Deposition velocities surfaces

Early models used a value for that remained constant throughout the day. However, measurements show that the deposition velocity increases during the day as surface heating increases atmospheric turbulence and hence diffusion, and plant stomatal activity increases (50—52). More recent models take this variation of into account. In one approach, the first step is to estimate the upper limit for in terms of the transport processes alone. This value is then modified to account for surface interaction, because the earth s surface is not a perfect sink for all pollutants. This method has led to what is referred to as the resistance model (52,53) that represents as the analogue of an electrical conductance... 

Although it does not physically explain the nature of the removal process, deposition velocity has been used to account for removal due to impaction with vegetation near the surface or for chemical reactions with the surface. McMahon and Denison (12) gave many deposition velocities in their review paper. Examples (in cm s ) are sulfur dioxide, 0.5-1.2 ozone, 0.1-2.0 iodine, 0.7-2.8 and carbon dioxide, negligible. 

The deposition velocities depend on the size distribution of the particulate matter, on the frequency of occurrence and intensity of precipitation, the chemical composition of the particles, the wind speed, nature of the surface, etc. Typical values of and dj for particles below about 1 average residence time in the atmosphere for such particles is a few days. 

Numerous atmospheric species react with the Earth s surface, mostly in ways that are not yet chemically described. The dissolution and reaction of SO2 with the sea surface, with the aqueous phase inside of living organisms or with basic soils is one example. Removal of this sort from the atmosphere usually is called dry removal to distinguish it from removal by rain or snow. In this case, the removal flux is often empirically described by a deposition 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). 

Data on the rate of attachment or deposition, i.e., plate-out of radioactive particles on walls can be used to calculate the particle deposition velocity. Deposition rates can be determined experimentally by measuring the surface activity on some samples... 

This paper deals with the plate-out characteristics of a variety of materials such as metals, plastics, fabrics and powders to the decay products of radon and thoron under laboratory-controlled conditions. In a previous paper, the author reported on measurements on the attachment rate and deposition velocity of radon and thoron decay products (Bigu, 1985). In these experiments, stainless steel discs and filter paper were used. At the time, the assumption was made that the surface a-activity measured was independent of the chemical and physical nature, and conditions, of the surface on which the products were deposited. The present work was partly aimed at verifying this assumption. 

The underlying physical and/or chemical mechanisms responsible for the differences observed between the radon progeny and the thoron progeny as related to different materials are not clearly understood. Finally, it should be pointed out that the main thrust in this paper was to determine differences in surface a-activity measured on different materials with the same geometrical characteristics exposed to identical radioactive atmospheres. The calculation of deposition velocities and attachment rates, although it follows from surface a-activity measurements, was not the intent of this paper. This topic is dealt with elsewhere (Bigu, 1985). 

Surface deposition is the most important parameter in reduction of the free and aerosol attached radon decay products in room air. If V is the volume of a room and S is the surface area available for deposition (walls, furniture etc), the rate of removal (plateout rate) q is vg S/V, always assuming well mixed room air. vg is the deposition velocity. 

Porstendorfer, 1984). Knutson et al. (1983) measured similar results in their chamber investigation. The results show that the values of the deposition velocity of the free radon daughters are about 100 times those of the aerosol radon progeny. But there are no information about the effective deposition surface S of a furnished room for the calculation of the plateout rates qf and qa by means of Vg and Vg. For this reason the direct measurements of the plateout rates in rooms are necessary. Only Israeli (1983) determined the plateout rates in houses with values between qf = 3-12 h"1 and qa = 0.4-2.0 h"1, which give only a low value of the... 

The deposition rate and the corresponding deposition velocity of the unattached daughters is found to have a value of 18/h and. 19 cm/s in the rooms and 8/h and. 08 cm/s in the cellar. The latter could be corrected to. 12 cm/s applying the same decrease in deposition velocity to the attached daughters as was found for the unattached daughters. The remaining difference is probably due to a smaller air velocity in the cellar and to a difference in roughness of the surface (concrete instead of wall paper and carpets). 

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). 

Environmental Fate. It can be concluded from the transport characteristics that surface water sediment will be the repository for atmospheric and aquatic thorium. Normally, thorium compounds will not transport long distances in soil. They will persist in sediment and soil. There is a lack of data on the fate and transport of thorium and its compounds in air. Data regarding measured particulate size and deposition velocity (that determines gravitational settling rates), and knowledge of the chemical forms and the lifetime of the particles in air would be useful. 

Dry deposition is usually characterized by a deposition velocity, V. The net flux (I ) of a species to the surface is proportional to the concentration of that species in air, [S] i.e., F a [S], The deposition velocity is just the proportionality constant relating flux and concentration, i.e.,... 

The gas-phase resistance depends on the height (z) above the earth s surface, as does the concentration of the pollutant as a result the deposition velocity is also a function of height. 

Muller, J.-F. Geographical Distribution and Seasonal Variation of Surface Emissions and Deposition Velocities of Atmospheric Trace Gases, J. Geophys. Res., 97, 3787-3804 (1992). 

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) ... 

The deposition rate depends also on the availability of surfaces. While gravimetric deposition takes place only on horizontal upward facing surfaces, thermokinetic deposition occurs also on downward facing and vertical surfaces. The more such surfaces are available for deposition, the faster the corresponding deposition rate is. Relationship of the deposition surface availability and the deposition velocity in a rectangular space is characterized by Lai and Nazaroff [12] as... 

Particle deposition velocities depend on a number of factors, including wind speed, atmospheric stability, relative humidity, particle characteristics (diameter, shape, and density), and receptor surface characteristics. Recent studies on dry particle deposition to surrogate surfaces and derived from atmospheric particle size distributions and micrometeorology suggest that a V equal to about 0.5 cm s 1 is applicable to urban/industrial regions [116-120]. 

Deposition velocities depend on the atmospheric stability, nature of the surface, nature of the chemical (for a gas-phase chemical), and (for a particle or particle-phase chemical) the size of the particle. For particle deposition, the deposition velocity is a minimum for particles with mean diameter in the range 0.3-0.5 pm, and it increases with both increasing and decreasing particle size (Eisenriech et al., 1981 Bidleman, 1988). 

By considering the mass balance of HT in the southern hemisphere, where there are no artificial sources other than through inter-hemispheric transfer, Mason Ostlund estimated that HT is removed from the atmosphere with a rate constant 0.155 a-1, giving a mean residence time of 6 a. The main sink for H2 or HT is oxidation by soil bacteria (Schmidt, 1974 Garland Cox, 1980 Sweet Murphy, 1981). Land comprises 29% of the earth s surface. The effective depth of the atmosphere (mass per unit ground area divided by density at ground level) is 8000 m and HT is well mixed. It follows that a deposition velocity to land of 0.135 mm s-1 would give a removal constant of 0.155 a-1. 

The deposition velocities to grass of particles with diameters between 1 and 5 jum were also found to be proportional to w (Chamberlain, 1967), but for these particles impaction has much less effect. Particles smaller than about 5 jum diameter are unlikely to bounce off surface in normal ambient conditions. The presence of micro-roughness elements on surfaces increases vg for these particles, and Chamberlain (1967) found more deposition to real leaves than to smooth sticky artificial leaves when the particle size was less than 5 jum. 

In the last two columns of Table 6.4 the turbulent deposition velocities of mass and momentum are shown. For droplets, vt and vm are approximately equal, indicating that capture by the surface is efficient, and aerodynamic resistance the limiting factor. 

Air-side deposition velocities have been studied extensively for the uptake of inorganic gases in plants. However, the deposition of PCDD/Fs introduces a new dimension to this subject, since these compounds are deposited primarily to the surface of the cuticle,45 not to the stomatal openings as in the case of most inorganic trace gases. Hence the receptor surface is fundamentally different for... 

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