Dry Deposition of Particles

TABLE 19.1 Typical Dry Deposition Velocities for Some Atmospheric Gases 

FIGURE 19.3 Particle dry deposition velocity data for deposition on a water surface in a wind tunnel (Slinn et al., 1978). 

To obtain field data to test and improve models of dry deposition of particles, improved techniques and/or new instrumentation appear to be critical (Nicholson, 1988). Particle-size-specific, eddy-flux measurements (see Section 19.5) of ambient particles appear promising (e.g., Neumann and den Hartog, 1985) but to date have suffered from poor statistics associated with few particles in specific size classes. Furthermore, similar studies at 

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Four processes are considered diffusion (absorption), dissolution in rain of gaseous chemical, and wet and dry deposition of particle-associated chemical. 

The last include wet and dry deposition of particles and solutes and gas exchange across the air-sea interface. Because of proximity to source, coastal waters tend to be more polluted than the open ocean. A notable exception is the worldwide acidification of surface waters caused by CO2 emissions. Of all the coastal waters, estuaries tend to be the most impacted. This is due to high rates of pollutant loading and to natural processes that act to concentrate these pollutants in the local sediments and biota. This is most unfortunate as estuarine waters support the world s largest fisheries and are where recreation is concentrated. 

Table 9.10 shows an estimate (Pandis et al., 1995) of the time scales for coagulation of smaller particles onto larger ones characteristic of various types of air masses. For comparison, typical time scales for condensation, dry deposition of the particles, and transport are also shown. (For discussions of dry deposition of particles, see, for example, Slinn (1982, 1993), Arimoto et al. (1987), and Main and Friedlander (1990)). As expected, condensation is fast, but coagulation is also significant on these time scales in some situations. 

Removal of particles by rainout is far more effective than dry deposition of particles, as illustrated in Example 4-f4. 

The rate of dry deposition of particles depends on the particle size. Particles are transported down to the quasi-laminar layer near the surface by the same mechanisms of turbulent transport as are gases. The rate of transfer across the laminar layer is determined by the particle diffusivity, which depends on particle size. Particles with sizes in the range of 0.1 to 1.0 /rni diameter have atmospheric lifetimes with respect to dry deposition of about 10 days. 

Ruijgrok, W., Davidson, C. I., and Nicholson, K. (1995) Dry deposition of particles Implications and recommendations for mapping of deposition over Europe, Tellus 47B, 587-601. 

Sehmel, G.A. and W.H. Hodgson, A Model for Predicting Dry Deposition of Particles and Gases to Environmental Surfaces. 1978. Battelle Pacific Northwest Laboratories. Report, Richland, WA. 

Ruijgrok, W., H. Tieben, and P. Eisinga, The dry deposition of particles to a forest canopy A comparison of model and experimental results. Atmospheric Environment, 1997.31(3) 399-415. 

Net transport of chemical to and within the canopy system is the result of the combined effects of a multitude of transport processes. These include advective processes such as wet deposition and dry deposition of particles from the atmosphere to the canopy, and from the canopy to the forest floor, as well as diffuse processes such as gaseous exchange between the atmosphere and the canopy, and between the subcanopy air and the forest floor. The relative importance of these processes depends on the chemical source situation, the physical chemical properties of the chemical, as well as the characteristics of the environment. 

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