Acidic air pollutants

The three ingredients—oxygen, water, and ions—needed for the corrosion of metals are present almost everywhere on Earth. Even pure rainwater contains a few HsO" and HCO3 ions from dissolved carbon dioxide. Higher ion concentrations—from airborne salt near the ocean, from acidic air pollutants, or from salts spread on icy roads—make corrosion worse in certain areas. 

Acidic air pollution is created also when nitrogen oxides from car exhausts combine with atmospheric moisture to form nitric acid. In either case, when this acidic moisture falls to Earth as rain or snow, it is known as acid rain. You can model the formation of acid rain in the minlLAB on this page. 

In the atmosphere, ammonia is estimated to have a half-life of several days. The primary fate process is reaction of ammonia with acid air pollutants and removal of the resulting ammonium compounds by dry or wet deposition. Rain washout and reaction with photochemically produced hydroxyl radicals are also expected to contribute to the atmospheric fate of vapor-phase ammonia. In water and soil, ammonia will volatilize to the atmosphere and be removed by microbial processes, by adsorption to sediment and soil matrices as well as by plant uptake. 

While the caveats listed above should not be unde rest i mated/ we believe that this analysis provides more oorvincing evidence than was heretofore available that damages to materials from acidic air pollutants are economically important. This outcome is attributed to improvements in the data describing the joint distribution of materials and air pollutants and in the damage functions. The non-comprehensive coverage of sectors and materials and the substantial uncertainty reported are strong arguments for further research. 

The earliest reports of acute environmental incidents were of air pollution emergencies. In the Meuse Valley of Belgium during 1930, 63 persons died and thousands became ill from sulfur dioxide and sulfuric acid air pollution (French 1989). The Donora, Pennsylvania, smog disaster in 1948 left 20 dead and more than 5,000 ill (French 1989). 

In air, a dominant fate process for ammonia is the reaction with acid air pollutants. Formation of particulate NH/ compounds by reactions with HNO3 and H2SO4 is rapid (Bouwman et al. 1997 Irwin and Williams 1988). The extent to which this process serves as a removal mechanism depends on the local concentrations of these acidic compounds (Goulding et al. 1998). Thus, it is likely more important in areas of high industrial activity, but of lesser importance over rural areas. These ammonium compounds can then be removed by dry or wet deposition. 

Granat s chemical relationship for European rainwater assumes the following sources and acidities/alkalinities sulfuric acid from air pollutants with two equivalents of acidity per mole, nitric acid from air pollutants with one equivalent of acidity per mole, ammonia from air pollutants with one equivalent of alkalinity per mole, sea salt with negligible alkalinity, calcium soil dust with two equivalents of alkalinity per mole, magnesium soil dust with two equivalents of alkalinity per mole, and potassium soil dust with one equivalent of alkalinity per mole(3). Cogbill and Likens added hydrochloric acid air pollutants with one equivalent of acidity per mole for the northeastern United States calculations(4). 

Ito K, Thurston GD, Hayes C, Lippmann M. Associations of London, England daily mortality with particulate matter, sulfur dioxide, and acidic air pollution. Arch Environ Health 1993 48 213-220. 

Receptors. The receptor can be a person, animal, plant, material, or ecosystem. The criteria and hazardous air pollutants were so designated because, at sufficient concentrations, they can cause adverse health effects to human receptors. Some of the criteria pollutants also cause damage to plant receptors. An Air QuaUty Criteria Document (12) exists for each criteria pollutant and these documents summarize the most current Hterature concerning the effects of criteria pollutants on human health, animals, vegetation, and materials. The receptors which have generated much concern regarding acid deposition are certain aquatic and forest ecosystems, and there is also some concern that acid deposition adversely affects some materials. 

The indirect hydration, also called the sulfuric acid process, practiced by the three U.S. domestic producers, was the only process used worldwide until ICI started up the first commercial direct hydration process in 1951. Both processes use propylene and water as raw materials. Early problems of high corrosion, high energy costs, and air pollution using the indirect process led to the development of the direct hydration process in Europe. However, a high purity propylene feedstock is required. In the indirect hydration process, C -feedstock streams from refinery off-gases containing only 40—60 wt % propylene are often used in the United States. 

Isopropyl Alcohol. Propylene may be easily hydrolyzed to isopropyl alcohol. Eady commercial processes involved the use of sulfuric acid in an indirect process (100). The disadvantage was the need to reconcentrate the sulfuric acid after hydrolysis. Direct catalytic hydration of propylene to 2-propanol followed commercialization of the sulfuric acid process and eliniinated the need for acid reconcentration, thus reducing corrosion problems, energy use, and air pollution by SO2 and organic sulfur compounds. Gas-phase hydration takes place over supported oxides of tungsten at 540 K and 25... 

Sulfuric acid may be produced by the contact process from a wide range of sulfur-bearing raw materials by several different process variants, depending largely on the raw material used. In some cases sulfuric acid is made as a by-product of other operations, primarily as an economical or convenient means of minimising air pollution (qv) or disposing of unwanted by-products. 

In the early 1970s, air pollution requirements led to the adoption of the double contact or double absorption process, which provides overall conversions of better than 99.7%. The double absorption process employs the principle of intermediate removal of the reaction product, ie, SO, to obtain favorable equiUbria and kinetics in later stages of the reaction. A few single absorption plants are stiU being built in some areas of the world, or where special circumstances exist, but most industriali2ed nations have emission standards that cannot be achieved without utili2ing double absorption or tad-gas scmbbers. A discussion of sulfuric acid plant air emissions, control measures, and emissions calculations can be found in Reference 98. 

Air pollution (qv) problems are characteri2ed by their scale and the types of pollutants involved. Pollutants are classified as being either primary, that is emitted direcdy, or secondary, ie, formed in the atmosphere through chemical or physical processes. Examples of primary pollutants are carbon monoxide [630-08-0] (qv), CO, lead [7439-92-1] (qv), Pb, chlorofluorocarbons, and many toxic compounds. Notable secondary pollutants include o2one [10028-15-6] (qv), O, which is formed in the troposphere by reactions of nitrogen oxides (NO ) and reactive organic gases (ROG), and sulfuric and nitric acids. 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

Air pollution problems and labor costs have led to the closing of older pyrometaHurgical plants, and to increased electrolytic production. On a worldwide basis, 77% of total 2inc production in 1985 was by the electrolytic process (4). In electrolytic 2inc plants, the calcined material is dissolved in aqueous sulfuric acid, usually spent electrolyte from the electrolytic cells. Residual soHds are generally separated from the leach solution by decantation and the clarified solution is then treated with 2inc dust to remove cadmium and other impurities. 

Biological processes are also being studied to investigate abiHty to remove sulfur species in order to remove potential contributors to acid rain (see Air pollution). These species include benzothiophene-type materials, which are the most difficult to remove chemically, as weU as pyritic material. The pyrite may be treated to enhance the abiHty of flotation processes to separate the mineral from the combustible parts of the coal. Genetic engineering (qv) techniques are being appHed to develop more effective species. 

Materials The damage that air pollutants can do to some materials is well known ozone in photochemical smog cracks rubber, weakens fabrics, and fades dyes hydrogen sulfide tarnishes silver smoke dirties laundry acid aerosols ruin nylon hose. Among the most important effects are discoloration, corrosion, the soiling of goods, and impairment of visibility. 

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