Soot is a fine, dark particulate matter created during the burning of carbon-containing materials like fossil fuels, wood, and biomass. It is a significant contributor to poor air quality both indoors and outdoors. Soot is a complex aerosol mixture suspended in the atmosphere, eventually settling onto surfaces.
The Mechanism of Soot Formation
Soot is formed through a process known as incomplete combustion, which occurs when a carbon-based fuel is burned with an insufficient supply of oxygen. Complete combustion converts hydrocarbon fuels entirely into carbon dioxide and water, but the lack of oxygen forces the fuel molecules to break down differently. This breakdown process, also called pyrolysis, involves the fuel molecules fragmenting at high temperatures.
The fragmented molecules then begin to form larger chemical structures, primarily polycyclic aromatic hydrocarbons (PAHs), which are ring-like organic compounds. These PAHs act as precursors, stacking together in a process called nucleation to form the initial soot particles, which are liquid-like at this stage. These nascent particles grow rapidly through surface reactions and coagulation with other particles, eventually forming the solid, carbonaceous material we recognize as soot.
Primary Sources of Soot Generation
Soot generation is tied to any activity that involves the incomplete burning of organic materials, making it a byproduct of both industrial and household activities. Transportation is a major contributor, particularly from older diesel engines that lack modern filtration systems. The burning of heavy oils and other less refined fossil fuels in industrial boilers and power plants also releases significant amounts of soot.
Residential heating is a major source, especially where wood-burning stoves and fireplaces are used. Common household activities also generate soot, including the use of candles, oil lamps, tobacco smoke, and certain types of cooking. Natural sources, such as wildfires and forest fires, contribute large amounts of soot into the atmosphere.
Physical and Chemical Makeup
The primary solid component of soot is referred to as black carbon, an impure form of elemental carbon. Soot particles are extremely small, falling into the category of fine particulate matter known as PM2.5, meaning they have a diameter of 2.5 micrometers or less.
The composition of soot is a complex mixture that varies based on the original fuel and combustion conditions. The carbon core is typically coated with adsorbed toxic compounds, primarily Polycyclic Aromatic Hydrocarbons (PAHs). These PAHs are known to be mutagens and the mixture can also include trace amounts of metals and sulfur compounds.
Health and Environmental Consequences
The small size of PM2.5 soot particles allows them to penetrate deeply into the lungs and even enter the bloodstream. Exposure to soot is linked to cardiovascular and respiratory problems, including aggravated asthma, chronic bronchitis, strokes, and heart attacks. The PAH compounds adsorbed onto the particles are classified as human carcinogens, increasing the risk of cancer with prolonged exposure.
From an environmental standpoint, the black carbon component of soot is recognized as a short-lived climate forcer with a warming effect. Black carbon is highly effective at absorbing solar radiation, which warms the surrounding atmosphere. When soot settles onto snow and ice, it darkens the surface, reducing its ability to reflect sunlight and accelerating melting. This accelerates the melt of glaciers and snowpacks, contributing to sea-level rise and affecting regional water supplies.