The formation of aerosol particles from ammonia-water vapour system by homogeneous binary nucleation has been studied. Numerical simulations based on the standard classical nucleation theory, self-consistent correction and the revised (original) classical nucleation theory are performed and they show that significant nucleation can occur in the boundary layer of an evaporating ammonia droplet in a humid atmosphere. The temperature of the ammonia droplet is below its boiling point. If the temperature of humid air is 40-50-degrees-C greater than the droplet temperature, significant droplet formation will occur.