Aerosols are not static; they are dynamic systems where particles constantly change.

3.1 Sticking Together: Coagulation

Coagulation is the process where aerosol particles collide and stick together to form larger particles. This reduces the total number of individual particles in the air but keeps the total mass or volume of the particulate matter constant.

This is similar to how small raindrops on a windowpane merge to form larger, heavier drops.

3.2 The Fight for Survival: The Kelvin Effect

The Kelvin effect describes a crucial property of droplets: the vapor pressure over a tiny, curved droplet’s surface is higher than it is over a flat liquid surface. In practical terms, this means a very small droplet requires a much higher level of ambient humidity to avoid evaporating compared to a larger drop. This delicate balance is a critical factor in the formation of clouds and fog, determining whether a microscopic water droplet will grow into a cloud droplet or simply vanish.

3.3 A Push from Heat: Thermophoresis

Thermophoresis is the movement of particles in response to a temperature gradient. When suspended in a gas that is hotter on one side and colder on the other, particles are pushed from the hotter region toward the colder region.

This is one reason why dust tends to collect on colder surfaces, like the characteristically dark streaks seen on a colder wall above a hot radiator, or the film of dust that accumulates on the cold side of a windowpane in winter.

These particle-level interactions scale up to create large, observable phenomena, most notably their effect on how we see the world.