Atmospheric corrosion aerosol particles

Similarly, SO2 and SO3 (SOJ compounds are produced in combustion by the oxidation of sulfur compounds within the fuel source. SO , emitted into the atmosphere can be incorporated into aerosol particles and wet-deposited as corrosive sulfuric acid. Both NO , and SO , emissions contribute to acid rain content from wet deposition, due to their participation in the formation of nitric and sulfuric acid, respectively. 

The study of the chemical composition of precipitation is of interest for the evaluation of many scientific and practical problems. Thus, a knowledge of the chemical composition makes it possible to evaluate the degree of air pollution, the sink terms in the atmospheric cycle of aerosol particles and water soluble gases, the corrosiveness of atmospheric waters and the effect of precipitation on the material balance of soils, waters and vegetation. For these reasons, many precipitation analyses have been done, mostly in the last three decades. In this book only those works are mentioned in which an extensive network were (or are) operated, as in the American, Swedish and Soviet programs. 

Aerosol particles are chemical mixtures of a number of ionic species, including ammonium, sulfate, chloride, nitrate, and hydrogen. The actual chemical mixture of each particle reflects the chemical conditions in which the particle was formed. Once adsorbed on a metal surface, the water-soluble part of each particle acquires water from the atmosphere and deliquesces, whereby it transforms into a concentrated aqueous solution. The ionic constituents that are liberated into the aqueous film may have a significant influence on the atmospheric corrosion processes. Moreover, many particles possess hygroscopic properties and retain water. Lienee, a typical particle may triple its volume when the relative humidity increases from dry to 90%. 

One excellent example of a multiana-lytical laboratory study is the influence of submicron sized particles of ammonium sulfate ((NH4)2S04) on the atmospheric corrosion of selected metals [22]. These particles were aerosolized and deposited under dry conditions on the metal surface, whereby the deposited amounts corresponded to up to 10 years of exposure in indoor locations of USA. By introducing humidity into the exposure... 

Atmospheric corrosion rates are commonly related to a critical relative humidity , above which the corrosion rate increases significantly and below which the rate is insignificant for many practical purposes. Depending on metal and exposure conditions, critical relative humidities have been reported in the range from 50 to 90%. The critical relative humidity is associated with the point of deliquescence of deposited aerosol particles, above which the aerosols rapidly absorb water until a saturated solution is obtained. For a single-phase aerosol, there is a well-defined critical relative humidity, whereas for a mixture of phases (the common situation in natural outdoor environments) the critical relative humidity is lower than those of the single phases. 

As discussed in Section 6.5, water vapor and the heat released and absorbed by transitions of water between the vapor state and the liquid or solid state are strongly involved in atmospheric energy transfer. Condensed water vapor in the form of very small droplets is of considerable concern in atmospheric chemistry. The harmful effects of some air pollutants—for instance, the corrosion of metals by acid-forming gases—requires the presence of water, which may come from the atmosphere. Atmospheric water vapor has an important influence on pollution-induced fog formation under some circumstances. Water vapor interacting with pollutant particulate matter in the atmosphere may reduce visibility to undesirable levels through the formation of very small atmospheric aerosol particles. 

Of utmost importance in atmospheric corrosion is the presence of particles and aerosols (an ensemble of particles suspended in the air) of mostly chlorides, sulfates, and nitrates. The size, shape, and chemical and physical properties of these particles and aerosols can vary widely. A more detailed description of particles and their role in (indoor) atmospheric corrosion is given elsewhere [20]. 

Sulfur dioxide stimulates oxidation. It is the major driving force for corrosion in metropolitan areas. Most of the sulfur acquired by surface is not in the form of gas but as dry deposition. In an urban atmosphere, SO is abundantly found in aerosol particles. Large particles containing ammonia are also found. H2S, SO2 and COS in all these participate directly in the corrosion process. The sulfur compound, COS, hydrolyzes to form H2S and it may form CU2S if the quantity of COS is abundant on the other hand, SO2 may hydrolyze to form a bisulfate ion. 

The oceans of the world are an important natural source of pollutant material. The ocean is continually emitting aerosols to the atmosphere, in the form of salt particles, which are corrosive to metals and paints. The action of waves on rocks reduces them to sand, which may eventually become airborne. Even the shells washed up on the beach are eroded by wave and tidal action until they are reduced to such a small size that they too may become airborne. 

Absorption route No data on absorption of particlss Into the body. Evaporation negligible at 20° C, but harmful atmospheric concentrations can build up rapidly in aerosol form, immediate effeete In high concentrations Is corrosive to the eyes and respiratory tract. Inhalations of high concentrations of particles can cause lung edema. Can affeot the respiratory tract. Exposure to high ooncentrations can be fatal. Keep under medical observation. ... 

Of course, breathing air containing these aerosol droplets is harmful because of the corrosive nature of the acids. The acids in turn can react with ammonia or metallic particles in the atmosphere to produce nitrate or nitrite salts. For example,... 

At an air temperature of 283 K (10 °C), an air pressure of 1,013 hPa and 60% relative humidity the water content is around 5.7 g/m. At 303 K (30 °C) and relative humidity of 100%, the water content rises to 31.4 g/m. The water film that condenses when the temperature drops or on relatively cold surfaces is always saturated with oxygen. Whereas corrosive action in a non-marine atmosphere is mainly determined by moisture content and potential industrial contaminations, the marine atmosphere is characterised by a raised content of salt particles carried on the wind from the sea spray. Since the salt particles deposited on the metal surface, or aerosols containing salt, also contain hygroscopic components, e.g. calcium and magnesium chlorides, liquid films form on the surface with very high salt content levels, even if the air is still above the dewpoint. 

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