The sulphur cycle and atmospheric acidity

If CO2 were the only atmospheric gas controlling the acidity of rain, then the pH of rainwater would be close to 5.6 (see Box 3.7). However, most pH measurements of rainwater fall below this value, indicating other sources of acidity. Much of this 'extra' acidity arises from the sulphur cycle, as shown in Fig. 7.18. Only two major routes give rise to the sulphur acidity. One is the burning of fossil fuels to produce the acidic gas SO2. The other is the production of the gas DMS by marine organisms, which then degases to the atmosphere across the air-sea interface (Fig. 7.23). Once in the atmosphere the DMS is oxidized by powerful oxidants, called free radicals (see Section 3.5). The two free radicals important for oxidation of DMS are hydroxyl (OH) and nitrate (NO3). The products of this oxidation are several, but the two most important are SO2 and methane sulphonic acid (MSA or CH3SO3H). The SO2 formed in this way is chemically indistinguishable from that coming from the burning of fossil fuels.

The SO2 from either source exists in the atmosphere either as a gas or dissolved in rain and cloud droplets, whose pH it lowers due to the acidity of the gas. However, within water drops SO2 can be quite rapidly oxidized to form sulphuric acid (H2SO4), which makes them much more acidic since H2SO4 is a strong acid (see Box 3.8). MSA formed by oxidation of DMS, via the OH/NO3 addition route (Fig. 7.18), also contributes to the acidity of atmospheric water. Since this

Atmosphere

H2SO4 (rain, aerosol)

OH and/or NO3

abstraction of H NO3 addition

OH and/or

Oceans

Land

Phytoplankton SO?- (dissolved)

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

compound can only be formed from DMS, in contrast to SO2, it is an unequivocal 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 H2SO4—for example, nitric acid (HNO3) arising from oxides of nitrogen (nitric oxide (NO) and nitrogen dioxide (NO2)) coming from combustion sources (see Section 3.6.2). Another of these substances, ammonium (NH+, produced by dissolution of ammonia (NH3) in water), is alkaline and so can partially counteract the acidity arising from the sulphur system. The NH3 is emitted by soil microbiological reactions (see Section 3.4.2), particularly 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 SO4- and alkaline NH+ in the atmosphere exist in small aerosol particles (size in the range 10-3 to 10 mm diameter), which have a chemical composition ranging from 'pure' H2SO4 to ammonium sulphate ((NH4)2SO4), depending on the relative strengths of the sources of SO4- and NH+. 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 integrated 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-SO2-SO4- route is likely to control, whereas close to urbanized/industrialized land,

Fig. 7.19 Time series (1987-1991) of weekly average concentrations of methane sulphonic acid (MSA), non-sea-salt-sulphate (nss-SO4-) and ammonium (NH+) measured at Mawson, Antartica. After Savoie et al. (1993), with kind permission of Kluwer Academic Publishers.

anthropogenic sources of SO2-SO4- will dominate. These contrasting situations are illustrated in Figs 7.19 and 7.20.

In Fig. 7.19 atmospheric measurements of particulate MSA, SO4- (after subtraction of the component coming from sea salt, the so-called non-sea-salt-sulphate (nss-SO42-)) and NH4+ made in air at Mawson in Antarctica are shown. This site is very remote from human activities and typically receives air which has blown over thousands of kilometres of the Southern Ocean before being

Fig. 7.20 Mean annual pH values of rain over Europe in 1985. The pH contours compare well with rates of acid deposition shown in Fig. 5.7. After Schaug et al. (1987). With permission from the Co-operative Programme for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe (EMEP) and the Norwegian Institute for Air Research (NILU).

Fig. 7.20 Mean annual pH values of rain over Europe in 1985. The pH contours compare well with rates of acid deposition shown in Fig. 5.7. After Schaug et al. (1987). With permission from the Co-operative Programme for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe (EMEP) and the Norwegian Institute for Air Research (NILU).

sampled. A clear seasonal cycle is apparent, with highest values of MSA and nss-SO^- in the austral spring and summer. This is exactly what would be expected if marine biological production of DMS was the dominant source of sulphur, since the phytoplankton are strongly seasonal in their production of DMS. For this site it is well established that marine plankton rather than anthropogenic emissions are the dominant source of sulphur acidity in the air. The same is true for most marine areas of the southern hemisphere. The very similar seasonal cycle seen for NH+ in Fig. 7.19 suggests the possibility of an analogous marine biological source for NH3 gas also.

Yearly averaged pH values of rain falling over Europe show a very different situation (Fig. 7.20). As might be expected for such a heavily developed area, it is anthropogenic sources which largely control the acidity of the rain. This is shown by the low pH values centred on the most heavily industrialized parts of the region (Germany, eastern Europe, the Low Countries and eastern Britain), with higher (less acidic) pH values to the north, south and far west of the area.

It is not possible to distinguish between SO2 and SO4- coming from fossil fuel burning or marine biogenic (DMS) sources by chemical means. However, recently a differentiation of these two sources has become possible by measuring the ratio of two stable isotopes of sulphur (34S/32S, expressed as 834S; Box 7.2) in rain and aerosol samples. Figure 7.21 illustrates the principle by which the technique works. The 834S of sulphur coming from power-station plumes (as SO2) has a value of between 0 and +5%o CDT (Canyon Diablo troilite), based on data from power plants in eastern North America and the UK. By contrast, the SO4- in seawater, from which phytoplankton make DMS, has a 834S value close to +20%o CDT. This large difference in 834S value (between 15 and 20%o CDT) between the two main sources of atmospheric sulphur is the basis of the method.

If a sample collected in the environment (aerosol, rain, surface water) has a 834S value of +20%o CDT, then it should have its sulphur essentially from the

Box 7.2 The delta notation for expressing stable isotope ratio values

Stable isotope (see Box 1.1) abundances cannot at present be determined with sufficient accuracy to be of use in studies of their natural variations. Mass spectrometers can, however, measure the relative abundances of some isotopes very accurately, resulting in stable isotope ratio measurements, for example oxygen—18O/16O, carbon — 13C/12C and sulphur—34S/32S. Stable isotope ratios are reported in delta notation (8) as parts per thousand (%o per mil) relative to an international standard, i.e.:

8 = | Rsample Rstandard |x 1000

where R represents a stable isotope ratio and 8 expresses the difference between the isotopic ratios of the sample and the standard. 8 is positive when the sample has a larger ratio than the standard, is negative when the reverse is true and is zero when both values are the same. The multiplication by 1000 simply scales up the numbers (which are otherwise very small) to values typically between 0 and ± 100.

For stable sulphur isotopes, the standard is an iron sulphide mineral (troilite) from the Canyon Diablo meteorite. It is known as CDT (Canyon Diablo troilite) and equation 1 becomes:

V S/ Sstandard J

Results are reported as 834S values relative to the CDT standard, for example, 834S = + 20%% CDT.

0 +10 +20 +30 534S%O CDT

Fossil fuel emissions

Seawater

UK urban aerosols

Mace Head aerosols

South Pacific aerosols

Fig. 7.21 Sulphur isotope ratios (S34S) for various sources of sulphur and in atmospheric aerosols for several localities. Mace Head (western Ireland) data, courtesy of Nicola McArdle, UEA.

Fig. 7.21 Sulphur isotope ratios (S34S) for various sources of sulphur and in atmospheric aerosols for several localities. Mace Head (western Ireland) data, courtesy of Nicola McArdle, UEA.

DMS route. On the other hand, if its measured 834S is close to the 0 to +5%o CDT range of fossil fuels, then its contained sulphur is likely to be from this source. Samples with intermediate values will have sulphur from both sources, the ratio being directly calculable by simple mass balance.

There are, of course, several assumptions behind this apparently simple description. One is that the d34S signal of all fossil fuel is in the above range. At the moment only a rather small number of samples of power-station flue gases from limited locations have been analysed. A second assumption is that the d34S signal of seawater SO4- is not altered significantly when DMS crosses the air-sea interface and is oxidized to SO2 and SO4- in the atmosphere. The evidence to date indicates that neither of these assumptions introduces much error, but more work is required to prove this approach.

As might be expected, urban aerosols have a 834S signature overlapping to somewhat higher than that from fossil fuels (Fig. 7.21). By contrast, the very few aerosol samples obtained from locations remote from human influence in the South Pacific have a 834S value which can approach that of seawater SO4-. Results from detailed sampling conducted over a full yearly cycle at Mace Head, a remote site on the west coast of Eire (Fig. 7.21), show almost the whole range of 834S. Because of the large number of samples collected it has been possible to calculate the percentage of sulphur from the two main sources for different seasons. Thus, in spring and summer approximately 30% of the sulphur in the aerosols at Mace Head comes from DMS (very probably produced by phytoplankton in the northeastern Atlantic, which are only active in any substantial way at these seasons), with the remaining 70% from fossil fuel sources (mainly in Europe, including the UK). In winter essentially all the sulphur is from this latter source. This is a good example of the utility of isotope measurements in environmental sciences, because it is possible to attribute sulphur to its sources without the need to know the strengths of those sources and without recourse to an atmospheric dispersion and deposition model. This avoids the considerable uncertainties associated with estimating these parameters.

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