Introduction
Black carbon (BC) has drawn considerable attention due to its key role in
climate and the atmospheric environment (Bond and Sun, 2005; Jacobson et al.,
2002, 2010). Because BC is the most efficient light-absorbing component in
ambient aerosols (Bond and Bergstrom, 2006; Ramanathan and Carmichael, 2008),
reduction measures targeting BC emissions have been recognized as a viable
way to mitigate global warming (Shindell et al., 2012; Jacobson et al., 2010)
and improve air quality in polluted regions (Ding et al., 2016; Z. Wang et
al., 2018). The benefits of BC emission reduction are mainly driven by more
solar radiation reaching the surface due to the reduction in BC light
absorption in the atmosphere.
The light absorption of ambient BC-containing particles can be reduced by
decreasing the BC mass concentration, weakening the BC light-absorption
capability or implementing both strategies. As primary aerosols, the mass
concentration of BC particles generally decreases with emission reduction.
When emission control measures were implemented, the mass concentration of
the BC present in the atmosphere was proven to decrease (Han et al., 2015;
Huang et al., 2010; Xu et al., 2015; J. K. Zhang et al., 2016). In terms of the
influence of emission reduction on the characteristics of BC aerosols,
previous studies usually highlighted the decrease in BC mass concentration
(Han et al., 2015; Huang et al., 2010; J. K. Zhang et al., 2016). However, few
studies have considered the change in the light-absorption capability of
BC-containing particles due to emission reduction.
The light-absorption capability of ambient BC-containing particles is closely
associated with their aging degree (Jacobson et al., 2001; Liu et al., 2017;
Moffet et al., 2009; Peng et al., 2016; Y. Zhang et al., 2016, 2018), i.e.,
the degree to which BC is internally mixed with other species (e.g., sulfate
and nitrate; Oshima et al., 2009). When fresh BC is emitted from incomplete
combustion (e.g., traffic emissions) other than biomass burning (Q. Wang et
al., 2018; Pan et al., 2017), they are most likely externally mixed with
other aerosol components (e.g., primary organic aerosol). These fresh BC
particles exist as almost bare particles with few other species condensed on
their surfaces and are called externally mixed BC particles (Jacobson et al.,
2001; Chung et al., 2005). During atmospheric transport, fresh BC particles
undergo aging, in which internally mixed BC particles form when other aerosol
components coat the bare BC surface (Cheng et al., 2006; Bond and Bergstrom,
2006; Peng et al., 2016; Zhang et al., 2018). The internally mixed BC
particles generally have a shell-and-core morphology, with the coating
materials and BC as the shell and core, respectively. This shell-and-core
morphology endows BC particles with a higher light-absorption capability
because the coating materials act as a lens to focus more photons on BC
(lensing effect, Lack and Cappa, 2010). Compared with externally mixed BC
particles (i.e., bare BC), the light absorption of internally mixed BC
particles (i,.e, coated BC) can be enhanced by a factor of 2–3 (Fuller et
al., 1999; Jacobson et al., 2001; Schnaiter et al., 2005; Y. Zhang et al.,
2016).
Emission reduction may affect the lensing effect by changing the amount of
coating materials for the BC-containing particles and consequently altering
the light-absorption capability of BC. Emission control measures can reduce
the concentrations of not only BC but also co-emitted gaseous pollutants
(e.g., volatile organic compounds (VOCs), SO2 and
NOx) present in the atmosphere (Tang et al., 2015; Huang et
al., 2015). The reduction in these secondary aerosol precursors can lower the
production of secondary components (e.g., secondary organic matter, sulfate
and nitrate) in aerosol particles (Cheng et al., 2008; Huang et al., 2010;
Han et al., 2015). This relationship implies that the interaction between BC
and secondary aerosol components via condensation and coagulation may be
impacted by primary emission reductions of both BC and co-emitted
pollutants (e.g., VOCs, SO2 and NOx); in other words,
emission control measures may influence BC aging in the atmosphere. As
mentioned above, the aging degree of BC-containing particles exerts a
substantial effect on their light-absorption capability. Less-aged BC is
expected as emission control measures are implemented to decrease BC
light-absorption capability. However, it is still unclear whether emission
control measures can lower the aging degree of BC-containing particles and
thus weaken their light-absorption capability.
In this work, we used the 2014 Asia-Pacific Economic Cooperation (APEC)
meeting in Beijing, China as a case study to investigate the effects of
emission control measures on the light absorption of ambient BC-containing
particles. This paper reports in situ measurements before, during and
after APEC and investigates how the concentrations of BC and coating
precursors, the BC aging degree and the BC light-absorption capability were
affected by emission reductions. Based on these results, we quantified the
impact of emission reduction during APEC on the light absorption of
BC-containing particles and further discuss the additional effect of
emission control measures on BC light absorption due to changes in the
coating materials of ambient BC particles.
Methods and data
Measurement location and period
The in situ measurements were carried out on the campus of Tsinghua University
(40∘00′17′′ N, 116∘19′34′′ E; Fig. S1 in the
Supplement). The observation site is located in downtown Beijing,
approximately 1 km from North 4th Ring Road, which has a high traffic
density. The air quality at this site is considered typical of the Beijing
urban environment. More details regarding the Tsinghua site can be found in
Zheng et al. (2015) and Zhang
et al. (2018).
The measurement period lasted from 28 October to 21 November 2014. A series
of aggressive measures were implemented from 3–12 November 2014 in Beijing
and the surrounding areas (i.e., Tianjin, Hebei, Shanxi, Shandong, Henan and
Inner Mongolia; shown in Fig. S1) to achieve good air quality during the APEC
meeting: mandatory restrictions on traffic flow in Beijing, limited or
arrested production from high-emitting factories, suspended construction
activities and bans on various outdoor burning practices (Gao et al., 2017;
Huang et al., 2015; Tang et al., 2015; J. K. Zhang et al., 2016; L. Zhang et
al., 2016). In this study, we classified the observation period into five
subperiods: before APEC (28 October–2 November 2014), which served as a
reference; during APEC (6–12 November 2014), which was characterized by the
enforcement of emission control measures; after APEC (17–21 November 2014),
which served as another reference; and two transition periods (3–5 and
13–16 November 2014), which are not discussed in this work considering that
we could not distinguish the BC particles transported to the site during
these days as characterized by the enforcement of emission control measures
or not (Fig. S2 and the associated discussion in the Supplement).
Instrumentation
A single-particle soot photometer (SP2) instrument (Droplet Measurement
Technologies, Boulder, CO, USA) uses a 1064 nm Nd:YAG laser to measure the
mass of a refractory BC (rBC) core (mrBC) and the scattering
cross section (Cs) of an individual BC-containing particle. As a
light-absorbing component, an rBC core is gradually heated by the continuous
laser beam and vaporizes at ∼4000 K, the temperature at which
detectable incandescent light is emitted (Schwarz et al., 2006; Moteki and
Kondo, 2010). The incandescence signal recorded by SP2 was used to determine
the mrBC of an individual BC-containing particle. The mass
concentration of rBC was calculated based on the mrBC and
sampling flow rate (∼0.12 L min-1, liters per minute). On the
other hand, we used the scattering signal from the SP2 measurement to
retrieve the Cs of an individual BC-containing particle
(including coating materials and rBC core) based on the leading-edge-only
(LEO) method developed by Gao et al. (2007). The validity of the LEO method
for ambient BC-containing particles observed in China has been evaluated by
Y. Zhang et al. (2016). More details on the SP2 technique have been reported
elsewhere (Gysel et al., 2011; Pan et al., 2017; Sedlacek et al., 2012;
Y. Zhang et al., 2016).
Data analysis
Aging degree of BC-containing particles
The aging degree of ambient BC-containing particles was retrieved by the SP2
measurements (i.e., the mrBC and the Cs of
BC-containing particles) and Mie calculation. To quantify the aging degree of
BC-containing particles, we assumed that a BC-containing particle was a
sphere with an rBC core and a non-refractory coating material (NR-CM) shell
(Moteki and Kondo, 2007; Subramanian et al., 2010; Y. Zhang et al., 2016).
The actual shape of BC-containing particles in the atmosphere is complex (He
et al., 2015; Scarnato et al., 2013; Z. Wang et al., 2017). In this study, we
focused on investigating BC-containing particles during pollution episodes.
Under polluted conditions, we have found fully aged BC-containing particles
in Beijing, China (Zhang et al., 2018). In our previous study (Y. Zhang et
al., 2016), we found that the thickly coated BC particles in the North China
Plain (including Beijing) exhibited a near-spherical shape, and the
core-shell structure used in the Mie calculation was reasonable.
In this study, the diameter of the rBC core (Dc) and the whole
particle diameter including the core and shell (Dp) were
calculated to retrieve the aging degree of BC-containing particles.
Dc was calculated from mrBC and the density of the
rBC core (ρc; here, a prescribed value of 1.8 g cm-3)
(Cappa et al., 2012; Pan et al., 2017; Laborde et al., 2013). Dp
was determined via the Mie calculation and was related to Dc, the
Cs of the BC-containing particle, and the refractive indices of
NR-CM (RINR-CM, 1.5–0i; Zhang et al., 2018) and rBC core
(RIc, 2.26–1.26i; Taylor et al., 2015). The uncertainty of
size information for the BC-containing particles from Mie calculation was
estimated to be ∼10 % in our previous work (Zhang et al., 2018).
More details regarding the calculation of Dp and Dc
for ambient BC-containing particles observed at the Tsinghua site can be
found in Zhang et al. (2018).
In this study, the aging degree of a BC-containing particle was characterized
by the mass ratio between NR-CM and rBC (mNR-CM/mrBC)
and was calculated by using Eq. (1):
mNR-CMmrBC=16×π×Dp3-Dc3×ρNR-CMmrBC,
where mNR-CM is the mass of the non-refractory coating materials;
ρNR-CM is the density of the non-refractory coating materials,
with a prescribed value of 1.4 g cm-3 in this study based on the
composition of submicron aerosols during APEC reported by J. K. Zhang et
al. (2016) and the densities of the various components (i.e., sulfate,
nitrate, ammonium and organic aerosol; Cappa et al., 2012).
Light absorption of BC-containing particles
In this study, the light-absorption capability of ambient BC-containing
particles was characterized by the light-absorption enhancement
(Eab) of BC from the lensing effect caused by the coating
materials. The Eab of BC-containing particles was retrieved using
a shell-and-core model based on Mie theory (Laborde et al., 2013; Metcalf et
al., 2013; Schwarz et al., 2008) and calculated by dividing the light-absorption
cross section of the whole BC-containing particle (Cab,p) by that
of the bare rBC core (Cab,c) at a certain wavelength (550 nm in
this study), as expressed in Eq. (2):
Eab=Cab,pDc,Dp,RINR-MC,RIcCab,cDc,RIc,
where Cab,c and Cab,p were determined from the Mie
calculation (uncertainty of ∼15% estimated in our previous study;
Zhang et al., 2018). Cab,c is related to Dc and
RIc. For Cab,p, we needed additional information on
the whole particle, i.e., Dp and RINR-CM.
The light-absorption coefficient (σab) of BC-containing
particles at a wavelength (550 and 670 nm used in this study) was determined
by the light-absorption capability of BC and the rBC mass concentration
(CrBC), as shown in Eq. (3):
σab=CrBC×MACp=CrBC×Eab×MACc,
where MACp and MACc are the mass absorption
cross section (MAC) of BC-containing particles and rBC cores, respectively,
which was calculated based on Mie theory and SP2 measurements. In this study,
the σab at 670 nm was also obtained by a multi-angle
absorption photometer (MAAP) measurement. The MAAP data were corrected using
the algorithm reported by Hyvärinen et al. (2013).
Results
Reduction in the concentrations of BC and coating precursors
Figure 1a shows the time series of the PM2.5 and rBC mass
concentrations during the campaign period. Three pollution episodes on 28
October–1 November and 6–11 and 17–21 November were observed before,
during and after APEC, respectively. Following the APEC study in Sun et
al. (2016), we focused on comparing the BC characteristics among the three
pollution episodes to investigate the effect of emission reduction. During
the three pollution episodes, the air masses over the site were mainly from
the south and east of Beijing (Fig. S3), where emission control measures were
implemented during APEC. On the other hand, the pollution episodes in Beijing
were characterized by low wind speed and planetary boundary layer (PBL), as
well as high relative humidity (Sun et al., 2016; Zheng et al., 2015).
Time series of (a) the mass concentrations of
PM2.5 and rBC and the number size distribution of (b) rBC
cores (Dc) and (c) whole BC-containing particles
(Dp).
The PM2.5 concentrations during the pollution episodes before and
after APEC were ∼127 and ∼213 µg m-3, respectively,
which were larger than that (∼66 µg m-3) during APEC.
The decrease in PM2.5 loadings revealed that the air quality was
improved during APEC. Similarly, the rBC mass concentration during APEC was
also smaller than those before and after APEC. However, the decreases in the
rBC concentration during APEC by ∼27 and ∼58 %, respectively,
compared with before and after APEC were smaller than the corresponding
decreases in the PM2.5 concentrations (∼48 and 69 %,
respectively), possibly indicating that more secondary aerosols (e.g.,
sulfate and nitrate) than primary aerosols (e.g., rBC) were reduced during
APEC, which could aid the decrease in coating materials on BC surfaces.
Figure 2 compares the mass concentrations of both rBC and the coating
precursors (i.e., NO2 and SO2) in the pollution episodes
before, during and after APEC. Compared with that before and after APEC, the
mass concentration of NO2 during APEC was decreased by ∼34 and
∼45 %, respectively, while the SO2 concentration was
reduced by ∼35 and ∼67 %, respectively. These results revealed
that the emission control measures implemented during APEC were a viable way
to reduce not only the rBC mass concentrations but also the concentrations of
secondary aerosol precursors present in the atmosphere. The
emission-control-caused reduction in secondary particle precursors (i.e.,
NO2 and SO2) during APEC could have reduced the secondary
aerosol formation in the atmosphere. Previous studies identified a reduction
in the concentrations of secondary components (e.g., sulfate and nitrate) in
aerosols during APEC compared to before and after APEC (J. K. Zhang et al.,
2016; Han et al., 2015). However, the change in coating materials on the BC
due to the reduction of secondary components was complex, which was not only
determined by the decrease in BC versus secondary components, but also
depends on secondary components condensed on BC-containing versus non-BC
particles.
Figure S4 shows the diurnal variations in the rBC, NO2 and
SO2 concentration and the PBL during the pollution episodes before,
during and after APEC. Comparing the diurnal variations between the rBC
concentration and the PBL revealed that the rBC concentrations during the
pollution episodes were dominated by the PBL. However, the precursor
concentration of secondary aerosol (i.e., NO2 and SO2)
during the pollution episodes exhibited different diurnal variations with a
peak at noontime and early afternoon, which was most likely attributed to
regional transport. The back-trajectory analysis (Fig. S3) revealed that the
air mass during the pollution episodes was mainly from polluted regions
(i.e., Hebei and Tianjin). This indicated that regional emission controls
would reduce the pollutant (i.e., rBC, NO2 and SO2)
concentration in Beijing under polluted conditions. Sun et al. (2016) have
demonstrated significant reductions in the precursors of secondary aerosol
during APEC compared to those in the non-APEC period due to emission controls
over a regional scale (i.e., Beijing and adjacent areas). The similar
PBL (Fig. S4) during the pollution episodes before, during and after APEC
further identified the important contribution of emission reduction to the
decrease in rBC, NO2 and SO2 concentration during APEC.
The mass concentrations of (a) rBC,
(b) NO2, (c) SO2 and
(d) O3 for the pollution episodes before, during and after
APEC. We separated the entire data sets into daytime (07:00 to 19:00 LT) and
nighttime (19:00 to 07:00 LT of the following day) sets.
Previous studies have pointed out the importance of photochemical reactions
in the BC aging process (Q. Wang et al., 2017; Metcalf et al., 2013; Zhang et
al., 2014; Peng et al., 2016), indicating that changing the daytime
concentrations of rBC and coating precursors might play a more important role
in affecting BC aging than altering the nighttime concentrations. We
separated the data sets for the pollution episodes before, during and after
APEC into daytime (07:00–19:00 LT) and nighttime
(19:00 to 07:00 LT of the following day) sets. Figure 2 shows that while the
emission controls were in place during APEC, a greater reduction in the rBC
and NO2 concentrations occurred during the day than at night.
Compared with those before and after APEC, the daytime reductions in the
NO2 concentration during APEC were reduced by as much as ∼40
and ∼51 %, respectively. By contrast, the daytime reduction
(∼25 %) in the SO2 concentration during APEC compared with
that before APEC was smaller than that at night, which might be attributable
to the high contribution of regional emissions (e.g., power generation and
industrial activities in Hebei Province) to the daytime SO2
concentration in Beijing (Guo et al., 2014; Tang et al., 2015). Meanwhile, a
similar reduction (∼67 %) in the daytime and nighttime SO2
concentrations during APEC compared with that after APEC was observed. In
summary, the significant reductions in the daytime levels of rBC and coating
precursors during APEC further indicated that BC aging in the atmosphere
might have been affected by the emission control measures.
Reductions in the aging degree of BC
Figure 1b and c show time series of the number size distribution of rBC cores
(Dc) and whole BC-containing particles (Dp),
respectively. The rBC cores observed before, during and after APEC exhibited
similar number size distributions, with a mode at ∼95 nm (Fig. 1b). The
similar modes of the rBC cores could have resulted from similar emission
sources for BC-containing particles observed before, during and after APEC.
However, the whole BC-containing particles (including coating materials and
rBC core) showed different number size distributions in the pollution
episodes before, during and after APEC (Fig. 1c), indicating different
amounts of coating materials on the BC surface during the three pollution
episodes. In the pollution episodes before and after APEC, the particle size
of the whole BC-containing particles exhibited sustained growth from
∼180 to ∼320 and ∼400 nm, respectively, which could be
attributed to the gradual condensation and coagulation of other species
(i.e., primary aerosol and secondary components) on the BC surface. However,
the continuous size growth of the whole BC-containing particles was not
observed in the pollution episode during APEC, in which the number particle
size distribution was with a mode no more than ∼280 nm (Fig. 1c),
significantly smaller than those before (∼320 nm) and after APEC
(∼400 nm). These results indicated that secondary formation during APEC
was insufficient to maintain continuous BC aging.
Figure 3 compares the mass ratio between the coating materials and rBC cores
(mNR-CM/mrBC) for BC-containing particles with
size-resolved rBC cores in the pollution episodes before, during and after
APEC. The mNR-CM/mrBC ratios of BC-containing particles
before, during and after APEC showed similar correlations with the rBC core
size; namely, the mNR-CM/mrBC ratio decreased with
increasing rBC core size (Fig. 3a). The size-dependent
mNR-CM/mrBC ratio of BC-containing particles indicated
that particle growth was more effective for smaller particles, which followed
the diffusion-controlled growth law (Seinfeld and Pandis, 2006). At a certain
size of rBC cores, Fig. 3a shows that the mNR-CM/mrBC
ratio of ambient BC-containing particles during APEC was significantly
smaller than those before and after APEC, revealing that the emission
restrictions during APEC weakened the condensation of other species on the BC
surface. For ambient BC-containing particles with ∼80–200 nm rBC
cores, the mNR-CM/mrBC ratios observed in the pollution
episodes before, during and after APEC were 4–22, 3–15 and 5–33,
respectively.
Comparison of the aging degree of BC-containing particles for the
pollution episodes before, during and after APEC:
(a) the mNR-CM/mrBC ratio of BC-containing
particles and (b) the reduction in the mNR-CM/mrBC
ratio of BC-containing particles during APEC relative to those before and
after APEC.
Figure 3b shows the reductions in the mNR-CM/mrBC ratios of
BC-containing particles for the pollution episodes during APEC compared with
those before and after APEC, which were also dependent on rBC core size.
Smaller rBC cores exhibited greater reductions in the
mNR-CM/mrBC ratio as a result of emission control
measurements during APEC. This indicated that in terms of BC aging, it was
more sensitive to emission levels for smaller rBC cores. This could be
explained by the diffusion-controlled growth law; i.e., the growth of smaller
BC particles was more effective (Metcalf et al., 2013; Seinfeld and Pandis,
2006), and thus the effect of emission reduction on BC aging was more
significant for smaller rBC particles. Compared with that before and after
APEC, the mNR-CM/mrBC ratio of ambient BC-containing
particles with ∼80–200 nm rBC cores during APEC was reduced by
∼10–30 and ∼31–53 %, respectively. The relationship between
the reduction in the mNR-CM/mrBC ratio of BC-containing
particles (Raging) during APEC and their rBC core size
(Dc) followed an exponential function (Fig. 3b), i.e.,
Raging=9.1+1576.6 exp(-0.055Dc) (relative to
that before APEC) and Raging=30.7+169.2 exp(-0.025Dc) (relative to that after APEC).
The reduction in the mNR-CM/mrBC ratio of BC-containing
particles for the pollution episode during APEC relative to that before and
after APEC showed pronounced diurnal cycles (Fig. 4). Compared with that
before APEC, the reduction in the mNR-CM/mrBC ratio of
BC-containing particles with 80–200 nm rBC cores during APEC showed maxima
in the afternoon (∼14:00–17:00 LT; Fig. 4a), consistent with the peak
time of the diurnal cycle of O3 concentrations before and during
APEC (Fig. 4c). This consistency indicated that the reduction in coating
materials on the BC surface during APEC compared to that before APEC was most
likely dominated by the lower photochemical production of secondary species.
Figure 5a1 shows that the reduction in the mNR-CM/mrBC
ratio of BC-containing particles during APEC relative to that before APEC
increases with the O3 concentration during the day
(07:00–19:00 LT), revealing that the effect of emission controls on BC
aging is associated with photochemistry. Moreover, Fig. 4a shows the diurnal
cycle of the reduction in the mNR-CM/mrBC ratio of
BC-containing particles during APEC compared to that before APEC with minima
during rush hour (∼06:00–08:00 LT), which can be due to a larger
contribution of primary emissions of fresh BC (namely, bare BC and
thin-coated BC particles) during rush hour than at other times for both
episodes before and during APEC.
Diurnal cycle of the normalized reduction in
the mNR-CM/mrBC ratio of BC-containing particles for the
pollution episode during APEC relative to those (a) before and
(b) after APEC. (c) Diurnal cycle of O3
concentration for the pollution episodes before, during and after APEC.
However, the reduction in the mNR-CM/mrBC ratio of
BC-containing particles for the pollution episode during APEC compared to
that after APEC showed a different diurnal cycle, with maxima at
∼10:00–12:00 LT and with minima at ∼15:00–17:00 LT (Fig. 4b).
Figure 4c shows that the daytime O3 concentrations after APEC are
significantly smaller than those during APEC, indicating a weakened
contribution from photochemistry after APEC. The increased amount of coating
materials of BC observed after APEC compared to that during APEC was mostly
likely attributed to enhanced other reactions (e.g., heterogeneous chemistry)
during haze episodes (Xie et al., 2015; Yang et al., 2015; Zheng et al.,
2015; Mu et al., 2018). Figure 5a2 shows that the variation in the reduction
in the mNR-CM/mrBC ratio of BC-containing particles during
APEC compared to that after APEC is poorly correlated with the O3
concentration. The diurnal trend of the reduction in
the mNR-CM/mrBC ratio of BC-containing particles during
APEC relative to that after APEC was likely driven by the simultaneous
effects of enhanced photochemistry and weakened other chemistry (e.g.,
heterogeneous reaction) contributions during APEC.
As discussed above, the reduction in the aging degree of ambient
BC-containing particles during APEC could have been caused by the decreased
chemical production (namely, weakened contributions from photochemical or
other reactions) of coating materials on the BC surface. Figure 5b shows that
the reduction in the mNR-CM/mrBC ratio of BC-containing
particles during APEC relative to that before and after APEC is associated
with a decrease of the concentrations of SO2 and NO2 due
to emission reduction. A greater decrease in the concentrations of
SO2 and NO2 corresponded to a greater reduction in the
mNR-CM/mrBC ratio of BC-containing particles during
APEC. The reduction in precursor emissions of secondary species (e.g.,
SO2 and NO2) could decrease the chemical production, and
therefore lower amounts of coating materials on the BC surfaces were
observed during APEC.
(a) Correlation between the reduction in the
mNR-CM/mrBC ratio of BC-containing particles for the
pollution episode during APEC relative to those (a1) before and (a2) after
APEC and the daytime (07:00–19:00) O3 concentration during APEC.
(b) Correlation between the reduction in
mNR-CM/mrBC during APEC relative to those before and
after APEC and the corresponding reduction in the concentrations of (b1)
NO2 and (b2) SO2.
Reduction in the light absorption of BC-containing particles
The reduction in the BC aging degree during APEC could weaken the
light-absorption capability of BC-containing particles owing to a decrease in
the lensing effect caused by less coating material on the BC surfaces (Fuller
et al., 1999; Lack and Cappa, 2010). Figure 6 compares the Eab of
BC-containing particles during the day for the pollution episodes observed
before, during and after APEC. The daytime Eab of BC-containing
particles with 80–200 nm rBC cores varied from ∼1.5 to ∼2.5
during APEC, values that were remarkably lower than before and after APEC
(i.e., Eab of 1.7–3.0 and 1.8–3.2, respectively; Fig. 6a);
these results reflected a weakened light-absorption capability of BC during
APEC. The reduction in the daytime Eab of BC-containing particles
(REab) during APEC compared with those before and after APEC
decreased with the rBC core size (Dc), and the relationship
followed an exponential function (REab=6.3+192.9 exp(-0.039Dc) relative to that before APEC and
REab=9.8+148.8 exp(-0.033Dc) relative to that
after APEC), as shown in Fig. 6b. Compared with before and after APEC, the Eab of BC-containing particles with ∼80–200 nm rBC cores
during the day decreased by ∼6–15 and ∼10–20 %, respectively.
Our results provide evidence that emission controls could weaken the
light-absorption capability of ambient BC-containing particles. This
weakening would have enhanced the effects of emission control measures during
APEC on BC light absorption.
Comparison of the light-absorption capability of BC-containing
particles during the day for the pollution episodes before, during and after
APEC: (a) light-absorption enhancement (Eab) of
BC-containing particles and (b) the reduction in Eab of
BC-containing particles during APEC relative to those before and after APEC.
Figure 7a shows the measured and theoretical light-absorption coefficient of
BC-containing particles during the campaign period. The measured
σab revealed that the daytime light absorption of
BC-containing particles in the pollution episode during APEC decreased by
∼42 and ∼68 % compared with those in pollution episodes before
and after APEC, respectively. This decrease could be attributed to the reduction in
both the rBC mass concentration and the light-absorption capability of
ambient BC-containing particles. In order to separate the contributions of a
decrease in rBC mass concentration and a weakening of BC light-absorption
capability to the reduction in light absorption during APEC, we calculated
the theoretical reduction in σab of BC-containing during
APEC with and without considering the weakened light-absorption capability of
BC-containing particles due to emission reduction (σab,with
and σab,without, respectively). When considering the
simultaneous reduction in the mass concentration and light-absorption
capability of BC, the calculated reduction in the daytime σab of
BC-containing particles during APEC related to non-APEC period showed a good agreement
with ones obtained from MAAP measurements (Fig. 7b). This agreement
demonstrated that the decrease in the light absorption of BC-containing
particles depended not only on the reduction of BC mass concentration, but
also on the weakening of their light-absorption capability.
Considering the reductions in both the mass concentration and
light-absorption capability of BC due to emission control measures, the
daytime light absorption of BC-containing particles (i.e.,
σab,with) decreased by ∼41 and ∼68 % during
APEC compared to those before and after APEC, respectively. However, the
σab,without of BC during APEC decreased by ∼34 and
∼62 % relative to that before and after APEC, respectively
(Fig. 7b). The difference between the reductions in σab,with
and σab,without indicated that the reduction in the rBC
concentration contributed ∼83 and ∼91 % of the reduction in BC
light absorption during APEC compared to before and after APEC,
respectively, while the weakening of the BC light-absorption capability
contributed ∼17 and ∼9 %, respectively. On average, the light
absorption of BC-containing particles in daytime during APEC decreased by
∼56 % compared with before and after APEC, of which ∼48 %
was contributed by the reduction in the mass concentration of rBC and the
remaining ∼8 % was controlled by the weakening of BC light-absorption
capability. These results imply that reductions in the emission of multiple
pollutants (i.e., BC and precursors of secondary species) in China could
benefit air quality and climate due to significantly lowering the light
absorption of BC, which was driven by reductions in both rBC mass
concentration and the light-absorption capability of BC-containing particles.
(a) The light-absorption coefficient (σab)
at 670 nm. (b) Reduction in the absorption coefficients
(σab) of BC-containing particles observed in the pollution
episode during APEC relative to those before and after APEC. The correlation
between the calculated σab (σab,calculated)
using Mie theory combined with SP2 measurements and the measured
σab (σab,measured) by MAAP is also
shown in (a). The σab,with and
σab,without values represent σab,calculated
values with and without, respectively, considering the differences in the
light-absorption capability of ambient BC-containing particles among the
episodes before, during and after APEC.
Conceptual scheme of the reduction in light absorption of
BC-containing particles due to multi-pollutant emission controls.
Discussion
Based on a comparison of the observations before, during and after APEC, we
found that the emission control measures successfully reduced both the rBC
mass concentration and the light-absorption capability (i.e.,
Eab) of BC-containing particles, resulting in a significant
decrease in the light absorption of BC. The mechanism underlying the effect
of the emission reductions during APEC on BC light absorption is summarized
in Fig. 8. Emission control measures reduce the amount of both BC and
co-emitted secondary aerosol precursors present in the atmosphere. The
presence of lower amounts of secondary particle precursors in the atmosphere
weakens the chemical formation of secondary aerosol components, suppressing
the condensation of secondary species on BC surfaces. Less coating material
on BC can weaken the lensing effect, which leads to a weakening of the
light-absorption capability for BC-containing particles. Simultaneous
reductions in the mass concentration and light-absorption capability of BC
can result in a much lower light absorption of BC during APEC compared to
before and after APEC.
In China, a series of emission controls measures have been implemented in
pollution regions (e.g., Jing-Jin-Ji region) aiming to increase the number
of clean days and decrease the number of haze days. This comparison between
periods with and without emission control measures may illustrate the
differences between clean and polluted periods. In terms of different
pollution levels in China, our findings imply that a clean period is
characterized by not only a lower BC mass concentration but also a weaker
light-absorption capability of BC-containing particles compared to that in
polluted periods. In our previous study (Zhang et al., 2018), we found that
the light-absorption capability of ambient BC-containing particles observed
in Beijing was enhanced by an increase in pollution levels, resulting in an
amplification of BC light absorption under polluted conditions. The present
work clearly demonstrates that emission control measures can reduce this
amplification effect by decreasing the light-absorption capability of
BC-containing particles. Moreover, this work can explain how emission control
measures reduce the amplification effect, namely by slowing the aging of BC
resulting from a reduction in co-emitted secondary aerosol precursors (e.g.,
SO2, NOx and VOCs).
The simultaneous reductions in the mass concentration and light-absorption
capability of BC due to emission controls confirmed the suggestions of
previous studies that BC emission reductions could achieve multiple benefits,
i.e., simultaneously controlling air pollution and protecting the climate
(Ding et al., 2016). Furthermore, our study implies that the air quality and
climate co-benefits of multi-pollutant emission controls are enhanced by the
weakened light-absorption capability of BC-containing particles. In terms of
air quality improvement, weakened light-absorption capability plays an
important role in both the direct and indirect effects of BC. Weakened
light-absorption capability can directly lower the light-absorbing efficiency
of BC aerosols in the atmosphere, resulting in more solar light radiation
reaching the surface; the weakened light-absorption capability of ambient
BC-containing particles can indirectly mitigate air pollution by improving
PBL suppression driven by the dome effect of BC (Ding et al., 2016; Z. Wang
et al., 2017). On the other hand, an enhanced reduction in climate warming
can be attributed to a smaller direct radiative forcing from BC aerosols due
to a weaker light-absorption capability of atmospheric BC-containing
particles. The importance of the weakened light-absorption capability of BC
highlighted in our study provides clues for the management of air quality and
climate change. The emission controls of multiple pollutants including BC and
co-emitted secondary aerosol precursors may be an efficient way to
simultaneously mitigate air pollution and climate warming.
Concluding remarks
The effects of emission reductions on the light absorption of BC-containing
particles are not only controlled by the reduction in the BC mass
concentration but also dependent on the change in their light-absorption
capability. The decrease in the BC mass concentration due to emission
control measures is well known. However, the impact of emission reduction on
the light-absorption capability of BC-containing particles remains unclear
due to a lack of available observations. The 2014 APEC meeting in Beijing,
China provides an invaluable opportunity to measure the variations in the
light-absorption capability of ambient BC-containing particles due to
emission reductions. In this work, based on in situ measurements at an urban
site in Beijing before, during and after APEC using an SP2 technique, we
explored whether and how emission control measures in China influence the
light-absorption capability of ambient BC-containing particles. Note that
this comparative study focused on the pollution episodes before, during and
after APEC.
We found that the emission control measures successfully lowered the aging
degree (i.e., mNR-CM/mrBC) of BC-containing particles.
The mNR-CM/mrBC ratio of BC-containing particles with
∼80–200 nm rBC cores during APEC decreased by ∼10–30 and
∼31–53 % compared to that before and after APEC, respectively. The
reduction in the mNR-CM/mrBC ratio of BC-containing
particles increased with decreasing rBC core size, following an exponential
function. The size-dependent reduction in the mNR-CM/mrBC
ratio of BC-containing particles indicated that emission reduction was more
effective for slowing the aging of smaller rBC particles. The reduction in
the mNR-CM/mrBC ratio of BC-containing particles during
APEC relative to those before and after APEC showed a pronounced diurnal
cycle, with maxima at ∼14:00-17:00 and ∼10:00–12:00 LT,
respectively. The decreased aging of BC-containing particles during APEC was
mainly driven by a reduction in chemical production (i.e., oxidation products
such as sulfate and nitrate) on the surface of BC due to lower amounts of
secondary aerosol precursors (e.g., the NO2 concentration during
APEC decreased by ∼34 and ∼45 % compared with those before and
after APEC, respectively, and the corresponding SO2 concentration
decreased by ∼35 and ∼67 % during APEC, respectively) present
in the atmosphere during BC aging. The reduction in
the mNR-CM/mrBC ratio of BC-containing particles during
APEC relative to those before and after APEC increased with the reduction in
the concentrations of NO2 and SO2.
Due to the lower amount of coating materials on BC surfaces during APEC, the
light-absorption capability (i.e., Eab) of BC-containing
particles with ∼80–200 nm rBC cores during the day decreased by
∼6–15 and ∼10–20 % compared to those before and after APEC,
respectively. The weakened light-absorption capability of BC-containing
particles enhanced the reduction in BC light absorption due to the emission
control measures. When considering the reduction in both the mass
concentration and light-absorption capability of BC-containing particles
during the day during APEC, the theoretical light absorption (i.e.,
σab) decreased by ∼41 and ∼68 % compared to
those before and after APEC, respectively. However, the reduced light
absorption of BC during the day caused by the decrease in the BC mass
concentration during APEC compared to before and after APEC was
estimated to be ∼34 and ∼62 %, respectively. Therefore,
∼10 –20 % of the reduction in the daytime light absorption of
BC-containing particles during APEC relative to those before and after APEC
could be attributed to the weakened light-absorption capability. Our study
revealed that reductions in the emissions of multiple pollutants (i.e., BC,
NO2 and SO2) could reduce the light-absorption capability
of BC. Weakened light-absorption capability of BC due to emission controls
further confirmed the suggestions of previous studies that BC emission
reductions can achieve multiple benefits, i.e., simultaneously controlling
air pollution and protecting the climate (Ding et al., 2016; Peng et al.,
2016; Zhang et al., 2018). Our study implies that the air quality and
climate co-benefits of multi-pollutant emission control could be enhanced
by the weakened light-absorption capability of BC-containing particles.