Anthropogenic black carbon, commonly known as soot, is a solid form of mostly pure carbon that results from the incomplete combustion of wood and fossil fuels.1 Black carbon belongs to a class of so-called “super pollutants” that simultaneously contribute to climate change, degrade the health of people and ecosystems, and reduce the productivity of agriculture.
All particle emissions from combustion are considered particulate matter (PM). Black carbon is the solid fraction of PM2.5 (2.5 micrometers in diameter) that strongly absorbs light and converts that energy to heat. When emitted into the atmosphere and deposited on ice or snow, black carbon causes global temperature change, snow and ice melting, and precipitation patterns.2 Black carbon has up to 1500 times the impact on global warming per unit mass compared to carbon dioxide.3
Global emissions of anthropogenic black carbon increased by a factor of 10 from 1750 through the early 2000s. Biomass burning includes residential cooking and heating with wood and dried animal dung, agricultural field burning, and deforestation fires. When biomass is burned in inefficient cookstoves, open fires, or poorly ventilated areas (common in many households in developing regions), it often does not burn completely due to limited oxygen and lower temperatures, resulting in black carbon emissions.
The introduction of fossil fuel combustion, coal in the nineteenth century and oil in the twentieth century accelerated the release of black carbon. Coal and oil are carbon-rich materials, and when burned in inefficient furnaces, engines, and stoves they release black carbon. Diesel and heavy fuel oils used in highway, rail, and ship transportation have especially high emission rates.
Process emissions refer to the black carbon emitted from a specific industrial or chemical process rather than direct fuel combustion. Examples include metal smelting, refining, and processing; cement and lime production; chemical manufacturing; and pulp and paper production.
Before the transition to fossil fuels that began in the mid-19th century, black carbon emissions were highly concentrated in the residential sector where the combustion of solid biomass fuels such as wood, dried animal dung, and agricultural residues were the principal sources of household energy use. These fuels were burned in very inefficient cookstoves and fireplaces.
Countries with the largest economies and largest populations account for most of historical and current black carbon emissions. China, India, and the United States account for nearly one-half of historic black emissions. However individual country emission profiles are very different.
In China, the residential use of fuels such as coal, firewood, dung cakes, and crop residues were the biggest contributor to black carbon emissions in China in recent years, and 95% of those occur in rural areas.4 Residential fuel use in India is also a major source of BC emissions, including large quantities of kerosene fuel used for lighting and cooking.5
In Brazil, about 21% of black carbon emissions in 2022 were from land use and land conversion associated with agriculture and forestry. Another 23% of emissions resulted from the combustion of gasoline and diesel fuels in road transportation.6 In the United States, more than 50% of anthropogenic black carbon emissions in 2022 resulted from the combustion of fossil fuels, especially diesel fuel, in road and rail transportation. Fuel use in the residential and commercial sectors account for about 14% of emissions.
Attention to reducing black carbon emissions has ramped up due to its effect on climate change via warming of the atmosphere and reducing the Earth’s albedo (reflectivity). But public health concerns are equally if not more important. Black carbon particles are among the smallest of all particulate matter particles and therefore can bypass the body’s defenses and become lodged in the lungs, bloodstream, and brain. Health impacts included respiratory and cardiovascular diseases, developmental issues in children, and increased mortality rates. The World Health Organization has identified black carbon and other forms of particulate matter as a public health imperative.7
The key to reducing black carbon emissions in developing nations is to increase access to clean cooking fuels such as electricity and liquified petroleum gas (LPG). About 2.3 billion people use polluting fuels and inefficient technologies for most of their cooking. Cleaner transportation fuels are needed to replace diesel and heavy fuel oil in transportation. These could include electric vehicles or combustion vehicles that run on compressed natural gas (CNG) or clean hydrogen. Improved efficiency of combustion engines and kilns, boilers, and other industrial equipment will reduce emissions, as will the reduced open burning of solid waste and agricultural residues.
1 Incomplete combustion occurs when there isn’t enough oxygen to fully oxidize a fuel. Instead of breaking down entirely into carbon dioxide (CO₂) and water (H₂O), the fuel is only partially burned, leading to the formation of other byproducts, such as carbon monoxide (CO), black carbon (soot), volatile organic compounds (VOCs), and other particulatesLink
2 International Council on Clean Transportation, A policy-relevant summary of black carbon climate science and appropriate emission control strategies, June 2009, Link
3 Climate and Clean Air Coalition, Black Carbon, accessed November 13, 2024, Link
4 Wang, Wenjun, Nina Khanna, Jiang Lin, and Xu Liu. “Black Carbon Emissions and Reduction Potential in China: 2015–2050.” Journal of Environmental Management 329 (March 1, 2023): 117087. https://doi.org/10.1016/j.jenvman.2022.117087.
5 Paliwal, U., Sharma, M., and Burkhart, J. F.: “Monthly and spatially resolved black carbon emission inventory of India: uncertainty analysis, Atmos. Chem. Phys., 16, 12457–12476, 2016, https://doi.org/10.5194/acp-16-12457-2016
6 Hoesly, Rachel, Steven J Smith, Noah Prime, Hamza Ahsan, Harrison Suchyta, Patrick O’Rourke, Monica Crippa, et al. “CEDS V_2024_07_08 Release Emission Data.” Zenodo, July 24, 2024. https://doi.org/10.5281/zenodo.12803197.
7 World Health Organization, “WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide,” 22 September 2021, https://www.who.int/publications/i/item/9789240034228
