Atmospheric particle deposition

The deposition of atmospheric particles occurs as dry deposition due to gravity and inertia with impaction and gravitational settling and wet deposition by rainfall events. 

The deposition velocity, Vd, concept has been applied in evaluations of aerosol transfer to the earth s surface. The deposition velocity represents the effective thickness of the atmosphere losing aerosols per unit time or the effective rate at which small aerosols are sedimenting. 

The deposition velocity of atmospheric particles is dependent on the particle diameter with large particles having higher deposition velocities according to the equation 

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Health effects attributed to sulfur oxides are likely due to exposure to sulfur dioxide, sulfate aerosols, and sulfur dioxide adsorbed onto particulate matter. Alone, sulfur dioxide will dissolve in the watery fluids of the upper respiratory system and be absorbed into the bloodstream. Sulfur dioxide reacts with other substances in the atmosphere to form sulfate aerosols. Since most sulfate aerosols are part of PMj 5, they may have an important role in the health impacts associated with fine particulates. However, sulfate aerosols can be transported long distances through the atmosphere before deposition actually occurs. Average sulfate aerosol concentrations are about 40% of average fine particulate levels in regions where fuels with high sulfur content are commonly used. Sulfur dioxide adsorbed on particles can be carried deep into the pulmonary system. Therefore, reducing concentrations of particulate matter may also reduce the health impacts of sulfur dioxide. Acid aerosols affect respiratory and sensory functions. 

The effect of oxidation pretreatment and oxidative reaction on the graphitic structure of all CNF or CNF based catalysts has been studied by XRD and HRTEM. From the diffraction patterns as shown in Fig. 2(a), it can be observed the subsequent treatment do not affect the integrity of graphite-like structure. TEM examination on the tested K(0.5)-Fe(5)/CNF catalysts as presented in Fig.2(b), also indicates that the graphitic structure of CNF is still intact. The XRD and TEM results are in agreement with TGA profiles of fi-esh and tested catalyst there is no obviously different stability in the carbon dioxide atmosphere (profiles are not shown). Moreover, TEM image as shown in Fig. 2(b) indicates that the iron oxide particle deposited on the surface of carbon nanofibcr are mostly less than less than 10 nm. 

An important aspect of atmospheric reactions is the possibility that tropospheric transformation products subsequently enter aquatic and terrestrial ecosystems through precipitation or by particle deposition. 

Interphase Material Transfer. In some cases there is unidirectional bulk transfer of material and associated chemical between compartments (e.g. sediment deposition or atmospheric particle fallout) in which case the rate is given by an expression similar to that for advection in which Gg (m3/h) is the rate of transfer of the material namely... 

The ash of peat forming plant species contains a predominant amount of silicon. This element is particularly abundant in the Sphagnum, where its content achieves 36% by ash weight. Iron and aluminum are the next abundant. The first is accumulated during the peat formation process. The accumulation of calcium and potash is more pronounced than sodium, and the sulfur content is also remarkable. A large amount of mechanically admixed mineral particles (40-80% by ash weight) is found in mosses. This is due to the deposition of fine dispersed mineral material from snowmelting waters and atmosphere dust deposition (Table 1). 

As a rule, simulations consider emissions of heavy metals from anthropogenic and natural sources, transport in the atmosphere and deposition to the underlying surface (Figure 6). It is assumed that lead and cadmium are transported in the atmosphere only as a part of aerosol particles. Besides, chemical transformations of these metals do not change removal properties of their particles-carriers. On the contrary, mercury enters the atmosphere in different physical and chemical forms and undergoes numerous transformations during its pathway in the atmosphere (Ilyn et al., 2002 2004 Ilyin and Travnikov, 2003). 

A different approach which also starts from the characteristics of the emissions is able to deal with some of these difficulties. Aerosol properties can be described by means of distribution functions with respect to particle size and chemical composition. The distribution functions change with time and space as a result of various atmospheric processes, and the dynamics of the aerosol can be described mathematically by certain equations which take into account particle growth, coagulation and sedimentation (1, Chap. 10). These equations can be solved if the wind field, particle deposition velocity and rates of gas-to-particle conversion are known, to predict the properties of the aerosol downwind from emission sources. This approach is known as dispersion modeling. 

Atmospheric PAH depositions are usually from very dispersed sources but they cover significant amounts of land surface. PAH concentrations from these sources are typically quite low in soil and they are adsorbed strongly to soil particles. Consequently, there is minimal leaching into the soil below and the adsorbed PAHs tend to resist biodegradation, volatilization, and/or photolysis. If low concentrations of HMW PAHs, such as benzoMpyrene, need to be reduced below some established risk threshold (often determined in a site-specific manner by the regulatory officials), bioremediation of these low concentrations will likely be a desired alternative because of the scale and magnitude of the problem. 

Nevertheless, these methods all result in a filter that captures the atmospheric particles. The mass loading can be large, the deposit uniform, and the filter reasonably stable under transport to a central analytical laboratory. Numerous papers have treated analysis of such filters, so this information is not repeated. This chapter focuses on the problems of chemical analyses of impactor substrates, for which the problems are more serious and the solutions elusive. 

Deposition of POPs may take place by (1) snow and rain scavenging of gases and aerosols (wet deposition), (2) dry particle deposition and (3) gas exchange with surfaces [61], The distributions of POPs between the gas and particle phases depend on their physical-chemical properties (Fig. 8) as well as the environmental conditions in the atmosphere, such as temperature, amounts and composition of particles [26, 35]. 

The production of arsenide semiconductors uses highly toxic arsine gas (ASH3) (Chein et al., 2006). Small amounts of the gas are released into the atmosphere at the manufacturing facilities. Once in the atmosphere, arsine oxidizes to AS4O6. The AS4O6 subsequently deposits on atmospheric particles that could be inhaled (Chein et al., 2006), 1901-1902. 

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]. 

Holsen, T.M. and K.E. Noll. 1992. Dry deposition of atmospheric particles application of current models to ambient data. Environ. Sci. Technol. 26 1807-1815. 

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). 

Slinn, W.G.N. (1982) Predictions for particle deposition to vegetative canopies. Atmospheric Environment, 16,1785-94. 

The size distribution of air particles not only influences the distribution and partitioning dynamics of POPs, but also determines dry and wet deposition flux of POPs. An interesting phenomenon was observed for relationship among atmospheric PAHs, particle size distribution, and the levels of PAHs in soil (Kim, 2004). For urban sites, the composition pattern and absolute concentrations of PAHs in soil were well correlated with those in air where the atmospheric particles size was distributed evenly among seasons with predominant amount of fine particles < 3 pm. Dry deposition flux of PAHs followed seasonal variation in atmospheric concentration in urban site. However, at a suburban site with large seasonal variation in particle size distribution, dry deposition flux and soil residue did not reflect the seasonal variation of atmospheric PAHs. From this result, site-specificity in atmospheric particle distribution may also influence the distribution and residues in the underlying soil. 

Zhu et al. (2004) reported the concentrations of 10 PAHs in four bodies of water in Hangzhou, China (July and November 1999 2002). The maximum levels of PAHs in the water bodies (34.4-67.7 pgl-1) were found in July, while significantly lower PAH concentrations (4.7-15.3 pgl-1) were measured in November. The measured PAH concentrations in sediments and soils, runoff water, and air particles were 224-4222 ngg-1, 8.3 pgl-1 and 2.3 pgm-3, respectively. Clearly, such substantial contamination may lead to acute toxic effects on aquatic organisms. However, the erosion of soil material does not contribute significantly to the contamination of sediments. The atmospheric PAH deposition to water bodies in the city area of Hangzhou was estimated to be 530tons/a, while the contribution of surface runoff water was... 

Degeneration of the tongue and oral mucosa of the cheek, gum, and hard palate was observed in rats exposed to the atmosphere in a furnace room or a phosphorus factory for an intermediate duration. These effects were most likely the result of a direct contact of white phosphorus with tissues of the mouth and/or indirect contact through particles deposited on the fur which are then ingested by preening (Ruzuddinov and Rys-Uly 1986). 

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