Deposition on surfaces

The concentration of indoor pollutants is a function of removal processes such as dilution, filtration, and destruction. Dilution is a function of the air exchange rate and the ambient air quality. Gases and particulate matter may also be removed from indoor air by deposition on surfaces. Filtration systems are part of many ventilahon systems. As air is circulated by the air-conditioning system it passes through a filter which can remove some of the particulate matter. The removal efficiency depends on particle size. In addition, some reactive gases like NOj and SOj are readily adsorbed on interior surfaces of a building or home. 

The rest of the less volatile fission products along with constituents of zircalloy, stainless steel, and the control rods are assumed to be in condensed form as inert aerosols that are treated together in TRAPMELT as "other aerosols." The aerosols are modeled as agglomerating and depositing on surfaces by several mechanisms (e.g., gravitational settling). 

Particles are present in outdoor air and are also generated indoors from a large number of sources including tobacco smoking and other combustion processes. Particle size, generally expressed in microns (10-6 m) is important because it influences the location where particles deposit in the respiratory system (U.S. Environmental Protection Agency 1995), the efficiency of particle removal by air filters, and the rate of particle removal from indoor air by deposition on surfaces. 

The majority of heterogeneous chemical and physical-chemical processes lead to formation of the intermediate particles - free atoms and radicals as well as electron- and oscillation-excited molecules. These particles are formed on the surface of solids. Their lifetime in the adsorbed state Ta is determined by the properties of the environment, adsorbed layer, and temperature. In many cases Ta of different particles essentially affects the rate and selectivity of heterogeneous and heterogeneous-homogeneous physical and chemical processes. Therefore, it is highly informative to detect active particles deposited on surface, determine their properties and their concentration on the surface of different catalysts and adsorbents. 

In recent studies, pesticides such as atrazine have been found in precipitation. Therefore volatilization and subsequent transport in the gaseous phase is an important environmental pathway. Vaporization rates of pesticides deposited on surface of soil and plant leaves depend on the physical-chemical properties of the substance. A useful physicochemical criterion is Henry s constant, Ku, which is defined as the equilibrium air-to-water partial pressure ratio of the substance (see Chapter 7). 

Optimization of the sampling of chemicals and mixtures deposited on surfaces that could be sources of human exposures. Specific collection methods have been assessed and laboratory based research work is underway to define optimization procedures for sampling. 

Increasing the size of PAHs makes their deposition on surfaces difficult because they can neither be sublimed nor made sufficiently soluble for solution processing. A precursor route has thus been designed according to which molecules are deposited on a surface and transformed into the final disc-type adsorbate structures in a thermal solid-state reaction with the substrate surface acting as a template.1261 An exciting example is the hexaether 41 (scheme 11) which is sublimed onto a Cu-(1U) sur-... 

Electron transfer between metal centers can alter the course of reaction in several ways (46). Thermal excitation may create especially reactive electron holes on the oxide surface, causing reductant molecules to be consumed at the surface at a higher rate. More importantly, electrons deposited on surface sites by organic reductants may be transferred to metal centers within the bulk oxide (47). This returns the surface site to its original oxidation state, allowing further reaction with reductant molecules to occur without release of reduced metal ions. Electron transfer between metal centers may therefore cause changes in bulk oxide composition and delay the onset of dissolution. 

This is a very broad conclusion, and additional measurements must be made. Some of this effort (which is current) should address the problem of other pollutants and condensation nuclei that accompany the nonurban oxidant. Interpretation of these measurements will increase the specificity of separating anthropogenic sources from natural background sources. Theoretical assessments of the existing observations will shed light on the relative roles played by stratospheric injection, plant emission, background methane, and diy deposition on surfaces in the natural portion of the tropospheric ozone cycle. 

Binns C (2001) Nanoclusters deposited on surfaces. Surf Sci Rep 44(1-2) 1 9 Andres R, Bein T, Dorogi M, Feng S, Henderson J, Kubiak C, Mahoney W, Osifchin R, Reifenberger R (1996) Coulomb staircase at room temperature in a self-assembled molecular nanostructure. Science 272 1323-1325... 

The mechanism we believe is responsible for the large SiOj-to-Si etch-rate ratios which have been obtained in fluorine-deficient discharges is based on several experimental observations. First of all, it has been shown that there are several ways in which carbon can be deposited on surfaces exposed to CF, plasmas. One way is to subject the surface to bombardment with CF ions which are the dominant positive ionic species in a CF plasma. The extent to which this can occur is shown by the Auger spectra in Fig. 3.3. Curve (a) is the Auger spectrum of a clean silicon surface and curve (b) is the Auger spectrum of the same surface after bombardment with 500 eV CFj" ions. Note that the silicon peak at 92 eV is no longer visible after the CFj bombardment indicating the presence of at least two or three monolayers of carbon. Another way in which carbon can be deposited on surfaces is by dissociative chemisorption of CFj or other fluorocarbon radicals. 

Although the term acid rain has been used extensively in the popular literature to describe the formation and deposition of acids at the earth s surface, the terminology acid deposition is more commonly encountered in the scientific literature. The reason for this is that deposition of acids can occur either as dry deposition or as wet deposition. The former refers to the direct transport of acidic gases or small particles to the surface, followed by adsorption, without first being dissolved in an aqueous phase such as rain, clouds, or fog. Wet deposition, on the other hand, refers to the transport of acids to, and deposition on, surfaces (including soil, trees, grass, buildings, etc.) after the acids have been dissolved in an aqueous medium. It should be noted that the surface itself can be either wet or dry the terms wet and dry deposition refer to the mechanism of transport to the surface, not to the nature of the surface itself. 

Plasma polymers are deposited on surfaces in contact with a glow discharge of organic or organometallic monomers, in the form of a thin film and/or as a powder. Such films find applications as surface modifiers and in applications where the bulk properties of extremely thin films are desirable. 

Specific adsorbents with electron densities on surface. Graphitized carbon blacks with dense monolayers of Group B molecules or macromolecules deposited on surface. Adsorbents with a functional group, for example, cyano, nitrile, or carbonyl, would also be included in this category. 

The next step in the experiment will be to incorporate mass analysis of material sputtered from the primary surface in order to reject neutrals and to be more selective in what is deposited on the secondary surface. It is hoped that catalytically useful materials, such as mass-selected small metal clusters, may eventually be deposited on surfaces. Furthermore, it may be possible to transfer reactive organic species (such as those in Table IV) to create new materials through control of the potential between the two surfaces. 

Hard amorphous carbon films may either be fully amorphous or contain tiny diamond crystallites. Hydrogen-free films may be deposited on surfaces from graphite by laser ablation or ion sputtering. Hydrogen-containing films are also possible. 

Table 1.7 shows examples from various types of location of the relative activity of 222Rn and its decay products, and the equilibrium ratio. Indoors, F depends on the ventilation and on the number of condensation nuclei in the air, since attachment reduces deposition on surfaces, as discussed below. 

Attachment of decay products to nuclei greatly affects the process of deposition, because the Brownian diffusivity of nuclei is typically about four orders of magnitude less than the molecular diffusivity of unattached decay products. The lifetime of decay products in air before deposition on surfaces is shorter if the air is clean than if it is dirty. 

Ventilation in centrally heated dwellings, though much less than in those with open fires, usually gives 0.1 to 0.3 air changes per hour, so ventilation and deposition on surfaces are about equally effective in limiting the concentration of attached decay products in dwellings. 

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