A mechanism for the formation of new particles in the atmosphere

The sensitivity of binary nucleation rates of sulfuric acid-water vapours in the atmosphere to changes in relative humidity and temperature has led to the suggestion that mixing processes which change either or both may be the cause of many nucleation events. Here it is pointed out that very sudden mixing events caused by breaking Kelvin-Helmholtz waves occur preferentially in just those regions where the process would be most effective. The necessary conditions for nucleation by this means are strong thermal and moisture stratification, wind shear and the presence of a gravity wave of longer period than the Kelvin-Helmholtz instabilities. Particle production by this process could proceed with lower unperturbed nucleation rates than with any slower mixing process. It would be sporadic in time, limited vertically to little more than the temperature inversion and horizontally to the limited region of turbulence set up by the breaking waves. Thin double layers of newly formed particles separated vertically by a few hundred metres would often result. Others could occur at about the same level at positions determined by the wavelength of the initiating disturbance.