Box 36 Reactions in photochemical smog

Reactions involving nitrogen oxides (NO and NO2) and ozone (O3) lie at the heart of photochemical smog.


where M represents a 'third body' (Section 3.10.1)

It is conventional to imagine these processes that destroy and produce nitrogen dioxide (NO2) as in a kind of equilibrium, which is represented by a notional equilibrium constant relating the partial pressures of the two nitrogen oxides and O3:

If we were to increase NO2 concentrations (in a way that did not use O3), then the equilibrium could be maintained by increasing O3 concentrations. This happens in the photochemical smog through the mediation of hydroxyl (OH) radicals in the oxidation of hydrocarbons. Here we will use methane (CH4) as a simple example of the process:


These reactions represent the conversion of nitric oxide (NO) to NO2 and a simple alkane (see Section 2.7) such as CH4 to an aldehyde (see Table 2.1), here formaldehyde (HCHO). Note that the OH radical is regenerated, so can be thought of as a kind of catalyst. Although the reaction will happen in photochemical smog, the attack of the OH radical is much faster on larger and more complex organic molecules. Aldehydes such as acetaldehyde (CH3CHO) may also undergo attack by OH radicals:



The methyl radical (CH3) in equation 13 may re-enter at equation 6. An important branch to this set of reactions is:




eqn. 14

leading to the formation of the eye irritant peroxyacetylnitrate (PAN).

There is no fog when Los Angeles smog forms, and visibility does not decline to just a few metres, as was typical of London fogs. Of course, the Los Angeles smog forms best on sunny days. London fogs are blown away by wind, but the gentle sea breezes in the Los Angeles basin can hold the pollution in against the mountains and prevent it from escaping out to sea. The pollution cannot rise in the atmosphere because it is trapped by an inversion layer: the air at ground level is cooler than that aloft, so that a cap of warm air prevents the cooler air from rising and dispersing the pollutants. A fuller list of the differences between Los Angeles-and London-type smogs is given in Table 3.5.

Table 3.5 Comparison of Los Angeles and London smog. From Raiswell et al. (1980).

Characteristic Los Angeles London

Table 3.5 Comparison of Los Angeles and London smog. From Raiswell et al. (1980).

Characteristic Los Angeles London

Air temperature

24 to 32°C

-1 to 4°C

Relative humidity


85% (+ fog)

Type of temperature inversion

Subsidence, at 1000 m

Radiation (near ground) at a few hundred metres

Wind speed





<30 m

Months of most frequent occurrence

Aug. to Sept.

Dec. to Jan.

Major fuels


Coal and petroleum products

Principal constituents

O3, NO, NO2, CO, organic matter

Particulate matter, CO, S compounds

Type of chemical reaction



Time of maximum occurrence


Early morning

Principal health effects

Temporary eye irritation (PAN)

Bronchial irritation, coughing (SO2/smoke)

Materials damaged

Rubber cracked (O3)

Iron, concrete

3.6.3 21st-century particulate pollution

It is valid to ask what might be

different about pollution

in the early 21st century.

One of the most notable issues in the last decade or so has been a rise in concern about fine particles (or aerosols) in the atmosphere. Some of this concern has come about because fine particles are now more noticeable because we have lessened the emission of many pollutant gases and smoke into the atmosphere. In some cases the concentrations of these particles have increased in urban air. There has also been a growing awareness that fine particles have a significant impact on health.

Fine particles are those that are respirable. Traditionally this would have been particles less than 10 mm in diameter that can make their way into the respiratory system. These particles, often referred to as PM-10 (PM is short for particulate matter), are usually accompanied by even finer particles about 2.5 mm called PM-2.5. These finer particles can go deep into the lung and become deposited in the alveoli, the terminal sacs of the airways, where gases are exchanged with the blood. Once in the alveoli various biochemical processes seek to combat the invading particles which ultimately place the individual under increased stress and at risk from a range of health effects.

These fine particles come from a range of sources including some that come from combustion processes. In the late 20th century the increasing importance of the diesel engine in vehicles added to the fine particle concentrations of European cities. The diesel engine can emit very small particles, perhaps only 0.1 mm across, but these readily coagulate into somewhat larger particles. These particles can also become coated with a range of organic compounds, which have the potential to be carcinogenic contributing to long-term health impacts.

Reactions in the atmosphere lead to the formation of secondary particles. The best known of these are sulphate particles from the oxidation of SO2 (eqns. 3.20 & 3.21). These particles are usually acid, although partial neutralization to ammonium bisulphate (NH4HSO4) is also possible. These particles have the potential to have additional irritant effects on the respiratory system. In recent years there has been a rising interest in the organic fraction of secondary aerosols with an awareness of the complexity of its chemistry. When volatile organic substances (see Box 4.14) react in the atmosphere they are typically converted to aldehydes, ketones (see Table 2.1) and organic acids. These more oxidized organic compounds are usually less volatile and can become associated with particles. Oxalic acid, a dicarboxylic acid (HOOC.COOH), is a highly oxidized small organic compound and is typical of the oxidation products found in the modern urban atmosphere. It is a relatively strong acid and not at all volatile, so readily incorporated into fine particles. This acid is also able to form complexes (see Box 6.4) with metals such as iron in the aerosol particles. Concern about the health impacts of small primary and secondary particles has driven much research into aerosols in urban air.

The 1990s was also a period when there was an increased awareness of the transport of pollutants from large-scale forest fires into areas with large populations. This was most notably reported in terms of smoke from tropical fires in South East Asia, although there were also worries about carbon monoxide from fires spreading into cities of the USA. In China, Korea and Japan there have been observations of increased haze in the air as particulates drift eastward from central China. Some of the particulate material is from wind-blown dust, but this is mixed with agricultural and industrial pollutants and even the soot from cooking stoves. Although there has been much discussion of the health effects of the smoke from such sources, studies have typically had to rely on information about urban aerosols, which are likely to be rather different. Biomass burning yields many millions of tonnes of soot, which has a graphitic structure and characteristic organic compounds such as abietic acid and retene (Fig. 3.4c) derived from plant resins. Potassium and zinc are also likely to be found in the particles from forest fires.

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