Mineral Dust and its Microphysical Interactions with Clouds

Abstract

Our understanding of the interactions of aerosols and clouds has a strong heritage in laboratory experiments, field measurements, and process modeling. We present a review on the state of knowledge for mineral dust emitted from major global dust source regions. Laboratory studies and field measurements have given insights on processes and mechanisms taking place when mineral dust is released into the atmosphere and reacts with the atmospheric constituents. Furthermore, theoretical approaches and parameterizations have been established to interpret the observations and quantitatively express the mechanisms by which dust can act as cloud condensation nuclei (CCN) and ice nuclei (IN). Finally, model simulations have been used in order to study the effects of dust particles to different aerosol-cloud-climate interactions. Dust can act as efficient CCN in clouds solely based on their relatively large size combined with the hydrophilicity from the adsorption of water vapor on their insoluble core. When mixed with even small fractions of hygroscopic material from emission or atmospheric processing, their hygroscopicity and CCN activity are significantly enhanced. The theoretical frameworks of adsorption activation and Kohler theory are presented to explain dust CCN activity, together with a summary on the potential contributions of dust to cloud droplet number concentration (CDNC), and its role in regulating supersaturation. Mineral dust aerosol is an effective IN and, combined with their concentration, can dominate ice production in cirrus and mixed-phase clouds even at great distances from source regions. The pathways to nucleation of ice are different for different cloud types and have distinct effects in those clouds. Our fundamental understanding of ice nucleation lags behind that for CCN activation, and a key challenge is that we cannot predict a priori which aerosol materials will make effective IN. Nevertheless, numerous field and laboratory studies have shown that mineral dust from deserts is one of the most important ice-nucleating aerosol types around the globe.