Measurement of number, mass and size distribution of particles in the atmosphere

Typical size distributions for airborne particles are described and the sig nificance of the ultrafine fraction highlighted. Size distributions may be expressed in terms of either mass (volume), surface area or number, and the interpretation of each is discussed together with appropriate measurement methods. The sources of ultrafine particles in the atmosphere include both primary emissions and secondary particles formed through homogeneous nuclea tion processes within the atmosphere. Examples of measurements of atmospher ic ultrafine particles are given, highlighting situations with high concent rations of primary ultrafine particles and also situations where gas-to-par ticle conversion through homogeneous nucleation gives rise to bursts of new particle production. Finally, the relationship between particle mass and n umber within the atmosphere at a polluted site is examined.