Washout acid rain

In the case of washout of sulfur dioxide (S02), a precursor of acid rain, the high solubility and the chemical reactivity of aqueous S02 result in nonattainment of equilibrium. Thus, semiempirical models have been proposed for... 

The contaminants may be deposited on the surfaces of the materials in the form of anhydrous or hydrated species. Some pollutants, like CO2, SO, NO, and HCl, are typical of urban and industrial areas, give rise to acid rains, and might contribute to the cathodic processes, while others, such as chlorides, are typical but not exclusive of marine and coastal areas and give rise to hygroscopic salts that increase the duration of wetting of surfaces, increase the conductivity of solutions, and make less protective the corrosion products. Some others, such as the sulfides, which can result from microbiological activity, alter the composition of the corrosion products, their protective capability, and the nobility of the metal often they are semiconductors, depolarize the cathodic process of hydrogen evolution, and may be oxidized to sulfuric acid by bacteria. Ammonia alters the composition of corrosion products and the solubility of metal ions it has particularly drastic effects on copper alloys and their corrosion forms. In the transport of these contaminants toward the surfaces, an important role is exerted by the wind and by the orientation of the surfaces, which can promote or hinder the washout by the rains. 

In the case of washout of sulfur dioxide (SO2), a precursor of acid rain, the high solubility and the chemical reactivity of aqueous SO2 result in nonattainment of equilibrium. Thus, semiempirical models have been proposed for the SO2 concentration in air beneath the rain-forming cloud. These models lump together all processes affecting SO2 removal from air (i.e., dissolution into water droplets, hydration, oxidation, and ionization). A first-order decay constant for the SO2 concentration. A, varies with the rainfall characteristics (rainfall rate and size of raindrops). Boubel et al. (1994) suggest a value of A equal to... 

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. 

Initial rainfall from a given event is the most acid, because of rain washout of atmospheric acid gases and aerosols. This effect produces initial rain pH s below 3 in central Pennsylvania, for example. In cities such as Los Angeles and London, when fog has been present and the air stagnant, pH values between 2 and 3 have been observed, due in part to evaporative concentration, with serious consequences for people with respiratory problems. 

In the atmosphere, ammonia can be removed by rain or snow washout. Reactions with acidic substances, such as H2SO4, HCl, or HNO3 (all produced in high concentrations from anthropogenic activities) produce ammonium aerosols, which can undergo dry or wet deposition. The gas phase reaction of ammonia with photochemically produced hydroxyl radicals is thought to contribute about 10% to the overall atmospheric removal process. The best estimate of the half-life of atmospheric ammonia is a few days. 

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