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Diesel particulate matter has two main components;

• Dry soot or solid carbon material

Dry soot is mainly the carbonaceous fraction of the particulate and its typical chemical formulae are C₈H, C₉H and C₁₀H. About 5 to 10 % by mass oxygen and 0.5% nitrogen are also present. Typical empirical formula of dry soot would be CH_0.11O_0.065N_0.005. Dry soot results from several processes like pyrolysis, dehydrogenation and condensation of fuel molecules.

• Soluble organic fraction (SOF) that can be extracted by a solvent like dichloromethane.

The soluble organic fraction originates from the fuel and oil hydrocarbons, and hence has H/C ratio ≈ 2, although depending upon engine operating conditions it may vary from 1.25 to 2.0. The hydrocarbons C₁₇ to C₄₀ are present in particulate SOF phase, the C₂₃ – C₂₄ being close to the mean.  Typically, SOF has an empirical formula CH_1.65O_0.1N_0.007. The soluble organic fraction is adsorbed on the solid soot core. The SOF also consists of  partial oxidation products and poly aromatic
hydrocarbons besides hydrocarbons originating from fuel and the lubricating oil.  The mass content of SOF varies significantly depending upon engine design and operating conditions, but mostly it is in the range from 20 to 45 percent.

In addition to SOF, sulphates originating from fuel sulphur, nitrogen dioxide and water are also absorbed on the particle core formed by soot. Other inorganic compounds of iron, silicon (fuel contamination), phosphorous, calcium, zinc (source is oil) etc. are also present in traces in the particulates.

Typical Composition of Diesel Particulate Matter

The content of different constituents of diesel particulate matter can be significantly different for different engine designs. These also vary with the emission control technology employed and the fuel quality particularly the sulphur content. Fuel sulphur content has been drastically reduced as the emission standards are becoming more stringent. The typical particulate composition for a Euro 3 turbocharged, after-cooled diesel engines is shown on Fig 2.21. Considerable reduction in carbon (dry soot) content, fuel and oil derived SOF and sulphates have taken place since early 1990s when PM emission standards were enforced for the first time and made further stringent during the following years. SOF content depends on engine design, operating conditions, lubricating oil consumption and fuel quality.  As regards sulphates, with the current low sulphur fuels (sulphur content  down to 50 to 350 ppm by mass  compared to 0.5 % prior to 1990), sulphate content would be less than 2 percent of total PM mass.