Air quality and climate responses to anthropogenic black carbon emission changes from East Asia, North America and Europe
Introduction
Black carbon (BC) aerosol, emitted from a variety of combustion processes, affects the Earth's climate system directly through absorbing and scattering the solar radiation, and indirectly through altering the cloud processes as well as the melting of snow and ice due to its unique physical properties (Bond et al., 2013). Though the freshly emitted BC (hydrophobic BC) is insoluble in water or common organic solvents, its aging processes in which water-soluble substances (e.g., sulfuric acid) accumulate on its surface would enhance its cloud condensation nuclei (CCN) activity. Aging (Onischuk et al., 2003, Oshima et al., 2009) complicates much of the issues of the lifetime and removal rate of BC.
The direct radiative forcing (DRF) is the most commonly studied climate forcing terms for BC (Chung and Seinfeld, 2005, Hansen et al., 2005, Sokolov, 2006), and the BC DRF from all present-day sources was estimated to be +0.88 W m−2 with 90% uncertainty bounds of +0.17 to +1.48 W m−2. In addition, it was found that BC contributed to the warming of Arctic regions (Sand et al., 2013, Shindell and Faluvegi, 2009). The semi-direct effect refers to the alteration of cloud distribution by DRF of BC. The RF of BC induces adjustments of different time scales within the climate system. Chung et al. (2002) found that radiation absorbing aerosol (e.g. BC) could warm the atmosphere while cool the land surface below, stabilize the boundary layer, reduce the evaporation and sensible heat flux from the land, and furthermore impact the monsoon dynamics in South Asia. Menon et al. (2002) and Ramanathan et al. (2005) found that regional climate trends of temperature fields and hydrological cycle could be attributed to lifting BC atmospheric burdens in Asia. The large uncertainty in modeled BC RF could be attributed to the diversity in estimating BC's atmospheric burden (e.g., emission, lifetime) and optical properties (e.g. mass absorption cross section and forcing efficiency) (Bond et al., 2013). Besides, the radiative forcing of BC depends highly on the altitude location (Ban-Weiss et al., 2012), vertical profile (Samset et al., 2013), and mixing state with co-emitted species (Cappa et al., 2012, Chung and Seinfeld, 2002, Haywood and Shine, 1995).
The cloud and ice/snow effects of BC even remain more uncertain (Denman et al., 2007, Flanner et al., 2007, Heintzenberg and Charlson, 2009). Nonetheless, the total climate forcing integrating all forcing terms was estimated to be +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. BC is potentially the second most important climate warming agent only inferior to carbon dioxide (Bond et al., 2013), whose RF was estimated to be 1.66 W m−2 (Forster et al., 2007). The predominant sources for BC include fossil fuels (38%), solid fuels (20%) and open burning (42%), and are largely contributed by anthropogenic combustion (Bond et al., 2004). This motivates the establishment of sound emission metrics for BC in mitigation of its adverse climate impacts. However, to our knowledge, little literature has investigated how BC emission in a certain region changes along the gradient (i.e., BC emissions are changed by −100%, −50%, 0%, 200% and 1000%, respectively) would impact the global climate.
In this study, we utilized a fully-coupled earth system model CESM to evaluate the global short-term climate response due to anthropogenic BC emitted from East Asia, North America and Europe, which are considered to be the biggest contributors in global anthropogenic BC emissions. Rather than identifying the equilibrium climate response from a BC emission perturbation (which may take more than century's period of simulation), this study focuses on the “closest to real” short-term effect (Jacobson, 2010) of BC on our environment. Specifically, we'd like to understand whether the disturbance of air quality and regional climate in different areas response linearly or nonlinearly to the fluctuation of BC emissions in a particular region. Only direct and semi-direct effects of BC on radiation are involved in this configuration. Therefore, the result reflects a short-term climate perturbation in a particular configuration of CESM. From there, we evaluate the extent that a robust emission-response relationship (i.e., air quality and climate perturbations) exists for BC from different regions. This may acknowledge policymakers the potential outcomes if BC emissions over the region are well controlled or uncontrolled.
The configuration of CESM is adapted in this study to allow chemistry processes coupled into general circulation model. That atmospheric component of CESM (i.e., CAM-Chem) utilized in this experiment has a comprehensive treatment of aerosol processes and its interaction with climate. We investigate various indicators related to climate perturbation, including black carbon burden, aerosol optical depth, radiation budget, cloud cover, the surface air temperature (SAT), surface pressure and wind fields, etc. We describe our experiment design and model configuration in Section 2, and show the air quality relevant results in Section 3 and climate relevant findings in Section 4. Finally, conclusions are drawn in Section 5.
Section snippets
Model description
The Community Earth System Model (CESM) version 1.1.2 (released in July 2013) was used in this study. CESM is a fully coupled climate model with components of an atmospheric model of Community Atmospheric Model Version 4 (CAM4), a land model of Community Land Model Version 4 (CLM4), an ocean model Parallel Ocean Program Version 2 (POP2), models of sea ice, land ice and river, and a high-performance coupler. The 1.1z release of CESM has been scientifically validated, and the outputs of
BC concentrations
The simulated 10 years annual mean surface BC concentrations are shown in Fig. 2. These results are calculated by subtracting the base simulation of each perturbation runs from sensitivity runs. The difference indicates the contribution of BC emissions from individual regions to the global BC burden. Thus, black carbon has its largest loading near the source region. Since BC has a relatively short lifetime of several days (Forster et al., 2007), the reach of regional influence is mainly
Radiative flux perturbation at top of the atmosphere
The net flux of incoming solar radiation and outgoing terrestrial radiation at top of the atmosphere (TOA) determines the global budget and associated energy balance of the earth system. Clouds and absorbing aerosols (e.g., BC) strongly influence both the shortwave and longwave transmissions. Therefore, any changes in BC burden even at the regional scale may disturb the radiation at TOA, consequently the earth energy balance. Fig. 6 shows the radiative flux perturbation (RFP, shortwave plus
Conclusions
This study employs a fully-coupled earth system model CESM to evaluate the sensitivity of global short-term response of air quality and climate to the change of BC emissions from East Asia, North America and Europe. Specifically, for each region all anthropogenic BC emissions are multiplied by a scaling factor of 0, 0.5, 1, 2, 5, or 10. In each sensitivity run, a 10-year fully-coupled simulation is conducted and the results are compared to the base simulation (i.e., BC emissions are unchanged)
Acknowledgment
We thank Louisa Emmons, Xiaoyuan Li as well as two anonymous reviewers for their helpful suggestions on this study. This work was supported by funding from the National Natural Science Foundation of China under awards 41222011 and 41390240, the Research Project of the Chinese Ministry of Education No. 113001A, the “863” Hi-Tech R&D Program of China under Grant No. 2012AA063303, the 111 Project (B14001) as well as the undergraduate student research training program.
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M. S. and W. T. contributed equally to this work.