Particulate Matter

Particulate matter (PM) consists of a complex combination of organic and inorganic substances in the form of extremely small particles and liquid droplets, including uncombusted fuel, acids (such as nitrates and sulfates), dust, and metal fragments, and represents a hazardous by-product of the combustion process. Particulate matter also varies in size, ranging from some particles that can be seen with the naked eye, such as dust and smoke, to particles so tiny that they can be identified only with an electronic microscope.

Particle size is an important factor because it is directly linked to PM's potential for causing health problems. Particles with a diameter smaller than 10 micrometers can have the most serious health effects because they can pass through the throat and nose and enter the lungs. The U.S. Environmental Protection Agency (EPA) classifies PM into two families, according to size: “inhalable coarse particles” and “fine particles.”

[Page 1047]The “inhalable coarse particles” group includes PM with a diameter that ranges between 10 micrometers (μm) and 2.5 μm and is generally referred to as PM10. The “fine particles” group includes PM with a diameter smaller than 2.5 μm and is designed PM2.5. In comparison, a human hair is on average 70 μm in diameter, making it seven times larger than the largest PM10 particle and 30 times larger than the largest PM2.5 particle.

These particles, as mentioned above, can also be made of a wide range of chemicals. The composition is determined by their origin, which brings about a further classification of particles: primary and secondary particles. Primary PM is emitted directly from a source, such as fires, fossil fuel burning, construction sites, etc., or is formed by abrasion. The particles' composition depends on their source; for example, particles originated by the interaction between the wind and rocks or the ground tend to consist of mineral oxides, while particles originated from sea spray are made of mostly magnesium, sulfate, organic compounds, calcium, or potassium, depending on the seawater composition.

Secondary PM is the result of the oxidation of primary gases in the atmosphere, which converts them from a gaseous phase to a subsequent condensed phase. Examples include nitrogen oxides and sulfur dioxides emitted from vehicles or power plants. Secondary particles may have an anthropogenic origin and form from gases derived by the combustion of fossil fuel or biomass burning, or a natural origin, such as from the photo-oxidation of natural volatile organic compounds (VOCs).

Other important constituents of primary and secondary particles can also be organic matter (OM) originated from anthropogenic (human-made) or biogenic sources, and elemental carbon (also known as “black carbon”). The secondary fraction of these two aerosols is a result of combustion products of internal combustion engines and has proven adverse effects on human health.