Sulphuric acid (H2SO4) is thought to be the main chemical trace gas responsible for nucleation in the atmosphere. Formation of pure water aerosol particles or droplets is kinetically hindered by an energy barrier due to the surface tension of water. Sulphuric acid has a low vapour pressure which means it condenses easily (boiling point is 337° C) and can act as a precursor gas for aerosols. In liquid phase it can be mixed in any fraction with water. A single sulphuric acid molecule in gas phase can already bind up to two water molecules by hydrogen bonds. These sulphuric acid/water cluster collide and form larger clusters which eventually grow spontaneously by further condensation.
Sulphuric acid can easily be charged by removing one proton (H+). This leads to an HSO4- ion that can form stable clusters together with a few neutral sulphuric acid and water molecules. Furthermore this leads to a higher collision rate with further uptake of sulphuric acid and water molecules.
In the CLOUD chamber sulphuric acid is produced by oxidation of SO2 with OH radicals, which are produced by reaction of water with singlet oxygen (O(1D)) that is produced by photolysis of ozone (O3).
Recent atmospheric measurements and theories suggest that other species may play an important role in nucleation as well. The influence of these species on nucleation and possible mechanisms are subject of further experiments.