In-cloud processes

The composition of liquid-water clouds and processes responsible for this composition are of obvious current interest in conjunction with the so-called acid precipitation phenomenon since clouds constitute the immediate precursor of precipitation. Additionally, cloud composition is of interest because impaction of cloud droplets on surfaces may directly deliver dissolved substances onto natural or artificial materials. In-cloud processes also influence clear-air composition since dissolved substances resulting from such reactions are released into clear air as gases or aerosol particles upon cloud evaporation. It is thus desired to gain enhanced description of the composition of clouds and the mecha-... 

In-cloud processes are a second major class of chemical transformations of aerosol particles (cf. Section 4.04.7.3). Clouds are technically aerosols, a special class in which the particles consist mainly of liquid or solid water and the gas phase generally exhibits slight supersaturation with respect to the condensed phase, i.e., an RH slightly greater than 100%. Clouds form in the atmosphere mainly as a consequence of air parcels being cooled below the dew-point of water, the temperature at which water vapor in a given air parcel is saturated with respect to the liquid. Generally as an air parcel rises to lower pressure. 

Finally, HC1 is soluble in water, and can therefore be removed from the atmosphere in cloud processes. 

Vapor cloud explosions. Explosions which occur in the open air are vapor cloud explosions. A vapor cloud explosion is one of the most serious hazards in the process industries. Although a large toxic release may have a greater disaster potential, vapor cloud explosions tend to occur more frequently. Most vapor cloud explosions have been the result of leaks of flashing flammable liquids. 

Oxidation of sulfur dioxide in aqueous solution, as in clouds, can be catalyzed synergistically by iron and manganese (225). Ammonia can be used to scmb sulfur dioxide from gas streams in the presence of air. The product is largely ammonium sulfate formed by oxidation in the absence of any catalyst (226). The oxidation of SO2 catalyzed by nitrogen oxides was important in the eady processes for manufacture of sulfuric acid (qv). Sulfur dioxide reacts with chlorine or bromine forming sulfuryl chloride or bromide [507-16 ]. 

Davenport, J. A. 1983. A Study of Vapor Cloud Incidents—An Update. Fourth Interna-tional Symposium on Loss Prevention and Safety in the Process Industries. European Federation of Chemical Engineering, Sept. 1983, Harrogate, England. 

Factory Mutual Research Corporation. 1990. Guidelines for the estimation of property damage bom outdoor vapor cloud explosions in chemical processing facilities. Technical Report, March 1990. 

The major mechanism of a vapor cloud explosion, the feedback in the interaction of combustion, flow, and turbulence, can be readily found in this mathematical model. The combustion rate, which is primarily determined by the turbulence properties, is a source term in the conservation equation for the fuel-mass fraction. The attendant energy release results in a distribution of internal energy which is described by the equation for conservation of energy. This internal energy distribution is translated into a pressure field which drives the flow field through momentum equations. The flow field acts as source term in the turbulence model, which results in a turbulent-flow structure. Finally, the turbulence properties, together with the composition, determine the rate of combustion. This completes the circle, the feedback in the process of turbulent, premixed combustion in gas explosions. The set of equations has been solved with various numerical methods e.g., SIMPLE (Patankar 1980) SOLA-ICE (Cloutman et al. 1976). 

Harrison, A. J., and J. A. Eyre. 1986. Vapor cloud explosions—The effect of obstacles and jet ignition on the combustion of gas clouds, 5th Int. Symp. Proc. Loss Prevention and Safety Promotion in the Process Industries. Cannes, France. 38 1, 38 13. 

Van Wingerden, C. J. M. 1988a. Experimental investigation into the strength of blast waves generated by vapor cloud explosions in congested areas. 6th Int. Symp. Loss Prevention and Safety Promotion in the Process Industries. Oslo, Norway, proceedings. 26 1-16. 

Maurer, B., K. Hess, H. Giesbrecht, and W. Leuckel. 1977. Modeling vapor cloud dispersion and deflagration after bursting of tanks filled with liquefied gas. Second Int. Symp. on Loss Prevention and Safety Promotion in the Process Irui., pp. 305-321. Heidelberg. 

FIGURE 2.14 When a small, highly charged cation is close to a large anion, the electron cloud of the anion is distorted in the process we call polarization. The green sphere represents the shape of anion in the absence of a cation. The gray shadow shows how the shape of the sphere is distorted by the positive charge of the cation. 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

Condensed phase interactions can be divided roughly into two further categories chemical and physical. The latter involves all purely physical processes such as condensation of species of low volatility onto the surfaces of aerosol particles, adsorption, and absorption into liquid cloud and rainwater. Here, the interactions may be quite complex. For example, cloud droplets require a CCN, which in many instances is a particle of sulfate produced from SO2 and gas-particle conversion. If this particle is strongly acidic (as is often the case) HNO3 will not deposit on the aerosol particle rather, it will be dissolved in liquid water in clouds and rain. Thus, even though HNO3 is not very soluble in... 

Adsorption of water on salt crystals plays a key role in many atmospheric and environmental processes. Alkah halides in particular play an important role in the first stages of drop growth in clouds. To understand the atomistic details of the wetting and dissolution processes that take place in these crystals, we apphed SPFM to the smdy of the adsorption of water vapor on single crystal surfaces and the role of surface defects, such as steps. 

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