Particulate matter is the name given to solid particles which are emitted from an engine.
Primarily, these are carbon-based and are a by-product of the combustion process, although a small proportion consists of engine debris.
Diesel engines are by far the biggest producers, although in contrast to oxides of nitrogen, production of diesel particulate matter is greatest at low and part-load conditions.
Particulates are often categorized by size, for example PM10’s are all those with a diameter of less than 10 microns, whilst PM2.5’s are all those with a diameter of less than 2.5 microns. In fact, most particulates are smaller than this – typically more than 90% range from a few tens of nanometres to a couple of microns in diameter. The smallest particles tend to have the worst health implications, because of their ability to penetrate deep into lung tissue.
Diesel particulate matter has two distinct components – at the core is a piece of carbon which is surrounded by a layer of adsorbed hydrocarbons.
This hydrocarbon layer is known as the ‘soluble organic fraction’ and may comprise up to 50% of the total by mass. The proportion comprised of the soluble organic fraction is greater when the engine is running under low load conditions.
The main health concern about particulate matter is that after inhalation it can penetrate deep into lung tissue, where it may lead to cancer. It is also implicated in cardiovascular disease.
In the wider environment it contaminates foliage and soils buildings. It can also contaminate products where it is present in the atmosphere in factories and warehouses.