How does cosmic rays influence aerosol growth? I have read about aerosol formation increasing due to ionisation but unable to understand what actually goes into the process? Can someone please explain how cosmic rays help in aerosol formation? From what i have read, it said that they help in forming clusters but how can cosmic rays help in forming clusters shouldn't they contribute more to the disintegration of clusters because of the immense energy they carry?
 A: For readability I'll talk about the formation of water droplets in air, but the fundamental points will hold for other systems as well (e.g. a bubble chamber where a superheated liquid evaporates along ionization trails).
For droplets to form, the air needs to be supersaturated in water. That is the humidity needs to be above 100%. Even with a humudity above 100%, no droplets will form in the homogeneous phase (even though a separate water and air phase is the equlibrium state).
The reason for that is a kinetic barrier due to the boundary energy between a water droplet and the air (that can be measured as surface tension). If you want to create a phase boundary, this takes a certain energy proportional to the area $A$ of the boundary, at the same time you only gain energy proportional to the volume $V$ of the droplet from condensation – therefore the formation of small droplets is energetically unfavourable (as $A/V \propto \frac 1 r \to \infty$ as $r \to 0$, where $r$ is the radius of the droplet). There will be a break even point in the radius beyond which further condensation is energetically favourable and condensation will progress.
This boundary can be overcome if there are so called nucleation sites. (Further Reading: Wikipedia on nucleation).
Nucleation sites can arise statistically due to fluctuations, but this becomes only relevant once the supersaturation is strong enough. Otherwise, fluctuations that create a sufficiently large droplet are simply too unlikely.
Typical nucleation sites for cloud formation are dust particles. If they are hydrophilic they modify the surface energy argument, by providing extra energy when water clings to them and if their radius is already larger than the limiting radius where further condensation is favourable, then the condensation will proceed and a droplet forms.
Now to finally answer the Question: Ionizing radiation will create nucleation sites, by inducing local fluctuations.

Regarding the disruption of clusters, there are several things to consider:

*

*Typically the volume of aerosol compared to the volume of the surrounding medium is negligible – so it is unlikely the high energy particles will actually hit the droplets, but they will rather have an effect on the surrounding medium.


*The particles won't give off their energy all at once, but along a trail. (For $\alpha$ particles this is typically a few centimetres in air, a lot more for $\beta^\pm$ particles, $\gamma$ rays don't create nucleation trails as they are not charged and thus interact much more weakly with the medium).


*Finally, it is instructive to compute the maximal size of droplet that can be evaporated by a high energy particle. We'll ignore the boundary energy and just look at the evaporation enthalpy (we are just interested in the rough scale).
Our example will be an $\alpha$-particle emitted by ${}^{210}\text{Po}$ which has a kinetic energy of $E = 5.4\,\text{MeV}$.
Evaporating a droplet of radius $r$ takes
$$ Q = q_E \rho \frac 4 3 \pi r^3 $$
Where $q_E$ is the enthalpy of evaporation, at $10\,\text{°C}$ it is about $q_E = 2500\,\frac{\text{kJ}}{\text{kg}}$, and $\rho = 1000 \frac{\text{kg}}{\text{m}^3}$ is the density of water.
With those numbers we get (from setting $Q = E$):
$$ r = \sqrt[3]{\frac{3E}{4\pi q_E \rho}} \approx 4 \cdot 10^{-8}\,\text{m} $$
That is $40\,\text{nm}$, while typical aerosol droplets have a size of several $\text{µm}$.
