Particulate matter
Air quality PM airborne particulate matter particulates
Particulate matter consists of small particles that remain suspended in air. PM10 and PM2.5 describe size groups; the finer PM2.5 fraction is included within PM10.
Small particles in different size groups
PM stands for Particulate Matter. In simplified terms, PM10 includes particles with aerodynamic diameters up to about 10 micrometres, and PM2.5 the subset up to about 2.5 micrometres. “Aerodynamic” accounts for their behaviour in air, not just their physical width.
What the concentration means
µg/m³ means micrograms of particle mass per cubic metre of air. A microgram is one millionth of a gram. The value measures mass, not particle number. Do not add PM10 and PM2.5: that would count the smaller particles twice.
Where the particles come from
- Directly from combustion, wear, resuspended dust or natural sources.
- Indirectly from gases reacting to form particles, including ammonia and nitrogen or sulphur compounds.
How weather changes the picture
Wind can dilute particles or bring them in; precipitation removes some. An inversion often restricts mixing. Looking at the sky still cannot reliably tell you the pollution level.
Fine particles can penetrate deep into the airways. Interpretation requires the measuring location, averaging period and particle group. A regional model forecast has limited ability to represent one heavily trafficked street.