Sulphuric acid is accepted as being a key element of the nucleation process, most likely due to its low saturation vapour pressure, which makes it less likely to evaporate than more chemicals. Some questions remain, however, as to whether other compounds contribute to nucleation, and to the growth of freshly nucleated particles.
The critical cluster size associated with nucleation is somewhere between 0.8 and 3.0 nm. Some studies have found that growth rates of particles from 3.0 nm to around 20 nm cannot be accounted for using condensation of sulphuric acid alone. Volatile organics compounds could account for some of this additional growth.
Secondary organic aerosol can also form via nucleation in the same way as sulphate aerosol.