Soot consists of particles of black carbon mixed with oxygen and various other chemicals. It is produced industrially for use as a black colorant in rubber tires, inks, and other products; it is also released by the incomplete burning of carbon-containing vapors. Soot is a type of aerosol, aerosols being liquid or solid particles small enough to float, at least for a time, in the air. Aerosols have a complex role in global warming: they may either scatter or absorb solar radiation (sunlight, visible and invisible), and different types may have either a cooling or warming effect on global or local climate. Soot, since it consists of black solid particles, is a strong absorber of sunlight and so has a warming effect on climate. As a form of toxic air pollution, it also harms human health directly.
Historical Background and Scientific Foundations
recognized as causes of global climate change, but in the 1990s and early 2000s, aerosols were increasingly recognized as having a role in global warming. By 2002, some scientists believed that black carbon particles, including soot and charcoal, might be the second-largest contributors to global warming after carbon dioxide.
Soot consists of microscopic particles formed when vapors containing carbon are imperfectly burned. This can happen around the edges of a flame, where temperatures are low, or in diesel or gasoline engines that do not provide the right conditions for complete combustion of fuel (combination of fuel with oxygen). Most atmospheric soot is produced by diesel and gasoline engines.
Soot is often distinguished from charcoal, which comes from the burning of organic matter such as wood rather than from the burning of fossil fuel. Charcoal particles—which, together with soot, form the category of aerosols called “black carbon particles”—are produced in large quantities in China, India, and South Asian countries by the burning of wood and other organic matter for cooking.
Because both soot and charcoal consist of solid particles, they tend to fall out of the atmosphere after a while or be carried to the ground by snow or rain. Nevertheless, small particles of black carbon can be carried thousands of miles, and are contributing significantly to Arctic and glacial melting by subtly darkening snow and ice. Falling snowflakes are efficient collectors of airborne particles, and a single flake may contain scores to thousands of aerosol particles. These particles cause snow to absorb solar energy that would otherwise be reflected back out into space, thus tending to melt the snow and adding to the warming of global climate.
According to James Hansen of the National Aeronautics and Space Administration (NASA) and Larissa Nazarenko of Columbia University, the darkening of snow albedo (reflectivity) by soot, especially in the Northern Hemisphere, may account for a fourth of observed global warming.
Impacts and Issues
Soot impacts human health directly, as a form of air pollution. For example, in 2000, the California Air Resources Board estimated that 70% of California's total cancer risk due to airborne toxins was from diesel soot. In 2004, the government of California estimated that soot caused 3,000 premature deaths in the state—more than the number of deaths due to homicide. Worldwide, about a million deaths a year are caused by airborne particle pollution, mostly by soot. Also, because soot absorbs sunlight, it darkens the ground below: the sky is darkened in India and China so much by charcoal and soot particles that agricultural productivity is reduced by 10–20% compared to what it would be under naturally clear skies.
In 2004, scientists attributed increased flooding in southern China and India and increased drought in the north in recent years to soot's regional affects on climate. In 2007, other scientists made measurements showing that brown clouds of soot over the Indian Ocean absorbed enough solar energy to account for up to half of the climate warming seen in Asia in recent decades, which has been causing glaciers to melt in the Himalayas.
WORDS TO KNOW
AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
BLACK CARBON: A type of aerosol (small, airborne particle) consisting mostly of carbon: includes soot, charcoal, and some other dark organic particles.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
Soot can respond quickly to mitigation efforts because individual soot particles do not stay in the atmosphere long. Asian scientists are urging the use of solar cookers and biogas plants that capture methane from animal waste and landfills to supply as much cooking heat as possible: this would reduce both deaths from pollution and the climate affects of black carbon. In industrialized countries, soot from gasoline engines has been greatly reduced by the use of catalytic converters on automobiles. Stricter soot-emissions standards for diesel trucks and cars could reduce soot emissions significantly.
Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Casey, Michael. “Indian Ocean Haze Adds to Global Warming.” The Washington Post (August 2, 2007). Chameides, William L., and Michael Bergin. “Soot Takes Center Stage.” Science 297 (2002): 2214-2215.
Hansen, James, and Larissa Nazarenko. “Soot Climate Forcing Via Snow and Ice Albedos.” Proceedings of the National Academy of Sciences 101 (2004): 423-428
Law, Kathy S. “Arctic Air Pollution: Origins and Impacts.” Science 315 (2007): 1537-1540.
soot / soŏt/ • n. a black powdery or flaky substance consisting largely of amorphous carbon, produced by the incomplete burning of organic matter. • v. [tr.] cover or clog (something) with soot. PHRASES: (as) black as soot intensely black.