus wind speed during the
southerly wind period and northerly wind period. The error bars correspond
to the standard deviations of MMD in each wind speed bin.
A pollutant rose plot of MMD versus wind speed and wind direction is
presented in Fig. 6a. The MMD of rBC was ∼160 nm at low-wind-speed conditions and exhibited a significant increase with increasing
southeastern wind speed. The maximum MMD exceeded 190 nm when the wind speed
was greater than 10 m s-1. Figure 6b presents the correlation between wind speed
and MMD. A southerly wind period was selected when the wind direction was
135–225∘, and a northerly wind period was the time when the wind
direction was 325–45∘. The MMD exhibited little correlation with
wind speed and varied little between the southern and northerly wind periods
when the wind speeds were less than 2 m s-1, as local rBC emissions were
predominant. An MMD of 150–160 nm during low-wind-speed periods may be
characteristic of the local sources. The MMD had a strong positive
correlation with the wind speed during the southerly wind period (r2=0.93), suggesting that the rBC from the south was larger, which may be
the result of the different rBC sources in the southern polluted region.
Since the air mass from the north is always clean, the local rBC emissions
may be the main contributors to the total rBC concentration in the northerly
wind period. Thus, the MMD may be more influenced by local emissions and
show a weak correlation with the wind speed during northerly wind periods.
Temporal variation in
Dp/Dc
The Dp/Dc for a given single rBC-containing particle was calculated
using the LEO fitting method. Herein, rBC cores with Dc=180±10 nm were selected because the low scattering signal of small rBC is easily
influenced by signal noise (Dp/Dc indicates the Dp/Dc with
Dc=180±10 nm in the following discussion if not specified).
The Dp/Dc variation during the investigation period is illustrated
in Fig. 7. In general, Dp/Dc was 1.20±0.05 on average
during the investigation, which is consistent with observations (1.15)
during the summer in Paris (Laborde et al., 2013). Black carbon sources
and the aging process significantly influenced the Dp/Dc of rBC. The
rBC from traffic is reported to be relatively uncoated (Liu et al.,
2014), whereas the rBC emitted by biomass burning is found to be moderately
coated, with Dp/Dc=1.20–1.40 (Pan et al., 2017). Moreover,
Dp/Dc increases with the aging process, and a larger
Dp/Dc (1.60) was found in an aged continental air mass (Shiraiwa
et al., 2008). The relatively low Dp/Dc value further supports the
argument that rBC was primarily emitted from on-road vehicles during the
summer in Beijing.
(a) Temporal variation in Dp/Dc, with Dc=180 nm; the
image plot denotes the frequency (dN/ dlogDp) of rBC-containing particles
with varied Dp/Dc values. The black line denotes the average
Dp/Dc for each hour. (b) The normalized frequency of
Dp/Dc for the clean period before the tandem experiment. (c) The
normalized frequency of Dp/Dc for the polluted period.
The Dp/Dc distributions for the two episodes before the tandem
CPMA-SP2–DMA-SP2 experiments are shown in Fig. 7. Episode 1 (7 June at 22:00 LST–8 June at 12:00 LST) occurred after a heavy rain period and is representative of
clean conditions. Episode 2 (11 June at 23:00 LST–12 June at 12:00 LST) was
characterized by the highest average Dp/Dc value (1.40) and the
highest PM2.5 concentration value (120 µg m-3) during the
observation period. The Dp/Dc exhibited a unimodal distribution
during episode 1 (Fig. 7b) and a clear bimodal pattern during episode 2
(Fig. 7c), which indicates that there may be one type of rBC in episode 1 and two
types of rBC with a clear coating thickness difference in episode 2. The coating
thickness of the first kind of rBC was relatively thin, with a
Dp/Dc value of ∼1.05, corresponding to the single peak
of the unimodal distribution in episode 1 and the left peak of the unimodal
distribution. This kind rBC may be freshly emitted by the local rBC sources
(such as traffic) and underwent a short aging time. However, the second kind
of rBC was thickly coated, with a Dp/Dc value of ∼1.80,
corresponding to the right peak of the bimodal distribution during episode 2. Zhang et al. (2018) demonstrated that 63 % of the rBC was estimated
to be transported from outside of Beijing during previous pollution events,
and the rBC-containing particles from regional transportation were
characterized by having more coating material. The rBC-containing particles
with Dp/Dc=1.80 in the right peak of the bimodal distribution
may be the result of transportation from polluted regions.
Diurnal variation in Dp/Dc for all periods, high-Ox
periods and low-Ox periods. The gray shaded area denotes the standard
deviation of Dp/Dc for all periods.
Diurnal variation in Dp/Dc
The temporal variation in Dp/Dc exhibited a clear day-high and
night-low pattern. Figure 8 exhibits the diurnal trend of Dp/Dc. The
mean Dp/Dc increased during the daytime, with a peak (1.20) at 14:00 LST and a minimum (1.12) at 06:00 LST. Coating thickness
(Dp/Dc) was controlled by the competing effects of emissions and
aging because freshly emitted thinly coated rBC tends to decrease
Dp/Dc and the aging process tends to increase Dp/Dc. The
increasing trend of Dp/Dc during the day could be explained by the
prevailing aging process, whereas the decreasing trend at night can be
explained by the prevailing emissions process, as the photochemical
condensation aging during the day was much faster than the coagulation aging
at night (Riemer et al., 2004; Chen et al., 2017). The advection of aged
rBC-containing particles from the upper boundary layer with the development
of boundary layer during daytime may be another reason for the increase in
Dp/Dc. By measuring the Dp from 06:00 to 14:00 LST, the Dp
growth rate was calculated to be 2.3 nm h-1. A larger Dp growth rate was
found in the period with a high Ox concentration, which may be
favorable for the formation of coating material on rBC. The photochemical
process and condensation aging have proven to be very efficient during the
day. Using a smog chamber, Peng et al. (2016) found that the Dp
growth rate of rBC-containing particles could reach 26 nm h-1 in Beijing's
urban area. Although the photochemical process and condensation may rapidly
increase the Dp, the difference between the present study and the smog
chamber results indicated that the “apparent” Dp growth rate in the
ambient measurement was relatively low given the continuous freshly emitted
rBC in urban Beijing. Thus, the Dp/Dc was always at a low level,
resulting in little light absorption enhancement during the summer.
Relationship between effective density and mobility diameter of
rBC-containing particles. The black circle and triangle denote the fresh
rBC-containing particles (Dp/Dc=1) measured on clean days and
polluted days in this study. Other markers denote the data from previous
research.
DiscussionMorphological evolution of rBC-containing particlesMorphology of bare rBC
By coupling DMA and SP2, the mass and the mobility diameter of bare rBC
(Dp/Dc≈1.0) can be obtained simultaneously, and,
therefore, the effective density (ρeff) can be calculated. The
ρeff of the ambient bare rBC was measured on a clean day (8 June)
and a polluted day (13 June). The ρeff of bare rBC at 200–300 nm
ranged from 0.41 to 0.29 g cm-3, which was much smaller than the material
density of rBC (1.8 g cm-3). This significant discrepancy indicates that
bare rBC was in a fractal structure consistent with the previous research
from electron microscopic images, which showed that bare rBC was in a
fractal chain-like structure (Adachi and Buseck, 2013; Li et al.,
2003; Wang et al., 2017). The effective densities showed no evident
difference between the polluted day and the clean day because the bare rBC
particles were freshly emitted and only affected by local sources. A power
law is always used to describe the fractal-like aggregates of particles:
Mp∝DmobDfm (Moteki and Kondo, 2010; Park et
al., 2004), where Dfm is defined as the mass fractal dimension that is
an indicator of particle compactness. The value of Dfm is 3 for ideal
spherical particles and less than 3 for fractal particles. Based on the
equation for ρeff, the following relationship can be found: ρeff∝DmobDfm-3. Thus, a larger bare rBC had a
smaller ρeff, which was consistent with the results in Fig. 9. A
power function was used to fit the observed data. ρeff∝Dmob-0.65 and ρeff∝Dmob-0.6 were
found separately on clean and polluted days, corresponding to the mass
fractal dimensions of 2.35 and 2.40, respectively. These mass fractal
dimensions from the summer in Beijing are similar to the observations
(Dfm=2.30) from urban Tokyo (Moteki and Kondo, 2010) and the
diesel exhaust measurement (Dfm=2.35) (Park et al.,
2004), suggesting that the freshly emitted bare rBC particles originated
primarily from traffic sources. Traffic may contribute a majority of the
fresh rBC during both polluted and clean periods in the summer.
(a) Scattering cross section of rBC-containing
particles as measured by SP2 (yellow line) and calculated by Mie theory
(blue line), assuming a core–shell structure. Bottom panel: the ratio (green
line) between these two scattering cross sections at a CPMA set point of 10 fg as a function of MR (the mass ratio of nonrefractory matter to rBC).
(b) The same as (a) but for a CPMA set point of 5 fg.
Morphology of rBC-containing particles with increasing coating
thickness
The morphological characteristics of rBC-containing particles were
investigated by comparing the σmeasured and σmodel
using a CPMA-SP2 system first proposed by Liu et al. (2017). The
comparison of σmeasured and σmodel as a function
of MR for a particle mass of 10 fg is illustrated in Fig. 10a. If the
ratio of σmeasured and σmodel equals 1, it implies
that the scattering cross section measured by SP2 is the same as the model
prediction under the assumption of a core–shell mixing structure; thus, the
rBC-containing particle was likely a core–shell structure. When the rBC was
bare (MR≈0), the rBC was in a fractal structure, as discussed
in Sect. 4.1.1. With increasing MR, the σmeasured/σmodel gradually decreased until MR=1.5, indicating that the
coating material may not be sufficient to encapsulate rBC and that the
rBC-containing particles tended not to be away from a core–shell structure.
Liu et al. (2017) showed that rBC-containing particles with
MR<1.5 primarily presented an external structure. When
1.5<MR<6, the σmeasured/σmodel steadily increased, which implied that the shape of
rBC-containing particles gradually transformed to become more compact, with
a core–shell-like structure, in this stage. When MR>6, the
σmeasured/σmodel approached 1, indicating that the
rBC-containing particles were in a core–shell-like structure in this stage.
Similar phenomena were found in the relationship of σmeasured/σmodel and MR for particle masses of 5 fg,
as illustrated in Fig. 10b. However, when MR≈0.1, σmeasured was consistent with the model prediction for a particle mass
of 5 fg. This is because the scattering signal was not sensitive to the
irregularity of smaller-sized particles (Moteki et al., 2010). Therefore,
a Mie theory-based core–shell model could capture the main morphological
features.
(a) The average MR (the mass ratio of nonrefractory matter to
rBC) under different CPMA set points (1, 2, 5, 10 fg). (b) The
fraction of different types of rBC-containing particles under different CPMA
set points under varied pollution conditions.
Different techniques have been used to explore the morphology of
rBC-containing particles in ambient and laboratory measurements (Zhang et
al., 2008; Peng et al., 2016; Pagels et al., 2009). It is generally agreed
that the morphology of rBC-containing particles will become more compact
with the aging process or with increasing coating thickness. However, this
study reveals that the morphology transform may only be true when the
coating is thick enough (MR>1.5), and the coatings may only
attach to rBC and slightly influence rBC-containing particles' morphology
when the coating is not thick enough (MR<1.5).
(a) Time series of Dp/Dc with Dc=180±10 nm
and Eabs at 550 nm wavelength using the
core–shell model and the morphology-dependent model. (b) Relationship
between Eabs and Dp/Dc. Circles denote the Eabs derived from
the core–shell model, and triangles denote the Eabs derived from the
morphology-dependent model.
Based on the relationship between the σmeasured/σmodel and MR, the rBC-containing particles are classified into
three groups: external stage (0<MR<1.5), transit stage
(1.5<MR<6) and core–shell stage (MR>6). A similar variation between the σmeasured/σmodel and MR was also found by Liu et al. (2017) and Wu et al. (2018). The MR transition point from the transit stage and core–shell
stage determined by Liu et al. (2017) is slightly lower than that in
this study. Liu et al. (2017) found that the MR transition point
varied among different rBC sources. In addition to rBC sources, the
environmental conditions during the aging process of rBC-containing
particles, such as temperature and humidity, may also influence the
rBC-containing particle morphology. We determined the MR transition
point in Beijing in summer. More work needs to be done in the future to
better quantify MR in different situations.
The combined CPMA and SP2 measurements were conducted separately on a clean
day (8 June) and a polluted day (13 June). Figure 11a presents the average
MR for different CPMA set points (1, 2, 5 and 10 fg) on 8
and 13 June. The average MR is 0.77 for Mp=1 fg and 5.29 for
Mp=10 fg on the clean day, whereas the average MR is 0.84 for
Mp=1 fg and 7.28 for Mp=10 fg on the polluted day. The
average MR values of the polluted day were all larger than those on the
clean day for the four Mp points. This result demonstrated that rBC had
more coating material on the polluted day than on the clean day. Based on
the MR transition points discussed above, the rBC-containing particles
were classified into three stages, as shown in Fig. 11b. The rBC-containing
particles with Mp=1 fg were primarily in the external mixing stage
regardless of the pollution conditions. With an increase in Mp, more
rBC-containing particles were in the transition or core–shell stage. On the
clean day, 28 % of the rBC-containing particles were in the core–shell
stage, when Mp=10 fg. However, on the polluted day, 45 % of the
rBC-containing particles were in the core–shell stage, when Mp=10 fg.
This phenomenon implied that most rBC-containing particles are not in an
ideal core–shell structure on clean days, whereas more rBC-containing
particles were in a core–shell structure with thicker coatings on the
polluted day.
Implications of rBC-containing particle morphology for light absorption
The morphology of rBC-containing particles varied with MR. A simple
core–shell model, as always used in the previous research to determine
optical properties, will certainly cause bias. Based on the classification
of the rBC-containing particles according to the relationship between
σmeasured/σmodel and MR, Liu et al. (2017)
proposed a simple morphology-dependent scheme in which the rBC-containing
particles at the external stage were considered to have no absorption
enhancement (Eabs) and the rBC-containing particles at the core–shell
stage were considered to have the same Eabs from the Mie theory under the
assumption of a perfect core–shell structure. The Eabs at the transit
stage was calculated by the interpolation of Eabs between the external
and core–shell stages. A graphical and detailed description of the
calculation of Eabs can be found in Fig. S6. Liu et al. (2017) proved
that this morphology-dependent scheme is in good agreement with the measured
Eabs. Thus, the Eabs at the 550 nm wavelength with
Dc=180±10 nm was calculated separately using the core–shell
model and the morphology-dependent scheme to quantify the uncertainty of
using a core–shell model, as shown in Fig. 12. The absorption enhancement
was 1.15, on average, using the core–shell model but was only 1.03 using the
new scheme. The Eabs determined by the core–shell model was
overestimated by 12 % because the observed averaged coating thickness
(Dp/Dc=1.2) determined from single-SP2 measurements corresponded
to MR=0.37, suggesting that the coating material was not sufficient
and most of the rBC-containing particles were not in a core–shell structure
in summer in Beijing. Thus, it is necessary to consider the morphology of
rBC-containing particles when calculating their optical properties.
Conclusions
The mixing characteristics of rBC-containing particles were investigated in
Beijing during the early summer of 2018 using a single-particle soot
photometer (SP2). The rBC had an approximately lognormal distribution as a
function of the mass equivalent diameter (Dc), characterized by a mass
median diameter (MMD) of 171 nm, which is consistent with previous urban
measurements. The mass size distribution was highly associated with the
meteorological conditions. Heavy rain events caused the rBC mass size
distribution to be smaller, indicating that wet scavenging may be a more
efficient removal mechanism for larger rBC-containing particles. The mass
size distribution of rBC shifted to larger sizes when southerly winds
prevailed, which was primarily caused by the different rBC sources in the
south.
The Dp/Dc was 1.20 on average, with Dc=180 nm during the
investigation period, indicating a low coating thickness of rBC during the
summer. The coating thickness exhibited a clear diurnal pattern with a peak
at 14:00 LST, increasing from 06:00 to 14:00 LST at a Dp growth rate of 2.3 nm h-1, with Dc=180 nm during the day. The growth rate was much higher
in high-Ox periods. However, this growth rate was significantly lower
than that in the smog chamber results, with a growth rate of 26 nm h-1
because the continuously emitted fresh rBC lowered the Dp/Dc in
ambient measurements. Although photochemical aging may be very efficient,
with continuously emitted fresh rBC, the Dp/Dc increase in the
ambient air was very slow, indicating that the rBC-containing particles were
primarily at a low Dp/Dc level in summer.
A tandem measurement system with a differential mobility analyzer (DMA) and
a centrifugal particle mass analyzer (CPMA) were coupled with an SP2 to
investigate the detailed characteristics of rBC-containing particles in
summer. The results showed that the effective density of bare rBC
(Dp/Dc=1.0) was determined to be 0.41–0.30 g cm-3 for
Dc=200–300 nm. These effective densities were significantly lower
than the rBC material density (1.8 g cm-3), suggesting that the bare rBC
was in a fractal structure. The corresponding mass fractal dimension
(Dfm) was 2.35, which agrees well with the Dfm of the direct
measurement from vehicles, and was unchanged regardless of pollution,
indicating that traffic emissions are a major source of fresh bare rBC on
both clean and polluted days during the summer in Beijing. With increasing
coating thickness, the morphology of rBC changed from a fractal structure to
a compact core–shell structure. When MR (Mcoat/MrBC) <1.5, rBC-containing particles were in an external structure. When
MR>6, rBC-containing particles were in a core–shell
structure. When 1.5<MR<6, the rBC-containing particles
were in a transition stage.
Based on the core–shell model and Mie theory, a new morphology-dependent
absorption enhancement (Eabs) scheme was proposed and applied to the
ambient measurements. A simulation showed that the Eabs averaged to 1.03
with Dc=180 nm at a wavelength of 550 nm in the summer. The core–shell
model overestimated the Eabs by 12 %.
Data availability
To request the data given in this study, please contact Xiaole Pan at
the Institute of Atmospheric Physics, Chinese Academy of Sciences, via email
(panxiaole@mail.iap.ac.cn).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-5771-2020-supplement.
Author contributions
HL and XP designed the research. HL, XP, XL, YT, YS, PF and
ZW performed the experiments. HL, XP, DL and XC performed the
data analysis. HL and XP wrote the paper.
Competing interests
The authors declare that they have no conflict of interest.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 41877314, 41675128).
Review statement
This paper was edited by Thomas Karl and reviewed by three anonymous referees.
ReferencesAdachi, K. and Buseck, P. R.: Changes of ns-soot mixing states and shapes
in an urban area during CalNex, J. Geophys. Res.-Atmos., 118, 3723–3730,
10.1002/jgrd.50321, 2013.Adler, G., Riziq, A. A., Erlick, C., and Rudich, Y.: Effect of intrinsic
organic carbon on the optical properties of fresh diesel soot, P. Natl. Acad.
Sci. USA, 107, 6699–6704, 10.1073/pnas.0903311106, 2010.Apte, J. S., Marshall, J. D., Cohen, A. J., and Brauer, M.: Addressing
Global Mortality from Ambient PM2.5, Environ. Sci. Technol., 49, 8057–8066,
10.1021/acs.est.5b01236, 2015.
Bond, T. C., Doherty, S. J., Fahey, D., Forster, P., Berntsen, T., DeAngelo,
B., Flanner, M., Ghan, S., Kärcher, B., and Koch, D.: Bounding the role
of black carbon in the climate system: A scientific assessment, J.
Geophys. Res.-Atmos., 118, 5380–5552, 2013.Cao, J. J., Lee, S. C., Ho, K. F., Zou, S. C., Fung, K., Li, Y., Watson, J.
G., and Chow, J. C.: Spatial and seasonal variations of atmospheric organic
carbon and elemental carbon in Pearl River Delta Region, China, Atmos.
Environ., 38, 4447–4456, 10.1016/j.atmosenv.2004.05.016, 2004.Cao, J. J., Lee, S. C., Chow, J. C., Watson, J. G., Ho, K. F., Zhang, R. J.,
Jin, Z. D., Shen, Z. X., Chen, G. C., Kang, Y. M., Zou, S. C., Zhang, L. Z.,
Qi, S. H., Dai, M. H., Cheng, Y., and Hu, K.: Spatial and seasonal
distributions of carbonaceous aerosols over China, J. Geophys. Res.-Atmos.,
112, D22s11, 10.1029/2006jd008205, 2007.Cappa, C. D., Onasch, T. B., Massoli, P., Worsnop, D. R., Bates, T. S.,
Cross, E. S., Davidovits, P., Hakala, J., Hayden, K. L., Jobson, B. T.,
Kolesar, K. R., Lack, D. A., Lerner, B. M., Li, S. M., Mellon, D., Nuaaman,
I., Olfert, J. S., Petaja, T., Quinn, P. K., Song, C., Subramanian, R.,
Williams, E. J., and Zaveri, R. A.: Radiative Absorption Enhancements Due to
the Mixing State of Atmospheric Black Carbon, Science, 337, 1078–1081,
10.1126/science.1223447, 2012.Chen, X. S., Wang, Z. F., Yu, F. Q., Pan, X. L., Li, J., Ge, B. Z., Wang,
Z., Hu, M., Yang, W. Y., and Chen, H. S.: Estimation of atmospheric aging
time of black carbon particles in the polluted atmosphere over
central-eastern China using microphysical process analysis in regional
chemical transport model, Atmos. Environ., 163, 44–56,
10.1016/j.atmosenv.2017.05.016, 2017.Cheng, Y. F., Su, H., Rose, D., Gunthe, S. S., Berghof, M., Wehner, B.,
Achtert, P., Nowak, A., Takegawa, N., Kondo, Y., Shiraiwa, M., Gong, Y. G.,
Shao, M., Hu, M., Zhu, T., Zhang, Y. H., Carmichael, G. R., Wiedensohler,
A., Andreae, M. O., and Poschl, U.: Size-resolved measurement of the mixing
state of soot in the megacity Beijing, China: diurnal cycle, aging and
parameterization, Atmos. Chem. Phys., 12, 4477–4491,
10.5194/acp-12-4477-2012, 2012.Ding, A. J., Huang, X., Nie, W., Sun, J. N., Kerminen, V. M., Petaja, T.,
Su, H., Cheng, Y. F., Yang, X. Q., Wang, M. H., Chi, X. G., Wang, J. P.,
Virkkula, A., Guo, W. D., Yuan, J., Wang, S. Y., Zhang, R. J., Wu, Y. F.,
Song, Y., Zhu, T., Zilitinkevich, S., Kulmala, M., and Fu, C. B.: Enhanced
haze pollution by black carbon in megacities in China, Geophys. Res.
Lett., 43, 2873–2879, 10.1002/2016gl067745, 2016.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.Gong, X. D., Zhang, C., Chen, H., Nizkorodov, S. A., Chen, J. M., and Yang,
X.: Size distribution and mixing state of black carbon particles during a
heavy air pollution episode in Shanghai, Atmos. Chem. Phys.,
16, 5399–5411, 10.5194/acp-16-5399-2016, 2016.Gysel, M., Laborde, M., Olfert, J. S., Subramanian, R., and Grohn, 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.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.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.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.Laborde, M., Crippa, M., Tritscher, T., Juranyi, Z., Decarlo, P. F.,
Temime-Roussel, B., Marchand, N., Eckhardt, S., Stohl, A., Baltensperger,
U., Prevot, A. S. H., Weingartner, E., and Gysel, M.: Black carbon physical
properties and mixing state in the European megacity Paris, Atmos.
Chem. Phys., 13, 5831–5856, 10.5194/acp-13-5831-2013, 2013.Lack, D. A., Langridge, J. M., Bahreini, R., Cappa, C. D., Middlebrook, A.
M., and Schwarz, J. P.: Brown carbon and internal mixing in biomass burning
particles, P. Natl. Acad. Sci. USA, 109, 14802–14807, 10.1073/pnas.1206575109,
2012.Lan, Z. J., Huang, X. F., Yu, K. Y., Sun, T. L., Zeng, L. W., and Hu, M.:
Light absorption of black carbon aerosol and its enhancement by mixing state
in an urban atmosphere in South China, Atmos. Environ., 69, 118–123,
10.1016/j.atmosenv.2012.12.009, 2013.Li, J., Posfai, M., Hobbs, P. V., and Buseck, P. R.: Individual aerosol
particles from biomass burning in southern Africa: 2. Compositions and aging
of inorganic particles, J. Geophys. Res.-Atmos., 108, 12, 10.1029/2002jd002310, 2003.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., Allan, J. D., Young, D. E., Coe, H., Beddows, D., Fleming, Z. L.,
Flynn, M. J., Gallagher, M. W., Harrison, R. M., Lee, J., Prevot, A. S. H.,
Taylor, J. W., Yin, J., Williams, P. I., and Zotter, P.: Size distribution,
mixing state and source apportionment of black carbon aerosol in London
during wintertime, Atmos. Chem. Phys., 14, 10061–10084,
10.5194/acp-14-10061-2014, 2014.Liu, D. T., Taylor, J. W., Young, D. E., Flynn, M. J., Coe, H., and Allan,
J. D.: The effect of complex black carbon microphysics on the determination
of the optical properties of brown carbon, Geophys. Res. Lett., 42,
613–619, 10.1002/2014gl062443, 2015.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–U132,
10.1038/Ngeo2901, 2017.
Menon, S., Hansen, J., Nazarenko, L., and Luo, Y.: Climate effects of black
carbon aerosols in China and India, Science, 297, 2250–2253, 2002.Moteki, N. and Kondo, Y.: Dependence of Laser-Induced Incandescence on
Physical Properties of Black Carbon Aerosols: Measurements and Theoretical
Interpretation, Aerosol Sci. Technol., 44, 663–675,
10.1080/02786826.2010.484450, 2010.Moteki, N., Kondo, Y., and Nakamura, S.: Method to measure refractive
indices of small nonspherical particles: Application to black carbon
particles, J. Aerosol. Sci., 41, 513–521, 10.1016/j.jaerosci.2010.02.013, 2010.Pagels, J., Khalizov, A. F., McMurry, P. H., and Zhang, R. Y.: Processing of
Soot by Controlled Sulphuric Acid and Water CondensationMass and Mobility
Relationship, Aerosol Sci. Technol., 43, 629–640,
10.1080/02786820902810685, 2009.Pan, X. L., Kanaya, Y., Taketani, F., Miyakawa, T., Inomata, S., Komazaki,
Y., Tanimoto, H., Wang, Z., Uno, I., and Wang, Z. F.: Emission
characteristics of refractory black carbon aerosols from fresh biomass
burning: a perspective from laboratory experiments, Atmos. Chem.
Phys., 17, 13001–13016, 10.5194/acp-17-13001-2017, 2017.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.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.Popovicheva, O. B., Persiantseva, N. M., Kireeva, E. D., Khokhlova, T. D.,
and Shonija, N. K.: Quantification of the Hygroscopic Effect of Soot Aging
in the Atmosphere: Laboratory Simulations, J. Phys. Chem. A, 115, 298–306,
10.1021/jp109238x, 2011.Qin, Y. and Xie, S. D.: Spatial and temporal variation of anthropogenic
black carbon emissions in China for the period 1980–2009, Atmos.
Chem. Phys., 12, 4825–4841, 10.5194/acp-12-4825-2012, 2012.
Ramanathan, V. and Carmichael, G.: Global and regional climate changes due
to black carbon, Nat. Geosci., 36, 335–358, 2008.Ramanathan, V., Crutzen, P. J., Kiehl, J. T., and Rosenfeld, D.: Atmosphere
– Aerosols, climate, and the hydrological cycle, Science, 294, 2119–2124,
10.1126/science.1064034, 2001.Riemer, N., Vogel, H., and Vogel, B.: Soot aging time scales in polluted
regions during day and night, Atmos. Chem. Phys., 4,
1885–1893, 10.5194/acp-4-1885-2004, 2004.Schnaiter, M., Linke, C., Möhler, O., Naumann, K. H., Saathoff, H.,
Wagner, R., Schurath, U., and Wehner, B.: Absorption amplification of black
carbon internally mixed with secondary organic aerosol, J.
Geophys. Res.-Atmos., 110, 11, 10.1029/2005jd006046, 2005.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., Moteki, N., Takegawa, N., Sahu, L., Takami, A.,
Hatakeyama, S., Yonemura, S., and Blake, D.: Radiative impact of mixing
state of black carbon aerosol in Asian outflow, J. Geophys.
Res.-Atmos., 113, 13, 10.1029/2008jd010546, 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.Taylor, J. W., Allan, J. D., Allen, G., Coe, H., Williams, P. I., Flynn, M.
J., Le Breton, M., Muller, J. B. A., Percival, C. J., Oram, D., Forster, G.,
Lee, J. D., Rickard, A. R., Parrington, M., and Palmer, P. I.:
Size-dependent wet removal of black carbon in Canadian biomass burning
plumes, Atmos. Chem. Phys., 14, 13755–13771,
10.5194/acp-14-13755-2014, 2014.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., Zhao, Z. Z., Cao, J. J., Ni, H. Y., Tie, X. X.,
Zhao, S. Y., Su, X. L., Han, Y. M., Shen, Z. X., Wang, Y. C., Zhang, N. N.,
Zhou, Y. Q., and Corbin, J. C.: Physicochemical characteristics of black
carbon aerosol and its radiative impact in a polluted urban area of China,
J. Geophys. Res.-Atmos., 121, 12505–12519, 10.1002/2016jd024748, 2016.Wang, Q. Y., Cao, J. J., Han, Y. M., Tian, J., Zhu, C. S., Zhang, Y. G.,
Zhang, N. N., Shen, Z. X., Ni, H. Y., Zhao, S. Y., and Wu, J. R.: Sources
and physicochemical characteristics of black carbon aerosol from the
southeastern Tibetan Plateau: internal mixing enhances light absorption,
Atmos. Chem. Phys., 18, 4639–4656, 10.5194/acp-18-4639-2018,
2018.
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 Structures of Soot Particles during Atmospheric Processing, Environ.
Sci. Tech. Let., 4, 487–493, 10.1021/acs.estlett.7b00418, 2017.Wu, Y., Cheng, T. H., Liu, D. T., Allan, J. D., Zheng, L. J., and Chen, H.:
Light Absorption Enhancement of Black Carbon Aerosol Constrained by Particle
Morphology, Environ. Sci. Technol., 52, 6912–6919, 10.1021/acs.est.8b00636,
2018.Wu, Y. F., Wang, X. J., Tao, J., Huang, R. J., Tian, P., Cao, J. J., Zhang,
L. M., Ho, K. F., Han, Z. W., and Zhang, R. J.: 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.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. J., Ho, K. F., Cao, J. J., Han, Z. W., Zhang, M. G., Cheng, Y.,
and Lee, S. C.: Organic carbon and elemental carbon associated with PM10 in
Beijing during spring time, J. Hazard. Mater., 172, 970–977,
10.1016/j.jhazmat.2009.07.087, 2009.Zhang, R. Y., Khalizov, A. F., Pagels, J., Zhang, D., Xue, H. X., and
McMurry, P. H.: Variability in morphology, hygroscopicity, and optical
properties of soot aerosols during atmospheric processing, P. Natl. Acad. Sci.
USA, 105, 10291–10296, 10.1073/pnas.0804860105, 2008.Zhang, Y. X., Zhang, Q., Cheng, Y. F., Su, H., Li, H. Y., Li, M., Zhang, X.,
Ding, A. J., and He, K. B.: Amplification of light absorption of black
carbon associated with air pollution, Atmos. Chem. Phys., 18,
9879–9896, 10.5194/acp-18-9879-2018, 2018.