Formation of High-Altitude Ice Clouds: Ice Nucleation or Freezing?

Upper tropospheric (UT) ice cirrus and type II ice PSCs are believed to be formed by freezing aqueous aerosol drops which contain up to 25–30 wt % of H2SO4 and HNO3. There are also reports that in UT aqueous drops, organic component may reach up to 50 wt %. Currently, atmospheric scientific community dealing with high-altitude ice clouds focuses only on the initial step of freezing, namely, on ice nucleation. How freezing proceeds after ice nucleation is not considered, although it is the freezing process itself which governs the phase state and surface properties of resulting cloud particles. Such limited consideration results in the lack of understanding of freezing process and, consequently, is a reason that some important problems of the formation and microphysics of UT cirrus and PSCs and the impact of these clouds on climate remain unsolved for decades. One of the important problems is whether freezing atmospheric aqueous drops produce completely solid ice particles, as is generally believed, or mixed-phased cloud particles in which an ice core is coated with a freeze-concentrated solution (FCS). About freeze-induced phase separation into pure ice and FCS1 and that mixed-phased cloud particles can be formed in UT2-5 and polar stratosphere6 have been reported some time ago. Unfortunately, atmospheric scientists dealing with UT cirrus and PSCs ignore these works that can stem from the fact that they cannot still comprehend and, consequently, accept the well-known fact of freeze-induced phase separation which occurs during the freezing of aqueous solutions, including atmospheric aqueous drops. The goal of this presentation is to present persuasive experimental results, including the visual demonstration of freezing process7, which would convince the atmospheric scientific community of the freeze-induced phase separation (Figure 1) and, consequently, of the formation of mixed-phase cloud particles at very beginning of UT cirrus and type II ice PSC development (Figure 1b). The knowledge of the phase state and surface properties of cloud particles (cloud microphysics) is important because they govern the rate of heterogeneous reactions destructing stratospheric and UT ozone and radiative properties of UT cirrus (absorption, reflection, and scattering of solar and terrestrial radiation). The FCS coating around UT cirrus particles strongly reduces the uptake of water vapor - the dominant greenhouse gas in UT - and, consequently, is responsible for the accumulation and persistence of elevated UT moisture.3,4

References:

1. Bogdan, A. Reversible formation of glassy water in slowly cooling diluted drops. J. Phys. Chem. B, 110, 12205-12206 (2006).

2. Bogdan, A., Molina, M. J., Sassen, K., Kulmala, M. Formation of low-temperature cirrus from H2SO4/H2O aerosol droplets. J. Phys. Chem. A, 110, 12541-12542 (2006).

3. Bogdan, A., Molina, M. J. Why does large relative humidity with respect to ice persist in cirrus ice clouds? J. Phys. Chem. A, 113, 14123-14130 (2009).

4. Bogdan, A., Molina, M. J. Aqueous aerosol may build up an elevated upper tropospheric ice supersaturation and form mixed-phase particles after freezing. J. Phys. Chem. A, 114, 2821-2829 (2010).

5. Bogdan, A., Molina, M. J. et. al. Solution coating around ice particles of incipient cirrus clouds. Proc. Natl. Acad. Sci. USA 11, E2439 (2013).

6. Bogdan, A., Molina, M. J. et. al. Formation of mixed-phase particles during the freezing of polar stratospheric ice clouds. Nature Chem., 2, 197-201 (2010).

7. Bogdan, A., Molina, M. J., et al. Visualization of freezing process in situ upon cooling and warming of aqueous solutions. Sci. Rep. 4, 7414 (2014).