Practical Paths towards Lowering Black Carbon Emissions

Abstract

Black carbon (BC) aerosol, accounting for a minor fraction of atmospheric aerosols, is attracting increased attention due to its impact on air quality, human health, and climate change. Focusing on BC emission reduction, this paper gives a brief introduction to the sources and global distribution of BC. Along with the decrease of BC emissions from such actions as the reduction of global greenhouse gases (GHGs) and regulating local air quality, it also highlights other BC reduction approaches such as control and improvement of combustion conditions, the elimination of open biomass burning, and the sequestration of BC by biomass pyrolysis. Finally, it is stressed that at this moment there is no enough reason to push BC reduction into any climate change related negotiations, although BC has been included in some of so-called win-win reduction targets for the quick response to both climate and non-climate appeals.

Keywords

black carbon ; reduction ; policy consideration

1. Introduction

Atmospheric black carbon (BC) aerosol originates primarily from the incomplete combustion of fossil fuels, biofuels, and biomass [ Bond et al ., 2004  ;  Penner et al ., 1993 ]. Depending on the sources, analytical methods, and fields of application, BC is also called elemental carbon, graphitic carbon, or soot, among other names [ Andreae and Gelencsér , 2006  ; Bond and Bergstrom, 2006 ]. BC has a graphitelike structure that enables it to be light-absorbing, oxidation-resisting and reagent-insoluble [ Andreae and Gelencsér, 2006  ; Bond and Bergstrom, 2006  ;  Watson et al ., 2005 ].

Although BC aerosol accounts for only a small portion of atmospheric components, it has significant effects on the atmospheric environment and human health. For example, ambient BC can catalyse the formation of nitrous acid and thus increase the production of photochemical ozone [ Ammann et al., 1998 ]. The direct light-absorbing and scattering properties of BC reduce atmospheric visibility, contributing to the formation of regional haze and affecting plant photosynthesis [ Chameides et al ., 1999  ; Wolff, 1981  ;  Wolff et al ., 1981 ]. As a constituent of airborne particulate matter, tiny BC particles can penetrate into the respiratory system, causing respiratory asthma, cardiovascular malfunction and certain types of cancer [ Armstrong et al ., 2004  ;  Mumford et al ., 1987  ; O’Neill et al., 2005 ].

In recent years, much attention has been devoted to global climate effects caused by BC aerosol, making it a worldwide focus of research [ Hansen et al ., 1998  ;  Highwood and Kinnersley, 2006 ; IPCC, 2007a]. In the latest IPCC report, direct radiative forcing (RF) by BC was determined to be 0.34 W m^–2 , which is approximately one-fifth of the RF of CO₂ [ IPCC, 2007b ]. Ramanathan and Carmichael [2008] assigned a value as high as 0.9 W m^–2 , which represents approximately 54% of the warming caused by CO₂ in the same period. In regions such as the Himalayas, the impact of BC on melting snowpack and glaciers may rival that of CO₂ [ Ramanathan and Carmichael, 2008 ]. BC emissions also significantly contribute to Arctic ice-melt, which is thought to be one of the tipping points in the Earth’s climate system [ Zender, 2007 ; Lenton et al ., 2008  ;  Shindell and Faluvegi, 2009 ].

In view of these adverse effects on the environment and health and the possible effects on global warming, BC and short-lived greenhouse gases (GHGs) reduction has been proposed as win-win strategy for the quick response to both climate and non-climate appeals [ Wallack and Ramanathan, 2009 ; Shindell and Faluvegi, 2009  ;  Kintisch, 2009 ]. This response was particularly highlighted by Nobel Laureate Dr. Mario Molina and his colleagues, who proposed a “fast-action” strategy to complement the slow development of CO₂ regulation. Such actions include reducing hydrofluorocarbons (HFCs), BC soot and tropospheric ozone, and expanding bio-sequestration through biochar [ Molina et al., 2009 ]. In this paper, we will present a brief summary of practical paths towards lowering BC concentrations in the atmosphere. We will also propose that even if reducing BC emissions is helpful for mitigation purposes, it can serve only as a supplement to rather than a substitution for the actions needed to control CO₂ and other GHGs; any attempt to put the BC reduction into climate-related discussions and negotiations may undermine global climate initiatives.

2. An overview of global BC emissions

Recent estimates of global BC emissions range from 8 to 24 Tg per year [ Penner et al ., 1993  ; Bond et al ., 2004  ; Chow et al ., 2010  ; Cooke et al ., 1999  ; Cooke and Wilson, 1996  ;  Liousse et al ., 1996 ]. According to Bond [2007], open biomass burning releases 42% of the total BC; domestic biofuel releases 18%; road transportation, 14%; non-road transportation, 10%; industry and power generation, 10%; and domestic fuel consumptions (coal and others), 6% ( Fig. 1 ). Developing countries contribute approximately 84% of the total, mainly from domestic biofuel and fossil fuel use, and developed countries account for only 16%, mainly from transportation and industrial activities. In terms of regional distribution, Africa is the largest emitter in the world, followed by China and Central and South America, with Oceania and the Middle East emitting the smallest amounts.

Figure 1. Sources of BC emissions in 2000 [ Bond, 2007 ]

Studies show an increasing historical trend for BC emissions in general (Fig. 2 ). Ito and Penner [2005] calculated the emissions of BC from biomass and fossil fuel burning for the period 1870–2000 and found an overall trend of gradual increase due to the expanding consumption of fossil fuel. Junker and Liousse [2008] developed a BC emission inventory for 1860–1997 from historical records of fossil fuel and biofuel consumption. Although the general trend is similar to that in Ito and Penner [2005] , the emissions decreased after the mid-1980s, possibly because of improvements in combustion technology. Novakov et al. [2003] also observed an increasing trend in BC emissions by fossil fuels since 1750, but BC emissions from fossil fuels alone exceeded the total emissions reported by Junker and Liousse [2008] , suggesting substantial uncertainty in the inventories of historical and current BC emissions.

Figure 2. Historical BC emissions

The high uncertainty in current inventories estimating aggregate BC emissions induces two problems. The first problem relates to climate models with carbonaceous aerosols as inputs: highly uncertain BC estimates will affect model results. The second problem relates to action: the differing estimates of BC emissions from various sectors or sources make target selection and analysis difficult [ Chow et al., 2010 ]. Thus, a precise and dynamic BC inventory (parameterised) based on the combination of source measurement, continuous ground monitoring and space sensing is necessary for understanding existing emissions, predicting future changes, and monitoring reduction effectiveness.

3. Practical paths towards lower BC emissions

Approaches towards lowering ambient BC concentrations are essentially associated with both BC sources and sinks. BC sinks rely on dry and wet depositions, which are generally independent of human manipulation, but the source is subject to human intervention through technological or behavioural changes. Fortunately, technologies for cutting BC emissions are already available. Cofala et al. [2007] projected that, based on the current worldwide expectations of economic development, there will be a 17% decrease of BC emissions in 2030 compared with 2000 through the implementation of the already adopted emission control legislation in each country. They stated that the full application of currently available emission control technologies could decrease BC emissions by up to 47% ( Fig. 3 ). Wallack and Ramanathan [2009] estimated that with existing technologies, BC emissions could be reduced by 50%, which is equivalent to the warming effect of CO₂ emitted in one or two decades.

Figure 3. Projected development of global anthropogenic emissions by economic sector for BC. Estimates for historic years (1990 and 2000) and for 2030 for the “current legislation” (2030_CLE) and the “maximum technically feasible reductions” (2030_MFR) cases [ Cofala et al., 2007 ]

In this section, we will compile some of the recognised approaches capable of preventing the release of BC into the atmosphere. In contrast to the usual sector-by-sector categorisation (e.g., residential and industrial), we will divide these approaches into five types based on their relevance to current or potential policies and technologies.

3.1. Benefits of GHG control

The extensive recognition and implementation of the Kyoto Protocol is indicative of the global desire to control GHGs. Fortunately, most of the measures taken to reduce GHGs automatically lead to reduction of BC emissions.

3.1.1. Promotion of carbon-free energy

Because BC is the result of incomplete combustion of carbon-containing materials, energy sources free of carbon cannot generate BC or CO₂ , and therefore, they are preferred. These energy sources include solar energy, wind power, hydropower, nuclear energy, geo-thermal energy, tidal energy, oceanic energy, and hydrogen energy. Today, these types of energy are also called clean energy, new energy, green energy, or renewable energy. According to a report published by the National Development and Reform Commission (NDRC) of China, by the end of 2005, the installed capacity of hydropower generation reached 117 GW, representing 23% of the total power generation capacity. The total capacity of photovoltaic generation was estimated to be around 70 MW, mainly serving as the power supply for remote area residents [ NDRC, 2007 ].

It is worth noting that the development and maintenance of the infrastructure for carbon-free energy involves a considerable pre-cost of energy, which may be supplied by fossil fuels with BC and CO₂ emissions. For example, although hydrogen energy itself is a clean energy without BC and CO₂ emissions, it is actually a secondary energy; for this reason, whether such energy is conducive to decreasing CO₂ and BC emissions depends on the origin of hydrogen (from fossil fuel or carbon-free energy).

3.1.2. Promotion of low-carbon energy

The molecular structure of fossil fuels such as diesel and coal abound in carbon, and their main product of combustion is CO₂ . In contrast, fuels with low amounts of carbon contain a high proportion of hydrogen; when burned, the released energy of these fuels comes from the formation of both CO₂ and H₂ O. As a result, low-carbon fuels emit less CO₂ than carbonintensive fuels and therefore contribute less to the global GHGs burden. Natural gas, marsh gas, coal gas, ethanol gasoline, and coal-bed gas, among others, can all be categorised as low-carbon energy. Because most of these fuels are in a gas state and can be easily burned, BC yields from these fuels are much less than from high-carbon fuels. A typical example is natural gas, which has a BC emission factor close to zero [ Bond et al., 2004 ].

3.1.3. Energy conservation and efficiency improvement

The improvement of energy conservation and efficiency helps to reduce energy consumption, resulting in less fuel burned and less CO₂ and BC generated. For example, in the 1980s, the Chinese government considered conservation a matter of strategic importance in its energy policy, and it adopted the principle of “equal treatment to development and conservation with immediate emphasis on the latter”. A striking decline in energy consumption per unit of energy-intensive products has been achieved in the industrial sector, especially in the thermal power, steel, and cement industries. From 1990 to 2005, China’s energy intensity decreased from 268 to 143 tons of coal equivalent, substantially reducing BC emissions [ NDRC, 2007 ].

3.1.4. Green behaviours

Modern concerns regarding the mitigation of carbon emissions extend to social behaviour, which includes significant changes in habits, ideology, and even infrastructure and institutions. For example, the reduction of pollution-causing activities requires changes in transportation demand and travel behaviour, which can be achieved by government investments in infrastructure to support greater use of mass transit, bicycling, walking, and other means [ ICCT, 2009 ]. The rejection of excessive packaging and once-off tableware as well as the application of energy-saving materials in home building are also signs of the adoption of green behaviours.

3.2. Benefits from air quality regulation

The interest in air quality is even greater than that in climate change. Airborne particulate matter (PM) is increasingly produced by rapid industrialisation and is heavily accumulated within limited areas by urbanisation. There is an increasing occurrence of urban haze in many cities of developing countries, where the major pollutant is inhalable particles (PM10) [ Chang et al., 2009 ]. BC is a constituent of indoor and outdoor PM, and efforts in air quality regulation can certainly lead to a direct reduction of BC concentration in the air. Deteriorating urban air quality also affects not only the viability of important natural and agricultural ecosystems in regions surrounding highly urbanised areas, also the energy budget from local to global scale, necessitating more robust international cooperation. For example, a bilateral workshop bringing together experts from China’s MEP (Ministry of Environmental Protection) and the United States’ EPA (Environmental Protection Agency) was held in Beijing in May 2010, addressing how to cobenefit from the integration of air quality management and climate change, with BC being an important topic.

3.2.1. Emission controls in industrial and power facilities

Emission control in industrial and power sectors is widely conducted to prevent dust from entering the atmosphere as pollutants. Typical control devices include electrostatic precipitators (ESPs), scrubbers, cyclones, and fabric filters that can intercept and collect particles through mechanical or non-mechanical processes [ Streets et al., 2001 ]. For example, the efficiency of PM collection devices in the power sector is roughly 98%–99% (ESPs), 90%–95% (wet particle scrubbers), 85%–93% (multicyclones), and 80%–85% (cyclones) [ Streets et al., 2001 ]. With the development of control technologies and the raising of air quality standards, an increased amount of particulate emissions from industrial-scale facilities will be captured.

3.2.2. Particle trapping in the transport sector

Although stringent emission controls in developed regions (e.g., the USA and EU) have reduced transportation-related BC emissions, the rapidly growing vehicle fleet may make BC emissions rise again, reaching levels 20% higher than in 2000 by 2050 [ ICCT, 2009 ]. In addition, there are numerous off-road conveyances, such as marine ships, trains, agricultural vehicles, and construction equipment; regulating these sources has even been extremely difficult due to their diversity and widespread use. Future regulations should try to cover these sources by appropriate means.

The most effective trapping technology is the installation of wall-flow filters in diesel vehicles. These filters function particularly well in eliminating BC emissions from ultra-low-sulphur fuel. For heavy-duty diesel vehicles as well as motorcycles, light-duty gasoline vehicles and light-duty diesel vehicles, the installation of such trapping devices results in immediate benefits [ ICCT , 2009 ].

3.2.3. Prevention of secondary rising of road dust

The secondary rising of road dust by factors such as wind or running vehicles makes an important contribution to ambient aerosol concentrations. Methods such as the expansion of vegetation coverage or the increase of watering on busy urban roads are recommended.

3.2.4. Promotion of collective heating

Collective heating (or central heating) in winter is an effective way to abate particulate pollution. By the application of large combustion facilities, such as industrial coal boilers, collective heating has good ventilation, excellent particle trapping and regular governance and thus produces less BC [ Streets et al., 2001 ]. Collective heating is profoundly different from traditional heating in Chinese households, which rely on coal stoves or mini-boilers without pollution control devices [ Bond et al ., 2004  ; Chen et al ., 2006  ; Streets and Aunan, 2005  ;  Zhi et al ., 2008 ].

3.3. Optimising combustion through technological means

The difference in formation mechanisms between CO₂ and BC allows BC emissions to be curbed by modifying combustion process. In other words, CO₂ yield depends on fuel quantity alone, while BC yield depends upon both fuel quantity and burning conditions, with the latter being more influential [ Zhi et al., 2008  ; 2009; Lan et al ., 2002  ;  Sinton et al ., 2004  ; Zuk et al., 2006 ]. The modification of combustion thus provides the possibility of cutting BC emissions without cutting down fossil fuel consumption and thus seems to be in the interest of major developing countries with rising energy demands.

Two sectors may make the best of such advantage, and they are discussed as follows.

3.3.1. Residential sector

The residential sector contributes about 24% of the total global BC emissions, including 18% from biofuel and 6% from fossil fuel [ Bond , 2007 ]. In China, the residential sector is the largest source of BC emissions due to the considerable amount of small cooking or heating stoves that burn raw coal or biofuels in households, without emission control [ Cao et al ., 2006  ;  Streets et al ., 2001 ]. Systematic experiments by Chen et al. [2006  ; 2009 ] and Zhi et al. [2008; 2009] demonstrated that BC emissions can be surprisingly reduced through the improvement of simple stoves and by using honey-comb coal instead of raw coal chunks (Fig. 4 ). More commendable is the significant decrease of BC/OC ratios caused by improvements in stoves and coal, which is thought to be more important to the net effect of carbonaceous aerosols on the Earth’s energy budget [ Zhi et al ., 2009  ;  Novakov et al ., 2005 ] (Fig. 5 ).

Figure 4. The effects of deploying improved stove and briquette coal on reducing BC emissions (case–1: traditional stove/coal chunk; case–2: improved stove/coal chunk; case–3: traditional stove/coal briquette; case–4: improved stove/coal briquette) [ Zhi et al., 2009 ]

Figure 5. Effects of improved stoves and coal briquettes on BC/OC ratios in flue particles [ Zhi et al., 2009 ]

It is accordingly reasonable to prompt governments to consider the possibility of phasing out the direct burning of bituminous raw coal chunks and polluting stoves and of financially supporting the research, production, and application of improved stoves and coal-briquettes [ Zhi et al., 2009 ]. In south Asia, a UNEP-supported project named Surya has been initiated by the mass introduction of more efficient biomass-burning stoves and cookers as well as solar cookers. Scientists engaged in the project hope to find win-win results from this large-scale experiment (http://www.unep.org/climateneutral/Portals/0/Image/BC%20e-Bulletin%20Sep%2009.pdf ).

3.3.2. Transport sector

In addition to the particle trapping described in Section 3.2.2 , the optimisation of combustion through technological innovation is also widely applied in oil combustors. Mature technologies for promoting engine efficiency are available, such as stratified charge combustion, homogeneous dilution, gasoline direct injection (GDI), electronically controlled engines, and improvement of ignition system and combustion chamber structre. Further measures for better combustion such as the utilisation of three-way catalysis (TWC) technology, thermal reactors and exhaust gas recirculation (EGR) are also noteworthy [ Wang , 2004 ]. The promotion of diesel desulphurisation and the enforcement of upgraded automobile emission limits are considered critical to realising more efficient combustion.

In developed nations, the transport sector plays a major role in BC emissions due to the large population of automobiles and other vehicles [ Bond , 2007 ]. Today, the booming economy in some major developing countries results in an expanding fleet of cars and trucks. It was reported that, in 2010, new car sales in Beijing reached 891,000, up 26% year on year (http://news.163.com/11/0110/02/6Q0KUKF400014AED.html ). This development implies a greater demand for oil, resulting in more pollutants and BC being released into the atmosphere.

3.4. Regulation of open biomass burning

Biomass burning adds no net CO₂ to the atmosphere, provided that the burned biomass is replanted; however, this is a futile ideal [ Searchinger et al., 2009 ]. Much of this burning is caused by natural events, such as forest and savannah fires; other biomass burning is the result of human activities, such as transforming prairie land to farmland or clearing agricultural residues for farming [ Bond, 2007  ;  Law and Stohl, 2007 ]. From the perspective of energy utilisation, biomass burning is a kind of energy waste due to the unmanageability of this energy for human purposes. From the perspective of environmental preservation, it results in air pollution and health risks. From the perspective of carbon emissions, it contributes nearly half of the global BC emissions [ Bond et al., 2004 ]; and from the perspective of global energy budgets linked to atmospheric aerosols, the estimated RF of biomass burning aerosol was updated from −0.20 W m⁻² (3 times uncertainty) in the IPCC Third Assessment Report to (0.02±0.12) W m^–2 in the IPCC Fourth Assessment Report. This update was due to improvements in the models in representing the absorption properties of the aerosol and the effects of biomass burning aerosol overlying clouds [ IPCC , 2007b ]. That is, biomass burning seems to exert a positive effect on the global climate as far as the direct effect alone is concerned. Thus, the previous notion that biomass burning aerosol may mitigate climate warming is challenged.

Considering that more than 40% of BC emissions originate from open biomass burning, actions for avoiding or lessening open biomass fires are critical. These actions include the suppression of all natural fires, the implementation of field burning bans, and the protection of forests and vegetation. To protect Arctic sea ice, countries of the Northern Hemisphere, particularly countries adjacent to the Arctic region, have proposed to join their efforts in decreasing their aerosol carbon emissions [ Zender, 2007 ].

3.5. Sequestration through biochar: A potential method for preventing BC from entering the atmosphere

BC is generally divided into two types: soot and char. The former is composed of submicron particles produced from the condensation of hydrocarbon radicals at high temperature (e.g., > 600 °C); the latter is formed when combustion occurs in some kinds of fuel (e.g., biomass) at relatively low combustion temperatures, with the original morphology of their source materials retained [ Han et al., 2007 ].

Sequestration through biochar captures carbon from the atmosphere and thus reduces atmospheric CO₂ concentrations. Controlled bio-charring begins with the high temperature decomposition of biomass in an oxygen-deprived environment and it ends in a stable charcoal-like substance called biochar [ Winsley, 2007 ]. The storage time of biochar in soil varies from hundreds to tens of thousands of years [ Sohi et al., 2009 ], during which additional benefits from soil amendment and fertilisation can be achieved [ Fowles, 2007  ;  Sohi et al ., 2009 ].

Biological sequestration through biochar promises a lower atmospheric BC concentration by preventing potential BC particles from entering the atmosphere. The most preferable action may be transitioning from slash-and-burn to slash-and-char for crop residues due to its feasibility in many countries [ Lehmann et al., 2006 ]. It is believed that both feedstocks and technologies for biochar production and deployment are available, including mobile or stationary units at local or regional levels [ Lehmann et al ., 2006  ; Sohi et al ., 2009  ;  IBI, 2008a ]. Even in households, fuel-efficient heating or cooking stoves can reduce BC emissions through biocharring. According to a report from the International Biochar Initiative, waste biomass alone has the potential to reduce 1 Gt carbon or 3.67 Gt CO₂ per year by 2040 [ IBI , 2008b ]. The biochar approach has been strongly recommended by scientists interested in carbon capture and storage (CCS) and has been included in a fast-action strategy proposed by Molia et al. [2009] for mitigating dangerous global changes.

Questions have also been raised regarding the biochar strategy. Some researchers have doubts concerning whether soil BC (in biochar form) can amend rather than harm the soil and whether biochar can be permanently or satisfactorily retained in soil rather than easily re-released into the atmosphere [ Hamer et al ., 2004  ;  Lehmann, 2007 ]. These concerns suggest more studies are needed in this area. Storing biochar in impermeable rock structures is scientifically more convincing than in soil, but it is more expensive in practice [ Fowles, 2007 ].

4. Further research is needed if relating BC reduction with climate change issues

Although the implementation of BC reduction policies in heavily polluted areas is always justified in the name of environmental protection and health security, attempts to duplicate this logic in the name of climate protection is not ensured. Further research in BC-climate science needs to be performed before BC can be undoubtedly designated as a climate mitigation target.

4.1. RF uncertainty

Ascertaining the RF of BC is critical to the appeal for globally concerted actions in reducing BC to protect climate. Current knowledge regarding BC’s RF is defective in two aspects. First, the IPCC has had difficulty in making a rough estimate of cloud-related BC forcing (indirect effect), leaving the RF characterisation of BC incomplete, which in turn affects the potential of persuasion regarding anti-global warming [ IPCC, 2007b ]. Second, when looking at the direct RF alone, the estimated RF since pre-industrial times given by the IPCC and other researchers differs by a factor of 3 [ ICCT , 2009 ]. Obviously, a consensus on the complete and proper RF value of BC is necessary to improve our understanding of the relative contribution of BC to global warming.

It is also important to find a proper metric for policy discussions. Although global warming potential (GWP) has been accepted as an excellent metric by the IPCC to compare the radiative effects of different species in reference to CO₂ [ IPCC , 2007a ], the GWP of BC was not definitely considered therein. Some researchers have tried calculating BC’s GWP according to the principles in the IPCC report, but there are large differences in the estimated GWP among different calculators, indicating the need for more refined calculations [ Berntsen et al ., 2006  ;  Bond and Sun, 2005 ; Hansen et al., 2007  ; Jacobson, 2002  ;  Jacobson, 2005 ].

The BC science suffers from the inconsistency of BC determination methods [ Watson et al ., 2005  ; Yttri et al ., 2009  ;  Cavalli et al ., 2010 ], and even the two conventional thermal/optical protocols, NIOSH and IMPROVE, cannot reach a similar OC/EC demarcation [ Birch and Cary, 1996 ; Birch et al ., 1999  ;  Chow et al ., 1993  ; 2001 ; 2007 ]. This is more complicated by the existence of brown carbon, which has been shown to be weakly light-absorbing (especially in the UV range) rather than non-light-absorbing, as previously thought [ Andreae and Gelencsér, 2006 ]. Consequently, BC measurements by thermal, thermal/optical or optical methods have become more operationally dependent. It is imperative to develop or designate a standard method for BC quantification or find an approach to relating one method with another [ Zhi et al., 2011 ].

4.2. Selective reduction difficulty

Recently, there is a suggestion that includes BC reduction in the priority for combating so-called irreversible climate change [ Molina et al ., 2009  ;  UNEP, 2009 ]. However, this is challenged by the difficulty in how to achieve a selective reduction of BC from the mixture of emissions because BC is always emitted in conjunction with scattering species [ Jacobson, 2002  ;  Penner et al ., 2003 ]. Therefore the action for the reduction of BC would usually cut off other aerosol components that are conducive to slow global warming. Ambient atmospheric aerosols are mainly composed of soil, dust, sulfate, nitrate, ammonium products, BC, OC, and sea salt, which impose a general “cooling” effect on climate system through scattering and absorbing solar radiation [ IPCC , 2007b ]. In addition, although BC can absorb part of solar radiation and has a warming effect, the aerosol absorption caused by natural processes-controlled soil dust can not be ignored either (5%–15%) [ Zhang et al., 2008 ]. Therefore, any attempt to put the BC reduction into climate-related global discussions and negotiations may undermine global climate initiatives. Fortunately, as we discussed previously, BC emissions can also be simultaneously reduced as a result of CO₂ control.

Acknowledgements

This work was co-supported by China 973 project of MOST (2011CB403401), and China Postdoctoral Science Foundation (20080440463, 200902157). We are grateful to Dagang Tang from Chinese Research Academy of Environmental Sciences (CRAES) for his help in translating some technical terms.

References

1. Ammann et al., 1998 M. Ammann, M. Kalberer, D.T. Jost, et al.; Heterogeneous production of nitrous acid on soot in polluted air masses; Nature, 395 (1998), pp. 157–160
2. Andreae and Gelencéer, 2006 M.O. Andreae, A. Gelencéer; Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols; Atmospheric Chemistry and Physics, 6 (2006), pp. 3131–3148
3. Armstrong et al., 2004 B. Armstrong, E. Hutchinson, J. Unwin, et al.; Lung cancer risk after exposure to polycyclic aromatic hydrocarbons: A review and metaanalysis; Environmental Health Perspectives, 112 (2004), pp. 970–978
4. Berntsen et al., 2006 T. Berntsen, J. Fuglestvedt, G. Myhre, et al.; Abatement of greenhouse gases: Does location matter? J; Climate, 74 (2006), pp. 377–411
5. Birch and Cary, 1996 M.E. Birch, R.A. Cary; Elemental carbon-based method for occupational monitoring of particulate diesel exhaust: Methodology and exposure issues; Analyst, 121 (1996), pp. 1183–1190
6. Birch et al., 1999 M.E. Birch, D. Dahmann, H.H. Fricke; Comparison of two carbon analysis methods for monitoring diesel particulate levels in mines; Journal of Environmental Monitor, 1 (1999), pp. 541–544
7. Bond, 2007 Bond, T. C., 2007: Testimony for the hearing on black carbon and climate change. in: House Committee on Oversight and Government Reform , accessedhttp://oversight.house.gov/images/stories/documents/20071018110647.pdf .
8. Bond and Bergstrom, 2006 T.C. Bond, R.W. Bergstrom; Light absorption by carbonaceous particles: An investigative review; Aerosol Science and Technology, 40 (2006), pp. 27–67
9. Bond and Sun, 2005 T.C. Bond, H. Sun; Can reducing black carbon emissions counteract global warming?; Environmental Science & Technology, 39 (16) (2005), pp. 5921–5926
10. Bond et al., 2004 T.C. Bond, D.G. Streets, K.F. Yarber, et al.; A technology-based global inventory of black and organic carbon emissions from combustion; J. Geophys. Res., 109 (2004) http://dx.doi.org/10.1029/2003JD003697 D14203
11. Cao et al., 2006 G. Cao, X. Zhang, D. Wang, et al.; Inventory of black carbon and organic carbon emissions from China; Atmospheric Environment, 40 (2006), pp. 6516–6527
12. Cavalli et al., 2010 F. Cavalli, M. Viana, K.E. Yttri, et al.; Toward a standardised thermal-optical protocol for measuring atmospheric organic and elemental carbon: The EUSAAR protocol; Atmospheric Measurement Techniques, 3 (2010), pp. 79–89
13. Chameides et al., 1999 W.L. Chameides, H. Yu, S. Liu, et al.; Case study of the effects of atmospheric aerosols and regional haze on agriculture: An opportunity to enhance crop yields in China through emission controls?; Proceedings of the National Academy of Sciences USA, 96 (1999), pp. 13626–13633
14. Chang et al., 2009 D. Chang, Y. Song, L. Liu; Visibility trends in six megacities in China 1973–2007; Atmospheric Environment, 94 (2) (2009), pp. 161–167
15. Chen et al., 2006 Y. Chen, G. Zhi, Y. Feng, et al.; Measurements of emission factors for primary carbonaceous particles from residential raw-coal combustion in China; Geophys. Res. Lett., 33 (2006) http://dx.doi.org/10.1029/2006GL026966 L20815
16. Chen et al., 2009 Y. Chen, G. Zhi, Y. Feng, et al.; Measurements of black and organic carbon emission factors for household coal combustion in China: Implication for emission reduction; Environmental Science& Technology, 43 (24) (2009), pp. 9495–9500
17. Chow et al., 2001 J.C. Chow, J.G. Watson, D. Crow, et al.; Comparison of IMPROVE and NIOSH carbon measurements; Aerosol Science & Technology, 34 (1) (2001), pp. 23–34
18. Chow et al., 2010 J.C. Chow, J.G. Watson, D.H. Lowenthal, et al.; Black and organic carbon emission inventories: Review and application to California; Journal of the Air & Waste Management Association, 60 (4) (2010), pp. 497–507
19. Chow et al., 1993 J.C. Chow, J.G. Watson, L.C. Pritchett, et al.; The DRI thermal/optical reflectance carbon analysis system: Description, evaluation and applications in U.S. air quality studies; Atmospheric Environment, 27A (8) (1993), pp. 1185–1201
20. Chow et al., 2007 J.C. Chow, J. Yu, J.G. Watson, et al.; The application of thermal methods for determining chemical composition of carbonaceous aerosols: A review; Journal of Environmental Science and Health, 42 (11) (2007), pp. 1521–1541
21. Cofala et al., 2007 J. Cofala, M. Amann, Z. Klimont, et al.; Scenarios of global anthropogenic emissions of air pollutants and methane until 2030; Atmopheric Environment, 41 (2007), pp. 8486–8499
22. Cooke et al., 1999 W.F. Cooke, C. Liousse, H. Cachier, et al.; Construction of a fossil fuel emission data set for carbonaceous aerosol and implementation and radiative impact in the ECHAM4 model; J. Geophys. Res., 104 (1999), pp. 22137–22162
23. Cooke and Wilson, 1996 W.F. Cooke, J.J.N. Wilson; A global black carbon aerosol model; J. Geophys. Res., 101 (1996), pp. 19395–19409
24. Fowles, 2007 M. Fowles; Black carbon sequestration as an alternative to bioenergy; Biomass and Bioenergy, 31 (5) (2007), pp. 426–432
25. Hamer et al., 2004 U. Hamer, B. Marschner, S. Brodowski, et al.; Interactive priming of black carbon and glucose mineralization; Organic Geochemistry, 35 (7) (2004), pp. 823–830
26. Han et al., 2007 Y. Han, J. Cao, J.C. Chow, et al.; Evaluation of the thermal/optical reflectance method for discrimination between char- and soot-EC; Chemosphere, 69 (2007), pp. 569–574
27. Hansen et al., 2007 J. Hansen, M. Sato, P. Kharecha, et al.; Climate change and trace gases; Philosophical Transactions of the Royal Society A, 365 (2007), pp. 1925–1954
28. Hansen et al., 1998 J.E. Hansen, M. Sato, A. Lacis, et al.; Climate forcings in the industrial era; Proceedings of the National Academy of Sciences USA, 95 (1998), pp. 12753–12758
29. Highwood and Kinnersley, 2006 E.J. Highwood, R.P. Kinnersley; When smoke gets into our eyes: The multiple impacts of atmospheric black carbon on climate, air quality and health; Environment International, 32 (2006), pp. 560–566
30. IBI, 2008a IBI (International Biochar Initiative), 2008a: Biochar: A soil amendment that combats global warming and improves agricultural sustainability and environmental impacts. Accessed http://www.carbon-negative.us/docs/IBI_BiocharWhitePaper.pdf .
31. IBI, 2008b IBI (International Biochar Initiative), 2008b: How much carbon can biochar systems offset-and when? Accessed http://www.biochar-international.org/images/final_carbon_wpver2.0.pdf .
32. ICCT, 2009 ICCT (The International Council on Clean Transportation), 2009: A policy-relevant summary of black carbon climate science and appropriate emission control strategies. Accessedhttp://www.theicct.org/pubs/BCsummary-dec09.pdf .
33. IPCC, 2007a IPCC; Core Writing Team, R.K. Pachouri (Eds.), et al. , Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press (2007), p. 104
34. IPCC, 2007b IPCC; S. Solomon (Ed.), et al. , Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press (2007), p. 996
35. Ito and Penner, 2005 A. Ito, J.E. Penner; Historical emissions of carbonaceous aerosols from biomass and fossil fuel burning for the period 1870–2000; Global Biogeochemical Cycles, 19 (2005) http://dx.doi.org/10.1029/2004GB002374 GB2028
36. Jacobson, 2002 M.Z. Jacobson; Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming; J. Geophys. Res., 107 (D19) (2002), p. 4410 http://dx.doi.org/10.1029/2001JD001376
37. Jacobson, 2005 M.Z. Jacobson; Correction to ‘control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming’; J. Geophys. Res., 110 (2005) D14105
38. Junker and Liousse, 2008 C. Junker, C. Liousse; A global emission inventory of carbonaceous aerosol from historic records of fossil fuel and biofuel consumption for the period 1860–1997; Atmospheric Chemistry and Physics, 8 (2008), pp. 1195–1207
39. Kintisch, 2009 E. Kintisch; New push focuses on quick ways to curb global warming; Science, 324 (5925) (2009), p. 323
40. Lan et al., 2002 Q. Lan, R.S. Chapman, D.M. Schreinemachers, et al.; Household stove improvement and risk of lung cancer in Xuanwei, China; Journal of the National Cancer Institute, 94 (11) (2002), pp. 826–835
41. Law and Stohl, 2007 K.S. Law, A. Stohl; Arctic air pollution: Origins and impacts; Science, 315 (2007), pp. 1537–1540
42. Lehmann, 2007 J. Lehmann; Bio-energy in black; Frontiers in Ecology and the Environment, 5 (7) (2007), pp. 381–387
43. Lehmann et al., 2006 J. Lehmann, J. Gaunt, M. Rondon; Bio-char sequestration in terrestrial ecosystems—A review; Mitigation and Adaptation Strategies for Global Change, 11 (2006), pp. 403–427
44. Lenton et al., 2008 T.M. Lenton, H. Held, E. Kriegler, et al.; Tipping elements in the Earth’s climate system; Proceedings of the National Academy of Sciences USA, 105 (6) (2008), pp. 1786–1793
45. Liousse et al., 1996 C. Liousse, J.E. Penner, C. Chuang, et al.; A global three-dimensional model study of carbonaceous aerosols; J. Geophys. Res., 101 (1996), pp. 19411–19422
46. Molina et al., 2009 M. Molina, D. Zaelke, K.M. Sarma, et al.; Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO₂ emissions  ; Proceedings of the National Academy of Sciences USA, 106 (49) (2009), pp. 20616–20621
47. Mumford et al., 1987 J.L. Mumford, X. He, R.S. Chapman, et al.; Lung cancer and indoor air pollution in Xuan Wei, China; Science, 235 (1987), pp. 217–220
48. NDRC, 2007 NDRC (National Development and Reform Commission), 2007: China’s National Climate Change Program (in Chinese). Accessed http://www.gov.cn/zwgk/2007-06/08/content_641704.htm .
49. Novakov et al., 2005 T. Novakov, S. Menon, T.W. Kirchstetter, et al.; Aerosol organic carbon to black carbon ratios: Analysis of published data and implications for climate forcing; J. Geophys. Res., 110 (2005) http://dx.doi.org/10.1029/2005JD005977 D21205
50. Novakov et al., 2003 T. Novakov, V. Ramanathan, J.E. Hansen, et al.; Large historical changes of fossil-fuel black carbon aerosols. Geophys; Res. Lett., 30 (6) (2003) doi: 1310.1029/2002GL016345
51. O’Neill et al., 2005 M.S. O’Neill, A. Veves, A. Zanobetti, et al.; Diabetes enhances vulnerability to particulate air pollution-associated impairment in vascular reactivity and endothelial function; Circulation, 111 (22) (2005), pp. 2913–2920
52. Penner et al., 1993 J.E. Penner, H. Eddleman, T. Novakov; Towards the development of a global inventory for black carbon emissions; Atmospheric Environment, 27A (1993), pp. 1277–1295
53. Penner et al., 2003 J.E. Penner, S. Zhang, C.C. Chuang; Soot and smoke aerosol may not warm climate; J. Geophys. Res., 108 (D21) (2003), p. 4657 doi: 4610.1029/2003JD003409
54. Ramanathan and Carmichael, 2008 V. Ramanathan, G. Carmichael; Global and regional climate changes due to black carbon; Nature Geoscience, 1 (2008), pp. 221–227
55. Searchinger et al., 2009 T.D. Searchinger, S.P. Hamburg, J. Melillo, et al.; Fixing a critical climate accounting error; Science, 326 (2009), pp. 527–528
56. Shindell and Faluvegi, 2009 D. Shindell, G. Faluvegi; Climate response to regional radiative forcing during the twentieth century; Nature Geoscience, 2 (2009), pp. 294–300
57. Sinton et al., 2004 J.E. Sinton, K.R. Smith, J.W. Peabody, et al.; An assessment of programs to promote improved household stoves in China; Energy for Sustainable Development, 8 (3) (2004), pp. 33–52
58. Sohi et al., 2009 Sohi, S., E. Lopez-Capel, and R. Bol, 2009: Biochar’s roles in soil and climate change: A review of research needs, CSIRO land and water science report 05/09. Accesssedhttp://www.csiro.au/files/files/poei.pdf .
59. Streets and Aunan, 2005 D.G. Streets, K. Aunan; The importance of China’s household sector for black carbon emissions; Geophys. Res. Lett., 32 (2005) http://dx.doi.org/10.1029/2005GL022960 L12708
60. Streets et al., 2001 D.G. Streets, S. Gupta, S.T. Waldhoff, et al.; Black carbon emissions in China; Atmospheric Environment, 35 (2001), pp. 4281–4296
61. UNEP, 2009 UNEP (United Nations Environmental Programme), 2009: Non-CO₂ emissions: Options for a way forward. Black Carbon E-Bulletin , accessed http://www.unep.org/dec/Information_Resources/black-carb-on.asp .
62. Wallack and Ramanathan, 2009 V. Wallack, V. Ramanathan; The other climate changers: Why black carbon and ozone also matter; Foreign Affairs, 88 (2009), pp. 105–113
63. Wang, 2004 X. Wang; Causes and purification measures for car exhaust; Science New Ground (in Chinese), 2 (2004), p. 41
64. Watson et al., 2005 J.G. Watson, J.C. Chow, L. Chen; Summary of organic and elemental carbon/black carbon analysis methods and intercomparisons; Aerosol and Air Quality Research, 5 (1) (2005), pp. 65–102
65. Winsley, 2007 P. Winsley; Biochar and bioenergy production for climate change mitigation; New Zealand Science Review, 64 (1) (2007), pp. 5–10
66. Wolff, 1981 G.T. Wolff; Particulate elemental carbon in the atmosphere; Journal of the Air Pollution Control Association, 31 (1981), pp. 935–939
67. Wolff et al., 1981 G.T. Wolff, R.J. Countess, P.J. Groblicki, et al.; Visibility-reducing species in the Denver “brown cloud”: II. Sources and temporal patterns; Atmospheric Environment, 15 (1981), pp. 2485–2502
68. Yttri et al., 2009 K.E. Yttri, C.L. Myhre, K. Tørseth; The carbonaceous aerosol-a remaining challenge; WMO Bulletin, 58 (1) (2009), pp. 54–60
69. Zender, 2007 Zender, C. S., 2007: Arctic climate effects of black carbon. Accessed http://dust.ess.uci.edu/ppr/ppr_hogrc_wrt.pdf .
70. Zhang et al., 2008 X. Zhang, Y. Wang, X. Zhang, et al.; Aerosol monitoring at multiple locations in China: Contributions of EC and dust to aerosol light absorption; Tellus, 60B (2008), pp. 647–656
71. Zhi et al., 2008 G. Zhi, Y. Chen, Y. Feng, et al.; Emission characteristics of carbonaceous particles from various residential coal-stoves in China; Environmental Science & Technology, 42 (9) (2008), pp. 3310–3315
72. Zhi et al., 2009 G. Zhi, C. Peng, Y. Chen, et al.; Deployment of coal-briquettes and improved stoves: Possibly an option for both environment and climate; Environmental Science & Technology, 43 (15) (2009), pp. 5586–5591
73. Zhi et al., 2011 G. Zhi, Y. Chen, J. Sun, et al.; Harmonizing aerosol carbon measurements between two conventional thermal/optical analysis methods; Environmental Science & Technology (2011) http://dx.doi.org/10.1021/es102803f
74. Zuk et al., 2007 M. Zuk, L. Rojas, S. Blanco, et al.; The impact of improved wood-burning stoves on fine particulate matter concentrations in rural Mexican homes; Journal of Exposure Science and Environmental Epidemiology, 17 (3) (2007), pp. 224–232