An Introduction to Environmental Chemistry

compound can only be formed from DMS, in contrast to SO 2 , it is an unequiv-

ocal marker for atmospheric acidity arising from marine biological activity.

The above description is, of course, a considerable simplification of the real

situation. For example, rain and cloud droplets contain other dissolved substances

important for pH control apart from H 2 SO 4 —for example, nitric acid (HNO 3 )

arising from oxides of nitrogen (nitric oxide (NO) and nitrogen dioxide (NO 2 ))

coming from combustion sources (see Section 3.6.2). Another of these substances,

ammonium (NH 4 +, produced by dissolution of ammonia (NH 3 ) in water), is alka-

line and so can partially counteract the acidity arising from the sulphur system.

The NH 3 is emitted by soil microbiological reactions (see Section 3.4.2), partic-

ularly areas of intensive agriculture, and, according to a recent suggestion, some

may come from the oceans (Fig. 7.19) in a cycle somewhat analogous to that of

DMS. Another factor is that some of the acid SO 42 - and alkaline NH 4 +in the atmos-

phere exist in small aerosol particles (size in the range 10-^3 to 10mm diameter),

which have a chemical composition ranging from ‘pure’ H 2 SO 4 to ammonium

sulphate ((NH 4 ) 2 SO 4 ), depending on the relative strengths of the sources of

SO 42 - and NH 4 +. These particles are formed in part by the drying out of cloud

droplets in the atmosphere.

The mass balance for sulphur in Fig. 7.17b represents the various fluxes inte-

grated over the whole globe. Because all the different sulphur compounds shown

in Fig. 7.18 have atmospheric residence times (see Section 3.3) of only a few days

and so are not well mixed, their distributions in the air are often inhomogeneous.

Indeed, for any particular region of the atmosphere, it is likely that one of

the major sulphur sources will dominate and thence determine the acidity of rain

and aerosols. In general, for remote—particularly marine—areas, the DMS–SO 2 –

SO 42 - route is likely to control, whereas close to urbanized/industrialized land,

266 Chapter Seven

Oceans

Atmosphere

Land

SO 4 2– (dissolved)

Phytoplankton

DMS (gas)

DMS (gas) MSA (rain,

aerosol)

OH and/or

NO 3 addition

OH (gas), O 3 ,

H 2 O 2 (rain)

H 2 SO 4 (rain, aerosol)

SO 2 (gas, rain)

OH and/or NO 3

abstraction of H

Fig. 7.18The main natural and anthropogenic routes for atmospheric sulphur dioxide

and sulphate.