The Complexity and Cloud Formation Potential of Atmospheric Mineral Dust (Invited Presentation)

Recent field studies have shown that mineral dust is abundant throughout the depth of the troposphere despite its large average size and surface source. Both laboratory and field studies also show that mineral dust can act as an effective cloud droplet and ice crystal nucleus. In the case of droplets, this is due to the large surface area and presence of soluble surface species. In the case of ice crystals, the reason for the high nucleation potential is more complicated. It is believed that a combination of the insoluble nature of the mineral dust, the presence of ‘active sites' (areas that have ice-like structure), and a large surface area (thereby increasing the probability of having an effective site for nucleation) are all factors in the efficacy. Despite the simple name, ‘atmospheric mineral dust' includes an infinite combination of mineralogies, surface morphologies, and coating states. Moreover, the location from which the mineral dust is emitted has an impact on the composition. Desert regions are generally believed to more readily emit ‘bare' particles whereas fertile regions, although less prone to emission of dust, result in internally mixed particles with e.g. organic components. The coating state, due either to material picked while on the surface of from gas-phase uptake when airborne, is a particularly important issue when considering the potential of mineral dust as a nucleus for cloud formation. A soluble surface coating, such as sulfates and nitrates, will enhance the droplet formation potential versus an identical insoluble mineral dust particle. Conversely, soluble surface coatings have been shown to ‘deactivate' potent ice nuclei, apparently by creating a barrier between gas phase water molecules and the active sites on the surface. The purpose of this presentation is to review the atmospheric importance of mineral dust, with a focus on cloud formation potential. A set of recent laboratory experiments have determined the extent of coatings on mineral dust – both as a fraction of particles that have coatings as well as the thickness of coatings when they are present. Finally, we compare these laboratory findings to field data and report the effect of these coatings on cloud formation with an emphasis placed on the ice nucleation process.