Binary homogeneous nucleation

We have seen that in homogeneous-homomolecular nucleation, nucleation does not occur unless the vapor phase is supersaturated with respect to the species. When two or more vapor species are present, neither of which is supersaturated, nucleation can still take place 

In classical homogeneous-homomolecular nucleation theory the rate of nucleation can be written in the form 

FIGURE 11.9 Clusters in binary nucleation. Proceeding vertically up the left side of the figure are clusters of pure water proceeding horizontally along the bottom of the figure are clusters of pure acid molecules. The darker circles denote water molecules the dashed lines indicate the boundary of the cluster. 

The free energy of formation of a nucleus containing nA molecules of species A and nB molecules of species B is given by 

Here psA and pB are the saturation vapor pressures of A and B over a flat surface of pure A and B, respectively. Thus AG can be written as 

We have seen that in homogeneous-homomolecular nucleation, nucleation does not occur unless the vapor phase is supersaturated with respect to the species. When two or more vapor species are present, neither of which is supersaturated, nucleation can still take place as long as the participating vapor species are supersaturated with respect to a liquid solution droplet. Thus heteromolecular nucleation can occur when a mixture of vapors is subsaturated with respect to the pure substances as long as there is supersaturation with respect to a solution of these substances. The theory of homogeneous-heteromolecular nucleation parallels that of homogeneous-homomolecular nucleation extended to include two or more 

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The term binary homogeneous nucleation is used to describe the formation of particles from two different gas-phase compounds such as sulfuric acid and water such nucleation can occur when their individual concentrations are significantly smaller than the saturation concentrations needed for nucleation of the pure compounds. It is believed that in the atmosphere, formation of particles from low-volatility gases occurs not by condensation of a single species but rather by the formation and growth of molecular clusters involving at least two, and as described shortly, probably three or more different species. 

Note that this predicts critical concentrations that are more than an order of magnitude below the experimentally observed concentrations of Wyslouzil et al. (1991) shown in Fig. 9.30 (see Problem 3). For a recent treatment of the binary homogeneous nucleation of H2S04 and H20, see Kulmala et al. (1998). 

The atmospheric situation is complicated by varying conditions of temperature, relative humidity, and concentrations of other gases such as NH3 which can enhance nucleation rates over those expected for a well-mixed air mass at a fixed temperature and RH (e.g., see Nilsson and Kulmala, 1998). However, there is a general consensus that the observed rates of nucleation of H2S04 often, indeed usually, exceed those expected from classical binary homogeneous nucleation theory. (Note that this is not always the case. For example, Pirjola et al. (1998) reported that the measured formation of nuclei in the Arctic boundary layer... 

Binary homogeneous nucleation of H2SO4 and H2O T- and RH-dependent sectional different eqs. in different aero modules... 

Mirabel, P., and J. L. Katz (1974). Binary homogeneous nucleation as a mechanism for the formation of aerosols. J. Chem. Phys. 60, 1138-1144. 

Mirabel and Clavelin (1978) ha ve derived the limiting behavior of binary homogeneous nucleation theory when the concentration of one of the vapor species becomes very small. For a low concentration of one species, say, B, the preexponential factor C simplifies from (11.90) but one has to distinguish two cases ... 

Mirabel and Clavelin (1978) have derived the limiting behavior of binary homogeneous nucleation theory when the concentration of one of the vapor species becomes very small. 

Zeng, X.C., and Oxtoby, D.W. (1991) Binary Homogeneous Nucleation Theory for the gas-liquid transition a nonclassical approach, J. Chem. Phys. 95, 5940. 

Gaman Al, Kuhnala M, Vehkamaki H, Napari I, Mircea M, Facchini MC, Laaksonen A (2004) Binary homogeneous nucleation in water-succinic acid and water-glutaric acid systems. J Chem Phys 120 282... 

In remote marine areas and the free troposphere, H2SO4 is formed by oxidation of SO2 in the presence of hydroxyl (OH) radicals. Ultrafine snlfuric acid particles are produced in presence of water vapor (24,25). This binary homogeneous nucleation process is strongly dependent on factors snch as temperature. 

Easter RC, Peters LK. Binary homogeneous nucleation temperature and relative humidity fluctuations, nonlinearity, and aspects of new particle production in the atmosphere. J Appl Meteorol 1994 33 775-784. 

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