According to Seinfeld , ‘particulate matter’ can be defined as ‘any substance, except of pure water, that exists as a liquid or solid in the atmosphere under normal conditions and is of microscopic or submicroscopic size but larger than molecular dimensions (about 2 Å)’. Based on this general definition, it is clear that particulate matter (PM) is a complex subject, with a variety of particles differing in size, origin, composition and morphology. Their (sub)microscopic sizes render individual atmospheric particles invisible to the naked eye. Though, collectively, particulates can become clearly visible (e.g., smoke, SMOG, haze, …). Apart from PM, different terminology exist referring to atmospheric particles, such as ‘environmental aerosol’, ‘aerosol’, ‘airborne particle’, ‘suspended particles’.
Due to the wide range of properties that has to be specified for the description of atmospheric particles, a strict or unique classification does not exist. However, a common way to categorise PM is according to particle size. Suspended PM usually entails particles with sizes from 0.001 to over 100 µm. Within this range, a coarse (> 2.5 µm) and fine (< 2.5 µm) fraction are generally discriminated, a fundamental distinction that is based on differences in chemical composition, origin, deposition mechanism etc.
Airborne particles of normal density in the micron and submicron size range are collected by using impactors (Harvard or Berner impactors - Figures). By generating an air stream with a certain velocity which is directed along a substrate, particles with well-defined mass and diameter will impact on the substrate, depending on the flow rate.
We use different analytical techniques to gather chemical information on fine and coarse dust fractions collected outdoors and indoors (e.g. workplaces, heritage monuments, museums…). The elemental composition is determined by Energy Dispersive X-ray Fluorescence (EDXRF) analysis whereas; the water soluble fraction is obtained by Ion chromatography (IC). Micro-Raman Spectroscopy is used to collect compositional information and individual particles’ size and shape is identified by Scanning Electron Microscopy (SEM).
 Seinfeld JH (1986) Air Pollutants, Atmospheric Chemistry and Physics of Air Pollution. John Wiley & Sons, New York, pp. 3-48.