Conservation of Historic Stone Buildings and Monuments (1982) / Chapter Skim
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Wet and Dry Surface Deposition of Air Pollutants and Their Modeling
Wet and Dry Surface Deposition of Air Pollutants and Their Modeling
Pages 183-196
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From page 183... ...
Small, submicron particles have difficulty penetrating the quasilaminar air layer adjacent to smooth surfaces, but once they contact the surface they are efficiently retained by van der Waals forces. All particles are susceptible to electrostatic forces that will encourage deposition if either the particles or the receptor surfaces carry an electrical charge.
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From page 184... ...
This work combines theoretical and laboratory research with field investigations of pollutant fluxes to provide a comprehensive understanding of the processes that determine the dry fluxes of many trace gases and small particles to uniform, natural surfaces. In the present context of deposition to stonework, the recent ecologically oriented work allows us to reconsider some of the formulations developed in earlier chemical engineering studies of the deposition to flat plates and to the surfaces of pipes.
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From page 185... ...
Once a pollutant particle or molecule contacts the surface, it is not necessarily captured (although van der Waals forces are usually considered sufficient to capture particles) .8 Thus, there is a surface resistance that quantifies the absorption of trace gases or the retention of particles at the surface.
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From page 186... ...
The similanty between deposition to flat smooth surfaces in con10-3 m ·~` 10-4 \ ~ O ~0 \ O 10-5~ 1 1 1 1 1 1 111 1 1 1 1 1o2 o 103 104 Sc 105 FIGURE 1 Variation of the surface boundary layer property B (see equation 1J with Schmidt number for transfer to smooth flat surfaces {after Lewellen and Sheng, 19801. Data are derived from Harriot and Hamilton {1965; open circlesI, Hubbard and Lightfoot {1966; triangles)
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From page 187... ...
Moller and Schumann, for example, find close to a SC-2/3 dependence in the case of small-particle transfer to water surfaces in wind tunnels.9 Figure 2 presents a familiar set of wind tunnel observations of the deposition velocity Ivy = F/C, where F is the flux density and C is the airborne concentration of the pollutant) to horizontal, flat surfaces, as a function of particle size.
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From page 188... ...
The remaining data refer to water surfaces, for friction velocities of 40 cm/s {Moller and Schumann, 1970; inverted triangles J and 11, 44, and 117 cm/s {Sehmel and Sutter, 1974; triangles, squares and circlesJ.
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From page 189... ...
The depth of this layer is usually assumed to be determined by viscosity and the friction velocity; ciassical studies of flow over sand in wind tunnels indicate layer thicknesses of the order of 50 Am in moderate velocities twind speed of a few m sod. Surface roughness elements of this characteristic size should therefore be suspected as sites for preferred deposition, especially if they are sharp and irregular.
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From page 190... ...
Such strong evaporation rates are not infrequent, but wet stone surfaces will be rapidly dried and the effect cannot persist for Tong periods. Stefan flow affects particles and gases alike; however, there are related mechanisms that act primarily on particles.
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From page 191... ...
A similar phoretic force is exerted on particles by water molecules Effusing past them. Whereas the Steen velocity considered above was a consequence of a mean displacement of the gas by evaporating water molecules, ~ffusiophoresis is a result of a net flux of water molecules past the particle in question and of a flux of heavier air molecules to replace them.
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From page 192... ...
The Urbana data show evidence that midwestern rural precipitation chemistry is somewhat different from that farther east; while sulfate and nitrate concentrations appear much the same, hydrogen ion concentrations seem to be substantially lower. The Beaverton, Oregon, data extend this trend to the West Coast, where the precipitation is cleaner in all aspects.
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From page 193... ...
The Urbana data are thought to be similarly affected, but clearly to a lesser extent. The MAPaS data provide excellent evidence that the Tog-normal frequency distribution usually associated with the precipitation process itself also provides a good representation of the chemical concentration data.
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From page 194... ...
Furthermore, it is evident that more detail is required than is obtained in most pollution monitoring programs, since the deposition processes vary with the time of day. It would be difficult to interpret daily averaged concentration data, whereas averaged daily cycles and frequency distributions would be quite informative.
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From page 195... ...
Both dry and wet fluxes will be greatest when air concentrations of pollutants are high. Although the relationship between air concentrations snot the chemical composition of precipitation is exceedingly complicated, rates of deposition by dry mechanisms are intimately related to air quality in the immediate vicinity of receptor surfaces.
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From page 196... ...
1974. Particle deposition rates on a water surface as a function of particle diameter and air velocity, I
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