Water nucleation on aerosol particles containing surface-active agents

Abstract

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