I mentioned in a previous post that over half of the aerosols in the atmosphere with the ability to act as cloud condensation nuclei (the seeds on which clouds form) are formed directly in the atmosphere from the gas phase in a process called nucleation. Here I will explain a bit about how that occurs.
Individual molecules collide as they move around in the atmosphere and, under certain conditions, they can stick together in a cluster. This cluster can then either continue to grow, from more molecules or other cluster sticking to it, or it can break up into individual molecules again. Whether it grows or breaks up is determined by the internal energy of system.
Internal energy is the kinetic energy associated with the random motion of molecules and the potential energy of the movement of electrons within the atoms. In this instance it is useful to express this energy in terms of the Gibbs Free Energy, a measurement of the amount of process-initiating work contained in the system. Here is a diagram of how the Gibbs Free Energy of a cluster of molecules varies with the number of molecules in the cluster
You can see that at first the internal energy increases as we add more molecules, but after while adding more particles decreases the internal energy of the cluster. We call this an energy barrier because we have to put energy into the system to get over the peak in the internal energy, but once we do, it is energetically favorable for the cluster keep growing. If this idea of internal energy is confusing, try thinking of it as analogous to pushing a ball up a hill. You have to put effort in to get the ball up to the top of the hill, but once it’s there it can roll down the other side without any further input from you.
Ok, so back to molecules in the air. Individual molecules are randomly colliding, sometimes sticking together and sometimes breaking apart. By chance, some of the might reach the size at the peak Gibbs Free Energy. We call this the critical size. From now on they will tend to grow more than they will tend to break apart. It is from here on in that we call it an aerosol particle.
The rate at which these aerosol particles form depends on the type of molecules that are colliding and the conditions such a temperature and relative humidity. I’ll go into all that in later post, as well a examining what controls how fast aerosols grow once they’ve formed.