Particulate Matter (DPM), as defined by the EPA regulations
and sampling procedures, is a complex aggregate of solid and liquid material. Its origin is carbonaceous particles generated in the engine
cylinder during combustion. The primary carbon particles form larger agglomerates and combine with several other, both organic and inorganic, components of diesel exhaust. Generally, DPM is divided into three basic fractions (Figure 1):
~ Solids - dry carbon particles, commonly known as soot,
~ SOF - heavy hydrocarbons adsorbed and condensed on the carbon particles, called Soluble Organic Fraction,
~ SO4 - sulfate fraction, hydrated sulfuric acid.
The actual composition of DPM will depend on the particular engine
and its load and speed conditions. "Wet" particulates can contain up to 60% of the hydrocarbon fraction (SOF), while "dry" particulates are comprised mostly of dry carbon. The amount of sulfates is directly related to the sulfur contents of the diesel fuel
Soot’s exact composition is difficult to characterize because different engine technologies and conditions produce different types of soot. Indeed, the smoke clouds coming from diesel engines can even have different colors. For example, blue smoke (mainly oil
and unburnt fuel) can indicate a poorly serviced and/or tuned engine; black smoke (soot, oil
and unburnt fuel) can indicate a mechanical fault with the engine; and white smoke (water droplets and unburnt fuel) is produced when the engine is started from cold and may disappear when the engine warms up.