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Typical size distributions for airborne particles are described and the signi cance of the ultra ne 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 ultra ne particles in the atmosphere include both primary emissions and secondary particles formed through homogeneous nucleation processes within the atmosphere. Examples of measurements of atmospheric ultra ne particles are given, highlighting situations with high concentrations of primary ultra ne particles and also situations where gas-to-particle conversion through homogeneous nucleation gives rise to bursts of new particle production. Finally, the relationship between particle mass and number within the atmosphere at a polluted site is examined.

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