Deposition velocities

It is generally impractical, in terms of the atmospheric models within which such a description is to be embedded, to simulate, in explicit detail, the microphysical pathways by which gases and particles travel from the bulk atmosphere to individual surface elements where they adhere. In the universally used formulation for dry deposition, it is assumed that the dry deposition flux is directly proportional to the local concentration C of the depositing species, at some reference height above the surface (e.g., 10 m or less) 

Atmospheric Chemistry and Physics From Air Pollution to Climate Change, Second Edition, by John H. Seinfeld and Spyros N. Pandis. Copyright 2006 John Wiley Sons, Inc. 

The advantage of the deposition velocity representation is that all the complexities of the dry deposition process are bundled in a single parameter, vd. The disadvantage is that, because vd contains a variety of physical and chemical processes, it may be difficult to specify properly. The flux F is assumed to be constant up to the reference height at which C is specified. Equation (19.1) can be readily adapted in atmospheric models to account for dry deposition and is usually incorporated as a surface boundary condition to the atmospheric diffusion equation. 

The process of dry deposition of gases and particles is generally represented as consisting of three steps (1) aerodynamic transport down through the atmospheric surface layer to a very thin layer of stagnant air just adjacent to the surface (2) molecular (for gases) or Brownian (for particles) transport across this thin stagnant layer of air, called the quasi-laminar sublayer, to the surface itself and (3) uptake at the surface. Each of these steps contributes to the value of the deposition velocity vd. 

Transport through the atmospheric surface layer down to the quasi-laminar sublayer occurs by turbulent diffusion. Both gases and particles are subject to the same eddy transport in the surface layer. Sedimentation of particles may also contribute to the downward flux for larger particles. 

The final step in the dry deposition process is actual uptake of the vapor molecules or particles by the surface. Gaseous species may absorb irreversibly or reversibly into the surface particles simply adhere. The amount of moisture on the surface and its stickiness are important factors at this step. For moderately soluble gases, such as SO2 and O3, the presence of surface moisture can have a marked effect on whether or not the molecule is actually removed. For highly soluble and chemically reactive gases, such as HNO3, deposition is rapid and irreversible on almost any surface. Solid particles may bounce off a smooth surface liquid particles are more likely to adhere upon contact. 

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Transport and Transformation. Once emitted into the atmosphere, the fate of a particular poUutant depends upon the stabihty of the atmosphere, which determines the concentration of the species, the stabihty of the poUutant in the atmosphere, which determines the persistence of the substance. Transport depends upon the stabUity of the atmosphere which, in turn, depends upon the ventilation. The stabUity of a poUutant depends on the presence or absence of clouds, fog, or precipitation the poUutant s solubUity in water and reactivity with other atmospheric constituents (which may be a function of temperature) the concentrations of other atmospheric constituents the poUutant s stabUity in the presence of sunlight and the deposition velocity of the poUutant. 

Many factors affect dry deposition, but for computational convenience air quaUty models resort to using a single quantity called the deposition velocity, designated or to prescribe the deposition rate. The deposition velocity is defined such that the flux T of species i to the ground is... 

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). 

Elicks BB, Baldocchi DD, Meyers TP, Hosker Jr RP, Matt DR. 1987. A preliminary multiple resistance routine for deriving deposition velocities from measured quantities. Water Air Soil Pollut 36 311-330. 

One major difference between pneumatic transport and hydraulic transport is that the gas-solid interaction for pneumatic transport is generally much smaller than the particle-particle and particle-wall interaction. There are two primary modes of pneumatic transport dense phase and dilute phase. In the former, the transport occurs below the saltation velocity (which is roughly equivalent to the minimum deposit velocity) in plug flow, dune flow, or sliding bed flow. Dilute phase transport occurs above the saltation velocity in suspended flow. The saltation velocity is not the same as the entrainment or pickup velocity, however, which is approximately 50% greater than the saltation velocity. The pressure gradient-velocity relationship is similar to the one for hydraulic transport, as shown in... 

Spherical polymer pellets with a diameter of 1/8 in. and an SG of 0.96 are to be transported pneumatically using air at 80°F. The pipeline is horizontal, 6 in. ID and 100 ft long, and discharges at atmospheric pressure. It is desired to transport 15% by volume of solids, at a velocity that is 1 ft/s above the minimum deposit velocity. 

Scott, A.G., Radon Daughter Deposition Velocities Estimated from Field Measurements, Health Physics 45 481-485 (1983). 

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). 

Bigu, J., Radon Daughter and Thoron Daughter Deposition Velocity and Unattached Fraction Under Laboratory-Controlled Conditions and in Underground Uranium Mines, Aerosol Sci., 16 157-165 (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. 

Taking into account the results of wind tunnel experiments the average deposition velocities for the free (vq = 2 m h 1) and the attached (v = 0.02 m h"1) radon decay products can be derived... 

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 velocities of the unattached daughters were calculated from the measurements in the radon chamber with the bare filter (Figure 2) and found to equal. 095+.007 cm/s for Po-218,. 085+.012 cm/s for Pb-214 and. 045+.015 cm/s for Bi-214. This decrease in deposition velocity is one of the most important sources of error in the room model. 

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). 

Table V. Summary of the deposition velocities of the attached and unattached radon daughters and the corresponding deposition rates from various researchers.

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