Abstract
Abstract. Dust particles represent a dominant source of particulate matter (by mass) to the atmosphere, and their emission from some source regions has been shown to be transported on regional and hemispherical scales. Dust particles' potential to interact with water vapor in the atmosphere can lead to important radiative impacts on the climate system, both direct and indirect. We have investigated this interaction for several types of dust aerosol, collected from the Southwestern United States and the Saharan region. A continuous flow diffusion chamber was operated to measure the ice nucleation ability of the dust particles in the temperature range of relevance to cirrus and mixed-phase clouds (−65
Highlights
The Saharan desert contributes the largest concentrations of dust to the atmosphere (Swap et al, 1996; D’Alemeida, 1986)
For dust particles to have a significant impact on cold cloud formation, it is necessary that the dust particles initiate the ice phase at lower humidities or warmer temperatures than are required for homogeneous nucleation of the background aerosol
At temperatures warmer than −40 ◦C, the shaded water-breakthrough region is well above water saturated conditions, so that if particles require these conditions for ice nucleation, they are unlikely to contribute to ice formation in the atmosphere
Summary
The Saharan desert contributes the largest concentrations of dust to the atmosphere (Swap et al, 1996; D’Alemeida, 1986). Both areas are frequently influenced by an elevated Saharan dust layer Their observations showed that liquid phase clouds were always required before significant ice crystal concentrations developed, suggesting that the impact of deposition freezing in this temperature regime is limited. At warmer temperatures, Lohmann and Diehl (2006), using their parameterization of heterogeneous ice nucleation, found that dust can have a significant impact on the liquid water path, cloud lifetime, precipitation rate and top of the atmosphere radiation They found significant differences in these properties if the dust was assumed to be composed of kaolinite or montmorillonite, two common mineral types found in atmospheric dust, due to small changes in the activation behavior between these species. Size-selected Arizona Test dust was coated with secondary organic material, and the influence of this coating on the freezing behavior of the dust was determined
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