Freezing and Microphysics of PSCs and Cold Cirrus Ice Clouds

The upper tropospheric (UT) cold cirrus ice clouds, covering up to 30% of the global sky, are important climate regulators, because they impact on the incoming solar and outgoing terrestrial radiation, and supply surfaces for heterogeneous reactions destructing UT ozone and regulating UT water vapor content, to name only a few. Polar stratospheric clouds (PSCs) play a crucial role in the ozone depletion events during polar winter/spring time. The observations and laboratory studies indicate that PSCs are formed by the freezing of aqueous aerosol containing inorganic H2SO4 and HNO3 species, whereas the UT ice cirrus clouds by the freezing of aqueous aerosol containing both inorganic and organic species. Despite the intensive studies of the past there remain large gaps in the understanding of the formation mechanisms and microphysics of UT cirrus clouds and PSCs. The microphysical parameters such as size, composition, phase state (solid, liquid, or mixed-phased), habit and orientation of cold cloud particles (ice crystals) can be used in general circulation models (GCMs) of Earth`s climate for the estimation of the stratospheric ozone loss, the predictions of its future recovery, and the impact of the UT ice clouds on climate. The output of GCMs is sensitive to the assumptions concerning the microphysical parameters of these clouds. In contrast to macrophysical characteristics such as the vertical and horizontal extensions of clouds, which can relatively easily be gained by satellite and ground-based sensing techniques, the knowledge of cloud microphysics requires in situ observations which are used complicated aircraft instrumentation. However, for the time being, the used instrumentation cannot determine with certainty the composition and phase state of cloud particles. For example, it cannot determine whether young and small cloud particles (~5 - 10 m) are liquid or mixed-phased: an ice core + a residual solution coating. This knowledge is important because the phase state of particles determine the rate of the stratospheric and UT ozone loss and the radiative properties of UT ice cirrus. Therefore, elaborated laboratory measurements, which can time-trace the phase transformations of aqueous aerosol droplets on microphysical level, are needed to fill in the gaps in our understanding of the microphysical properties of PSCs and UT cirrus ice clouds. Such measurements are going to be performed during the accomplishment of this project.

Although freezing phenomenon has been investigated for more than a century, its understanding is far from complete that can stem from the fact that freezing process itself has never been visualized before. The projects most significant result is that we developed of a novel, ground-breaking approach for the in situ visualization of the freezing process, nascent ice/FCS (freeze-concentration solution) morphology and its subsequent change with the aging of frozen solutions. Water, being a ubiquitous natural solvent, rarely exists in pure state but as a component of various aqueous solutions. In contrast, ice is highly intolerant to impurities. Hence, upon freezing aqueous solutions separate into pure ice and FCS. An example of this freeze-induced phase separation and ice/FCS morphology is shown in Figure 1 for bulk and finely dispersed aqueous solutions. Our novel approach can be used for the study of the freezing process and ice/FCS morphology of dispersed and bulk aqueous in/organic solutions relevant for (i) the atmosphere - the formation and microphysical properties of upper tropospheric (UT) cirrus and polar stratospheric clouds (PSCs) and (ii) lyophilization (freeze-drying) in biotechnology, pharmaceutics, food industry, tissue engineering scaffolds etc. Figure 1. Images of frozen bulk 20 wt % citric acid (a) and dispersed 15 wt % H2SO4 (b). The projects results will benefit both atmospheric sciences and the optimization of time- and energy-consuming lyophilization process. For example, the freeze-induced phase separation leads to the formation of mixed-phase UT cirrus and PSC particles (Figure 1b). The inclusion of mixed-phased cloud particles into climate models will allow us to obtain more realistic conclusions about the impact of these clouds on climate, the pace of stratospheric ozone depletion and duration of its future recovery. In pharmaceutics, our novel approach can be used for the determination of formulation critical parameters needed for the optimization of lyophilization. The optimized lyophilization will improve the quality attributes of lyophilized products, for example, drugs containing therapeutic proteins, and reduce pharmaceutical waste and, consequently, reduce the cost of drugs.