London smogprimary pollution

Urban pollution is largely the product of combustion processes. In ancient times cities such as Imperial Rome experienced pollution problems due to wood smoke. However, it was the transition to fossil fuel burning that caused the rapid development of air pollution problems. The inhabitants of London have burnt coal since the 13 th century. Concern and attempts to regulate coal burning began almost immediately, as there was a perceptible and rather strange smell associated with it. Medieval Londoners thought this smell might be associated with disease.

Fuels usually consist of hydrocarbons, except in particularly exotic applications such as rocketry, where nitrogen, aluminium (Al) and even beryllium (Be) are sometimes used. Normal fuel combustion is an oxidation reaction (see Box 4.3) and can be described:

fuel + oxygen ^ carbon dioxide + water

This would not seem an especially dangerous activity as neither CO2 nor water is particularly toxic. However, let us consider a situation where there is not enough O2 during combustion, i.e. as might occur inside an engine or boiler. The equation might now be written:

coal + oxygen ^ carbon monoxide + water

Here we have produced carbon monoxide (CO), a poisonous gas. With even less oxygen we can get carbon (i.e. smoke):

At low temperatures, in situations where there is relatively little O2, pyrolysis reactions (i.e. reactions where decomposition takes place as a result of heat) may cause a rearrangement of atoms that can lead to the formation of polycyclic aromatic hydrocarbons (see Section 2.7) during combustion. The most notorious of these is benzo[a]pyrene (B[a]P; see Fig. 2.4), a cancer-inducing compound.

Thus, although the combustion of fuels would initially seem a harmless activity, it can produce a range of pollutant carbon compounds. When the earliest steam engines were being designed, engineers saw that an excess of oxygen would help convert all the carbon to CO2. To overcome this they adopted a philosophy of 'burning your own smoke', even though this required considerable skill to implement and was consequently of only limited success.

In addition to these problems, contaminants within the fuel can also cause air pollution. The most common and worrisome impurity in fossil fuels is sulphur (S), partly present as the mineral pyrite, FeS2. There may be as much as 6% sulphur in some coals and this is converted to SO2 on combustion:

Table 3.4 Sulphur content of fuels.

Fuel

S (% by weight)

Coal

7.0-0.2

Fuel oils

4.0-0.5

Coke

2.5-1.5

Diesel fuel

0.9-0.3

Petrol

0.1

Kerosene

0.1

Wood

Very small

Natural gas

Very small

There are other impurities in fuels too, but sulphur has always been seen as most characteristic of the air pollution problems of cities.

If we look at the composition of various fuels (Table 3.4), we see that they contain quite variable amounts of sulphur. The highest amounts of sulphur are found in coals and in fuel oils. These are the fuels used in stationary sources such as boilers, furnaces (and traditionally steam engines), domestic chimneys, steam turbines and power stations. Thus, the main source of sulphur pollution, and indeed smoke, in the urban atmosphere is the stationary source. Smoke too is mainly associated with stationary sources. Steam trains and boats caused the occasional problem, but it was the stationary source that was most significant.

For many people, SO2 and smoke came to epitomize the traditional air pollution problems of cities. Smoke and SO2 are obviously primary pollutants because they are formed directly at a clearly evident pollutant source and enter the atmosphere in that form.

Classical air pollution incidents in London occurred under damp and foggy conditions in the winter. Fuel use was at its highest and the air near-stagnant. The presence of smoke and fog together led to the invention of the word smog (sm[oke and f]og), now often used to describe air pollution in general (Fig. 3.5). Sulphur dioxide is fairly soluble so could dissolve into the water that condensed around smoke particles.

Traces of metal contaminants (iron (Fe) or manganese (Mn)) catalysed the conversion of dissolved SO2 to H2SO4 (see Box 4.4 for a definition of catalyst).

Sulphuric acid has a great affinity for water so the droplet absorbed more water. Gradually the droplets grew and the fog thickened, attaining very low pH values (see Box 3.5).

Terrible fogs plagued London at the turn of the last century when Sherlock Holmes and Jack the Ripper paced the streets of the metropolis. The incidence of bronchial disease invariably rose at times of prolonged winter fog —little wonder, considering that the fog droplets contained H2SO4. Medical registrars in

Fig. 3.5 The London smog of 1952. Photograph courtesy of Popperfoto Northampton, UK.

Victorian England realized that the fogs were affecting health, but they, along with others, were not able to legislate smoke out of existence. Even where there was a will, and indeed there were enthusiasts in both Europe and North America who strove for change, the technology was far too naive to achieve really noticeable improvements. The improvements that did come about were often due to changes in fuel, in location of industry or in climate.

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