Cloud ice is an important factor in climate and in the atmospheric self-cleansing capacity because it affects precipitation and thus the lifetime of clouds and trace gases. Ice crystals in the atmosphere may form by homogeneous nucleation of cloud droplets and aqueous aerosol particles or by heterogeneous nucleation on surfaces of so-called ice nuclei (IN). Field measurements show that ice formation in cumulus and stratiform clouds begins at temperatures much warmer than those associated with homogeneous ice nucleation in pure water, and the partial glaciation of these clouds is ascribed to heterogeneous ice nucleation. Various insoluble particles such as mineral dust, soot, metallic particles, volcanic ash, or primary biological particles have been suggested as IN. In terms of the efficacy of individual particles, mineral dusts seem to play a predominant role. Mineral dust aerosols, e.g. from deserts, contain mainly the clay minerals kaolinite, montmorillonite, and illite, which have frequently been used as mineral dust surrogates in ice freezing experiments. All of these studies do report heterogeneous ice nucleation; however, there are large uncertainties in the observed freezing temperatures, prevalent modes of nucleation, and the influence of aging and coatings of mineral dust particles.
The objectives of the proposed project are:
(1) To perform experiments with emulsified and bulk suspensions of clay minerals using differential scanning calorimetry, where freezing is observed via the released latent heat. Experiments with emulsified suspensions reflect the IN activity of average clay mineral particles, while freezing in bulk experiments occurs on the best IN present in the solution.
(2) To investigate how surface modifications and coatings influence the IN activity of the clay minerals. This part of the project aims at elucidating, which surfaces of the clay minerals are involved in the freezing process and how aging may influence the IN ability of airborne clay minerals.
(3) To observe crystal growth and the preferred location of nucleation of aqueous droplets deposited on hydrophobically coated slides with a high-speed camera attached to an optical microscope. This part of the project aims at resolving whether ice nucleation occurs preferentially surface close or in the volume of the droplet.
We will use the clay minerals kaolinite, montmorillonite, and illite for these investigations as well as natural desert dust samples. The results of this study can be cast into a form that can be used in numerical models of cloud formation and precipitation to parameterize the IN number concentration as a function of temperature and/or ice saturation (from box models to large-scale climate models).