B. Gorbunov

Ice nucleation on soot particles

Abstract The ice-forming activity of soot particles of various sizes has been studied in a cloud chamber under temperatures ranging from −5 to −20°C. It was found that the fraction of aerosol particles forming ice crystals was influenced by the temperature, the mean radius of aerosol particles and the degree of oxidising of the soot particle surface. It was suggested that oxidising affected the concentration of surface chemical groups that could form hydrogen bonds with water molecules. A decrease in the temperature and an increase in the radius of particles led to an increase in the number of ice crystals. Data obtained were parameterised and an expression was derived that enables the concentration of ice crystals to be calculated for conditions in a low cloud. Based on these experiments, this particular soot is a very potent source of ice nuclei. Data obtained were compared with Fletcher theory. It was shown that the theory contradicts experimental data and cannot be recommended for evaluation of the number of ice crystals in clouds. An application of the data obtained to aircraft condensation trail formation is discussed.

Water nucleation on aerosol particles containing both soluble and insoluble substances

A theory of heterogeneous water nucleation on aerosol particles consisting of soluble and insoluble substances has been developed. The case of partially wettable spherical insoluble core is considered. The theory links interfacial free energies of the surfaces, size of an insoluble core and chemical characteristics of the soluble substances with the ability of the aerosols to form water droplets. It is shown that water nucleation on such aerosols is strongly influenced by insoluble substances. The theory is compared with Kohlers theory and major differences in equilibrium pressure of water vapour and nucleation rate were found. These differences are explained by the influence of the insoluble substances on the free energy of embryo formation.

Water nucleation on aerosol particles containing both organic and soluble inorganic substances

A theory of the formation of water droplets on aerosol particles consisting of soluble inorganic salts and organic (soluble, insoluble and surface active) substances has been developed. The case of partially wettable spherical insoluble core is considered. The theory links the interfacial free energies of the surfaces, the size of the insoluble core, and the chemical characteristics of the soluble substances with the ability of the aerosols to form water droplets. The theory is compared with Kohler theory and major differences in the equilibrium pressure of water vapour and nucleation rate were found. The influence of various types of organic material on CCN activation is considered. It is shown that the presence of both soluble and insoluble organic substances in CCN can affect their ability to form cloud droplets. Surface active substances are found to be the most important components of organic matter that are able to considerably affect CCN activation. A CCN containing ammonium sulphate, sodium chloride or another salt and surface active agent is activated under lower supersaturation than CCN containing only salts. The presence of surface agents in CCN could lead to greater concentration of cloud droplets. Thus, varying the concentration of surface active substances in atmospheric aerosols may alter the number concentration of cloud droplets and the radiative properties of clouds.

Studies of silver iodide ice-forming activity—II. influence of aerosol particle size and structure on the ice-forming activity

Abstract The fraction of silver iodide aerosol particles active at −5 and −10°C has been found not to be a monotonic function of the radius of aerosol particles. To explain this phenomenon, silver iodide aerosols were supposed to be mixtures of particles of two structural modifications. Electron diffraction studies showed silver iodide particles with radii less than 150–200 A to have the hexagonal modification, whereas larger particles to have the cubic low-temperature modification. The anomaly in the dispersity dependence of the ice-forming activity gave the same phase transition radius.

Kinetics of ice nucleation on aerosol particles in supercooled fog—I. The influence of water vapour depletion

Abstract The kinetics of heterogeneous nucleation of ice on aerosol particles has been experimentally studied in supercooled fog. Two cloud chambers of 0.018 and 0.25 m 3 , constructed so as to avoid local supersaturations of hot vapour in cold air, have been employed to study kinetics dependencies. Particles of silver iodide, mass-mean radii of 4–100 nm, and of copper acetyl acetonate, 80–120 nm, have been used as ice-forming nuclei. At the supercooled fog temperatures of 268, 263 and 253 K the ice nucleation constant has been shown to be practically independent on the aerosol particles size. The effect of ice crystal concentrations in fog on the frequency of ice nucleation on particles has been observed.

Water nucleation on aerosol particles containing surface-active agents

A theory of heterogeneous water nucleation on aerosol particles, which consist of soluble and surface-active substances has been developed. A similar approach has been used by Gorbunov and Hamilton (1996). The theory links interfacial free energiy of the surface of an embryo and chemical characteristics of aerosol particles with the ability of the aerosols to form water droplets under tropospheric conditions. The embryo formation free energy ΔG is a basis to calculate the water nucleation rate (Pruppacher and Klett, 1980) and probability of cloud droplet formation. An expression for ΔG was obtained as a function of a number of water molecules Nw in an embryo: Formula omitted (1) Here Rg is the universal gas constant. σeg is the interfacial free energy of the border between the surface of the embryo and gas phase. Sw is supersaturation of water vapour above a flat surface of pure water. Seg is the values of the surface area of the embryo that contains Nw moles of water. aw is the water activity in a solution of the same composition as the embryo (Pruppacher and Klett, 1980). Water nucleation rate and free energies in the case of aerosol particles that contain NaCl and NaOOCC15H31 have been calculated. It was shown that water nucleation on such aerosols is strongly influenced by the surface-active agent, see Figure 1. The presence of a small amount of the surface-active agent (the mass fraction of the surface-active agent in dry particles is about 10−2) considerably reduces the free energy of embryo formation. Calculation of the influence of the surfactant on the nucleation rate has shown a substantial increase in ability of an aerosol particle of cloud droplet production. An aerosol particle that contains a surface active substance needs less NaCl to become a cloud droplet. The presence of about 3% of the surfactant leads to a decrease in mass of NaCl particles by a factor of 4. Thus, atmospheric aerosol particles with surface active substances will produce more cloud droplets than particles without surfactants. Surface active substances that contain both hydrophobic and large hydrophilic groups exist in sea water and can be formed in the result of biomass decay. They can accompany sea salt fraction in atmospheric aerosols. Another source of substances that contain both hydrophobic and large hydrophilic groups is combustion, for example traffic emission or biomass burning. Thus, it is likely to expect the presence of surface active substances in the atmospheric aerosol. Figure omitted