Reactivity of trace substances in the atmosphere

Gases with short residence times in the atmosphere are clearly those that can be removed easily. Some of these gases are removed by being absorbed by plants or solids or into water. However, chemical reactions are the usual reason for a gas having a short residence time.

What makes gases react in the atmosphere? It turns out that most of the trace gases listed in Table 3.3 are not very reactive with the major components of air. In fact, the most important reactive entity in the atmosphere is a fragment of a water molecule, the hydroxyl (OH) radical. This radical (a reactive molecular fragment) is formed by the photochemically initiated reaction sequence, started by the photon of light, hv:

The OH radical can react with many compounds in the atmosphere and thus it has a short residence time. The rates are faster than with abundant gases such as O2.

The reaction between nitrogen dioxide (NO2) and the OH radical leads to the formation of HNO3, an important contributor to acid rain.

By contrast, kinetic measurements in the laboratory (which aim at determining the speed of reaction) show that gases that have slow rates of reaction with the OH radical have a long residence time in the atmosphere. Table 3.3 shows that

Table 3.3 Naturally occurring trace gases of the atmosphere. From Brimblecombe (1986).

Residence time Concentration (ppb)

Table 3.3 Naturally occurring trace gases of the atmosphere. From Brimblecombe (1986).

Residence time Concentration (ppb)

Carbon dioxide

4 years

360000

Carbon monoxide

0.1 year

100

Methane

3.6 years

1600

Formic acid

10 days

1

Nitrous oxide

20-30 years

300

Nitric oxide

4 days

0.1

Nitrogen dioxide

4 days

0.3

Ammonia

2 days

1

Sulphur dioxide

3-7 days

0.01-0.1

Hydrogen sulphide

1 day

0.05

Carbon disulphide

40 days

0.02

Carbonyl sulphide

1 year

0.5

Dimethyl sulphide

1 day

0.001

Methyl chloride

30 days

0.7

Methyl iodide

5 days

0.002

Hydrogen chloride

4 days

0.001

OCS, N2O and even CH4 have long residence times. The CFCs (chlorofluoro-carbons, Fig. 3.4b: refrigerants and aerosol propellants) also have very limited reactivity with OH. Gases like these build up in the atmosphere and eventually leak across the tropopause into the stratosphere. Here a very different chemistry takes place, no longer dominated by OH but by reactions which involve atomic oxygen (i.e. O). Gases that react with atomic oxygen in the stratosphere can interfere with the production of O3:

and can be responsible for the depletion of the stratospheric O3 layer. This means that CFCs are prime candidates for causing damage to stratospheric O3 (Section 3.10).

We should note that nitrogen compounds are also damaging to O3 if they can be transported to the stratosphere, because they are involved in similar reaction sequences. We have already seen that tropospheric NO2 is unlikely to be transferred into the stratosphere (eqn. 3.14). It was, however, nitrogen compounds from the exhausts of commercial supersonic aircraft flying at high altitude that were the earliest suggested contaminants of concern. In this case the gases did not have to be unreactive and slowly transfer to the stratosphere, but were directly injected from aircraft engines. A large stratospheric transport fleet never came about, so attention has now turned to N2O, a much more inert oxide of nitrogen produced at ground level and quite capable of getting into the stratosphere. This gas is produced both from biological activities in fertile soils (see Section 3.4.2, 5.5.1) and by a range of combustion processes—most interestingly, automobile engines with catalytic converters.

Finally, we should note that some reactions lead to the formation of particles in the atmosphere. Most particles are effectively removed by rainfall and thus have residence times close to the 4-10 days of atmospheric water. By contrast, very small particles in the 0.1-1 mm size range are not very effectively removed by rain droplets and have rather longer residence times.

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