Box 38 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 (SO2) at a concentration of 5 x 10-9atm. This means that a cubic metre of air contains 5 x 10-9m3 of SO2. We can convert this to moles quite easily because a mole of gas occupies

0.0245 m3 at 15°C and atmospheric pressure. Thus our cubic metre of air contains 5 x 10-9/0.0245 = 2.04 x 10-7mol of SO2. In a rain-laden cloud we can expect one cubic metre to contain about 1 g of liquid water,

If the SO2 were all removed into the droplet and oxidized to sulphuric acid

(H2SO4), we would expect the 2.04 x 10-7mol to dissolve in 0.001 dm3 of liquid water, giving a liquid-phase activity of 2.04 x 10-4moll-1. The H2SO4 formed is a strong acid (Box 3.3), so dissociates with the production of two protons under atmospheric conditions:

Thus the proton activity will be 4.08 x 10-4mol l-1, or the pH 3.4. Evaporation of water from the droplet and removal of further SO2 as the droplet falls through air below the cloud can lead to even further reduction in pH.

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 stratosphere. This includes OCS, N2O and to a lesser extent CH4. Once in the stratosphere 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 (O3). The discovery in 1984 that there was a hole in the ozone layer over Antarctica emphasized the threat imposed by these gases.

Although O3 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 O3 in the upper atmosphere. If all the O3 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 O3 only 3 mm thick. The tenuous nature of the O3 layer means that for some decades scientists have been concerned that

O3 in the stratosphere could be damaged by the presence of CFCs. However, calculations of gas-phase chemistry suggested that changes in the atmosphere as a whole would be small. This explains why the detection of an O3 hole over Antarctica in 1984 came as a surprise (Fig. 3.6). The rapid destruction of O3 in the polar stratosphere in the 1970s and 1980s (Fig. 3.6) proved the chemistry of the O3 layer to be much more complex than had previously been thought.

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