Different heterogeneous routes of the formation of atmospheric ice Anatoli Bogdan Institute of Physical Chemistry, University of Innsbruck Austria and Department of Physics, University of Helsinki Finland

Contents Introduction - ice in the atmosphere Heterogeneous formation of ice - contact freezing - immersion freezing - classical notions of heterogeneous freezing - a new hypotheses of heterogeneous freezing

What are high-altitude cirrus clouds? - sub-visible cirrus (SVC) clouds with optical depth< thin cirrus clouds with optical depth < 0.3

(From Winker and Trepte, GRL, 25, (1998).) SVC cloud in tropical tropopause region at about 17 km

Short information about SVC clouds Location of the SVC clouds: - globally widespread near the tropopause region Temperature range: - between 210 and 185 K Formation: - freezing diluted H 2 SO 4 /H 2 O aerosol droplets Origin of aerosol droplets in the upper troposphere: - in situ gas-to-particles conversion - deposition processes from the lower stratosphere (Minnis, P. et al., Science, 259, (1993); McCormick, M. P., et al., Nature, 373, (1995)) Observed size of aerosol droplets: - depending on background or volcanic conditions diameter can be in the range of about 0.2 – 3 m m (Hofmann & Rosen, Science, 222, (1983). Composition of aerosol droplets: - < 30 – 35 wt % H 2 SO 4

Why are SVC clouds important for the climate? - they reflect and scatter incoming solar radiation (cooling effect) - they trap outgoing terrestrial infrared radiation (warming effect) - they supply surface for heterogeneous loss of ozone that is an important greenhouse gas at high altitudes (cooling effect) Both radiative properties and the rate of ozone loss depend on the microphysics of SVC clouds

Microphysical characteristics of SVC clouds - Small ice water content (IWC) : – g/m 3 - Small effective ice diameter : < 20 – 30 m m - Small ice particle density : < 1 cm -3 - Different shape (habits) of ice crystals

Polar stratospheric clouds (PSCs)

Formation of ice in the atmosphere

Nucleation occurs in the interior of a uniform pure substance (vapor of liquid), by a process called homogeneous nucleation. The creation of a nucleus implies the formation of an interface at the boundaries of a new phase.

Heterogeneous freezing in the atmosphere

Nucleation induced by a foreign insoluble particle (nuclei) is called by hetergogeous nucleation.

Fumed silica (SiO 2 ) is fractal object - SiO 2 is widely encountered in meteoritic smoke particles (MSPs) and combustion origin Fumed silica (SiO 2 ) can be considered as a representative of atmospheric ice nuclei

Using of fumed silica in heterogeneous freezing measurements of aqueous droplets

Weight (g) of H 2 SO 4 in diluted H 2 SO 4 /H 2 O drops as a function of size ( m m ) and composition (10 – 25 wt % H 2 SO 4 ) Weight of H 2 SO 4 is in the range 5* – 1.2* g

Initial thickness of an over-layer of initial composition of 38 wt % H2SO4

Different routes of the formation of ice in the atmosphere

Contact freezing occurs when a super-cooled drop comes in "contact" with an ice nuclei

Immersion freezing occurs when an ice nuclei is inside of a super-cooled drop

When an ice crystal falls through super-cooled droplets then accretion or riming occurs, i.e., super-cooled drops freeze onto ice crystal. The resultant particle is often referred to as graupel

When an ice crystal falls through other ice crystals then aggregation occurs which produce a snowflake

As temperature decreases, ice is formed in a super-cooled water droplet when the pressure of ice becomes less than the pressure of water, i.e., when a super-saturation with respect to ice appears. This brings about a change in Gibbs free energy per unit volume, G v, between the water and ice phase. This change in free energy is balanced by the energy gain to create a new volume (negative change), and the energy cost due to creation of a new interface (positive change). When the overall free energy change, ΔG is negative, nucleation is favored.

If a formed ice nucleus is too small (known as an unstable nucleus or "embryo"), the energy that would be released by forming its volume (negative change) is not enough to create its surface (positive change) then nucleation does not proceed. The formed nucleus should reach some critical size (or radius), in order to be stable and the growth of the ice phase proceeds. In the classic theory, for a spherical ice cluster we cam write for the overall Gibbs energy change where the first term is negative and the second one positive.

The Gibbs free energy needed to form the ice cluster of critical radius can be found from the conditions

The Gibbs free energy change needed to form the ice cluster of critical radius is where G v is a change in free energy per unit volume. and the critical radius

Large supercooling favors the freezing of water. We can relate ΔG to supercooling ΔT = T – T o, where = K, and find r* and ΔG * as a function of ΔT and The larger the supercooling ΔT = T – To, the smaller the critical radius and the less energy needed to form it.

Heterogeneous nucleation Heterogeneous nucleation occurs much more often than homogeneous nucleation. The free energy needed for heterogeneous nucleation is equal to the product of homogeneous nucleation and a function of contact angle : where

The barrier energy needed for heterogeneous nucleation is reduced, and less super-cooling is needed.

In nature freezing usually occurs below the maximum heterogeneous freezing temperature (which is melting temperature of ice, K) but above the homogeneous freezing temperature which is about 233 K. Water in between these two temperatures is said to be in super-cooled state.

A new hypotheses of heterogeneous freezing