Airborne particulate matter is complex by origin and composition

Particulate Matter (PM) is an air pollutant consisting of a mixture of solid and liquid particles suspended in the air. These suspended particles vary in size, composition and origin. Particles are often classified by their aerodynamic properties because (a) these properties govern the transport and removal of particles from the air; (b) they also govern their deposition within the respiratory system; and (c) they are associated with the chemical composition and sources of particles.

PM can be either directly emitted into the air (primary PM) or be formed in the atmosphere from gaseous precursors (mainly sulfur dioxide, oxides of nitrogen, ammonia and non-methane volatile compounds). Particle mass concentration is usualy expressed as μg/m3. Mostly, particles are sampled and characterized on the basis of their aerodynamic diameter, usually called particle size. PM can be measured with gravimetric devices (measuring the mass of PM per volume unit sampled air) or with devices using light scattering, oscilation frequency or electron attenuation techniques.

Airborne particles are classified by size and/or sampling method

Different names are being used for types or fractions of particulate matter, defined either by particle size or by sampling method. The most commonly used names include:

• TSP (total suspended particulates), comprising all airborne particles;
• PM10, particles with an aerodynamic diameter of <10 μm;
• PM2.5, particles with an aerodynamic diameter of <2.5 μm;
• coarse particles, comprising particles with an aerodynamic diameter between 2.5 μm and 10 μm;
• ultrafine particles, comprising particles with an aerodynamic diameter of <0.1 μm;
• BS (black smoke), a widely used indicator of the “blackness” of aerosols (and therefore as a surrogate for soot);
• BC (black carbon), which is also used as a surrogate for soot.

Smaller particles have more health effects

In general, smaller particles (PM10 and smaller) and combustion-related particles are more important for health effects than larger and mechanically-formed particles. Because PM2.5 can deeper penetrate the airways than PM10, it is assumed that PM2.5 is the more health-threatening fraction. However, both PM2.5 and PM10 fractions have been associated with health effects. Recent studies suggest that BS and BC are also important health-related components.