Introduction to Aerosol Science

Aerosols are powders, or droplets, suspended in a gas, with a typical particle diameter of about one micrometer. They are important in many fields of science and engineering, and recently it has been proposed that they might be preventing catastrophic global warming.

The atmosphere is filled with huge amounts of particulate matter, from the surface right up to the stratosphere and higher. Being so small, the particles are not usually visible. They can occasionally grow large enough to scatter light noticeably: this is then perceived as 'haze'. Or they can act as a nucleus for the condensation of water to make a relatively large cloud droplet. But even light scattering that is almost imperceptible to our eyes is enough to affect the climate. The reflection of sunlight from aerosol cools the planet to offset the more familiar Greenhouse Effect, the warming due to certain pollutant gases. A more subtle cooling effect is the influence the particles have on the size, density and lifetime of atmospheric clouds. These modified clouds then reflect sunlight differently. The effects are only now being fully understood, in an attempt to build a complete picture of the influence of aerosol particles upon atmospheric physics and chemistry.

Particles can be directly injected into the atmosphere, or can form by condensation from a vapour. Once formed, aerosol particles can collide and stick together, or they can grow by further condensation from the vapour phase. Aerosol particles can interact with thermal, acoustic and optical radiation, gas-phase chemical species (including ions), electric and gravitational fields, can be transported in gas flows and deposited onto surfaces. Many of these aspects of aerosol behaviour can be measured experimentally or modelled. The results then tell us something about the atmosphere, certain industrial processes, or even the behaviour of powders inhaled into the lung.

Aerosol science is a young, interdisciplinary field, with broad participation from physicists, chemists, engineers and biologists. Studies can often address very practical needs, involving both environmental and technological issues, or can focus on very fundamental aspects. It is definitely not just about spray cans!