Experimental and Theoretical Studies of Binary Nucleation and Condensation

Citation

Kreidenweis-Dandy, Sonia Maria (1989) Experimental and Theoretical Studies of Binary Nucleation and Condensation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pmy6-3d94. https://resolver.caltech.edu/CaltechETD:etd-02132007-092817

Abstract

Many gas-to-particle conversion processes occurring in the atmosphere (and in technological applications) involve more than one gaseous species. An understanding of how gas-to-particle conversion occurs in multicomponent systems is necessary to predict the evolution of atmospheric aerosols. Of particular interest is the validity of binary nucleation theory in describing particle formation from two interacting vapors.

Chapter II presents a modeling study of heat and mass transfer to aqueous droplets dried under various conditions, and discusses the applicability of common assumptions in describing such processes. A method for the separation, into droplets containing different solutes, of an aerosol composed of two types of aqueous droplets is proposed.

Next, in Chapter III, an experimental study of binary nucleation theory using two similar organics (dibutylphthalate and dioctylphthalate) is presented, and compared with the predictions of an integral model that describes particle formation using binary nucleation theory. It was found that the number concentrations of particles formed in the presence of both vapors was higher than could be attributed to single-component nucleation of either organic, suggesting that binary nucleation was the mechanism for particle formation. Model predictions using the theoretical binary nucleation rates, modified by suitable (species-dependent) enhancement factors, were able to represent the data well.

Attention was next focused on an environmentally-important organosulfur compound, dimethylsulfide, and its oxidation under atmospheric-type conditions. In particular, the aerosol-forming ability of the two major sulfur containing products, methanesulfonic acid and sulfuric acid, was investigated theoretically. Binary nucleation and multicomponent condensation theories were used to predict particle formation and growth in the chemically reacting system at 36% relative humidity, and model predictions were compared with published experimental smog chamber measurements of dimethylsulfide photooxidation. It was found that the experimental results could be well represented by a model that allowed for binary nucleation of aqueous sulfuric acid droplets, and ternary growth of these droplets by condensation of water, methanesulfonic acid, and sulfuric acid vapors. This investigation is presented in Chapter IV.

The calculations presented in Chapter IV are some of the first estimates of particle formation in the methanesulfonic acid/water binary vapor system. In order to assess the validity of binary nucleation theory in describing this particle formation, an experimental program was initiated for the investigation of binary nucleation phenomena in this system. A continuous-flow, mixing-type device was proposed that would yield information not only on the critical saturation ratios required for observable particle formation, but the actual variation of nucleation rate with the gas-phase concentration of each species. The experimental apparatus that was constructed and used for this purpose and a summary and analysis of the experimental results are found in Chapter V. Particle formation was observed at moderate relative humidities and undersaturated acid vapor concentrations, demonstrating that methanesulfonic acid is able to undergo binary nucleation with water vapor. The adequacy of classical binary nucleation theory in predicting the nucleation rates is discussed in detail. The second major goal of the experimental program that was realized was the demonstration of the usefulness of this device in the investigation of binary nucleation phenomena, particularly for corrosive materials, which are difficult to work with in conventional systems.

Because of its successful application to the methanesulfonic acid/water vapor system, this device shows great promise for future applications in the study of binary nucleation phenomena. Suggestions for the modification and improvement of the apparatus that emerged from laboratory experience and from the data analysis are presented in Chapter VI.

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