Droplet activation measurements for dry and wet generated mineral dust aerosol: The significance of soluble material

This study examines the interaction of clay mineral particles and water vapor to determine the conditions required for cloud droplet formation. Droplet formation conditions are investigated for three clay minerals: illite, sodium-rich montmorillonite, and Arizona Test Dust. Using wet and dry particle generation coupled to a differential mobility analyzer (DMA) and cloud condensation nuclei counter, the critical activation of the clay mineral particles as cloud condensation nuclei is characterized. Electron microscopy (EM) is used to determine non-sphericity in particle shape. EM is also necessary to classify particles by their surface area and account for transmission of multiply charged particles by the DMA. Ion chromatography is used to investigate soluble material in wet-generated samples and indicates that wet and dry generation yield different particles. Activation results are analyzed in the context of both κ-Köhler theory and Frenkel, Halsey, and Hill (FHH) adsorption activation theory and comparisons are made to literature values. This study has two main results: 1. κ-Köhler theory is a suitable framework to describe clay mineral nucleation activity despite apparent differences in κ with respect to size. For dry-generated particles the size dependence is a likely an artifact of the shape of the size distribution: there is a sharp drop-off in particle concentration at ~300 nm, and a large fraction of particles classified with a mobility diameter less than ~300 nm are actually multiply charged, resulting in a much lower critical supersaturation than expected. For wet-generated particles, deviation from κ-Köhler theory is likely a result of the dissolution and redistribution of soluble material. κ-Köhler theory adequately describes the activation potential of mineral dust without the additional complexity of FHH theory. 2. Wet-generation is found to be unsuitable for simulating the lofting of fresh dry dust because it drastically change the size-dependent critical supersaturations by fractionating and re-partitioning soluble material.