A soot particle is born, matures through its life in the flame, and either dies in the flame or escapes death by exiting the flame. The volume fraction of soot particles within a laminar nonpremixed methane-air jet flame is shown in Figure 4. At atmospheric pressure there is relatively low loading throughout the flame, but with increases in the ambient pressure the soot loading increases dramatically. Part of the problem in eliminating soot results from not knowing its origin. Birth for a soot particle is not well understood.
Various theories exist for the nucleation processes that allow soot to be produced.
Fuel-rich parts of the flame generate PAHs and then coagulate into larger structures which condense into clear, tarry substances thought to be nascent soot particles. Over time, surface growth kinetics convert these particles into black, spherical, soot primary particles with diameters of approximately forty nanometers. Newly-formed soot particles are especially hazardous because they have very high PAH loadings. Part of the overall concern about soot as a carcinogen is that PAHs remain attached to the soot particles and are transported into the lungs along with the soot. The environmental conditions a soot particle navigates strongly shape its characteristics.