The impacts of black carbon (BC) aerosols on air quality,
boundary layer dynamics and climate depend not only on the BC mass
concentration but also on the light absorption capability of BC. It is well
known that the light absorption capability of BC depends on the amount of
coating materials (namely other species that condense and coagulate on BC). However, the difference of light absorption capability of
ambient BC-containing particles under different air pollution conditions
(e.g., clean and polluted conditions) remains unclear due to the
complex aging process of BC in the atmosphere. In this work, we investigated
the evolution of light absorption capability for BC-containing particles
with changing pollution levels in urban Beijing, China. During the campaign
period (17 to 30 November 2014), with an increase in PM
Black carbon (BC) is an important aerosol component that absorbs visible sunlight and contributes to heating of the atmosphere (Menon et al., 2002; Bond and Bergstrom, 2006; Gustafsson and Ramanathan, 2016). Atmospheric BC can impact climate through radiative effects, which are strongly associated with the optical properties of BC, especially light absorption (Jacobson, 2000; Lesins et al., 2002; Cheng et al., 2006; Ramanathan and Carmichael, 2008). Estimating the climate effects of BC is one of the major challenges in climate change research, partly due to large uncertainties in the light absorption capability of BC-containing particles under ambient conditions (Cappa et al., 2012; Liu et al., 2015; Gustafsson and Ramanathan, 2016; Liu et al., 2017). The light absorption capability of atmospheric BC is complex and poorly quantified, and it changes with the morphology, density and mixing state of the BC-containing particles (Schnaiter et al., 2005; Zhang et al., 2008; Knox et al., 2009; Peng et al., 2016). Previous theoretical (Jacobson, 2001; Moffet et al., 2009; Zhang et al., 2016) and observational (Knox et al., 2009; Cappa et al., 2012; Peng et al., 2016) studies showed a broad range of absorption enhancements (1.05–3.05) of BC-containing particles during the atmospheric aging process. To date, conflicts remain between model- and observation-based studies of the light absorption capability of atmospheric BC-containing particles (Jacobson, 2001; Cappa et al., 2012; Liu et al., 2015; Liu et al., 2017).
The light absorption capability of BC-containing particles depends strongly on the particle mixing state (Liu et al., 2015; Liu et al., 2017), i.e., the degree of internal mixing between BC and other particle species (i.e., non-BC components) by the atmospheric aging process (i.e., condensation, coagulation and heterogeneous oxidation). The non-BC species (i.e., coating materials) on the surface of BC cores can enhance BC light absorption via the lensing effect (namely, the coating materials act as a lens to focus more photons on BC; Fuller et al., 1999; Jacobson, 2001; Bond et al., 2006; Lack and Cappa, 2010). In terms of individual BC-containing particles, more coating materials result in stronger light absorption capability. The coating materials on the BC surface are controlled by secondary processes (e.g., photochemical production) (Metcalf et al., 2013).
The production of secondary aerosols in the atmosphere varies significantly with pollution levels (Cheng, 2008; Yang et al., 2015; Zheng et al., 2016; Mu et al., 2018), indicating that BC-containing particles most likely exert different light absorption capability values under different pollution levels. Compared with clean air conditions, polluted periods feature more secondary aerosols, especially secondary inorganic species such as sulfate (Guo et al., 2014; Sun et al., 2014; Zheng et al., 2015). Whether the changes of secondary aerosols with air pollution will affect the coating materials on the BC is complex, and not only depends on the increase in BC vs. secondary aerosols amount but also controlled by secondary material condensation on BC- vs. non-BC-containing particles. Recent BC aging measurements in Beijing and Houston using an environmental chamber (flowing ambient air fed with lab-generated fresh BC) have revealed that a clear distinction in the light absorption capability of BC-containing particles exists between urban cities in developed and developing countries (Peng et al., 2016), and this difference is likely due to the differences in air pollution levels.
To date, whether and how the aging degree and light absorption capability of BC-containing particles will change with air pollution development is still unclear. Although the enhancement of BC light absorption due to coating materials on BC surfaces has already been intensively investigated (Schnaiter et al., 2005; Moffet et al., 2009; Shiraiwa et al., 2010; Zhang et al., 2016), there are few studies on the evolution of the light absorption capability of BC-containing particles with changing air pollution levels. The variation in the light absorption capability of BC-containing particles associated with air pollution can lead to different effects of BC aerosols on air quality and climate under different pollution levels. To improve the evaluation of BC-related effects on air quality and climate, some models have considered BC internally mixed with other species (namely coating materials on BC surface), which can affect the light absorption capability of BC-containing particles. However, the difference of coating materials on BC under different air pollution conditions remains unclear.
In this work, we conducted an intensive field measurement campaign in urban
Beijing, China, to investigate the difference of the theoretical light
absorption capability of atmospheric BC-containing particles under different
pollution levels. First, we analyzed the evolution of theoretical light
absorption capability of BC with increasing air pollution levels and
estimated the relationship between the changing rate in the theoretical
light absorption capability of BC and that in the PM
The in situ measurements were conducted on the campus of Tsinghua University
(Tsinghua site, 40
Ambient aerosol particles were collected by a PM
The SP2 instrument measures a single BC-containing particle using a 1064 nm
Nd
Based on the rBC core mass (
The RI
The
In terms of the
Based on the size information on BC-containing particles (i.e.,
The
The MAC of BC-containing particles (MAC
To evaluate the impact of regional transport on BC-containing particles, we used a variant of the effective emission intensity (EEI) defined by Lu et al. (2012) to quantify the amounts of BC over the observation site from different source regions. In this study, the spatial origin of the BC observed at our site was divided into local sources in Beijing and regional sources in other areas (i.e., Hebei, Tianjin, Shanxi and Inner Mongolia; Fig. S1). The EEI takes into account emission, transport, hydrophilic-to-hydrophobic conversion and removal processes (i.e., dry and wet deposition) of BC throughout the whole atmospheric transport process from the origin of the BC emission to the receptor site. A novel back-trajectory approach was developed by Lu et al. (2012) to calculate EEI values.
In this study, the back-trajectory analysis was performed by the Hybrid
Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model to obtain
the transport pathways of BC to the observation site (40
We calculated the EEI of BC at a resolution of 0.25
The total EEI of trajectory
Time series of
In this study, we use a parameter of the simple forcing efficiency (SFE) to
roughly evaluate the radiative forcing of BC-containing particles. The SFE is
defined as normalized radiative forcing by BC mass, which is
wavelength dependent (Chylek and Wong, 1995; Bond and Bergstrom, 2006; Chen and Bond, 2010; Saliba et al., 2016). The wavelength-dependent SFE for
BC-containing particles is determined by Eq. (
Figure 1 shows the time series of the PM
To validate SP2 measurements and Mie calculation used in this study, we
compared the calculated light absorption coefficient (
The increase in the
Previous theoretical studies reported that the coating materials on the BC
surface can significantly enhance the light absorption of BC via the lensing
effect (Fuller et al., 1999; Jacobson, 2001; Moffet et
al., 2009; Lack and Cappa, 2010). In other words, the aging degree of BC-containing particles
(characterized by the
Figure 2a shows the
Figure 3 shows the increase of the
Meanwhile, following a semiquantitative analysis used in Metcalf et al. (2013), we calculated the IR
According to the diffusion-controlled growth law (Seinfeld and Pandis, 2006),
the evolution of the size of BC-containing particles (
Following Eq. (11), the
The IR
The IR
Figure 4a explores the relationship between the changing rate of calculated
Figure 4b shows frequency distribution of
Moreover, we found that the growth rate of
The evolution of theoretical light absorption of BC with pollution levels
depends on the change in both rBC mass concentrations and calculated
The average PM
BC aging in the atmosphere, namely BC internally mixing with other aerosol components, is associated with atmospheric transport (Gustafsson and Ramanathan, 2016). In Beijing, the rapid increase in aerosol particle concentrations during pollution episodes is most likely caused by regional transport of polluted air mass (Yang et al., 2015; Zheng et al., 2015). Therefore, regional transport of pollution may play an important role in the enhancement of BC light absorption capability associated with air pollution. In this study, we used the EEI analysis (Lu et al., 2012) to explore the effects of regional transport on the increase in theoretical light absorption capability of BC with increasing pollution levels.
Spatial distribution (0.25
Figure 5 shows the spatial distribution (0.25
Due to the increase in the regional contribution, the total BC amount
transported to the observation site, characterized by the EEI
(EEI
Variations in MAC, SFE and DRF of
BC-containing particles under different pollution levels. The DRF values for
BC-containing particles were obtained by scaling the average DRF
(0.31 W m
Under different pollution levels, regional transport not only influenced the
BC mass concentrations but also the BC aging process and timescale. Table 1
shows that the mass-average value of the
Conceptual scheme of the amplification effect on BC light absorption associated with air pollution.
To further explore the importance of aging during regional transport, its
contributions were compared with those of local chemical processes with
respect to increases in
According to the evolution of the EEI
The increase in BC light absorption capability with increasing air pollution
levels suggests that greater solar absorption (i.e., direct radiative
forcing, DRF) by atmospheric BC-containing particles occurs under more
polluted conditions. The DRF of atmospheric BC-containing particles depends
not only on the BC mass concentrations but also on the BC forcing
efficiency, which strongly depends on the light absorption capability of
BC-containing particles. In this study, the forcing efficiency of
BC-containing particles was estimated based on a simple radiation transfer
model (Eq.
Considering the increase in both the light absorption capability
(
The enhanced climate effects of BC aerosols in Beijing could be taken to be
representative of polluted regions in China. Previous measurements of BC
aerosols in China (Andreae et al., 2008; Huang et al., 2013; Wang et al., 2014b; Zhang et
al., 2014; Zheng et al., 2015; Gong et al., 2016; Zhao et al., 2017) showed that the BC mass concentrations in different regions
(e.g., Beijing, Xi'an, Nanjing, Shanghai and Guangzhou) reached values of
Our results reveal that under a more polluted environment, the BC-containing
particles are characterized by more BC mass concentrations and more coating
materials on the BC surface and therefore a higher light absorption capability. As
shown in Fig. 8, this amplification effect on BC light absorption associated
with air pollution is caused by increasing BC concentration and at the same
time enhanced light absorption capability of BC-containing particles by more
coating production in the more polluted air. Variation in both the mass
concentration and light absorption capability of BC associated with air
pollution strongly depends on the air pollutant emission (e.g., BC,
Air pollution control measures may, however, break this amplification effect by reducing BC concentration and at the same time lowering the light absorption capability of BC-containing particles by slowing down the coating processes with cleaner air (Fig. 8). Take air pollution controls during the 2014 Asia-Pacific Economic Cooperation (APEC) meeting in Beijing, China, as an example; we found that as a result of emission controls on local Beijing and areas adjacent to Beijing (i.e., Hebei, Tianjin, Shanxi, Henan, Shandong and Inner Mongolia), light absorption of BC-containing particles decreased significantly during APEC compared t before APEC (Zhang et al., 2018). This is not only contributed by a reduction of BC mass concentration but also by lower light absorption capability of BC-containing particles with less coating materials on the BC surface in cleaner atmosphere conditions, indicating that synergetic emission reduction of multi-pollutants could achieve co-benefits for both air quality and climate.
The light absorption of BC-containing particles depends not only on the BC
mass concentration but also on their light absorption capability
(characterized by the calculated MAC and
The relationships between the changing rate of calculated
The increase in BC light absorption capability with increasing pollution
levels in Beijing was controlled by aging during regional transport. The EEI
analysis showed that
Due to the increase in BC light absorption capability with increasing air
pollution levels, stronger forcing efficiency of the BC-containing particles
was found under more polluted conditions. During the campaign period, the BC
forcing efficiency increased by
The amplification effect on BC DRF due to the increase in BC light absorption capability introduced in this work not only concerns Beijing but is also likely to operate in other polluted regions in China. The amplification effect could not only increase the direct contribution of BC to air pollution and climate change due to more light absorption but would also enhance the indirect contribution by stronger aerosol–meteorology and aerosol–climate feedbacks. Our finds in this work can provide some implication in the difference of BC-related effect on air quality and climate under different air pollution conditions (e.g., clean air and polluted environment) due to change in BC light absorption capability associated with air pollution.
The air pollution control may break the amplification effect by reducing BC concentration and at the same time lowering the light absorption capability of BC-containing particles by slowing down the coating processes with cleaner air. Therefore, breaking the amplification effect through emission controls would achieve a co-benefit effect with simultaneous mitigation of air pollution and climate change. Further study will focus on if and how emission reduction of BC and other pollutants in China will break the amplification effect.
The observational data used in this study can be provided upon request to Yuxuan Zhang and Qiang Zhang (yuxuan.zhang@mpic.de and qiangzhang@tsinghua.edu.cn).
The supplement related to this article is available online at:
YZ and QZ designed the research. YZ and HL performed the field measurements. YZ analyzed the data. YZ, QZ, YC and HS interpreted the data. YZ and QZ wrote the manuscript with input from all co-authors.
The authors declare that they have no conflict of interest.
This work was supported by the National Natural Science Foundation of China (41625020, 41571130032 and 91644218) and the Guangdong “Pearl River Talents Plan” (2016ZT06N263). Edited by: Hailong Wang Reviewed by: three anonymous referees