thumb|Emission of soot in the [[exhaust gas of a large diesel truck, without particle filters]]

Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. Soot is considered a hazardous substance with carcinogenic properties. Most broadly, the term includes all the particulate matter produced by this process, including black carbon and residual pyrolysed fuel particles such as coal, cenospheres, charred wood, and petroleum coke classified as cokes or char. It can include polycyclic aromatic hydrocarbons and heavy metals like mercury.

Soot causes various types of cancer and lung disease.

Terminology

Definition

Among scientists, exact definitions for soot vary, depending partly on their field.

Terms like "soot", "carbon black", and "black carbon" are often used to mean the same thing, even in the scientific literature, but other scientists have stated this is incorrect and that they refer to chemically and physically distinct things.

Carbon black is a term for powdery carbonaceous matter whose industrial production has been underway since the 19th century. Carbon black is composed almost entirely of elemental carbon. Carbon black is not found in regular soot—only in the special soot that is intentionally produced for its manufacture, mostly from specialised oil furnaces. Unlike carbon black, black carbon is produced unintentionally. The chemical composition of black carbon is much more varied, and typically has a much lower proportion of elemental carbon, compared with carbon black.

Sources

Soot as an airborne contaminant in the environment has many different sources, all of which are results of some form of pyrolysis. They include soot from coal burning, internal-combustion engines, and defective furnaces. Soot in very low concentrations is capable of darkening surfaces or making particle agglomerates, such as those from ventilation systems, appear black. Soot is the primary cause of "ghosting", the discoloration of walls and ceilings or walls and flooring where they meet. It is generally responsible for the discoloration of the walls above baseboard electric heating units.

The formation and properties of soot depend strongly on the fuel composition, but may also be influenced by flame temperature. Regarding fuel composition, the rank ordering of sooting tendency of fuel components is: naphthalenes → benzenes → aliphatics. However, the order of sooting tendencies of the aliphatics (alkanes, alkenes, and alkynes) varies dramatically depending on the flame type. The difference between the sooting tendencies of aliphatics and aromatics is thought to result mainly from the different routes of formation. Aliphatics appear to first form acetylene and polyacetylenes, which is a slow process; aromatics can form soot both by this route and also by a more direct pathway involving ring condensation or polymerization reactions building on the existing aromatic structure.

Description

The Intergovernmental Panel on Climate Change (IPCC) adopted the description of soot particles given in the glossary of Charlson and Heintzenberg (1995), "Particles formed during the quenching of gases at the outer edge of flames of organic vapours, consisting predominantly of carbon, with lesser amounts of oxygen and hydrogen present as carboxyl and phenolic groups and exhibiting an imperfect graphitic structure".

Formation of soot is a complex process, an evolution of matter in which a number of molecules undergo many chemical and physical reactions within a few milliseconds. The PAHs in soot are known mutagens and are classified as a "known human carcinogen" by the International Agency for Research on Cancer (IARC). Soot forms during incomplete combustion from precursor molecules such as acetylene. It consists of agglomerated nanoparticles with diameters between 6 and 30 nm. The soot particles can be mixed with metal oxides and with minerals and can be coated with sulfuric acid.

Soot formation mechanism

Many details of soot formation chemistry remain unanswered and controversial, but there have been a few agreements:

Among these diesel emission components, particulate matter has been a serious concern for human health due to its direct and broad impact on the respiratory organs. In earlier times, health professionals associated PM<sub>10</sub> (diameter < 10&nbsp;μm) with chronic lung disease, lung cancer, influenza, asthma, and increased mortality rate. However, recent scientific studies suggest that these correlations be more closely linked with fine particles (PM<sub>2.5</sub>) and ultra-fine particles (PM<sub>0.1</sub>).

Diesel exhaust (DE) gas is a major contributor to combustion-derived particulate-matter air pollution. This serves as a plausible mechanistic link between the previously described association between particulate-matter air pollution and increased cardiovascular morbidity and mortality.

Soot also tends to form in chimneys in domestic houses possessing one or more fireplaces. If a large deposit collects in one, it can ignite and create a chimney fire. Regular cleaning by a chimney sweep should eliminate the problem.

Soot modeling

Soot mechanisms are difficult to model mathematically because of the large number of primary components of diesel fuel, complex combustion mechanisms, and the heterogeneous interactions during soot formation.

See also

  • Activated carbon
  • Atmospheric particulate matter
  • Bistre
  • Black carbon
  • Carbon black
  • Coal
  • Colorant
  • Creosote
  • Diesel particulate matter
  • Dust
  • Fullerene
  • Health effects of coal ash
  • Health effects of wood smoke
  • Indian ink
  • Joss paper
  • Open burning of waste
  • Rolling coal
  • Soot blower
  • Spheroidal carbonaceous particles
  • Sulfur dioxide

References