Aerosol physics - Part 1

Invented by the German Schmauss in 1920, the word aerosol designates the suspension, in a gaseous medium (air, in most cases), of liquid or solid particles, or both, with a negligible falling velocity. In air, under normal conditions, this corresponds to particles ranging in size from fractions of a nanometer to 100 µm.

Strictly speaking, an aerosol is a two-phase system formed by particles and the carrier gas. In practice, however, and as we shall see in this article, "aerosol" is often synonymous with "particle".

The science of aerosols, which began in the 1950s, has developed considerably over the last twenty years. Considered to be one of the major atmospheric pollutants involved in global warming and the possible ozone hole, aerosols are involved in many natural phenomena.

These include the atmosphere's radiation balance, their influence on visibility, cloud and precipitation formation, and ocean-atmosphere exchanges. They are also vectors of atmospheric radioactivity.

So, in order to gain a better understanding of atmospheric phenomena and their dynamics, the "gas" model initially used is tending to give way to an "aerosol" model, even if the latter is more complicated.

They are also found in many industrial fields, in the world of cleanrooms, filtration, air purification and air conditioning. They are even more common in cutting-edge industries that use technologies involving some of their physical properties, and whose manufactured products are highly dependent on the transfer of contamination (aerospace, electronics, integrated circuits, food processing, pharmaceuticals, nanotechnologies linked to metallurgy).

They are also taken into account in nuclear reactor safety (Phébus experiment at IRSN Cadarache, for example).

Their effects on health are the subject of numerous investigations and measurements in the case of industrial hygiene. And let's not forget the 15,000 deaths a year and the 500,000 cases of various infections in French hospitals, for which aerosols, poorly controlled, are partly responsible.

In fact, the science of aerosols has now become a "big science", fuelled by substantial financial investment, with consequent economic spin-offs [1]. The field encompasses physics, chemistry and, increasingly in recent years, biology, with what are known as bioaerosols.

But whatever their origin and nature, aerosol behavior is highly dependent on their physical properties. That's why we're going to describe their main characteristics. We won't go into the complicated issue of aerosol shape and size. Readers are referred to reference [2].

First, we'll look...