Abstract
Abstract. Mineral dusts are well known to be efficient ice nuclei, where the source of this efficiency has typically been attributed to the presence of clay minerals such as illite and kaolinite. However, the ice nucleating abilities of the more minor mineralogical components have not been as extensively examined. As a result, the deposition ice nucleation abilities of 24 atmospherically relevant mineral samples have been studied, using a continuous flow diffusion chamber at −40.0 ± 0.3 °C and particles size-selected at 200 nm. By focussing on using the same experimental procedure for all experiments, a relative ranking of the ice nucleating abilities of the samples was achieved. In addition, the ice nucleation behaviour of the pure minerals is compared to that of complex mixtures, such as Arizona Test Dust (ATD) and Mojave Desert Dust (MDD), and to lead iodide, which has been previously proposed for cloud seeding. Lead iodide was the most efficient ice nucleus (IN), requiring a critical relative humidity with respect to ice (RHi) of 122.0 ± 2.0% to activate 0.1% of the particles. MDD (RHi) 126.3 ± 3.4%) and ATD (RHi 129.5 ± 5.1%) have lower but comparable activity. From a set of clay minerals (kaolinite, illite, montmorillonite), non-clay minerals (e.g. hematite, magnetite, calcite, cerussite, quartz), and feldspar minerals (orthoclase, plagioclase) present in the atmospheric dusts, it was found that the feldspar minerals (particularly orthoclase) and some clays (particularly kaolinite) were the most efficient ice nuclei. Orthoclase and plagioclase were found to have critical RHi values of 127.1 ± 6.3% and 136.2 ± 1.3%, respectively. The presence of feldspars (specifically orthoclase) may play a significant role in the IN behaviour of mineral dusts despite their lower percentage in composition relative to clay minerals.
Highlights
Ice clouds affect the earth’s energy budget and hydrological cycle, with their impact on climate representing one of the largest uncertainties in the forecasting of future climate (Baker and Peter, 2008; DeMott et al, 2010; Forster et al, 2007; Ramanathan et al, 2001)
It can be seen that more inactive species (e.g. ZnS) have a better defined critical relative humidity which is close to water saturation, whereas the better ice nuclei (IN) have an activation spectrum spread across a wider range of RHi, reflecting a range of active sites on the particles that have differing ice nucleating efficiencies
While the discussion will focus primarily on the mineral components of Mojave Desert Dust (MDD) and Arizona Test Dust (ATD), several other pure minerals are included in Table 1 to make a more complete comparison of minerals that may be present in other types of mineral dusts
Summary
Ice clouds affect the earth’s energy budget and hydrological cycle, with their impact on climate representing one of the largest uncertainties in the forecasting of future climate (Baker and Peter, 2008; DeMott et al, 2010; Forster et al, 2007; Ramanathan et al, 2001). These clouds influence the radiative properties of the earth by trapping its outgoing infrared radiation and reflecting incoming visible solar radiation, having both a warming and cooling effect on the earth (Baker and Peter, 2008). A variety of heterogeneous ice nucleation mechanisms has been identified (Vali, 1985): (i) deposition nucleation, where water vapour deposits directly onto a solid as ice; (ii) condensation freezing, which occurs when liquid water condenses on ice nuclei (IN) to form a liquid droplet at temperatures where it rapidly freezes; (iii) immersion freezing, where an ice nucleus becomes immersed in a liquid droplet within which ice formation eventually occurs; and (iv) contact freezing, in which ice nuclei
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