An Introduction to Environmental Chemistry

there is another source that is related to the low reactivity of some gases. Gases

in the atmosphere tend to react with the OH radical. Gases that do not react with

OH in the troposphere can survive long enough to be transferred into the stratos-

phere. This includes OCS, N 2 O and to a lesser extent CH 4. Once in the stratos-

phere these gases become involved in reactions involving atomic oxygen (O). In

addition to these natural trace gases there are a number of anthropogenic trace

gases that are resistant to attack by OH. Among these the CFCs have become

infamous because of their effects on stratospheric chemistry, particularly that of

ozone (O 3 ). The discovery in 1984 that there was a hole in the ozone layer over

Antarctica emphasized the threat imposed by these gases.

Although O 3 is a toxin in the troposphere (Section 3.6.2), it plays a vital role

in shielding organisms on the Earth from damaging UV radiation. There are only

very small amounts of O 3 in the upper atmosphere. If all the O 3 in the Earth’s

atmosphere, most of which is found in the stratosphere, were brought to ground

level it would constitute a layer of pure O 3 only 3 mm thick. The tenuous nature

of the O 3 layer means that for some decades scientists have been concerned that

O 3 in the stratosphere could be damaged by the presence of CFCs. However, cal-

culations of gas-phase chemistry suggested that changes in the atmosphere as a

whole would be small. This explains why the detection of an O 3 hole over Antarc-

tica in 1984 came as a surprise (Fig. 3.6). The rapid destruction of O 3 in the polar

stratosphere in the 1970s and 1980s (Fig. 3.6) proved the chemistry of the O 3

layer to be much more complex than had previously been thought.

3.10.1 Stratospheric ozone formation and destruction

The formation of ozone is a photochemical process that uses the energy involved

in light. The shorter the wavelength of light, the larger the amount of energy it

The Atmosphere 59

Box 3.8 Removal of sulphur dioxide from an air parcel

A parcel of air over a rural area of an
industrial continent would typically be
expected to contain sulphur dioxide (SO 2 ) at
a concentration of 5¥ 10 -^9 atm. This means
that a cubic metre of air contains 5¥ 10 -^9 m^3
of SO 2. We can convert this to moles quite
easily because a mole of gas occupies
0.0245 m^3 at 15°C and atmospheric pressure.
Thus our cubic metre of air contains 5¥
10 -^9 /0.0245=2.04¥ 10 -^7 mol of SO 2. In a
rain-laden cloud we can expect one cubic
metre to contain about 1 g of liquid water,
i.e. 0.001 dm^3.
If the SO 2 were all removed into the
droplet and oxidized to sulphuric acid

(H 2 SO 4 ), we would expect the 2.04¥ 10 -^7 mol

to dissolve in 0.001 dm^3 of liquid water,

giving a liquid-phase activity of 2.04¥

10 -^4 mol l-^1. The H 2 SO 4 formed is a strong acid

(Box 3.3), so dissociates with the production

of two protons under atmospheric conditions:

eqn. 1

Thus the proton activity will be 4.08¥

10 -^4 mol l-^1 , or the pH 3.4. Evaporation of

water from the droplet and removal of

further SO 2 as the droplet falls through air

below the cloud can lead to even further

reduction in pH.

HSO 24 Æ+ 2 H SO+- 42