Thermodynamics of Atmospheric Aerosol Systems

4 THERMODYNAMICS OF ATMOSPHERIC AEROSOL SYSTEMS 10.4.1 The H2S04-H20 System 

The importance of sulfuric acid in aerosol formation in a humid environment has been emphasized in a number of studies (Kiang et al. 1973 Mirabel and Katz 1974  

Jaecker-Voirol and Mirabel 1989 Kulmala and Laaksonen 1990 Laaksonen et a). 1993). It is instructive to study first this simplified binary system before starting the discussion of more complicated atmospheric aerosol systems. 

Thus (ph2so4/ h2so4) — 1 0017, and in this case the Kelvin effect is negligible the increase in water vapor pressure due to the droplet curvature is only 0.166% above that of a flat surface. The particle size growth factor at 90% relative humidity is 2.12. Thus the 1 pm diameter drop at 50% RH will grow to become a drop of (2.12/1.48) (1) = 1.43 pm at 90% RH. 

Particle Size Growth Factor without Kelvin Effect, Dp / Dp0 

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THERMODYNAMICS OF ATMOSPHERIC AEROSOL SYSTEMS 529 9.4.2 The Sulfuric Acid-Ammonia-Water System... 

To calculate gas solubility in natural geochemical systems, basic thermodynamic properties such as the Henry s law constant and, in the case of weak electrolytes the dissociation constant, must be combined with a thermodynamic model of aqueous solution behavior. An analogous approach has been used to predict mineral solubilities in concentrated brines (1). Such systems are also relevant to the atmosphere where very concentrated solutions occur as micrometer sized aerosol particles and droplets, which contain very small amounts of water relative to the surrounding gas phase. The ambient relative humidity (RH) controls solute concentrations in the droplets, which will be very dilute near 1(X)% RH, but become supersaturated with respect to soluble constituents (such as NaCl) below about 75% RH. The chemistry of the aerosol is complicated by the non-ideality inherent in concentrated electrolyte solutions. 

The total aerosol mass during the variation of total ammonia in the system is also shown in Figure 10.17. One would expect that an increase of the availability of NH3, an aerosol precursor, would result in a monotonic increase of the total aerosol mass. This is not the case for at least the ammonia-poor conditions (ammonia/sulfuric acid molar ratio less than 1). The increase of NH3 in this range results in a reduction of the H2S04(aq) and the accompanying water. The overall aerosol mass decreases mainly because of the loss of water, reaching a minimum for an ammonia concentration of 1.8 pg m-3. Further increases of ammonia result in increases of the overall aerosol mass. This nonlinear response of the aerosol mass to changes in the concentration of an aerosol precursor is encountered often in atmospheric aerosol thermodynamics. 

Stelson and Seinfeld (1981) have shown that solution concentrations of 8-26 M can be expected in wetted atmospheric aerosol. At such concentrations the solutions are strongly nonideal, and appropriate thermodynamic activity coefficients are necessary for thermodynamic calculations. Tang (1980), Stelson and Seinfeld (1982a-c), and Stelson et al. (1984) have developed activity coefficient expressions for aqueous systems of nitrate, sulfate, ammonium, nitric acid, and sulfuric acid at concentrations exceeding 1M. 

Reliable prediction of thermodynamic and kinetic properties for chemical processes (e.g., reactions of chlorinated hydrocarbons on surfaces and in aqueous systems, atmospheric oxidation of organic precursors to ozone and aerosols) as well as for designing green chemical manufacturing processes. 

In this section we will discuss the chemical potentials of species in the gas, aqueous, and aerosol phases. In thermodynamics it is convenient to set up model systems to which the behavior of ideal systems approximates under limiting conditions. The important models for atmospheric chemistry are the ideal gas and the ideal solution. We will define these ideal systems using the chemical potentials and then discuss other definitions. 

Values of Henry s law constant k =plc, where p is the partial pressure of the solute in the gas above the solution and c is the concentration of the solute) is a quantity frequently apphed in the thermodynamic description of dilute aqueous solutions, which is used in environmental chemistry and atmospheric physics as a major criterion for describing air-water partitioning of solutes at near ambient conditions. It plays amajor role in evaluating the transport of pollutants between atmosphere and aquatic systems, rainwater and aerosols. The octanol-water partition coefficient is a dimensionless number defined as the ratio of the compound s concentration in a known volume of octan-l-ol (Cq) to its concentration in a known volume of water (c ) after the octan-l-ol and water have reached equihbrium. It has been found to be related to water solubility, soil/sediment absorption coefficients and bioconcentration factors of pollutants for aquatic life. The adsorption coefficient normalised to the organic carbon content of the soil (sediment) is a useful indicator of the binding capacity of... 

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