Introduction
Black carbon (BC) is generated from incomplete combustion of carbon-based
fuels (Ramanathan and Carmichael, 2008) and can exert significant impacts on
global and regional climate, planetary boundary layer height (PBLH), air
quality and human health, etc. (Lee et al., 2017; Bond et al., 2013; Ding et
al., 2016). BC can strongly absorb solar radiation and warm up the atmosphere
directly. By internally or externally mixing with non-BC materials (coatings,
including co-emitted primary organic/inorganic and secondary materials that
associate with BC) (C. Chen et al., 2016; Lee et al., 2017; J. Wang et al.,
2017), the properties and morphologies of BC might be altered greatly (Liu et
al., 2013, 2015, 2017; Cappa et al., 2012; Peng et al., 2016; Y. Wang et al.,
2017b; Li et al., 2016). Thick coating can increase the mass absorption cross
section of BC, thus enhance the light absorption of BC core via “lensing
effect” (Jacobson, 2001; Liu et al., 2015; Pokhrel et al., 2017). However,
coating thickness of BC-containing particles significantly depends on
sources/chemical compositions and aging processes; thus there are great
uncertainties in light absorption enhancement (Eabs) of BC as
well as its global radiative forcing (Cappa et al., 2012; Liu et al., 2015,
2017; Cui et al., 2016). For instance, the mass ratio of coatings to BC core
(RBC, an analog of coating thickness) from biomass burning is
usually greater than 3 (Liu et al., 2017) and can be larger than 10 in remote
sites (J. Wang et al., 2017). Normally, when RBC is less than
1.5, it is probably from traffic sources, whereas secondary organic aerosol
(SOA) dominant BC-containing particles is usually with a RBC
greater than 4 (Lee et al., 2017). Moreover, the coating species can modify the
hygroscopicity of BC-containing particles (Liu et al., 2013) when associated
with hydrophilic materials, and some of them can be activated as cloud
condensation nuclei (CCN), therefore altering the albedo and precipitation of
clouds indirectly (Dusek et al., 2010, 2006).
In the past decades, a number of field studies on BC have been conducted in
the winter of Beijing and have mainly focused on BC mass loadings, mixing
states, optical properties, human health impacts and sources (coal
combustion, biomass burning and vehicles, etc.) (Wu et al., 2017, 2016; Cheng
et al., 2017; Ji et al., 2017; Y. Wang et al., 2017a; Q. Wang et al., 2016;
Y. Chen et al., 2016; Meng et al., 2016; Liu et al., 2016; Yang et al., 2014;
Schleicher et al., 2013a, b; Song et al., 2013; Zhang et al., 2017). There
were real-time studies on BC and on the chemical characteristics of total
fine particles (including particles with and without BC) in Beijing. However,
to the best of our knowledge, no study was conducted in real time to
characterize the chemical compositions exclusively of BC-containing particles
in Beijing despite the aforementioned important effects of coating materials
on BC properties. Currently, a few studies have explored BC-containing
particles in other locations, e.g., Toronto (Willis et al., 2016; Lee et al.,
2015), California (Lee et al., 2017; Massoli et al., 2015; Cappa et al.,
2012), London (Liu et al., 2015) and Tibet (J. Wang et al., 2017) by using
the Aerodyne soot-particle aerosol mass spectrometer (SP-AMS) (Onasch et al.,
2012; Lee et al., 2015; J. Wang et al., 2016; Ge et al., 2017b). The SP-AMS
physically combines the 1064 nm laser vaporizer of single-particle soot
photometer (SP2) into a high-resolution aerosol mass spectrometer (HR-AMS).
After removal of the AMS tungsten vaporizer and by operating the instrument
with laser vaporizer only, refractory BC as well as its associated coating
can be evaporated, since the 1064 nm laser can selectively heat the BC
(Massoli et al., 2015). In other words, laser-only SP-AMS can exclusively
measure BC cores and the species coated on BC cores. This unique technique
allows us to explore the characteristics of BC-coating species in detail with
no perturbations from other co-existing non-BC containing particles in
ambient air.
Beijing, as the most reprehensive megacity with a large population in
developing countries, the BC-containing particles may have specific source
profiles and physiochemical properties; therefore elucidation of its
characteristics is important to understand the haze formation and improve air
quality in such regions. In this work, as part of the UK–China Atmospheric
Pollution & Human Health (APHH) study (Shi et al., 2018), we report for the
first time the real-time measurement results on the chemical composition,
mass loading, size distribution and sources/processes of BC-containing
particles during the wintertime of 2016 in urban Beijing. Results regarding
physical properties and optical properties are presented in Liu et al. (2018)
and Xie et al. (2019) of this special issue, respectively.
Experiments
Sampling site and instrumentation
As part of the APHH winter campaign, we conducted measurements at the
Tower Division of Institute of Atmospheric Physics (IAP), Chinese Academy of
Science (39∘58′ N, 116∘22′ E) in Beijing
(Fig. S1 in the Supplement), from 15 November to 13 December of 2016. The
site was surrounded by residential infrastructures and a freeway in the east
(360 m).
The SP-AMS was deployed on the rooftop of the Herong
building (∼8 m above the
ground), with a PM2.5 cyclone (model URG-2000-30EN) and a diffusion
dryer in front of the inlet. The single-particle soot photometer (SP2,
Droplet Measurement Technology, Inc., Boulder, CO, USA) was operated
simultaneously inside another container nearby (∼20 m away) on the
ground. The SP2 incandescence signal was calibrated for BC mass by using
Aquadag® black carbon standard (Aqueous
Deflocculated Acheson Graphite, Acheson Inc., USA) (Laborde et al., 2012).
For the SP-AMS, since the filament that ejects electrons can still heat the
tungsten vaporizer up to ∼200 ∘C (Willis et al., 2014) even if
it is turned off, the tungsten vaporizer was thus physically removed to make
sure only BC and its associates were vaporized by the laser and to eliminate
the influence of uncoated species on BC cores.
The tuning and calibration procedures of SP-AMS followed the procedures
described previously (Lee et al., 2015; Willis et al., 2016; Massoli et al.,
2015; J. Wang et al., 2017). During the campaign, the SP-AMS was run with a
10 min cycle: one W mode with high chemical resolution (2.5 min) and two
mass sensitive V modes, including one with particle time of flight (PToF) mode
(2.5 min) and another one (5 min) with a large mass-to-charge (m/z) range
(up to 2000) (J. Wang et al., 2016). The filtered air measurement was
performed for a day to determine the detection limits (DLs) of various
aerosol species and to adjust the fragmentation table. The ionization
efficiency (IE) and relative ionization efficiency (RIE) of sulfate and
nitrate were calibrated by using pure ammonium nitrate and ammonium sulfate
according to Jayne et al. (2000), respectively. RIE of BC was calibrated by
using REGAL black particles (RB, REGAL 400R pigment black, Cabot Corp.) (Onasch et al.,
2012), and the average ratio of C1+ to C3+ was
determined to be 0.53 to minimize the influence of C1+ from
non-refractory organics. However, it should be aware that the laser-only SP-AMS
cannot vaporize ammonium nitrate or sulfate if they do not coat BC; thus the
IE and RIE calibrations were done before removal of the tungsten vaporizer
and the values were assumed to be unchanged after the tungsten heater's
removal (Willis et al., 2016). Note that the RIE of BC was calibrated before the
campaign and was repeated in the middle and end of the campaign. RIEs of
nitrate, ammonium, sulfate and BC were determined to be 1.1, 3.82, 0.82 and
0.17, respectively. The default value of 1.4 was used as the RIE of organics
(Canagaratna et al., 2007). Polystyrene latex (PSL) spheres (100–700 nm)
(Duke Scientific Corp., Palo Alto, CA) were used to calibrate the size before
the campaign (Canagaratna et al., 2007) .
Data analysis
Standard AMS data analysis software (Squirrel and Pika) based on Igor Pro
6.37 (Wavemetrixs, Lake Oswego, OR, USA) were used to obtain the
concentrations, mass spectra and size distributions of BC and its coating
species. All data were calculated based on high-resolution fitting results.
Due to different vaporization schemes between the SP-AMS and HR-AMS, the mass
spectra from these two instruments even for the same population of particles
are not entirely the same. Laser-only SP-AMS can result in less
fragmentation overall; therefore the mass profile may contain more large m/z fragments
and less small m/z fragments compared with that from HR-AMS (Massoli et al.,
2015). Therefore, here the elemental ratios of organics, i.e.,
oxygen-to-carbon, hydrogen-to-carbon and nitrogen-to-carbon ratios (O/C,
H/C and N/C) were determined by the Aiken approach first (Aiken et al., 2008),
and then O/C and H/C were corrected by using factors of 0.83 and 1.16,
respectively (Canagaratna et al., 2015).
Source apportionment for organics coated on BC was conducted by using
positive matrix factorization (PMF) (Paatero and Tapper, 1994) evaluation
tool written in Igor (Ulbrich et al., 2009). In this study, high-resolution
mass spectra (HR-MS) of organic (including BC) and inorganic species were
combined together to perform the PMF analyses (Sun et al., 2012; J. Wang et
al., 2017, 2018). It should be noticed that only fragment ions from
polycyclic aromatic hydrocarbons (PAHs) were included for m/z range of
∼150 to ∼250 in the PMF analysis because of the limited mass
resolution of SP-AMS. All PMF solutions were evaluated following the standard
instruction (Zhang et al., 2011). Finally, four types of organic aerosol (OA)
associated with BC were determined eventually, including a fossil fuel
combustion OA (FFOA), a biomass-burning OA (BBOA) and two oxygenated OA (OOA1
and OOA2) (a diagnostic plot was provided in Fig. S2).
Supporting data such as meteorological parameters including relative humidity
(RH), wind speed (WS), wind direction (WD) and temperature (T), as well as
concentrations of gaseous species such as O3, SO2,
NO, NO2, NOx, NOy,
NOz, and CO were measured in parallel. All
data here are reported in local time (Beijing Time, UTC+8).
Temporal variation in (a) relative humidity (RH) and
temperature (T, ∘C), (b) wind speed (WS, m s-1) and
wind direction (WD) and (c, d) mass loadings of CO, SO2,
NOx and O3.
Results and discussion
Overview of BC-containing aerosol characteristics
Figures 1 and 2 show the temporal variations in meteorologic parameters, mass
loadings of gaseous pollutants (CO, NOx, SO2 and
O3), BC and its associated coating components (sulfate, nitrate,
ammonium, chloride, total OA and four PMF-resolved OA factors). The
campaign-averaged composition of BC-containing particles and mass
contributions of the four OA factors to total OA were also displayed in
Fig. 2. Overall, wind directions and speeds had close associations with
overall mass loadings of BC-containing particles. The polluted periods
(characterized by concentrations of BC-containing particles above
10 µgm-3) were accompanied by relatively low wind speeds
(< 4 m s-1) and in a relatively large part from southern air
masses, since Beijing is at the foot of the mountains, which facilitates the
accumulation of pollutants from the southern North China Plain (NCP). The clean
periods (characterized by the concentrations of BC-containing particles below
10 µgm-3) were mainly under the control of northwesterly strong
winds (>4 ms-1) (Fig. S3). During the campaign,
the mass loadings of BC cores and BC-containing particles ranged from
0.11 to 26.54 µgm-3 and 0.71 to 174.40 µgm-3,
with averages of 4.9 µgm-3 and 29.4 µgm-3,
respectively. We also compared BC concentrations determined by the SP-AMS
with those from SP2, and they correlated quite well with each other (r2
of 0.93; Fig. S4), indicating that the quantification of BC by the SP-AMS is
reliable.
(a) Temporal variations in mass loadings of inorganic
coating components (sulfate, nitrate, ammonium and chloride) and BC cores,
and (b) temporal variations in mass loadings of organic coating
(Org) and PMF-separated OA factors (inset pie charts show the average
composition of total BC-containing particles and organics, respectively).
The coating species occupied on average about 83.4 % of the mass of
BC-containing particles, indicating that BC was generally thickly coated
throughout the whole campaign, with an average mass ratio of coatings to BC
(RBC) of ∼5.0. Organic aerosol (OA) was the most abundant
coating component, taking up 59.4 % of the total mass, followed by
nitrate (8.8 %), sulfate (6.5 %), ammonium (4.7 %) and chloride
(4.0 %). OA correlated quite well with BC (r2 of 0.97), suggesting
that many OA species were co-emitted and mixed with BC, and indeed, primary
OA (POA = FFOA + BBOA) was found to dominate the OA mass
(66.3 % = 43.9 % + 22.4 %). Chloride (Cl-) had a great
correlation with BC (r2 of 0.94), suggesting it was mainly associated
with primary emissions, for example, gasoline, diesel and coal combustion
during wintertime in urban Beijing. Sulfate and nitrate are typically
secondarily formed; therefore their correlations with BC were relatively
weak (r2 of 0.64 for SO42- vs. BC and 0.60 for
NO3- vs. BC). Their properties are discussed in detail in the following sections.
Chemically resolved size distributions of BC-containing
particles
Figure 3a shows the campaign-averaged mass-based size distributions of major
BC-coating species, including organics (BC-org), sulfate (BC-sulfate),
nitrate (BC-nitrate), chloride (BC-Chl) and BC core itself. It should be
noticed that the size distribution of BC was scaled from that of m/z 24
(C2+), as other major carbon cluster ions might be significantly
affected by other ions; for example, C1+ at m/z 12 can be influenced
by fragments from non-BC organics, C3+ at m/z 36 by HCl+,
C4+ at m/z 48 by SO+ and C5+ at m/z 60 by
C2H4O2+, etc. Similarly, the size distribution of BC-Chl
was scaled from Cl+ signal at m/z 35. As shown in Fig. 3a, on average,
size distributions of BC-sulfate, BC-nitrate and BC-org displayed similar
patterns with a major peak at ∼550 nm (vacuum aerodynamic
diameter, Dva), suggesting that they were relatively well
mixed internally. However, the BC presented a remarkably different pattern with a much
broader distribution and smaller peak sizes than its coating species, and in
particular, relatively small particles tended to have thin coatings.
Mass-based campaign-averaged size distributions: (a) major
coating components and BC cores, and (b–f) image plots of size
distributions of sulfate, nitrate, BC, organics and chloride as a function
of RBC (mass ratio of coating to BC). (Note that size distributions
of BC and chloride were scaled from those of m/z 24 and m/z 35,
respectively.)
Figure 3b–f further present image plots of size distributions of the major
aerosol components as a function of RBC (a surrogate of coating
thickness). In contrast to the average data shown in Fig. 3a, the coating
species can be roughly classified into two modes separated by RBC of
∼4.5. Most sulfate and nitrate concentrated at
RBC>4.5 (Fig. 3b and c): sulfate peaked in a narrow RBC
range of 5.5–6.5, while significant nitrate mass could
distribute across a wider RBC range (even to RBC of
∼8.0). Only organics and chloride had a significant portion of mass
distributed on relatively thinly coated BC-containing particles at
RBC<4.5 (Fig. 3e and f). Specifically, they both showed a
submode locating in the regime with RBC of ∼3.5–4.5 and
Dva of ∼200–700 nm. These submodes suggest that organics
or chloride are partially from primary sources as freshly emitted BC are
more likely thinly coated. This is consistent with organics including
species from fossil fuel and biomass-burning combustion, revealed by the PMF
analysis. Similarly, coal burning might contribute to chloride during
wintertime in Beijing (Sun et al., 2016). As for sulfate and nitrate, since
they are predominantly secondary species, they would coat BC cores due to
chemical aging and are therefore mostly distributed at higher RBC.
High-resolution mass spectra of (a) fossil fuel combustion
OA (FFOA + BC), (b) biomass-burning OA (BBOA + BC),
(c) OOA1 + BC, (d) OOA2 + BC, (e) mass
fractions of the BC fragments apportioned in different OA factors, and
(f) diurnal cycles of the four OA factors relative to BC.
Sources of organic-coating species
The high-resolution mass spectra of different factors of the organic coating, resolved from PMF
analyses, their relative contributions and diurnal cycles of temporal
variations relative to BC, are shown in Fig. 4. Figure 4a illustrates the mass
profile of the fossil fuel combustion OA with BC carbon clusters (FFOA + BC).
This factor had a low O/C ratio of 0.16. In this work, this factor might
include emissions from both traffic and coal combustion, as it contained a
series of significant PAH ion fragments in the mass spectrum (PAH
fragments are negligible in other factors), indicative of coal burning (Sun
et al., 2014, 2016), and presented a good correlation with
C4H9+ (r2 of 0.72) – an AMS tracer ion of vehicle
emissions (Zhang et al., 2005). Temporal variations in FFOA also correlated
well with C9H7+ (m/z 115, r2 of 0.92) and
Cl- (r2 of 0.60), which have been proposed as possible coal combustion
tracer species (Yan et al., 2018; Sun et al., 2014). The FFOA / BC (Fig. 4f)
appeared to be higher at nighttime than during the daytime. Note that the
diurnal pattern of BC itself (Fig. 5c) was similar to that of FFOA / BC. The
diurnal variations in BC might be influenced by both fossil fuel combustion
activities and relatively low PBLH at nighttime. The fossil fuel
combustion included coal burning and vehicle emissions (gasoline cars and
the heavy-duty diesel vehicles that are only allowed to enter the city
late at night). The mass ratios of different factors to BC have
a smaller influence from PBLH; therefore high levels of FFOA / BC strongly
indicate that co-emitted organic species with BC from fossil fuel combustion
were enhanced at nighttime.
Diurnal cycles of (a) T and RH, (b) wind
direction and wind speed, (c) mass ratio of coatings to BC
(RBC) and BC, (d) org / BC, SO42-/BC,
NO3-/BC and Cl-/BC, (e) mass loadings of
gaseous species (CO, SO2, NOx) and (f)
O/C and oxidation state (OSc=2×O/C-H/C).
Figure 4b shows the mass spectrum of BBOA and related BC clusters. One
feature of this factor is that it had relatively high fractional
contributions of C2H4O2+ (1.47 % of total) and
C3H5O2+ (0.95 %), which are often regarded as AMS
marker ions from levoglucosan emitted from biomass-burning (Cubison et al., 2011;
Mohr et al., 2009). Note that the FFOA also contained appreciable
C2H4O2+ and C3H5O2+ signals,
partially because coal burning (such as lignite) can emit some
levoglucosan as well (Yan et al., 2018). Nevertheless, the mass fractions of
C2H4O2+ and C3H5O2+ in FFOA were
smaller than those in BBOA, and they correlated much better with BBOA than
those with FFOA (for example, r2 of 0.90 for BBOA vs. C2H4O2+,
and 0.72 for FFOA vs. C2H4O2+). The BBOA correlated very
well with another biomass-burning tracer – K+(r2 of 0.90). In
addition, BBOA had negligible PAH ion fragments, while the FFOA contained
remarkably high PAH signals. Such characteristics are generally in
agreement with previous AMS findings at the same location during wintertime
in Beijing (Sun et al., 2016). For these reasons, the second factor was
identified as BBOA. The diurnal pattern of BBOA / BC reached minimum during
afternoon and was high overall at nighttime, similarly to FFOA / BC,
indicating the nighttime enhancement of BB-related organics emissions in
wintertime Beijing.
Besides the two POA factors, we also identified two secondary OA factors
(OOA1 and OOA2), the O/C ratios of which were 0.45 and 0.28. OOA1 was
the most oxidized OA factor that had a higher
CO2+/C2H3O+ ratio than that of OOA2. The
correlation between OOA1 and sulfate was better than with nitrate
(r2 of 0.99 vs. 0.86). As a comparison, the less oxygenated OOA2 correlated
better with nitrate than with sulfate (r2 of 0.59 vs. 0.34). These
characteristics are consistent with previous AMS–PMF results (Zhang et al.,
2011). In contrast to the diurnal cycles of FFOA / BC and BBOA / BC, the OOA2 / BC ratio
rose significantly from early morning and peaked in the afternoon
(∼ 15:00). The diurnal pattern of OOA1 / BC presented a similar
peak at ∼ 15:00. This result demonstrates clear evidence of the
important role of afternoon photochemical reactions to the formation of
secondary organic species. However, the precursors leading to the formations
of OOA1 and OOA2 remain to be elucidated. Interestingly, for OOA1 / BC, in
addition to the peak during the afternoon, it increased during early evening and
remained at high levels until early morning. This result indicates that
nighttime aqueous-phase processing (high levels of RH at nighttime shown
in Fig. 5a) can also contribute to OOA1 production. As such behavior was not
observed for OOA2 / BC, it agrees with previous field and laboratory findings
that aqueous-phase reactions tend to produce more highly oxygenated species
(Ervens et al., 2011; Ge et al., 2012; Herrmann et al., 2015; Xu et al.,
2017).
Overall, the mass fractions of BC cores that were associated with fossil
fuel combustion, biomass burning, less and more oxygenated secondary
processes were 32.7 %, 31.8 %, 18.7 % and 16.9 % (Fig. 4e).
The organic coating of BC was predominantly primary species.
(a, b) Average compositions of BC-containing particles
during clean and polluted periods, (c, d) mass fractions of the
non-BC coating components (left y axis) and OSc (right
y axis) during clean and polluted periods as a function of
RBC, box plots of BC mass loadings (e) and
RBC (f) during clean and polluted periods (colors of the components
are consistent with those in Fig. 2).
Diurnal patterns of BC and coating species
Figure 5 presents the diurnal cycles of meteorological parameters (T, RH,
WS and WD), BC concentrations and RBC, mass ratios of major
species to BC, gaseous species (CO, SO2 and NOx),
O/C and OSc (oxidation state, defined as 2×O/C-H/C) (Kroll et al., 2011). Note that BC did not present a peak at 08:00,
yet RBC, org/BC, SO42-/BC,
NO3-/BC and Cl-/BC were all low at ∼08:00. This
was likely attributed to an increase in the mass fractions of fresh and
barely coated BC-containing particles (rather than the increase in absolute
concentrations of fresh BC-containing particles) emitted during morning rush
hours from traffic emissions, etc. This was consistent with the decreases in
O/C and OSc and increases in CO and NO2 at 08:00
on the day. On the contrary, the RBC drop at ∼16:00 was
unlikely due to the influence of the afternoon rush hour, as there were no
increases in CO, NO2, and both O/C and OSc were
at high levels. In fact, the 16:00 RBC drop was mainly caused by
the large decrease in org / BC (as SO42-/BC,
NO3-/BC and Cl-/BC did not decrease at 16:00,
Fig. 5d), which were mainly the portions of fossil fuel and biomass-burning
OA (Fig. 4f).
The diurnal variation in NO3-/BC peaked at ∼ 15:00–16:00,
consistent with the variation in T and similar to those in the previous
reports during wintertime in Beijing (Ge et al., 2017a; Sun et al., 2016),
reflecting the dominant contribution of photochemical formation of nitrate.
SO42-/BC showed a relatively small afternoon increase, indicating
partial sulfate was produced from photochemical activities; it also
presented a nighttime enhancement, similar to OOA1 / BC, suggesting the
sulfate formation in aqueous-phase, consistent with the nighttime increase in RH and decrease in temperature (Fig. 5a). Due to increases in FFOA / BC,
BBOA / BC and OOA1 / BC (the portion likely from aqueous-phase production),
org / BC remained at high levels at nighttime. All these increases were added
together, leading to high RBC at nighttime. In addition,
Cl-/BC varied generally similar to those of FFOA / BC and BBOA / BC, again
indicating its strong association with primary emissions.
Characteristics of coating species during different periods
Coating compositions during clean and polluted periods
Figure 6 shows the variation in BC-coating composition as a function of
RBC during clean periods (CPs) and polluted periods (PPs) (divided by the
concentration of 10 µgm-3). Contrasting
differences in the coating composition during these two cases was observed: primary OA
(especially FFOA) appeared to be the most abundant component during CPs, while
mass contributions of secondary organic and inorganic species were
remarkably high during PPs (Fig. 6a and b), and the average RBC during
PPs (∼5.1) was also higher than that during CPs
(∼4.5) (Fig. 6f). These results again reinforce the
importance of secondarily formed species to the heavy haze pollution in
urban Beijing (Huang et al., 2014). Furthermore, the BC-coating composition
and OSc were both relatively stable compared to RBC during CPs
(Fig. 6c). On the contrary, during PPs, with the increase in RBC, the
mass fractions of secondary species (OOA1, nitrate and sulfate) clearly increased, especially at RBC>5; consistently, OSc of
organic coating increased from ∼-0.85 to >-0.70.
Such behavior again highlights the contribution of the chemical aging process to
the heavy haze pollution.
High-resolution mass spectra of the average OA at different
episodes: (a) first episode (FE), (b) second episode (SE)
and (c) whole campaign (inset pies show the average compositions
during corresponding episodes; colors of different components are consistent
with those in Fig. 2).
Relative to other observations (J. Wang et al., 2017; Massoli et al., 2015;
Cappa et al., 2012), the levels of RBC during both CPs and PPs are
much smaller than those for highly aged BC, which might have RBC>10. As BC-containing particles in urban Beijing were influenced
by multiple local and regional primary sources, the relative amount of secondarily
formed coating species would be less than those of highly aged BC; therefore
this lower RBC is expected. On the other hand, the
RBC levels are generally higher than those found for the
BC-containing particles in Los Angeles where the average RBC was
typically smaller than 4 due to the direct and prominent influence of vehicle
emissions (Lee et al., 2017). Regarding the variations in coating composition
in relation to RBC, the behavior during PPs is in fact consistent with a few
previous field measurement results in American and European urban locations
(Massoli et al., 2015; Liu et al., 2017; Lee et al., 2017; Cappa et al.,
2012; Collier et al., 2018), indicating a general trend for more aged BC-containing
particles in urban areas to have a thicker coating. Yet
this property can be altered if significant POA emissions exist, such as in the
case during CPs in this work, and a case with heavy BBOA influences observed
in the Tibeten Plateau (J. Wang et al., 2017).
Coating compositions during two polluted episodes
Although we demonstrated in Sect. 3.5.1 that the heavy pollution of BC-containing
particles was on average associated with more secondary species, the
underlying governing factors of individual pollution events might vary. Here we investigated the characteristics of BC-containing
particles in two most polluted episodes occurring during the campaign. The
first episode (FE) was accompanied with relatively high RH (from 18:00 of
3 December to 08:00 of 4 December 2016), while the second episode (SE)
was dominated by primary emissions (from 00:00 to 06:00 of 11 December
2016). The average mass loadings of BC cores and BC-containing particles were
18.1 and 123.1 µgm-3 during FE and 14.4 and 80.0 µgm-3 during SE, respectively – both were much higher than the
campaign-averaged BC of 4.9 µgm-3 and BC-containing particles
of 29.4 µgm-3. Back trajectories, wind rose plots and
distributions of the wind speeds and directions of these two episodes were
provided in Fig. S5, showing that these two episodes had remarkably different
air mass origins and sources.
For FE, the average T and RH were ∼4.2 ∘C and
∼78 %. The average T was close to the
campaign-averaged value of 4.8 ∘C, but the air was more humid than the
campaign-averaged RH of ∼50 %. Correspondingly, we observed
remarkable elevations of the mass contributions of sulfate from 6.5 % to
10.3 %, nitrate from 8.8 % to 10.2 % and OOA1 from 7.5 % to 11.5 %
(Fig. 7a and c). Such enhancements were very likely linked with
aqueous-phase processing as this episode occurred at nighttime and was
characterized with high RH conditions. During FE, nitrate and sulfate also
correlated very well (r2 of 0.94; Fig. S6); therefore the formation of
nitrate would also be related to aqueous-phase processing in this episode.
Consistently, nitrate and sulfate formations driven by high RH in the North
China Plain have been proven previously (Kuang et al., 2016; Sun et al.,
2018; Wu et al., 2018). As a comparison, the mass fraction of
photochemical-relevant OOA2 decreased significantly from campaign-averaged 13.3 %
to 9.8 %. In addition, the mass fraction of Cl- also increased
from campaign-averaged 4.0 % to 5.3 %; meanwhile, we found that relative
to the campaign-averaged values, the KCl+/BC ratio decreased 14 %, the
K3SO4+/BC ratio increased 28 %, possibly indicating that
the heterogeneous replacement reactions of coal-burning-related Cl- by
SO42- during FE (Fig. S6). Overall, mainly due to the
aqueous-phase production of secondary coating components, compared to
campaign-averaged values, the average RBC became larger during FE (5.5
vs. 5.0), OA became more oxygenated (O/C of 0.18 vs. 0.15), and size distributions
of OA, sulfate and nitrate all shifted to larger peak sizes (Fig. S7a).
On the other hand, for SE, even though it also occurred at nighttime,
the average RH was significantly low (∼47 %), and it was
overwhelmingly dominated by primary species (50.6 % of FFOA, 15.2 % of
BBOA and 18 % of BC). Secondary sulfate and nitrate only took up 2.5 %
and 2.2 % of the total mass of BC-containing particles. Nighttime
aqueous-phase-related OOA1 contribution was nearly negligible (only
0.8 %), which, in another way, manifests that at nighttime OOA1
production was strongly associated with high RH conditions. Due to the
contribution of fresh primary emissions, the coating of OA was less oxygenated
than that of campaign-averaged OA (O/C of 0.12 vs. 0.15), and the average
RBC during SE was consistently smaller (4.5 vs. 5.0). Mass spectrum of
BC-org (Fig. 7b) also contained significant PAH fragments, in line with the
large contribution from FFOA (mainly coal combustion). The average size
distribution of OA during SE was broader and peaked in a smaller diameter
(< 500 nm Dva) (Fig. S7b) in response to the dominance of POA.
Occurrence of the highly polluted SE demonstrates that, even though the
pollution of BC-containing particles in urban Beijing during winter are on
average governed by secondary species, local primary emissions can sometimes
lead to serious and short-term pollution events as well.