Black carbon (BC), a distinct type of carbonaceous material formed from the
incomplete combustion of fossil and biomass based fuels under certain
conditions, can interact with solar radiation and clouds through its strong
light-absorption ability, thereby warming the Earth's climate system. Some
studies have even suggested that global warming could be slowed down
in the short term by eliminating BC emission due to its short lifetime. In
this study, we estimate the influence of removing some sources of BC and
other co-emitted species on the aerosol radiative effect by using an
aerosol–climate atmosphere-only model BCC_AGCM2.0.1_CUACE/Aero with prescribed sea surface temperature
and sea ice cover, in combination with the aerosol emissions from the
Representative Concentration Pathways (RCPs) scenarios. We find that the
global annual mean aerosol net cooling effect at the top of the atmosphere
(TOA) will be enhanced by 0.12 W m
Aerosols in the atmosphere can alter the amount of sunlight reaching the Earth, perturb the temperature structure of the atmosphere, and influence cloud cover by directly scattering sunlight (e.g., sulphate, organic carbon (OC) and nitrate) or absorbing it (e.g., black carbon (BC) and dust) (Boucher et al., 2013). Aerosol particles can also change cloud microphysical and optical properties by acting as cloud condensation nuclei (CCN) or ice nuclei (Twomey, 1977; Albrecht, 1989; DeMott et al., 1997). These changes due to aerosols will directly or indirectly affect the climate. Since the start of the industrial era, an increase in atmospheric aerosol emissions has likely led to a net cooling of the Earth's climate system (Boucher et al., 2013).
BC has a special role in the climate system, although it accounts for less
than 5 % of the mass of atmospheric aerosol in most areas of the world (X. Y. Zhang et al., 2012). BC can increase the amount of solar radiation
absorbed within the Earth's climate system and heat the atmosphere or
surface by directly absorbing sunlight in the visible to infrared wavebands
(Hansen et al., 2000; Ramanathan and Carmichael, 2008), changing the cloud
amount and its brightness due to embedding into clouds (Chuang et al., 2002;
Koch and Del Genio, 2010; Jacobson, 2012; Wang et al., 2013a), or by
reducing the surface albedo due to deposition onto snow and ice surfaces
(Wang et al., 2011; Lee et al., 2013). BC has even been considered as a
potential cause of global warming (Hansen et al., 2000; Jacobson, 2010; Bond
et al., 2013). Ramanathan and Carmichael (2008) compared the radiative
forcings of greenhouse gases and BC, suggesting that the direct radiative
forcing due to BC was larger than that due to any other greenhouse gas
except CO
Reducing the emissions of absorptive aerosols (e.g., BC) would decrease the
absorption of solar radiation by atmospheric aerosols, thereby enhancing the
aerosol net cooling effect. However, BC, OC, sulphate, and some other
aerosols have many common emission sources (e.g., in the emission sectors of
transportation, industrial, residential, and commercial energy consumption,
etc.), and they are generally co-emitted into the atmosphere (Lamarque et
al., 2010). A technology-based global emission inventory of BC and OC showed
that BC and primary OC particles were co-emitted from combustion including
fossil fuels, biofuels, open biomass burning, and urban waste burning (Bond
et al., 2004). An inventory of air pollutant emissions in Asia supporting
the Intercontinental Chemical Transport Experiment-Phase B showed that
sulfur dioxide (SO
Sulphate, BC, and OC are the main aerosol species in the atmosphere, and the emissions of sulphate and OC will be reduced accordingly if the emission of BC is reduced. Both sulphate and OC are strongly scattering and hygroscopic aerosols, and they can cool the climate system by directly scattering solar radiation and increasing the cloud albedo and lifetime by acting as CCN (Boucher et al., 2013). Therefore, would global warming necessarily be slowed down by reducing BC emission in the future? This is the point of this study.
Focusing on the issue mentioned above, the impact of removing some BC sources and other co-emitted species on the aerosol radiative effects was studied in this paper by using an aerosol–climate atmosphere-only model BCC_AGCM2.0.1_CUACE/Aero (Atmospheric General Circulation Model of Beijing Climate Center, BCC_AGCM2.0.1, coupled with the aerosol model of China Meteorological Administration Unified Atmospheric Chemistry Environment for Aerosols, CUACE/Aero) (Z. L. Wang et al., 2014) with prescribed sea surface temperature (SST) and sea ice cover (SIC), in combination with the Representative Concentration Pathways (RCPs) emission scenarios (van Vuuren et al., 2011) underpinning the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). In Sect. 2, we introduce the aerosol–climate model and simulation details. In Sect. 3, we present the effects of reducing only BC emission and then of the simultaneous reduction of BC and other co-emitted aerosol emissions on aerosol direct, semi-direct and indirect, and net radiative effects. Finally, our discussion and conclusions are presented in Sect. 4.
We use the aerosol–climate atmosphere-only model BCC_AGCM2.0.1_CUACE/Aero developed by Zhang et al. (2012a), and improved by Jing and Zhang (2013), Zhang et al. (2014), and Z. L. Wang et al. (2014) in this study. The aerosol direct, semi-direct, and indirect effects (albedo and lifetime indirect effects on stratiform clouds) have been included in BCC_AGCM2.0.1_CUACE/Aero. The model has been used to study the impact of aerosol direct radiative effect on East Asian climate (Zhang et al., 2012a), direct radiative forcing of anthropogenic aerosols (Bond et al., 2013; Myhre et al., 2013a), climate response to the presence of BC in cloud droplets (Wang et al., 2013a), effect of non-spherical dust aerosol on its direct radiative forcing (Wang et al., 2013b), anthropogenic aerosol indirect effect (Z. L. Wang et al., 2014), and direct effect of dust aerosol on arid and semi-arid regions (Zhao et al., 2014).
A detailed description of BCC_AGCM2.0.1 was given by Wu et al. (2010). The
model employs a horizontal resolution of T42 (approximately
2.8
The aerosol model CUACE/Aero is a comprehensive module incorporating
emission, gaseous chemistry, transport, removal, and size-segregated
multi-component aerosol algorithms based on the Canadian Aerosol Module
developed by Gong et al. (2002, 2003). A detailed description of CUACE/Aero
was given by Zhou et al. (2012). The mass concentrations of the main five
aerosols in troposphere, i.e., sulphate, BC, OC, dust, and sea salt, can be
calculated. Each aerosol type is divided into 12 bins as a geometric series
for a radius between 0.005 and 20.48
Six simulations were run in this study. In all simulations, the model
settings were the same, whereas aerosol emissions were different. All
simulations kept greenhouse gas concentrations fixed at year 2000 levels in
order to obtain the effect of change in aerosol emissions exclusively. Table 1 gives the emission setups in all simulations. As a base case, the first
simulation (SIM1) used emissions of SO
The difference between SIM2 and SIM1 shows the impact on aerosol radiative
effects (AREs) of reducing only BC emission maximally in the four RCPs
scenarios. The difference between SIM3 and SIM1 indicates the effect of
maximally reducing the emission of absorbing BC, combined with the least
reduction in the emissions of precursor (SO
Simulation setups.
Global distributions of simulated and observed annual
mean aerosol optical depth (AOD) at 550 nm.
The aerosol direct effect (ADE) was obtained by calling the radiation
routine two times (Ghan et al., 2012):
Global amounts of aerosol emissions and annual means of aerosol burdens.
Global annual mean differences of aerosol direct (DRF),
semi-direct and indirect (CRF), and net effect at the TOA (FNT) (positive
values mean incoming, units: W m
The simulation performance of BCC_AGCM2.0.1_ CUACE/Aero has been given by Z. L. Wang et al. (2014) in detail. They demonstrated that the model has a good ability to simulate aerosols, cloud properties, and meteorological fields. However, we replace the aerosol emission from AeroCom with those given by Lamarque et al. (2010) for present-day conditions in this work. Thus, a comparison of simulated annual mean aerosol optical depth (AOD) with satellite retrievals is shown in Fig. 1. The simulated AODs range from 0.3 to 0.6 over the Sahara Desert and are from 0.15 to 0.3 in nearby Arabian areas due to the large dust loading. The AODs are mainly between 0.2 and 0.4 in eastern China, and exceed 0.15 in eastern North America and West Europe due to the large emissions of anthropogenic aerosols. The AODs are above 0.1 over most subtropical oceans because of the contribution of sea salt and sulphate. The model generally reproduces the geographical distribution of AOD well, but it significantly underestimates the AODs over South Asia, eastern China, and tropical oceans. These errors could be caused by several factors such as uncertainties in the aerosol sources, coarse model resolution, the uncertainties of physical processes, and the absence of nitrate, ammonium and secondary organic aerosols in the model (Zhang et al., 2012a).
Global distributions of simulated annual mean aerosol
column burdens (units: mg m
Global distributions of difference in simulated annual
mean aerosol direct effect (units: W m
Global distributions of difference in simulated annual
mean CCN concentration at surface (units: cm
Tables 2 and 3 show the global emission amounts and annual mean column
burdens of aerosols in all simulations and differences in AREs among them.
The global emission amount of BC is reduced from 7.8 Tg yr
Many previous studies mentioned in Sect. 1 have indicated that there are
several common sources of SO
Global distributions of difference in simulated annual
mean column CDNC (units: 10
Figure 2 shows the global distributions of simulated annual mean sulphate,
BC, and OC burdens under all six simulations. As can be seen from Fig. 2b,
the BC column burdens are significantly decreased in areas with high BC
emission such as East Asia, South Asia, central Africa and South America,
eastern North America, and Western Europe compared with recent past levels
when the emission of only BC is reduced. Changes in other aerosol burdens
are not obvious. The reduction in the BC concentration weakens the direct
absorption of solar radiation by atmospheric aerosols, leading to a cooling
effect at the TOA in these regions. The largest cooling exceeds 1 W m
Global distributions of difference in simulated annual
mean SWCF and LWCF (units: W m
Global distributions of difference in simulated annual
mean aerosol net effect (units: W m
Figure 2c–f show that there are different levels of reduction in the
annual mean sulphate, BC, and OC burdens in SIM3 to SIM6, with decreases of
up to 2.0–5.0 mg S m
It has been argued that eliminating BC emission would be an
effective measure to slow down global warming and environmental pollution.
In this study, we assess the impact of removing some sources of BC and other
co-emitted species on aerosol radiative effects by using an aerosol–climate
atmosphere-only model BCC_AGCM2.0.1_CUACE/Aero
with prescribed SST and SIC, in combination with the RCP scenarios.
Compared with the aerosol effect for recent past, the global annual mean
aerosol net cooling effect at the TOA is enhanced by 0.12 W m
However, our results suggest that associating with the reduction of net
cooling effects directly from aerosols, the aerosol indirect effect is also
weakened when emissions of SO
This study highlights that reducing only BC emission could play a positive role in mitigating global warming and environmental pollution, and would be beneficial to human health. However, the emissions of some co-emitted scattering aerosols and their precursor gases will be inevitably reduced when BC emission is reduced due to their homology. Therefore, reducing BC emission could lead to unexpected warming on the Earth's climate in the future, unless certain technical advances in emission reduction technology are available for removal of the BC exclusively without influencing the other co-emitted components.
There exists large uncertainty in BC radiative forcing (Bond et al., 2013;
Boucher et al., 2013; Myhre et al., 2013a, b). One reason for the
uncertainty is from the biases of current emission inventories of BC, mostly
obtained from the so-called bottom-up approach (Cohen and Wang, 2014). Cohen
and Wang (2014) provided a global-scale top-down estimation of BC emissions,
a factor of more than 2 higher than commonly used global BC emissions data
sets, by using a Kalman Filter method. If present-day BC emissions have been
substantially underestimated, increase in aerosol net cooling effect may be
larger due to only reduction in BC emission. Furthermore, co-emissions of
other compounds with BC, such as CO
Because of the potential uncertainties mentioned above, we need to continuously improve our understanding on emissions of BC and its co-emitted species through a lot of observation and analysis. We also encourage other modeling groups to perform similar simulations to check the robustness of these results.
This work was supported by the National Basic Research Program of China (2011CB403405), National Natural Science Foundation of China (41205116), Public Meteorology Special Foundation of MOST (GYHY201406023), MOST (2014BAC16B01), and CAMS Basis Research Project (2012Y003). Edited by: K. Tsigaridis