Diffusive flux atmospheric deposition

A few of the U/Th nuclides are supplied to the sea via atmospheric deposition and diffusion through sediment pore waters. Decay of Rn in the atmosphere to °Pb and its subsequent removal by wet and dry deposition is an important source of dissolved °Pb to the sea. As the bulk of the Rn in the atmosphere is of continental origin, the flux of °Pb via this route depends on factors such as distance from land and aerosol residence times. °Po is also deposited on the sea surface through this source, but its flux is <10% of that of °Pb. Leaching of atmospheric dust by sea water can also contribute to nuclide fluxes near the air-sea interface, this mechanism has been suggested as a source for dissolved Th. 

Nitrate flux from aerobic zones to anaerobic sites For nitrate reduction to occur in wetlands, nitrate must be present in anaerobic zones. Thus, nitrate reduction rates are regulated by transport of nitrate either by diffusion or by mass flow from aerobic zones to anaerobic portions of the soil. Similarly, the rate of nitrification and the oxygen availability in the soil regulate nitrate concentrations in aerobic zones of the soil. In wetlands with limited inputs of nitrate from external sources, nitrification and atmospheric deposition are the primary sources of nitrate. In these systems, denitrification rates are tightly coupled to nitrification rates. 

The flux of trace gases and particles from the atmosphere to the surface is calculated by multiplying concentrations in the lowest model layer by the spatially and temporally varying deposition velocity, which is proportional to the sum of three characteristic resistances (aerodynamic resistance, sublayer resistance, and surface resistance). The surface resistance parametrization developed by Wesely (1989) is used. In this parametrization, the surface resistance is derived from the resistances of the surfaces of the soil and the plants. The properties of the plants are determined using land-use data and the season. The surface resistance also depends on the diffusion coefficient, the reactivity, and water solubility of the reactive trace gas. 

Dry gaseous deposition is a complex process which depends on the physical-chemical properties of the PCBs, characteristics of the adsorbing surface, and environmental conditions (e.g., windspeed). In the ambient atmosphere, dry particulate deposition is predominantly in the form of fine aerosols (<1 pm), which deposit on surfaces by rapid, vibratory (Brownian) diffusion (Holsen and Noll 1992). However, in urban areas, PCBs are associated with course aerosols (>1 pm), and these particulates represent the majority of the dry deposition flux even though PCBs are largely in the vapor phase (Holsen et al. 1991). 

The estimate for sea salt goes back to a detailed study of Eriksson (1959) of the geochemical cycles of chloride and sulfur. He calculated the rate of dry fallout of sea-salt particles from a vertical eddy diffusion model and then existing measurements of sea-salt concentrations over the ocean. This led to a global rate for dry deposition of 540 Tg/yr. Eriksson then argued that wet precipitation would remove a similar amount annually. It is now known, however, that wet precipitation is more effective than dry deposition in removing aerosol particles from the atmosphere, so that Eriksson s value must be an underestimate. The discussion in Section 10.3.5 suggests a flux rate for sea salt of about 5,000 Tg/yr. 

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. 

It is likely that natural ecosystems (forest, grassland) emit no or only small amounts of ammonia because normally there is a deficit of fixed nitrogen in landscapes. Reported emissions factors over forests span three orders of magnitude and are likely be influenced by re-emission of wet deposited ammonium. Older publications considerably overestimated emission by using simple models considering soil ammonium concentrations obtained from relative decomposition and nitrification rates, where Henry s law gives the equilibrium concentration of ammonia gas in the soil, and a simplified diffusion equation yields the flux to the atmosphere, for example, Dawson (1977) calculated it to be about 47 Tg N yr b... 

HICKS At night, flow might be highly stable. We have measured the ozone deposition as a function of time of day the flux goes down to zero or very close to zero at night. Even though the flux is low, you can still get very large concentration differences because the atmospheric diffusivity is also very low. The basic question is how do you interpret these concentration differences The problem is one of stability and... 

Particles of >10 nm diameter are subject to significant sedimentation rates as a result of gravitational forces (the velocity may be estimated crudely by use of Stoke s Law). Consequently their atmospheric lifetime is severely limited by the gravitational settling process, and by impaction upon surfaces. Particles of <10 jum diameter are removed only relatively slowly from the atmosphere, those greater than ca. 0.3 /im by impaction processes and the smaller particles by diffusive deposition. The deposition flux may be estimated from the deposition velocity and some data are presented in Section 4.2.2. 

Bhardwaj et al. [20] solved numerically a very complex problem of drying colloidal droplets and deposits formation. Their model takes into account the interaction of the free surface with the peripheral deposit and eventual depinning. The diffusion of vapour in the atmosphere is solved numerically, providing an exact boundary condition for the evaporative flux at the droplet-air interface. The formation of different deposit patterns obtained experimentally is explained well by their simulations. 

Three cases must be differentiated evaporation-equilibration takes place (i) in a sealed chamber, (ii) in an open chamber, or (iii) in a chamber where air of controlled composition constantly flows. In a sealed chamber, the escaping molecules accumulate in the vapor phase up to saturation, and the net flux of evaporation thus stops. During thin film formation in a sealed chamber, only a certain quantity of liquid is deposited on the substrate and evaporates. If the quantity of the volatile molecules is lower than that needed to saturate the volume of atmosphere, evaporation proceeds and the quantity of molecules (S) remaining in the film is fixed by the relative vapor pressure. Eventually, the film composition will include a proportion of volatile species that is in equilibrium with the quantity that could evaporate. In open chambers equipped with exit windows, the composition of the chamber atmosphere is in equilibrium with the external atmosphere. Assuming that the diffusion in the vapor phase is very fost, the relative vapor pressure inside the chamber (Pa) is constant, and the rate of evaporation is governed by the difference of vapor pressure in the chamber (PJ and at the solution surfece (Pg) (that is to say, at a distance of A from the surface as a first approximation). The net flux of evaporating species across the border, in the x-direction that is normal to the surfece, is then given by the Knudsen equation ... 

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