8/3/2019 Assess the Evidence That Indicates the Increase in Atmospheric Concentration of Sulphur and Nitrogen Oxides

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NSBHS Y12 Chemistry Alexander Peng

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Assess the evidence that indicates the increase in atmospheric

concentration of sulfur and nitrogen oxidesNatural sources of sulfur dioxide include the decomposition of organic matter by bacteria to produce

H2S which then is oxidised according to the equation 2H2S(g) + 3O2(g) 2SO2(g) + 2H2O(l). It is also

released by volcanic events such as eruptions from volcanos and geysers and is also released in

smoke from bushfires. As there has been no significant increase in volcanic eruptions or bushfires

then it is unlikely that this has caused an increase in atmospheric concentration of sulfur oxides.

Human sources of SO2(g) include the burning of fossil fuels, particularly coal and petroleum, and

industrial plants involving the smelting of sulfide ores.

Sulfide ores are smelted according to the equations:

Copper (Chalcopyrite): 2CuFeS2(s) + 5O2(g) + 2SiO2(s) 2Cu(l) + 2FeSiO3(l)+ 4SO2(g)Zinc: 2ZnS(s) + 3O2 2ZnO(s) + 2SO2(g)

This is because fuels often contain compounds of sulfur as impurities. For example, in coal, iron

sulfide (FeS2) is often found as an impurity, and when burnt it is oxidised to produce sulfur dioxideaccording to:

4FeS2(s) + 11O2(g) 8SO2(g) + 2Fe2O3(s)

As the amount of fuel combusted and ores smelted has been steadily increasing, then it is probable

that atmospheric concentration of sulfur oxides has been increasing too.

Similarly, nitrogen oxides are also naturally formed in the atmosphere during thunderstorms due to

the extreme heat of lightning. The heat causes the nitrogen molecules to break into their atomic

states and bond with either nitrogen or oxygen gases as the heat from the lightning dissipates. The

reactions can be represented as N2(g) + O2(g) 2NO(g) for nitric oxide and 2NO(g) + O2(g) 2NO2(g) for

nitrogen dioxide. As the number of lightning strikes is not increasing, this does not indicate an

increase in nitrogen oxides.

However, nitrogen oxides are also formed under high

temperatures found in the combustion of fuels, particularly in

car exhausts and thermal power stations. The high

temperatures, like those caused by lightning, provide the

activation energy for nitrogen and oxygen to react and form

NOx compounds. The comparative abundance of NO2 gases in

regions where millions of these engines exist indicates that

they have a significant effect on atmospheric concentration.

Quantitative analysis of Antarctic ice core samples by the CSIRO has shown steady increases in

concentrations of these oxides in recent times.

While the annual averages in cities has not increased significantly, the number of days where

concentrations have been above safe levels has increased dramatically.

However levels of these oxides are below 0.01ppm and difficult to detect and measure. There is no

reliable data before the 1950s, and it wasnt until the 1970s that sensitive gas analysis techniques

Figure 1Nitrogen Dioxide 2009

tropospheric density

8/3/2019 Assess the Evidence That Indicates the Increase in Atmospheric Concentration of Sulphur and Nitrogen Oxides

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NSBHS Y12 Chemistry Alexander Peng

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allowed accurate and close monitoring of levels. Both SO2 and NO2 form sulphate and nitrate ions

which are hard to detect and measure.

Localised evidence also suggests increasing concentrations of SO2 and NO2, as the incidence of acid

rain has been increasing along with photochemical smog especially in Europe and China. Although

this is likely due to increasing concentrations of SO2 and NO2 there are other variables that could

cause or affect these events.

While some sulfur and nitrogen oxides are produced naturally, the amount released from natural

sources is not sufficient to cause a noticeable increase atmospheric concentration of these oxides.

The increase in acid rain in many industrial areas indicates a rise in acidic atmospheric non-metal

oxides due to emissions produced by industrial processes and analysis of artic core samples shows a

general trend worldwide of increasing concentrations of SOx and NOx gases. It is therefore probable

that the increase in SOx and NOx gases is largely human caused.