>>>Analyse information from secondary sources to summarise the industrial origins of sulfur dioxide and oxides of nitrogen and evaluate reasons for concern about their release into the environment
Sulfur Dioxide
The industrial origins of sulfur dioxide are from power plants burning fossil fuels, refineries, some chemical plants, pulp mills, smelters and marine vessels. It also is emitted when coal and oil is burnt or processed. These amounts of different emissions are shown in the pie graph, though it does not include natural sources. Nevertheless, two thirds of the release of sulfur dioxide comes from natural sources such as geothermal hot springs, volcanoes, bushfires and bacterial processes.
Coal generally contains 0.5-6% sulfur, which are mainly metallic sulfides. So in the combustion process sulfur is converted to sulfur dioxide. This reaction releases vast quantities of sulfur dioxide into the atmosphere.
S(in compound) + O2(g )→ SO2(g)
When metal is extracted from sulfide ores the first step is to actually roast the sulfide ore in air. This produces sulfur dioxide along with the metal or metal oxide. An example of this is when zinc is extracted from a sulfide ore:
When iron sulfide is reduced by heat, the sulfide ion is oxidised to produce sulfur dioxide. This release of sulfur dioxide may be collected or just released into the atmosphere.
4FeS2(s) + 11O2(g) → 2Fe2O3(s) + 8SO2(g)
Sulfur dioxide emitted into the atmosphere by industrial processes is ultimately converted into dilute sulfurous acid, returning to Earth as acid rain. For this reason, sulfur dioxide is a major cause of air pollution. Though the concentrations of sulfur dioxide do not increase dramatically..
The main health effects experienced are most related with the irritation of the respiratory system. Moderate exposure causes an irritation of to the lungs and throat, can decrease the effectiveness of the respiratory system’s defences against foreign particles and bacteria, and in some cases can causes a constriction of airways making it difficult to breathe. Still, sulfur dioxide can have increased toxicity when acidic pollutants, liquid or solid aerosols, and particulates are also in the ambient air. These effects are more commonly experienced by people who have head colds or who are exercising, especially along roads where the concentration of sulfur dioxide is high. When sulfur dioxide is inhaled the main health affects episodes of bronchitis, chronic coughing, difficulty in breathing, increased respiratory tract infections and symptoms of headaches, dizziness and nausea.
Plant species are easily injured by sulfur dioxide and effects include visible injury to sensitive plants (such as valuable pines, legumes, red and black oaks, white ash, alfalfa and blackberry). This is at low exposure of 0.12 ppm for 8 hours. At higher levels of exposures such as 0.30 ppm for 8 hours visible injury can be obtained on plants of intermediate sensitivity. However, sometimes it can have a positive benefit from low levels for very few species growing on sulfur deficient soils.
The other possible effects of sulfur dioxide include accelerating corrosion of metals through the formation of sulfuric acid, damaging stone and masonry, paint, various fibres, paper, leather, and electrical components. Also increased sulfur dioxide contributes to impaired visibility especially in the cities.
Nitrous Oxides
There are three common oxides of nitrogen, nitrous oxide, N2O, nitric oxide, NO, which are both neutral, and nitrogen dioxide, NO2, which is acidic.
Nitric oxide is naturally produced through lightning. Very high-localised temperatures generated by lightning, atmospheric oxygen and nitrogen gases combine to form nitric oxide:
2N2(g) + O2(g) → 2NO(g)
The main human activity that generates large amounts of nitric oxide is combustion in power plants, cars and trucks. At the high temperatures in the combustion chambers, oxygen and nitrogen combine to form nitric oxide (as demonstrated in the equation above). The natural source of nitrogen dioxide comes from nitric oxide slowly reacting with oxygen in the atmosphere to form it:
2NO(g) + O2(g) → 2NO2(g)
The natural source of nitrous oxide is formed by the action of certain bacteria on nitrogenous material in soils. This has increased with human activity through the increased use of nitrogenous fertiliser, which provides more raw materials for bacteria. This is a problem because it adds to the green house effect.
The main concern for the build up of the concentration of nitrous oxide is the physical and environmental factors. The health affects of nitrous oxide when inhaled include disorientation, fixated vision, throbbing or pulsating auditory hallucinations, similarly pulsating visual hallucinations, and an increased pain threshold. This however, only lasts for a little while before dissipating but it also depends on the amount inhaled. Also nitrous gas permeates the lipid membranes and produces anaesthetic effect, which is why it is commonly used in dental procedures. Although this procedure is only suitable for small quantities and a short period of time as larger doses can prove to be dangerous and harmful to one’s health.
Environmental factors mainly link to the enhanced greenhouse effect. As nitrous oxide concentrations have grown by 15% in the 1990’s and can remain in the atmosphere for several decades. This in turn adds to the greenhouse effect by keeping the earth warmer by retaining the warmth from the sun within the atmosphere and consequently leading to the increase in global temperatures. In turn, this has lead to increased concern as the rising temperatures affect the earth and show the need for a monitoring of greenhouse gases including nitrous oxide.
The release of these dangerous oxides must be minimised as they both contribute to the degradation of air quality and cause respiratory difficulties in humans. Both cause acid rain and, which leads to the destruction of buildings and monuments on a global scale that, are highly valued among some cultures. The environmental concern is that the acid rain caused by these two oxides can significantly lower the pH of some soils that plants are dependent on, and die out because of it. Fish and other aquatic life are also threatened as their lakes and rivers acidify.
>>>Assess the evidence which indicates increases in atmospheric concentrations of oxides of sulfur and nitrogen.
The main evidence that indicates an increase in atmospheric concentration of oxides of both sulfur and nitrogen is:
• The increase in the burning of fossil fuels that have high sulfur content, especially coal.
• The increasing number of cars in use
• The increase in factory gaseous discharges
• An increase in thunderstorm activity
It has become apparent that over the past two centuries, the world’s consumption of coal and other fossil fuel based products has increased dramatically. Statistics complied from across the planet show that the number of cars in use around the world is climbing exponentially, and that high temperatures inside internal combustion engines lead to inert nitrogen reacting with oxygen to produce nitric oxide (NO) which can react slowly with oxygen in the air to produce nitrogen dioxide (NO2).
Though the causes of these oxides can be measured, the oxides themselves in the atmosphere are hard to monitor. The technology for measuring the concentration of these oxides in such small a quantities (about 0.001 ppm) has only been available since the 1970’s, so atmospheric concentrations before then can only be assumed. NO2 and SO2 are very soluble, so a build up of these oxides isn’t possible (unlike CO2) as they are constantly dissolving in rain and washing out of the atmosphere that way.
There is currently not sufficient reliable evidence to make a statement about the concentrations of nitrogen and sulfur oxides in the atmosphere, as they are so small, and there is nothing to compare results to even if we did get an accurate measurement. More research over a more expansive period of time is required before any conclusions can be made about the concentrations of NO2 and SO2 in the atmosphere.
Figure 1 An estimate of the concentration of Nitrogen oxides in present in the atmosphere over the last two centuries.
In most large cities the annual average concentration of sulfur dioxide and nitrogen dioxide is about 0.01 ppm for each. (This is 10 time the value for clean air) but it is not harmful. Since the industrial revolution (early 1800s) sulfur dioxide emissions increased greatly due to the burning of coal (luckily Australian coal has a low percentage of sulfur 0.5%- 6%) unlike China. Air quality has decreased- indicated by pollution events across the world, for example the USA in the 1950’s - 60’s.
The fitting of gas absorbers onto factory flues helped to curb such emissions. But because sulfur dioxide and nitrogen dioxide are washed out of the atmosphere by rain, there appears to not have been any significant build-up of their concentrations over the last century (unlike carbon dioxide and 30% increase and nitrous oxide- which has increased by an even greater percentage). However, it is difficult to be sure about sulfur dioxide and nitrogen dioxide because there is a lack of reliable data prior to 1950. It has only been in the last few decades that we have been able to monitor concentrations of these gases below 0.1ppm.
Sulfur dioxide will always form from volcanic eruptions. Oxides of nitrogen will always form in the atmosphere by nitrogen and oxygen and lightening reactions. More research is required into collating and summarizing old events prior to the 1800s such as fish deaths, damage to pin forest or erosion of marble/limestone (all due to acid rain). It must also be noted that the heat from cities and the effects of global warming is spawning more thunderstorms, which translates to an increase in the production of nitrogen oxides.
