Deposition, dry velocity

These gases deposit rapidly due to their reactivity with surfaces, and exhibit elevated dry deposition velocities rapid dry deposition has been confirmed in recent field studies in forests and the Arctic (Lindberg and Stratton 1998 Lindberg et al. 2002). At concentrations typical of raral or remote ecosystems, the dry deposition of RGHg and Hg(0) are far greater than PHg, although this species may be of importance under dry conditions near sources (Pirrone et al. 2000). 

At present evaluation of POP depositions to various types of the underlying surface are under investigations. The spatial distribution of PCB-153 depositions to areas covered with forests, soil and seawater in 2000 is demonstrated in Figure 13. Depositions of this pollutant to forests, soil and seawater were estimated using different parameterizations of dry deposition velocities for different types of underlying surfaces. This resulted in considerable differences in depositions to the considered areas. As seen from the maps, the highest levels of PCB-153 depositions were characteristic of forested areas (Dutchak et al., 2004). 

HCHO (0.33 cm s-1) (Brasseur et al., 1998) and it has been assumed that the dry deposition velocity for CH3CHO and other aldehydes is the same as that for HCHO. 

The dry deposition velocity of lead-212, a thoron (thoron or radon-220 itself originating from thorium-232) decay product has been reported to be in the range 0.03-0.6 cm/sec (Bigu 1985 Rangarajan et al. 1986). These low deposition velocities indicate that the thoron daughter, stable lead, may have a long residence time in the atmosphere with respect to dry deposition. 

Rangarajan C, Eapen CD, Gopalakrishnan SS. 1986. Measured values of the dry deposition velocities of atmospheric aerosols carrying natural and fallout radionuclides using artificial collectors. Water Air Soil Pollut 27 305-314. 

The particulate flux to a surface (F(i part) and the particle-bound atmospheric concentration of a compound (Cail part) at a given height are related to the dry deposition velocity (Fd part) ... 

Andersen HV, Hovmand MF, Hummelshoj P, Jensen NO (1999) Measurements of ammonia concentrations, fluxes and dry deposition velocities to a spruce forest 1991-1995. Atmos Environ 33 1367-1383... 

Measurements of activity in grass and soil in areas where no rain fell at the relevant time have been used to estimate dry deposition after the Chernobyl accident. In Denmark and in southern England, vg for137Cs was about 0.5 mm s 1 (Roed, 1987 Clark Smith, 1988). In Stockholm, however, where the Chernobyl fallout arrived several days earlier, and the particle size was larger, the dry deposition velocity of caesium was 5 mm s 1 (Persson et al., 1987). Refractory elements such as 95Zr had dry deposition velocities about 20 mm s 1. 

Tucker, W.A., Lyman, W.J., Preston, A.L. (1983) Estimation of the dry deposition velocity and scavenging ratio for organic chemicals. In Precipitation Scavenging, Dry Deposition, and Resuspension. Pruppacher, et al., Editors, pp. 1242-1256, Elsevier Science Publishing Co., New York. 

C is a term representative of the photochemistry (production or destruction), Ey the entrainment velocity, [03] the concentration of free tropospheric ozone, the dry deposition velocity and H the height of the boundary layer. The ozone budget shown in Table 8 has been calculated both in the summer and winter using the ozone continuity equation for a site in the marine boundary layer. 

Assuming a typical dry deposition velocity of 1 cm s and a boundary layer height of 1000 m, (H),... 

The effects of dry deposition are included as a flux boundary condition in the vertical diffusion equation. Dry deposition velocities are calculated from a big leaf multiple resistance model (Wesely 1989 Zhang et al. 2002) with aerodynamic, quasi-laminar layer, and surface resistances acting in series. The process assumes 15 land-use types and takes snow cover into account. 

FIGURE 4-32 Diurnal cycle of Vd for ozone for a deciduous forest, averaged over the period July 7 to August 30, 1988. Observed values of Vd and values estimated by four models are shown. (Reprinted from Atmospheric Environment, Vol. 30, L. Zhang, J. Padro, and J. L. Walmsley, A Multi-Layer Model vs. Single-Layer Models and Observed 03 Dry Deposition Velocities, pp. 339-345, Copyright 1996, with permission from Elsevier Science.)... 

Despite the different mechanisms involved, deposition velocities for absorption often are found to be similar in magnitude to values for impaction (i.e., approximately 0.1 to 1 cm/sec for gases whose absorption into a surface is limited by the rate of Fickian transport through a stagnant boundary layer (Table 4-9). For additional information on the estimation of dry deposition velocities, the reader is referred to Brook et al. (1996, 1997), Hicks et al. (1987), Wesely (1989), and Bennett (1988). 

Assume the dry deposition velocity over a New England forest for particles emanating from a midwestern U.S. power plant averages 0.5 cm/sec on a certain day. The scavenging coefficient in a rain-forming cloud over the forest is 10 3/sec. If the cloud is 500 m in vertical extent, what is the equivalent wet deposition velocity, Vw, for rainout of the particles ... 

Brook, J. R., Di-Giovanni, F., Cakmak, S., and Meyers, T. P. (1997). Estimation of Dry Deposition Velocity Using Inferential Models and Site-Specific Meterology-Uncertainty Due to Siting of Meterological Towers. Atmos. Environ. 31(23), 3911-3919. 

Hicks, B. B., Baldocchi, D. D., Meyers, T. P., Hosker, R. P., Jr., and Matt, D. R. (1987). A Preliminary Multiple Resistance Routine for Deriving Dry Deposition Velocities from Measured Quantities. Water, Air, Soil Pollu. 36, 311—330. 

Padro, J. (1996). Summary of Ozone Dry Deposition Velocity Measurements and Model Estimates over Vineyard, Cotton, Grass and Deciduous Forest in Summer. Atmos. Environ. 30(13), 2363-2369. 

Zhang, L., Padro, J., and Walmsley, J. L. (1996). A Multi-Layer Model vs. Single-Layer Models and Observed 03 Dry Deposition Velocities. Atmos. Environ. 30(2), 339-345. 

Finally, we discuss the relationship between the residence time and the dry deposition velocity, vj, defined as the ratio of the particle flux ( /cm sec) toward the surface to... 

Figure 11. Dry Deposition Velocities of SO for shielded and unshielded Teflon plates (20) Reproduced with permission from Ref. 20. Copyright 1985, American Geophysical Union.

The inability to estimate dry deposition velocities and the lack of knowledge of the compounds that are likely to contribute to corrosion makes Equation 3 difficult to apply in evaluating corrosion data. The results of the experiments reported here will address both of these issues and will be used to develop the framework for a model for analysis of galvanized steel corrosion data obtained from field studies. 

Use of Equation 7 is limited by the accuracy of the measurement of the decrease in the gas phase concentration and in practice can only be used to calculate dry deposition velocities of compounds that readily absorb onto the surface. 

Approximate values for the dry deposition velocities can be obtained by equating the two expressions for the deposition (Equations 8 and 9) and solving for the deposition velocity. The result is... 

NQp Exposure Experiment. The experimental conditions and results for the short-term NO2 exposure experiment are given in Table I. The average concentration of NO2 during the dry periods was 649 ppb which decreased to 627 ppb in the presence of dew, thus yielding (according to Equation 7) a value of 0.05 cm/s for the dry deposition velocity. Both N02 and N03 were detected in the dew with the NO2" concentration (203 nmol/ml) representing 95 of the NOx concentration. Trace amounts of S04 were found in the dew. A value of 149 nmol/ml was found for the Zn dew concentration. Equation 10 yields a value of O.O6 cm/s for the NO2 dry deposition velocity, in reasonable agreement with the value found from the decrease in gas phase concentrations. 

Comparison of the NO2 dry deposition velocity with that found for SO2 under similar conditions (vd (SO2) = 0.8 cm/s Table IV) indicates a substantial difference in the processes of uptake of the two gases on the test panels. The gas phase resistances for both compounds are expected to be essentially the same. Therefore, the difference between the dry deposition velocities is likely due to a difference in the aqueous reaction and diffusive mechanisms that take place on the metal surface covered by a thin film of moisture. 

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