Apart from cloud and precipitation particles, which have relatively large sizes, the Earth's atmosphere consists a mixture of gases, small solid particles, and small liquid droplets. Mixtures of air with small solid particles and small droplets are called aerosols The small particles and droplets themselves (called aerosol particle, but often loosely referred to as simply aerosols) are important not only in air chemistry but in determining visibility, the formation of cloud particles, atmospheric radiation, and atmospheric electricity. In this chapter we will be concerned primarily with chemical aspects of atmospheric aerosols, but we will start by describing two of their important physical attributes, namely, their concentrations and size distributions.

Aerosol concentrations and size distributions

Total number and mass concentrations

One of the oldest and most convenient techniques (which in various modified forms is still in use) for counting the number concentrations of atmospheric aerosols is the Aitken1 nucleus counter. In this instrument, humid air is expanded rapidly so that it cools and becomes supersaturated by several hundred percent with respect to water. At these high supersaturations, water condenses onto virtually all of the particles in the air to form a cloud of small water droplets in the chamber of the counter. The number concentration of droplets in this cloud (which is close to the total number concentration of aerosol particles) can be determined automatically by optical techniques or by allowing the droplets to settle onto a substrate where they can be counted. The concentration of aerosol particles measured with an Aitken nucleus counter is referred to as the Aitken (or condensation) nucleus count (CN count for short).