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.
black carbon ; reduction ; policy consideration
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  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.
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 , 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.
Sources of BC emissions in 2000 [ Bond, 2007 ]
Studies show an increasing historical trend for BC emissions in general (Fig. 2 ). Ito and Penner  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  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  , the emissions decreased after the mid-1980s, possibly because of improvements in combustion technology. Novakov et al.  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  , suggesting substantial uncertainty in the inventories of historical and current BC emissions.
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.
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.  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  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.
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.
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.
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).
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 ].
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 ].
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.
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.
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.
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 ].
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.
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 ].
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.
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 ).
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 ]
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 ).
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.
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 ].
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.  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 ].
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.
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 ].
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.
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.