15.2: Mitigating Black Carbon

Black carbon, a major component of soot, consists of small particles of carbon that are mainly produced by incomplete combustion of fossil fuels or biomass, such as wood or other organic materials. These particles are classified as aerosols because they are light enough to remain in the atmosphere for anywhere from several hours to a few weeks.

Black carbon impacts

While many aerosols reflect solar radiation (sunlight) and have a cooling effect, black carbon absorbs sunlight and radiates infrared heat, significantly contributing to the anthropogenic greenhouse effect. Black carbon is estimated to be the second or third most important warming agent behind CO₂, with an impact comparable to that of methane.

Black carbon has additional negative impacts beyond its direct warming effect. In particular, when black carbon particles “rain out” of the atmosphere, some land on surfaces covered with snow or ice. There, the dark particles absorb sunlight, reducing the reflectivity of these surfaces and accelerating melting of snow, ice, and underlying permafrost. This in turn amplifies warming, particularly in the Arctic. It’s estimated that implementing the black carbon and methane mitigation measures discussed in this chapter could reduce Arctic warming between 2005 and 2040 by nearly two-thirds, compared with a business-as-usual scenario.

Moreover, black carbon has serious negative impacts on air quality and human health. Black carbon particles are classified as PM_2.5, which refers to particles that are less than 2.5 micrometers in size (PM stands for “particulate matter”). A micrometer is one-thousandth of a millimeter. (For comparison, most bacteria are between half a micrometer and 5 micrometers long, so we’re considering particles that are quite small indeed.)

Because they are so small, PM_2.5 particles like black carbon can be inhaled deep into the lungs, where they are difficult to dislodge, and even into the bloodstream. This results in significant negative impacts on human health, including premature deaths from lung cancer and heart disease. A recent study by UNEP estimated that measures to reduce black carbon could avoid 2.4 million premature deaths annually by 2030 (within a range of 0.7 to 4.6 million annual deaths).

Black carbon sources

In combustion (burning), carbon-rich molecules such as those in plant matter or fossil fuels combine with oxygen to produce carbon dioxide and water vapor. In actual combustion, not all of the carbon is converted to carbon dioxide. Some of it remains in complex interlinked molecules that form black carbon aerosols. Major sources of black carbon pollution include a variety of types of combustion, as detailed in Figure 15.2.1. The largest category is residential and commercial combustion. About 75% of this is from cooking with solid fuels (coal, firewood, and dung) by the world’s poorest 3 billion. The next largest source is transport, with more than 90% of the emissions due to diesel vehicles. Next is industrial processes (8%) where solid fuels are used for combustion in boilers, kilns, and furnaces. Agriculture contributes 7% of black carbon emissions, mainly through burning of agriculture residues and waste.

Fortunately, a range of policy measures and off-the-shelf technologies are already available to address many of the major sources of black carbon, as we’ll see in the next section.

Black carbon mitigation

Table 15.2.1 summarizes the nine most important black carbon mitigation measures, as identified by UNEP in 2011. In this section, we will highlight a few of these key measures.

Table 15.2.1 Black carbon mitigation measures affecting BC and other co-emitted compounds

Sector	Measure
Transport	Diesel particle filters for road and off-road vehicles Elimination of high-emitting vehicles in road and off-road transport
Residential	Replacement of coal by coal briquettes in cooking and heating stoves Pellet stoves and boilers, using fuel made from recycled wood waste or sawdust, to replace current wood-burning technologies in the residential sector in industrialized countries Introduction of clean-burning biomass stoves for cooking and heating in developing countries Substitution of clean-burning cookstoves using modern fuels for traditional biomass cookstoves in developing countries
Industry	Replacement of traditional brick kilns with vertical shaft kilns and Hoffman kilns Replacement of traditional coke ovens with modern recovery ovens
Agriculture	Ban on open field burning of agricultural waste

Source: Adapted from United Nations Environment Programme and World Meteorological Organization. 2011. Integrated Assessment of Black Carbon and Tropospheric Ozone. UNEP, Nairobi, Kenya.

Figure 15.2.1 Black carbon percentage contribution from various sources, 2005. Data from UNEP and WMO 2011.

Figure 15.2.2 Woman cooking in Mukteshwar, Indian Himalayas, India. Photograph by V. Ramanathan.

As shown in Figure 15.2.1, the largest single source of black carbon is residential and commercial combustion, and this is primarily for heating and cooking of food. In particular, cooking with traditional, inefficient stoves fueled by wood, dung, or agricultural waste (Figure 15.2.2) is a major source of black carbon emissions worldwide, second only to burning of biomass. Household members, mostly women, must in many cases walk several kilometers each day to obtain firewood and are exposed to high levels of indoor particulates while cooking. Indoor air pollution is estimated to cause about 3 million premature deaths each year.

Pollution from cooking stoves is a particularly acute problem for the 3 billion global poor, many of whom lack the infrastructure and financial resources for gas or other fossil fuel stoves. An estimated 38% of households worldwide lack access to efficient, low-emission cookstoves, particularly in developing regions of Asia and sub-Saharan Africa.

Replacing biomass fuels such as wood or dung with fuels such as liquified petroleum gas or kerosene can significantly reduce black carbon emissions, but this is only possible in areas with developed fossil fuel and transportation infrastructures. In less-developed areas, inefficient traditional cooking methods can be replaced by cleaner forced-draft biomass stoves, which use a small fan to increase oxygen flow, promoting more efficient and complete combustion and cutting black carbon emissions by 80%. Because the stoves require less fuel, they can also cut CO₂ emissions by 50%. This is a win-win solution that not only reduces global emissions but provides a significant improvement in quality of life.

Improving stoves is the single most effective SLCP mitigation measure and actually saves money over the long run in reduced fuel costs. Considering the benefits, why haven’t these stoves been more widely adopted? Paradoxically, one obstacle is the initial cost to buy a stove, which may be moderate by the standards of developed nations but can amount to a month’s earnings or more for a low-income household. However, each stove is estimated to reduce warming emissions by the equivalent of 5.3 tons of CO₂ per year.

Compensating householders for these reduced emissions based on a reasonable carbon price is one way to pay back the cost of the stove and even provide a small income stream. This kind of bottom-up mechanism that directly rewards individuals for their actions to protect climate may prove to be a key strategy for reducing SLCPs and is the approach taken by Project Surya (www.projectsurya.org). Other pilot programs to promote and fund wider use of clean cookstoves are also underway, under the auspices of organizations such as the Climate and Clean Air Coalition (Box 15.1.1) and the Clean Cooking Alliance (cleancookingalliance.org).

In the transportation sector, a variety of measures are also available to mitigate black carbon emissions. Diesel engines in particular emit much higher levels of black carbon than gasoline engines. However, diesel particle filters are available that can eliminate up to 95% of black carbon particulate emissions from each vehicle. Cities, states, and regions around the world, including Santiago (Chile) and New York City, have implemented regulations requiring the use of diesel particle filters. The State of California requires particulate filters for commercial vehicles such as heavy-duty trucks, and the European Union has required filters on all new diesel engines since 2009.

In addition to capturing diesel particles with filters, reductions in black carbon and other emissions from vehicles can be achieved through many of the same transportation measures discussed in Chapter 14. More fuel-efficient vehicle use, electrification of transportation systems (including use of hydrogen cells in larger trucks), smart transportation systems, and reduction of vehicle miles traveled all reduce total emissions of black carbon as well as CO₂.

Elimination of the most heavily polluting vehicles, mostly older vehicles with poor emission controls, is another effective measure to reduce black carbon emissions from transportation. However, this can be difficult to implement in countries with weak governance and inadequate enforcement systems.

Policies to reduce or ban agricultural waste burning can also contribute to black carbon mitigation. Such policies have already been enacted in the European Union and California. By requiring diesel particle filters and phasing out agricultural waste burning, the State of California succeeded in reducing black carbon concentrations by 50% between 1990 and 2010.

Figure 15.2.3 Nine measures that aim to reduce black carbon emissions. Reproduced with permission from Climate and Clean Air Coalition; data from UNEP and WMO 2011.

Figure 15.2.3 displays some of the measures that are currently available to mitigate black carbon emissions.