Nucleation and particle formation in the upper atmosphere

Abstract

In the high latitudes of the summer mesopause, temperatures can be sufficiently cold that the region becomes strongly supersaturated in water at ambient vapor concentrations around a part per million by volume. With supersaturation, water vapor can be removed from the vapor phase to form and grow cloud particles. Meteoric dust and ions are suspected to be the dominant sources of nuclei upon which water can collect. The development of ice particles from cores of meteoric dust involves a process known as heterogeneous nucleation. Particle formation promoted by ionic nuclei is called ion‐induced nucleation. The formalisms for these two competitive mechanisms and their application to conditions of the summer polar mesopause are reviewed. Results suggest that both processes are feasible mechanisms for initiating noctilucent cloud formation. However, because calculated nucleation rates are highly uncertain, the predominance of one or the other nucleation mechanism for noctilucent cloud formation cannot be unambiguously established.