Particulate matter is defined as anything that can accumulate on a special filter paper through which diesel exhaust passes during a defined test procedure.
Diesel particulate matter consists of hydrocarbons from the fuel and the lubricating oil, coal and inorganic topics such as sulphur and ashes.

A particulate filter must regularly regenerated.
This can be done passively where the filter is regenerated during normal operation or actively where the temperature in the filter is increased in different ways.
For this to work better oxidation temperatures of soot can be reduced by using additives in the fuel or by increasing the proportion nitrogen dioxide in the exhaust gases.
Nitrogen dioxide oxidizes foolish at lower temperature than oxygen and can be formed by oxidation of nitric oxide over a catalyst before the filter.
The particulate filter can also be coated with catalytic material that contributes to the oxidation of nitrogen oxide.

In active regeneration, the temperature in the filter is increased to accelerate the oxidation of soot.
This can be done by additional injection of fuel, either into the cylinder to increase the exhaust gas temperature or through an oxidation catalyst mounted before the filter in the exhaust gas stream.
The oxidation of the fuel over the catalyst leads to an increase in temperature which accelerates soot combustion.
For fuel to be oxidized in the catalyst, the catalyst temperature must exceed about 250°C, which can be a problem in some applications where the exhaust gas temperature is low for a long time.
Other ways to regenerate the filter are by using a burner before the filter or by electrical heating, either of the filter or the exhaust gas.