Rural atmosphere

Chromates are very effective inhibitors of the corrosion of magnesium alloys by saline and other waters, and many treatments have been developed by means of which substantial hlms containing slightly soluble chromate are formed in the metal surface. Except on parts which are to be exposed only to a rural atmosphere, chromate treatment must be supplemented by paint, for which it provides a good base. 

Care in designing and conducting the test in no way reduces the need for discrimination on the part of the person using the test data in the selection of a coating for a particular purpose. Test environments must reflect the deteriorating influences of the service for which they are applicable. A coating system cannot reliably be selected for service in a chemical plant on the basis of performance determined in a rural atmosphere. 

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.

Organic aerosols formed by gas-phase photochemical reactions of hydrocarbons, ozone, and nitrogen oxides have been identified recently in both urban and rural atmospheres. Aliphatic organic nitrates, such dicarboxylic acids as adipic and glutaric acids, carboxylic acids derived from aromatic hydrocarbons (benzoic and phenylacetic acids) and from terpenes emitted by vegetation, such as pinonic acid from a pinene, have been identified. The most important contribution in this held has been that of Schuetzle et al., who used computer-controlled... 

Rasmussen, R. A., and M. W. Holdren. Analyses of Cj to Cm Hydrocarbons in Rural Atmospheres. Paper No. 72-19 Presented at the 65th Annual Meeting of the Air Pollution Control Association, Miami Beach, Florida, June 18-22, 1972. 

Reaction of DEHP vapor with hydroxyl radicals in the atmosphere has been predicted, with an estimated half-life of about 6 hours using the Atmospheric Oxidation Program (Meylan and Howard 1993). The atmospheric half-life, however, is expected to be longer for DEHP adsorbed to atmospheric particulates. Based on the estimated half-life alone, extensive transport of DEHP would not be expected and concentrations in Antarctic snow would not be predicted. Nonetheless, DEHP appears to be present in urban and rural atmospheres (see Section 6.4), and its transport might be mainly in the sorbed state. Data confirming this degradation pathway have not been located. Direct photolysis and photooxidation are not likely to be important (Warns 1987). 

The corrosion rates of various metals in industrial, marine and rural atmospheres are given in Table 4.76. Zinc has higher corrosion resistance than cadmium and iron in all the three atmospheres. Zinc has higher corrosion resistance than copper in industrial... 

It is obvious from the data that galvanic corrosion of zinc in rural atmosphere can be five times the rate in corresponding atmospheric corrosion and three times that in marine atmospheres. Mild steel appears to be the most efficient cathode among the metals studied. More detailed discussions are given in the literature.92... 

Benzene in the atmosphere exists predominantly in the vapor phase (Eisenreich et al. 1981). The most significant degradation process for benzene is its reaction with atmospheric hydroxyl radicals. The rate constant for the vapor phase reaction of benzene with photochemically produced hydroxyl radicals has been determined to be 1.3 10"12 cm3/molecule-second, which corresponds to a residence time of 8 days at an atmospheric hydroxyl radical concentration of 1.1 x 106 molecules/cm3 (Gaffney and Levine 1979 Lyman 1982). With a hydroxyl radical concentration of 1 x 108 molecules/cm3, corresponding to a polluted atmosphere, the estimated residence time is shortened to 2.1 hours (Lyman 1982). Residence times of 472 years for rural atmospheres and 152 years for urban atmospheres were calculated for the reaction of benzene with ozone (03) using a rate constant for 03 of 7 /1 O 23 cm3/molecule-second (Pate et al. 1976) and atmospheric concentrations for 03 of 9.6/1011 molecules/cm3 (rural) and 3/ 1012 molecules/cm3 (urban) (Lyman 1982). 

Dry deposition is frequently the main sink for ozone in the rural atmospheric boundary layer. What is the lifetime of ozone with respect to this process ... 

Zinc belongs to the materials that exerts a strong dependence of the corrosion rate on the concentration of sulphur pollutants. In several investigations the corrosion rate in urban atmospheres was found to be 2-6 times higher than in rural atmospheres (5,... 

It is a well known fact that formation of the green patina takes a substantially shorter time in urban than in rural atmosphere, where often very long time elapses before the surface is covered by patina or in very pure atmospheres the surface remains covered by a black oxide layer. Also the corrosion rate in rural atmosphere is usually lower (<1 um/year) than in urban or industrial atmospheres (1-3 /Lim/year) (8 . 

The associated time constants are obtained from the respective rate or photodissociation coefficients and reactant concentrations for ground-level rural atmospheric conditions. Data are based in part on a compilation of Ehhalt and Drummond (1982). Orders of magnitude are shown in parentheses. 

Sexton, K., and H. Westberg (1984). Nonmethane hydrocarbon composition of urban and rural atmospheres. Atmos. Environ. 18, 1125-1132. 

Methanol, ethanol, and methyl tertiary butyl ether (MTBE) are OxHCs added to fuels to decrease tailpipe emissions of NMHCs and CO. Some alcohols, aldehydes, and ketones are emitted from biogenic sources others are also produced in the atmosphere through photochemical oxidation. The levels of some of the light OxHCs (e.g., acetaldehyde, methanol, ethanol, and acetone) are substantial. In the rural atmosphere, these can dominate the VOCs distribution (see,... 

SOA compounds formed by gas-phase photochemical reactions of hydrocarbons, ozone, and nitrogen oxides have been identified in both urban and rural atmospheres. Most of these species are di- or polyfunctionally substituted compounds. These compounds... 

Montzka, S. A., Trainer, M., Goldan, P. D., Kuster, W. C., and Fehsenfeld, F. C. (1993) Isoprene and its oxidation products, methyl vinyl ketone and methacrolein, in the rural atmosphere, J. Geophvs. 

Japar, S. M., Brachaczek, W. W., Gorse, R. A., Norbeck, J. M., and Pierson, W. R. (1986) The contribution of elemental carbon to the optical prop>erties of rural atmospheric aero.sols, Atmos. Environ., 20, 1281-1289. 

In wet atmospheres, nickel initially forms NiO and (NiOH)2 [35,36]. Nickel sulfates are present as corrosion products on the surface in outdoor exposures [37].Jouenefatmospheric corrosion of nickel in industrial, urban, and rural atmospheres. Nickel corrodes through a pitting corrosion process. The highest corrosion rates were observed in industrial areas. The corrosion products were mainly sulfates, chlorides, and n ligible amounts of nitrates surrounded by carbonate species. The pitting corrosion process occurs in two steps on nickel surfaces exposed to an outdoor atmosphere, as shown in Fig. 10.9 [38]. 

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

A metal that resists corrosion in one atmosphere may lack effective corrosion resistance elsewhere hence, relative corrosion behavior of metals changes with location for example, galvanized iron performs well in rural atmospheres, but is relatively less resistant to industrial atmospheres. On the other hand, lead performs in an industrial atmosphere at least as well as, or better than, elsewhere because a protective film of lead sulfate forms on the surface. 

A high sulfuric acid content of industrial and urban atmospheres shortens the life of metal structures (see Tables 9.2 and 9.3). The effect is most pronounced for metals that are not particularly resistant to sulfuric acid, such as zinc, cadmium, nickel, and iron. It is less pronounced for metals that are more resistant to dilute sulfuric acid, such as lead, aluminum, and stainless steels. Copper, forming a protective basic copper sulfate film, is more resistant than nickel or 70% Ni-Cu alloy, on which the corresponding hlms are less protective. In the industrial atmosphere of Altoona, Pennsylvania, galvanized steel sheets [0.381 kg zinc per m, 0.028 mm thick (1.25 oz zinc per ft, 1.1 mil thick)] began to rust after 2.4 years, whereas in the rural atmosphere of State College, Pennsylvania, rust appeared only after 14.6 years [19]. 

Nickel is quite resistant to marine atmospheres, but is sensitive to sulfuric acid of industrial atmospheres (Table 9.2), forming a surface tarnish composed of basic nickel sulfate. Corrosion in the industrial atmosphere of New York City is about 30 times higher than in the marine atmosphere of La Jolla, California, and about 20 times higher than in the rural atmosphere of State College, Pennsylvania (Table 9.2). 

Stainless steels and aluminum resist tarnish in industrial, urban, and rural atmospheres, as is apparent from their satisfactory use over many years as... 

Zinc coatings are relatively resistant to rural atmospheres and also to marine atmospheres, except when seawater spray comes into direct contact with the surface. Table 14.1 lists the ranges of typical atmospheric corrosion rates in each of the three types of atmospheres, rural, marine, and urban/ industrial [18]. 

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