Nucleation is indeed a fascinating and essential part of the process by which raindrops form. The particles you mention—dust, pollen, salt, or even tiny aerosols—are collectively known as condensation nuclei. These particles are critical because they provide a surface on which water vapor can condense into liquid droplets.

Why are condensation nuclei essential?

1. Energy Barrier: For water vapor to transition into a liquid state, it must overcome a significant energy barrier. Without a surface to condense on, the vapor molecules would require extremely low temperatures to spontaneously condense in midair, as the surface tension of a droplet forming in pure air is incredibly high.
2. Stability: The presence of a nucleus reduces this barrier, making it energetically favorable for water molecules to aggregate and form stable liquid droplets.

Can raindrops form without particles?

In the absence of these particles, condensation could theoretically occur, but it would require super-saturation—a condition where the air holds much more water vapor than it normally can at a given temperature. This would demand conditions that are far more extreme than what typically exists in Earth’s atmosphere. Even then, the process would be inefficient and rare.

Practical Reality

As you noted, it’s virtually impossible to have an atmosphere completely devoid of particles. Natural sources like soil erosion, ocean spray, and biological processes constantly release particles into the air. Additionally, human activities (like combustion) contribute aerosols.

In essence, every raindrop owes its existence to these tiny particles. Without them, precipitation as we know it would not occur, emphasizing the interconnectedness of Earth’s systems.