Atmosphere sulphur dioxide

The European Commission has adopted a Proposal for a Directive on national emissions ceilings for certain atmospheric pollutants and a Proposal for a Directive relating to ozone in ambient air. The national emissions ceilings Directive will set individual limits for each Member State s total emissions in 2010 of the four pollutants responsible for acidification, eutrophication and ozone formation in the lower atmosphere sulphur dioxide, nitrogen oxides, VOCs and ammonia. The EU Solvents Directive has been formally adopted by the Commission. 

Bluth G. J. S., Schnetzler C. C., Krueger A. J., and Walter L. S. (1993) The contribution of explosive volcanism to global atmospheric sulphur dioxide concentrations. Nature 366, 327-329. 

As to its ejection to the atmosphere, sulphur dioxide exceeds all other pollutants (more than 130 ml. t. per year). The main quantity of sulphur dioxide is ejected into the atmosphere by power plants (50.3%), automotive transport (20.0%), ferrous and nonTerrous metallurgy (25.8%), and the petrochemical industry... 

Scaringelli, F.P., Saltzman, B.E. and Frey, S.A., 1967. Spectrophotometric determination of atmospheric sulphur dioxide. Anal. Chem., 39 1709-1719. 

Far-infrared absorption spectra of H02 in the gas phase have been detected with a water-vapour laser magnetic resonance spectrometer.40 The identification of H02 as the absorbing species is based on a partial analysis of the spectra, and on a variety of different chemical methods used to produce the radical. Using the photochemical 1802 competitive isotopelabelling technique, rate constants have been determined for the reaction of the hydroperoxyl radical with atmospheric sulphur dioxide.41 All measurements were made relative to the disproportionation reaction ... 

Fig. 7.18 The main natural and anthropogenic routes for atmospheric sulphur dioxide and sulphate.

The OECD project showed that the acidification of precipitation was due to an increased content of sulphuric and nitric acid. The main cause was identified as the increased use of fossil fuels, including motor vehicle traffic. In the atmosphere, sulphur dioxide and nitrogen oxides from combustion processes are oxidized to sulphuric acid and nitric acid, which are taken up and deposited by the precipitation. 

As already discussed, we can identify the major substances which pollute the atmosphere sulphur dioxide and nitrogen oxides, and products of their transformation. During the interaction with water droplets and falling rain, the acids and their salts (even more toxic) are formed from these gases. Acid rain leads to considerable negative ecological effects. 

Sulphur dioxide - sulphur dioxide (SO2) is a colourless gas it reacts on the surface of a variety of airborne solid particles. It is soluble in water and can be oxidised within airborne water droplets. The major health hazards associated with exposure to high concentrations of sulphur dioxide include affects on breathing, respiratory illness, alterations in pulmonary defences and aggravation of existing cardiovascular disease. In the atmosphere, sulphur dioxide mixes with water vapour produoing sulphuric acid. This acidic pollution can be transported by wind over many hundreds of miles, and when present in sufficient quantities can be deposited as acid rain. 

On the industrial scale it is produced in large quantities for the manufacture of sulphuric acid and the production methods are dealt with later. It was once estimated that more than 4 000 000 tons of sulphur dioxide a year entered the atmosphere of Britain from the burning of coal and oil. 

The UK Environment Agency deals with over 6000 oil pollution incidents each year. One estimate suggests tliat tlie cheiTtical industry contributes to 50% of all ah pollution witli proportions approximating to sulphur dioxide (36%), carbon dioxide (28%), nitrogen oxides (18%), carbon monoxide (14%) and black smoke (10%). Motor spirit refining is responsible for ca 26% of emissions of volatile organic compounds to the atmosphere. In 1996 there were over 20 000 reports of water pollution incidents with 155 successful prosecutions. 

Sulphur dioxide plays such an important role in the corrosion of metals in the atmospheres of industrialised countries that detailed consideration of its action seems justified. For all metals SO appears to be selectively adsorbed from the atmosphere, less so for aluminium than for other metals, and for rusty steel it is almost quantitatively adsorbed even from dry air at 0°C Under humid conditions sulphuric acid is formed, the oxidation of SOj to SOj being catalysed by metals and by metallic oxides. 

This important fact was first demonstrated by Vernon in a series of classical experiments, some of which are summarised graphically in Fig. 3.1. He showed that rusting is minimal in pure air of less than 100 l o relative humidity but that in the presence of minute concentrations of impurities, such as sulphur dioxide, serious rusting can occur without visible precipitation of moisture once the relative humidity of the air rises above a critical and comparatively low value. This value depends to some extent upon the nature of the atmospheric pollution, but, when sulphur dioxide is present, it is in the region of 70-80%. Below the critical humidity, rusting is inappreciable, even in polluted air. 

In most districts, however, sulphur dioxide and dust particles are the main corrosive pollutants. It has been demonstrated that there is a direct relationship between sulphur dioxide in the atmosphere and the corrosion of steel exposed to it (see Fig. 3.2). In a series of tests carried out in the Sheffield area, sulphur dioxide accounted for about 50% of the variations in corrosion rate at the different sites. ... 

The significance of the amount of sulphur dioxide rather than the concentration has been demonstrated by other workers who have studied the effects of atmospheric flow rate. An increase in steel corrosion with increase in atmospheric flow rate at a constant volume concentration of sulphur... 

Sulphur dioxide in the air originates from the combustion of fuel and influences rusting in a number of ways. For example, Russian workers consider that it acts as a cathodic depolariser , which is far more effective than dissolved oxygen in stimulating the corrosion rate. However, it is the series of anodic reactions culminating in the formation of ferrous sulphate that are generally considered to be of particular importance. Sulphur dioxide in the air is oxidised to sulphur trioxide, which reacts with moisture to form sulphuric acid, and this in turn reacts with the steel to form ferrous sulphate. Examination of rust Aims formed in industrial atmospheres have shown that 5% or more of the rust is present in the form of iron sulphates and FeS04 4H2 0 has been identified in shallow pits . 

Aggressive environments include marine conditions and particularly industrial atmospheres containing high concentrations of acid gases such as sulphur dioxide rain washing is benehcial in both environments, while dampness and condensation alone can accentuate the rate of attack in the presence of chlorides and acidic sulphates. 

Studies have been carried out of the effects at high temperatures of sulphurand of atmospheres containing hydrogen sulphidesteam sulphur dioxide and hydrogen chloride . 

The atmospheric pollution prevailing in special industrial or laboratory locations may induce more severe corrosion, e.g. the vapours from concentrated hydrochloric or acetic acid will etch tin, and moist sulphur dioxide will produce a sulphide tarnish, as will hydrogen sulphide at temperatures above about 100°C. The halogens attack tin readily. The commonly used volatile corrosion inhibitors are without adverse action although the benefit derived from their use is doubtful. 

Industrial atmospheres usually accelerate the corrosion of zinc. When heavy mists and dews occur in these areas, they are contaminated with considerable amounts of acid substances such as sulphur dioxide, and the film of moisture covering the metal can be quite acid and can have a pH as low as 3. Under these conditions the zinc is dissolved but, as the corrosion proceeds, the pH rises, and when it has reached a sufficiently high level basic salts are once more formed and provide further protection for the metal. These are usually the basic carbonate but may sometimes be a basic sulphate. As soon as the pH of the moisture film falls again, owing to the solution of acid gases, the protective film dissolves and renewed attack on the metal occurs. Hudson and Stanners conducted tests at various locations in order to determine the effect of atmospheric pollution on the rate of corrosion of steel and zinc. Their figures for zinc are given in Table 4.34 and clearly show the effect which industrial contamination has on the corrosion rate. 

Ambler has attempted to find a relationship between the corrosion of zinc and iron and atmospheric salinity in the UK. This followed previous tests in Nigeria, when it was concluded that the governing factor in the corrosion of steel and zinc was airborne salt and that there was a relationship between corrosion and the distance from the sea. In the UK, however, no such relationship was found to exist, and the governing factor in the corrosion of zinc in the atmosphere is confirmed to be the amount of sulphur dioxide pollution. 

The composition of the atmosphere to which components at high temperature may be exposed varies very widely, and most work on these aspects has accordingly been carried out in clean air. The aggressiveness of air is considerably enhanced by the presence of trace amounts of other reactive gases such as steam, carbon dioxide and sulphur dioxide thus the figures subsequently quoted may in fact be appreciably lower than those encountered in specific atmospheres. The data presented should, however, prove an adequate guide to the order of the effect to be expected. 

The atmosphere most commonly encountered by irons at elevated temperatures is probably air contaminated with traces of carbon dioxide, sulphur dioxide and steam. Because of the difficulty of defining such an atmosphere, most of the work on scaling has been carried out with relatively... 

In most of its uses, e.g. the external surfaces of tinplate cans, tinned steel has only to resist condensed moisture. In the absence of pollution of the atmosphere by unusually large amounts of sulphur dioxide or chlorides, or of several days of continuous wetting, tinned steel remains unrusted even the thin porous coatings on the common grades of tinplate remain bright and unmarked over the periods involved in the commercial handling and domestic storage of cans, and the domestic use of kitchenware. When... 

The deposit resists atmospheric tarnish even in the presence of high pollution by sulphur dioxide (in contrast to nickel) and hydrogen sulphide, and coatings exposed to the outdoor atmosphere remain bright indefinitely, sometimes taking on a slightly more pink colour as the oxide film thickens. 

Atmospheric exposure trials, carried out in Cambridge, established the fact that when rusty specimens were painted in the summer, their condition, after some years exposure, was very much better than that of similar specimens painted in the winter It was found that steel weathered in Cambridge carried spots of ferrous sulphate, deeply imbedded in the rust, and that the quantity of ferrous sulphate/unit area was very much greater in the winter than in the summer this seasonal variation was attributed to the increased sulphur dioxide pollution of the atmosphere in the winter, caused by the combustion of coal in open grates. It was concluded that there was a causal relationship between the quantity of ferrous sulphate and the effective life of the paint. It was suggested that these soluble deposits of ferrous sulphate short-circuit the resistance of the paint film and, since paint films are very permeable to water and oxygen, the ferrous sulphate will become oxidised and hydrolysed with the production of voluminous rust, which will rupture the film at numerous points, thus giving rise to the characteristic type of failure seen on painted rusty surfaces. 

Atmospheres polluted by oxidising agents, e.g. ozone, chlorine, peroxide, etc. whose great destructive power is in direct proportion to the temperature, are also encountered. Sulphuric acid, formed by sulphur dioxide pollution, will accelerate the breakdown of paint, particularly oil-based films. Paint media resistant both to acids, depending on concentration and temperature, and oxidation include those containing bitumen, acrylic resins, chlorinated or cyclised rubber, epoxy and polyurethane/coal tar combinations, phenolic resins and p.v.c. 

Exposure of the samples to a controlled moist atmosphere containing sulphur dioxide, as recommended in BS 1615 1972, Method H, is an example of a test bridging the gap between sealing tests and accelerated corrosion tests. After exposure for 24 h at 25 2°C, poorly sealed films show a persistent heavy white bloom, while good sealing produces at the most a slight superficial bloom. 

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