Rural atmospheric dusts

Relationship between Log (EF) for elements in rural atmospheric dust and the melting point (K) of the elements. 

Since the majority of the elements in surface dust arise from deposited aerosol and added soil it is not surprising to find strong linear relationships between the concentrations of the elements in an atmospheric dust and street or house dust. This is illustrated by the two examples given in Fig. 8 for remote house dust vs urban atmospheric dust and street dust vs rural atmospheric dust. As discussed above crustal/soil material is a major component of atmospheric dust and the soil based elements in the atmospheric dust are Al, Ca, Fe, Mg, Mn, Ni, K, Si and Ti. The elements As, Br, Cd, Cl, Co, Cu, Pb, Rb, Se, V, and Zn are, on the other hand, enriched in atmospheric dust. The same elemental distribution applies to surface dust, but in this case their concentrations (compared on a mass basis) are reduced presumably due to dilution with soil. However, the elements enriched in the atmosphere remain enriched in the surface dusts. 

Fig. 8. Relationship between the concentration of elements in surface and atmospheric dusts (a) house dust vs urban atmospheric dust, (b) street dust vs rural atmospheric dust [M] is the concentration of element M.

It is remarkable that, except for local hot-spots such as around industrial sites, mining areas and volcanoes, the elemental compositions of atmospheric dust in similar locations, such as remote or rural or urban are relatively constant over the world. This suggests either common sources, or a dominant source, or good mixing and transport of the dust around the globe. In fact all three factors have a role in determining the uniformity. Because of the consistent composition it is possible to estimate the median concentrations of the elements in atmospheric dusts in similar, but widely separated, locations. These estimates are given in columns 2 to 7 in Table n. The concentrations of the elements in the atmospheric dust are expressed as mass per volume of air. For remote locations (columns 2 to 5) the concentrations are in ng m 3, whereas for rural and urban areas (columns 6 and 7) the elemental concentrations are in xg m-3. 

Diverse techniques have been employed to identify the sources of elements in atmospheric dust (and surface dust) (Table V). Some involve considering trends in concentration and others use various statistical methods. The degree of sophistication and detail obtained from the analyses increases from top left to bottom right of the Table. The sources identified as contributing the elements in rural and urban atmospheric dusts are detailed in Table VI. The principal sources are crustal material, soil, coal and oil combustion emissions, incinerated refuse emissions, motor vehicle emissions, marine spray, cement and concrete weathering, mining and metal working emissions. Many elements occur in more than one source, and they are classified in the... 

Basically, a complete HVAC system is called upon to induct atmospheric air, clean and heat or cool it, and then circulate the treated air throughout the premises. The amount and type of contaminants present in the air will vary widely with the site of the system. In rural areas, dust concentration is likely to be of the order of 0.05 to 0.5mg/m and to comprise mainly soil erosion particles, vegetable matter, seasonal pollens and a minimum of carbonaceous matter. In metropolitan areas dust concentration is likely to be 0.1 to 1 mg/m, with a high proportion of carbonaceous matter, ash, silicon and other granular products. In industrial areas, the figure can be expected to be the order of 2 to 5mg/m, with a large content of carbonaceous matter, as well as tarry oils and waxes, mineral and chemical dusts, sulphurous gases and acids. 

A rural atmosphere is normally classified as one that does not contain chemical pollutants but does contain organic and inorganic dusts. Its principal corrodents are moisture and, of course, oxygen and carbon dioxide. Arid or tropical atmospheres are special cases of the rural environment because of their extreme relative humidities and condensations (Ref 4). Hie rural atmosphere is generally the least corrosive. 

Despite the low average consumption of pesticides, in the sporadic reports available, it could be seen that even the roadside dusts, rural and urban soils and the underwater sediments are contaminated. Many pesticides are degrading the Indian environment, even though faster dissipation and possible degradation of POPs chemicals like HCHs and DDTs were observed in Indian soils by the tropical climate of India (Pillai, 1986). Such a phenomenon of dissipation in the dry season was substantiated by Ramesh et al. (1991) in the river sediments (Fig. 9.2). Further, the relative flux of residues into the aquatic environment is smaller than the amount volatilized to the atmosphere in tropical countries like India (Tanabe et al., 1991). 

The aerial transport of pollen and microorganisms has received some attention (Gregory, 1973, 1978 R. Campbell, 1977). Bacteria (size < 1 p.m) are difficult to discern directly, and their study requires cultural growth techniques. In contrast to fungi spores, they usually occur attached to other aerosol particles because they are mobilized together with dust. Concentrations number several hundred per cubic meter in rural areas and several thousand in the cities. Air is not their natural habitat, so that multiplication does not take place. On the contrary, the atmospheric aerosol appears to have definite germicidal qualities (Riiden and Thofern, 1976 Riiden et al., 1978). 

Millions of tons of soot, dust, and smoke particles are emitted into the atmosphere of the United States each year. The average suspended particulate concentrations in the United States vary from about 0.00001 g/m of air in rural areas to about six times as much in urban locations. In heavily polluted areas, concentrations of particulates may increase to 0.002 g/m . 

Atmospheric corrosion is an electrochemical process with the electrolyte being a thin layer of moisture on the metal surface. The composition of the electrolyte depends on the deposition rates of the air pollutants and varies with the wetting conditions. The factors influencing the corrosivity of atmospheres are gases in the atmosphere, critical humidity and dust content. Two rural environments can differ widely in average yearly rainfall and temperature and can have different corrosive... 

Atmospheric contaminants often responsible for the rusting of structural stainless steels ate the chlorides and metallic iron dust. Chlorides can originate from concrete (CaCl2) and spraying of salt on the road or from exposure to industry and marine locations. Chlorides promote pitting or crevice attack on stainless steels. As discussed above, the corrosivity of different atmospheres can differ and must be considered when materials are selected. Rural and urban environments without pollutants (chloride) do not corrode stainless steel, even in areas with high humidity. 

Continental soil dust is usually contained in the size fraction between 1 and 20 pm in diameter, with the mass median diameter about 2-4 pm. Larger particles fall out quickly because of their large sedimentation velocities, although individual dust particles of several 100 pm have been collected in South America that have been transported from the Sahara (46). The reported size spectra of mineral particles (41,47) indicate that the mass of fine clay particles is one to two orders of magnitude smaller than that of coarse silt particles (1-10 pm). Noll et al. (48) compared the composition of coarse atmospheric particles at a urban and rural site. At the rural site, the coarse particles contained predominantly cmstal material (limestone 89% and silicates 6%) and biogenic material. Limestone had a mean mass diameter of about 20 pm, and silicates 12 pm. In urban samples, a significant fraction (25%) of rubber tire was found with a mean mass diameter of 25 pm. 

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