Black Carbon: A Deadly Air Pollutant

We explain the formation, composition and health effects of black carbon (BC) particles typically emitted during the incomplete combustion of fossil fuels, wood and other forms of biomass. We also look at the effects of these soot-like particulates on clouds as well as glacier melt and ice/snow albedo in the Arctic. BC is a significant component of emissions from ships and of the huge Asian brown cloud that appears over parts of China and the Indian subcontinent.
Forest fire with plumes of smoke, Canada
Photo: © B.C. Wildfire Service

What is Black Carbon?

The term “black carbon” (BC) refers to particles emitted during the incomplete combustion of fossil fuels, wood and other forms of biomass. It is one of the most damaging forms of air pollution for two reasons: (a) It is a major driver of climate change around the world. (b) It has a number of serious impacts on human health. 1

Black carbon is a climate forcing agent. It’s the most heat-absorbent form of particulate matter, able to absorb one million times more solar energy than carbon dioxide (CO2). In fact, it’s the second largest contributor to climate change after CO2.

(Note: BC particles and aerosols can also exert a cooling effect on the ground beneath them. This is well documented in studies of the Asian brown cloud, as well as satellite studies conducted during the Iraqi occupation of Kuwait in 1991. However, research continues.)

Luckily, unlike CO2 gas, which stays active in the atmosphere for centuries even millennia, black carbon – being a particle – stays aloft only for a week or so before it is washed out of the atmosphere by rain or snow.

As well as being a major cause of rising temperatures, black carbon is one of the most deadly forms of airborne pollution because of its extremely small size. A significant proportion of black carbon aerosols are less than 2.5 microns in diameter – 30 times smaller than the width of a human hair. 2 These PM2.5 pollutants can penetrate the deepest regions of the lungs and pass into the bloodstream, damaging the internal organs with fatal results. 3

Previously, medical experts associated PM10 (diameter 10 microns) with serious conditions like chronic lung disease, lung cancer, influenza, asthma, and increased mortality rate. However, evidence now suggests that fine particles (PM2.5) and ultra-fine particles (PM0.1) are the main culprits. 4

Black carbon is one of the most common types of such particles. It’s also worth noting that fossil-fuel combustion and biomass burning causes 85 percent of all breathable airborne particulate pollution – meaning PM2.5. 5

According to the “State of Global Air (2019)” a key report compiled by the Health Effects Institute (HEI), PM2.5 pollution contributed to nearly 3 million deaths in 2017, more than half of these fatalities occurred in India and China. 6

Where Does Black Carbon Come From?

In developed countries, black carbon is typically formed by the incomplete pyrolysis of coal in industrial plants and houses, or petroleum in cars, trucks and ships. The largest single source in developed countries, for example, is diesel fuel emissions in traffic exhaust. Which is why black carbon is a significant contributor to urban smog in warmer months. 7

In developing countries, emissions of black carbon also come from wood-burning or liquid natural gas stoves, as well as the traditional burning of agricultural residues and other biomass waste.

In addition, another growing source of BC emissions are forest fires. For example, the recent Arctic fires which scorched millions of hectares across Alaska, Canada and Siberia and the Australian bushfires which destroyed large areas of Victoria, both released vast amounts of carbon black, soot and other particles.

Commercial deforestation in Brazil, the Philippines and Indonesia (among other areas) also adds to the amount of black carbon released into the atmosphere, which is currently estimated at around 6.6 million tonnes. 8

According to one study, roughly 40 percent of black carbon is emitted from fossil fuels, 40 percent from open biomass burning (including forest fires), and 20 percent from burning biofuels (in stoves). 9

Note: Black carbon is not to be confused with carbon black, which is distinguished by a high EC (elemental carbon) content and well-controlled properties. 10

How is Black Carbon Produced?

As mentioned, BC particles are the result of incomplete combustion of hydrocarbon fuels (e.g. in internal combustion engines) which is a normal occurrence – combustion is rarely if ever complete. Complete combustion would convert all the carbon into carbon dioxide (CO2).

As it is, other gases and aerosol components are also given off during the burning process. These co-emitted gases include: carbon monoxide (CO), volatile organic compounds (VOCs), and nitrogen oxides (NOx) – all of which are precursors of ground level ozone (O3) – as well as other greenhouse gases, such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).

In addition, black carbon can be co-emitted with other gases – such as sulfur dioxide (SO2) or ammonia (NH3) – that react to form secondary particulate matter in the atmosphere. Some of these chemical compounds warm the atmosphere (BC, CO2, CH4, N2O), whereas others cool the atmosphere (SO2, NOx, NH3). Some are relatively long-lived (CO2 and N2O), the others, including black carbon, are short-lived.

As well as gases and black carbon, other particulates are also emitted, such as particles of charred wood, cenospheres, cokes, char, polycyclic aromatic hydrocarbons (PAHs) and heavy metals like mercury, as well as those formed in the atmosphere as a result of complex reactions of compounds like sulfur dioxide and nitrogen oxides. These particles are collectively known as soot.

The Sulfur Cycle
What Are the Environmental Effects of Fossil Fuels?

What’s the Difference Between Soot and Black Carbon?

Soot is the collection of different microscopic particles emitted during the incomplete combustion of hydrocarbons. Black carbon is simply one of the main particles found in that collection.

Soot, notably from coal-fired power-plants and diesel exhaust pollution, accounts for over 25 percent of the total hazardous pollution in the troposphere. 11

Asian Brown Cloud

Black carbon pollution emitted by diesel combustion, and wood fires is believed to be a significant component of the huge Asian brown cloud that materializes over parts of China and the Indian subcontinent, during the dry season, which scientists have linked to the southward shift of the Asian monsoon. The cloud is believed to be responsible for a marked rise in respiratory problems among the people of the region.

Black carbon from diesel engines of ships
Marine diesel engines are a significant source of black carbon emissions. Photo: © Roberto Venturini/ eea.europa.eu

BC Emissions From Ships

Ships discharge an estimated 67,000 tonnes of black carbon into the atmosphere annually. It accounts for more than a fifth of all emissions from ships over a 20-year timescale, which makes BC a major contributor to the sector’s impact on climate warming. Overall, container ships are the biggest culprits, accounting for more than one quarter of emissions. But cruise ships also account for a disproportionate share, averaging 10 tonnes of black carbon per ship in 2015, almost three times more than the average container ship. 12

Following the publication of a recent study which revealed that most black carbon present in the Arctic is from fossil fuel combustion, the International Maritime Organization (IMO) agreed that ships can make use of cleaner energies like distillate fuels, which cut BC emissions by one third compared to heavy fuel oil (HFO), or liquefied natural gas (LNG), which emits nearly no black carbon at all. Less common fuels, like methanol or biodiesel could cut BC emissions from an individual ship by as much as 75 percent.

Alternatively, the IMO suggested ships use diesel particulate filters (DPFs) or electrostatic precipitators (ESPs), which can reduce BC by a whopping 90 percent.

BC in the Soil

As much as 60 percent of the entire organic carbon reservoir found in soils is contributed by black carbon. 13 In the tropics, BC also serves as a reservoir for nutrients. Indeed, tests have shown that soils without high amounts of BC are significantly less fertile than soils with BC.

A good example of how BC contributes to the efficiency of the global carbon cycle, is the “black soil” (terra preta) of the Amazon basin, which is famous for its high concentrations of charcoal residues. These were added to the earth by the slash-and-char agricultural practices of pre-Columbian native populations.

These black soils contain three times more organic carbon, which helps to retain other nutrients at the same time. The carbon actually remains in the soil for thousands of years, giving terra preta a far better nutrient retention capacity than the surrounding soils. 14 See also: Why is Soil So Important to the Planet?

A change in agricultural methods may also reduce global warming. One study has shown that switching to slash-and-char from slash-and-burn methods, could reduce carbon emissions from land use change by a full 12 percent. 15

How Does Black Carbon Affect Climate Change?

Black carbon has a direct effect on global warming, due to the fact that it absorbs incoming sunlight in the atmosphere and turns it into heat. As stated above, it absorbs around a million times more solar radiation than carbon dioxide. That said, the warming process involved is not fully understood, and almost certainly involves a contribution from other particles. 16

Do Black Carbon Particles Exert a Warming or Cooling Effect?

It seems that black carbon particles have a warming and a cooling effect. On the one hand, they absorb solar radiation from the sun and infrared radiation rising from the surface of the Earth. This warms the atmosphere.

On the other hand, by blocking and absorbing solar radiation (sunlight), the particles shade the Earth’s surface from the sun’s heat, which has a cooling effect. The sunlight-blocking power of black carbon was illustrated by a satellite study in 1991, during the burning of oil fields in Kuwait. The study revealed that surface temperatures beneath the burning oil fires were as much as 10 degrees Celsius cooler than temperatures of other local areas under clear skies. 17

Another study shows that despite the known heat-warming effects of black carbon particles, simply eliminating all BC from the atmosphere is unlikely to bring about the huge cooling effect intended. 18 This is because when black carbon is created from incomplete combustion of certain hydrocarbons, other aerosols (e.g. from sulfur dioxide and nitrogen dioxide) are created that have cooling effects, due to the atmospheric haze they create.

The hydrocarbons that emit higher levels of cooling aerosols include those contained in biomass and biofuels. In contrast, coal, diesel and other fossil fuels tend to emit significantly more black carbon but fewer cooling aerosols. The focus, therefore, should be on reducing BC emissions from fossil fuels.

Clouds

Black carbon also affects the reflectivity and duration of clouds, which affects both heat and rainfall in the atmosphere. The exact effects vary according to several factors including how much black carbon is in the air and at what altitude it is.

BC suspended inside clouds tends to make them evaporate, and thus unable to reflect sunlight (warming effect). BC above low-lying clouds has a completely different effect, at least over the oceans. It seems to stabilize the layer of air on top of the clouds, causing them to grow and last longer (cooling effect).

Note, however, that this impact has been observed over the ocean but not over dry land, where the presence of BC above low-level cloud appears to inhibit cloud development. 19

Albedo and the Arctic

White surfaces (e.g. ice, snow) reflect sunlight back into space. Dark surfaces (e.g. oceans, dark green forests) absorb sunlight. So, when black carbon falls onto ice reducing its whiteness, it reduces the ice’s ability to reflect sunlight which causes a warming effect and speeds up melting. (For more on this, see: What is the Albedo Effect?) Deposition of black carbon is one reason for the rapid melting of glaciers around the globe.

Black carbon found on ice in Alaska
Dark carbon dust found on a glacier in Alaska. These deposits reduce the ability of ice to reflect sunlight back into space. Photo: © Ins1122/Flickr

Black carbon emissions from northern Eurasia, North America, and Asia are a major contributor to the warming of the Arctic and the retreat of sea ice across the region.

One particular problem in the Arctic is gas flaring. Gas flaring is a traditional method for the disposal of gaseous and liquid hydrocarbons through combustion at oil/gas drilling and processing sites, due to a shortage of pipelines etc., as well as to safeguard against the dangers of high-pressure build-ups. Unfortunately, gas flaring releases large amounts of BC pollution, which may account for up to 42 percent of BC deposition. 20

International bodies like The Global Gas Flaring Reduction Partnership (GGFR) and The Arctic Council Expert Group on Black Carbon and Methane are pushing for a reduction in routine gas flaring operations, although such reductions may be outweighed by the overall growth in oil and gas extraction from the Arctic region. Moreover, as Arctic sea ice melt increases, it is bound to lead to more shipping in the region, which means more BC emissions from heavy fuel oil combustion. For example, according to the Committee on the Marine Transportation System, U.S. shipping alone could increase five-fold in Arctic waters by 2025.

Sea Levels

Black carbon deposition in the Arctic Circle, is also having an impact on sea level rise due to the melting of the ice sheets. The main victim is the Greenland Ice Sheet which is losing ice at a record rate.

Black carbon is also having a small impact on Antarctica but only from local BC pollutants. 21 Whether the recent Australian bushfires will change matters, remains to be seen.

Glacier Melt Causes Shortage of Freshwater

Traditional glaciers in the Himalayas and on the high Tibetan Plateau feed the main rivers of China and India and thus play a critical role in the supply of fresh water for crop-irrigation and other purposes. Without this supply of fresh water, ecosystems will be severely damaged, and food production will fall, affecting literally billions of people.

Because of black carbon’s very short life span in the atmosphere, it has an immediate effect on global warming. As a result, any reduction in BC emissions will have an equally immediate impact on climate change. It will also limit the impact of climate feedbacks such as the reduction of albedo. More reasons why a switch to renewable energy can provide immediate benefits.

What Are the Health Effects of Black Carbon?

The health effects of air pollution are responsible for millions of premature deaths each year, raising serious concern among medical professionals around the globe. Air quality in the large manufacturing regions of India and China – where most of the planet’s most polluted cities are found – is especially worrisome.

The most severe health impacts are mainly caused by the inhalation of particulate matter like black carbon. BC happens to be one of the most common and ubiquitous atmospheric pollutants, and is particularly unhealthy because of its tiny size: many times smaller than a grain of table salt.

BC aggravates the respiratory system, causing distress to anyone with asthma or allergenic breathing difficulties. Its microscopic size (PM2.5 or smaller) allows it to penetrate into the deepest recesses of the lungs and from there into the bloodstream, and even into the brain.

PM2.5 like black carbon is closely associated with a number of health impacts, including premature death, from lung disease, bronchitis, pneumonia, chronic obstructive pulmonary disease (COPD), heart disease and stroke.

Woman cooking with wood on open fire, India
Open fires burning wood for cooking and heating are still widely used in homes across Asia. The resulting indoor pollution causes millions of premature deaths across the region. Photo: © Engineering for Change/Flickr.

Solutions

As we have seen, sources of black carbon particles in the developed world are different from those in the developing world, so the solutions to the problem will vary, too. However, all solutions must incorporate a reduction in the global consumption of fossil fuels, which is essential to lower our emissions of black carbon.

In the West, for example, the spread of electric vehicles (EVs) and the rapid decarbonization of the energy sector are essential steps to lower the amount of BC in the atmosphere.

In China and India, a reduction in coal-fired power plants is essential. In addition, traditional biofuels – such as wood, charcoal, coal – that are burned in crude stoves or open fires by more than 3 billion people around the world – need to be replaced by cleaner distillate fuels, like kerosene and LPG. Switching to EVs is also important, as studies show that highway proximity worsens the inhalation of black carbon. 22

In the Arctic, the banning of routine gas flaring in the Siberian, Norwegian, Alaskan and Canadian oil & gas sectors, is equally necessary, while the shipping sector needs to start moving away from heavy fuel oils towards low-emission fuels.

References

  1. “Black Carbon Briefing Report.” Climate and Clean Air Coalition Scientific Advisory Panel 2018 Annual Science Update. []
  2. “Particulate Matter (PM) Basics.” []
  3. “Black Carbon Research.” []
  4. Omidvarborna; et al. (2015). “Recent studies on soot modeling for diesel combustion”. Renewable and Sustainable Energy Reviews. 48: 635–647. []
  5. “Pollution from Fossil-Fuel Combustion is the Leading Environmental Threat to Global Pediatric Health and Equity: Solutions Exist.” Frederica Perera. Int J Environ Res Public Health. 2018 Jan; 15(1): 16. []
  6. “State of Global Air (2019)” []
  7. See for instance: “Health Impact of PM10, PM2.5 and Black Carbon Exposure Due to Different Source Sectors in Stockholm, Gothenburg and Umea, Sweden.” David Segersson, et al. Int J Environ Res Public Health. 2017 Jul; 14(7): 742. (2017) []
  8. “Climate & Clean Air Coalition to remove short-lived climate pollutants” []
  9. “Global and regional climate changes due to black carbon.” V. Ramanathan and G. Carmichael. 1 Nature Geoscience, March (2008) []
  10. “Carbon black vs. black carbon and other airborne materials containing elemental carbon: Physical and chemical distinctions.” Christopher M.Long, et al. Environmental Pollution Volume 181, October 2013, Pages 271-286. []
  11. Omidvarborna; et al. (2014). “Characterization of particulate matter emitted from transit buses fueled with B20 in idle modes”. Journal of Environmental Chemical Engineering. 2 (4): 2335–2342. []
  12. “Black Carbon Emissions and Fuel Use in Global Shipping, 2015 Report.” International Council on Clean Transportation (ICCT) []
  13. Gonzalez-Perez, et al, 2004 []
  14. “The bright prospect of biochar: Nature Reports Climate Change”. Kleiner, Kurt (2009). Nature.com. 1 (906): 72–74. []
  15. “Bio-Char Sequestration in Terrestrial Ecosystems – A Review.” Lehmann, et al. 11 Mitigation and Adaptation Strategies for Global Change. Pages 407-8. (Springer 2006) []
  16. See, for example: “Radiative absorption enhancements by black carbon controlled by particle-to-particle heterogeneity in composition.” Laura Fierce et al. Proceedings of the National Academy of Sciences, 2020; 201919723 []
  17. The Kuwait oil fires as seen by Landsat.” Robert F. Cahalan. Journal of Geophysical Research: Atmospheres. 97 (D13): 14565. May 30, 1991. []
  18. “Weak global warming mitigation by reducing black carbon emissions.” Takemura, T., Suzuki, K. Sci Rep 9, 4419 (2019). []
  19. “Black carbon absorption effects on cloud cover: Review and synthesis.” Koch, D., and A.D. Del Genio, 2010: Atmos. Chem. Phys., 10, 7685-7696 []
  20. “A Missing Component of Arctic Warming: Black Carbon from Gas Flaring.” Mee-Hyun Cho et al. Environmental Research Letters 14, no. 9 (September, 2019): 094011 []
  21. “Local Emissions and Regional Wildfires Influence Refractory Black Carbon Observations Near Palmer Station, Antarctica.” Alia L. Khan et al. Front. Earth Sci. April 2019. []
  22. “Highway proximity and black carbon from cookstoves as a risk factor for higher blood pressure in rural China.” by Jill Baumgartner, Yuan-Xun Zhang, et al. Proceedings of the National Academy of Sciences (PNAS), Aug. 25. []
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