Atmospheric sulphur: Natural and man-made sources

ATMOSPHERIC SULPHUR : NATURAL MAN-MADE SOURCES

AND

C. F. CULLIS and M. M. HIRSCHLER Department of Chemistry, The City University, London, England (First received 18 June 1979 and jn~nul~or~

25 April 1980)

Abstract - The principal processes by which sulphur compounds are emitted into the atmosphere are reviewed in the light of the most recent data. The main natural source of the atmospheric sulphur is biogenic activity, although considerable uncertainty still exists regarding both the nature and the amounts of the princi& reduced sulphur compounds generated in this way. The combustion ofcoal and wtroleum accounts for ca 90X of the total sulphur emitted by man, the only other large source being the smelting of copper ores. Comparisons are made between-the man-made emissions calculated on a per unit area and a per capita basis for the whole Earth and for some industrialized countries. Estimates for 1976, made with the aid of emission factors, indicate that man’s activities generate a total of 104 Tg S a- ‘. This already represents over 40 % of all atmospheric sulphur emissions and, if it continues to increase at the present rate, will exceed Nature’s contribution well before the end of the present century.

1. I~ODUCTION Sulphur is a relatively abundant element which plays an essential part in the environmental cycle. On land, it is found mainly as sulphide and sulphate ores and in the oceans it is present predominantly as dissolved sulphate. In the atmosphere, however, the principal

sulphur compounds are hydrogen sulphide (probably together with other reduced sulphur species), sulphur dioxide and sulphate aerosols and mists. Sulphur compounds are not accumulating in the atmosphere. A cycle operates whereby sulphur is continuously transported between the different phases; and there is a delicate balance between the refease of sulphur into the atmosphere and its return to the Earth’s surface, although over the last 100 y or so, the increasing amounts of atmospheric sulphur generated by man may have shifted the balance point. This is shown by recent changes in the sulphur-content of polar ice, which had previously remained constant over the centuries (Koide and Goldberg, 1971). This paper summarizes the present state of knowledge regarding the nature and the amounts of sulphur compounds generated into the atmosphere from various sources. Recent quantitative estimates for the main removal processes will be mentioned briefly. In particular, an attempt will be made to assess the present extent of man-made sulphur emissions into the atmosphere and their impact on the global picture. The latest estimates hitherto available of man’s emissions correspond to 1965 data. 2. THE SULPHUR CYCLE

Figure 1 shows diagrammatically the principal processes by which sulphur compounds are both

emitted into and removed from the atmosphere; these will be considered in turn and estimates will be made of the annual amounts of sulphur generated by each of them. These estimates range from those which can be determined with a reasonable degree of accuracy, such as man’s contributions, to those which are not yet amenable to direct measurement or calculation, such as biogenic emissions. Removal of sulphur from the atmosphere takes place by precipitation processes (involving mainly sulphate) and by dry deposition (principally of sulphur dioxide). However, the overall sulphur cycle is complicated by transfers of sulphur from land to oceans, and vice-versa, and indeed also between the various land phases. No attempt will be made here to discuss either the further reactions of emitted sulphur compounds within the atmosphere or the interactions or exchanges between sulphur compounds in the oceans and land-phases, which do not directly involve the atmosphere. Authoritative reviews of the sulphur cycle have been given elsewhere (Junge, 1960, 1963a; Eriksson, 1963; Robinson and Robbins, 1970,1972,1975; Kellogg et ai., 1972; Friend, 1973; Granat et al., 1976, Whelpdale, 1978a; Bolin, 1979).

3. NATURAL SOURCES OF ATMOSPHERIC SULPHuR

3.1. Non-biogenic sources 3.1.1. Geothermal emissions. Various sources of geothermal activity such as sulphur springs are responsible for the emission of sulphur compounds into the atmosphere (Natusch and Slatt, 1978). However, by far the greatest amount of sulphur generated in this way is derived from volcanoes. Here the predominant

1263

1264

Reduced 12S,S02, so:

SOa-

S cpds

Reduced SO,SOJ

S cpds

Reduced SO,,SO:

SO,

so,,so:

S cpds

Fig. 1. The atmospheric sulphur cycle.

sulphur com~und emitted is generally sulphur dioxide (Gerlach and Nordlie, 1975). Hydrogen sulphide may however rival it at relatively low temperatures and under reducing conditions (Qzawa, 1966; Ross, 1968), and smaller amounts of sulphur trioxide, sulphate and elemental sulphur are often also present. Oxidation of the more reduced sulphur compounds may occur as the hot eruption clouds mix with oxygen, so that the composition of the sulphur-containing gases can vary widely (Cadle et al., 1971). Some calculations of mean annual sulphur emissions are based on the premise that the gases emitted contain a constant known proportion of sulphur and that their total weight bears a fixed ratio to the associated weight of lava, which can be measured directly. On the assumption that the gas generated is 0.5 wt% of the lava (Macdonald, 1972), the total emission of sulphur dioxide from volcanoes over the last 400~ has been estimated as 1.5 Tg a- ’ (Kellogg et al., 1972). During many eruptions, however, a considerably higher proportion of gas (2.5 wt %) may be associated with the lava (Anderson, 1974), so that the amount of sulphur dioxide emitted into the atmosphere may be as high as 7.8Tga-’ (Cadle, 1975). Another estimate has been made from studies of sulphate aerosols in the stratosphere (upper atmosphere) (Friend, 1973). If the average annual emission from volcanoes is 2.2 x 103 Tg of magma and the volcanic gases contain 1~01% (3.3 wt %) of suiphur dioxide, the annual emission rate is 4Tg SOz a-‘. Recently, remote-sensing correlation spectrometry has been extensively used to measure discharges of sulphur dioxide, in Japan (Okita, 1971; Moffat et al., 1972; Okita and Shimozuru, 1975), in Central America (Stoiber and Jepsen, 1973), in Hawaii (Stoiber and Malone, 1975) and from Mount Etna in Italy (Haulet et al., 1977; Zettwoog and Haulet, 1978). This tech-

nique generally shows signi~~ntly larger emissions than others. ExtrapoIation, to cover the whole of the Earth’s surface, of the total daily emissions of sulphur dioxide from Central America (1.3 x 10e3 Tg), excluding large eruptions, suggests that the tota volcanic contribution to atmospheric sulphur dioxide emissions is 10 Tg SO, a-’ (Stoiber and Jepsen, 1973) or perhaps slightly more (Davey, 1978). Doubt must be placed however on the extraordinarily high figures found for Mount Etna, where an emission of 3.7 x 10m3Tg SO, d-’ has been found (Haulet et al., 1977). This value alone would account for 1.3 Tg SOz a- ‘. Simul~~usly, however, only lo-’ Tg of magma has been found to be emitted per day, i.e. the ratio of magma to SO2 is only 2.7. The amount of magma needed to account for the high SO2 emission found would be cu. 2.7 Tg of magma d - I, i.e. corresponding to a magma/SO, ratio of 738. Thus, although the reported value of SOz emissions is probably too high, perhaps because a rather severe eruption at Mount Etna had not yet completely subsided at the time of the measurements (June 1975), the amount of magma is consistent with general estimates of average annual magma emission for volcanoes (Friend, 1973). Data found for May 1977 at Mount Etna (Zettwoog and Haulet, 1978) are much lower, ahhough they are still very high (1.1 x 10-3Tgd-‘). Volcanism has been found to affect significantly the sulphate concentration in polar ice (Delmas and Boutron, 1978; Delmas, 1979), especially as a result of precipitation following stratospheric transport. Nevertheless, it must be concluded that the amount of sulphur released into the atmosphere by volcanoes is unlikely to be significantly greater than 5 Tg S a- ’ and is thus small in comparison with that from other sources. 3.1.2. &u-spray. A more important source of atmos-

Atmospheric sulphur : natural and man-made sources pheric sulphur is the fine spray, formed above the oceans, the individ~l droplets in which evaporate to leave even smaller solid particles. Sodium sulphate is the second most abundant constituent of sea-water, the SOi-/Clratio being about 0.14 (Riley and Chester, 1971). The amount of sulphur emitted into the atmosphere depends on the sulphate content of seawater (known to be reasonably constant at 0.27 %) and on the extent to which sulphate ions are enriched relative to sodium and chloride ions by fractionation during spray formation (Sugawara, 1965). The major part of the emission is returned to the oceans as a result of sedimentation and precipitation but about 10 y0 of the spray-generated sulphate is carried over and deposited on land surfaces (Eriksson, 1959, 1960). More recent estimates based on sulphate in rivers (Garrels and Mackenzie, 1971) and on sub-micron particles on coasts (Johansson et al., 1974) give results between 7 and 10%. The generally accepted figure for the total emission of sulphur from sea-spray still remains at 44 Tg S a- 1 (Eriksson, 1959, 1960). 3.2. Biogenic sources The biological reduction of sulphur compounds constitutes by far the most important natural source of atmospheric sulphur. Such reduction occurs most readily in the presence of organic matter and under oxygen-deficient conditions. The sulphur compounds entering the atmosphere are derived from the nonspecific reduction of sulphur in marine algae, soils and decaying vegetation (Rasmussen, 1974; Hitchcock, 1976a) and from bacteria, which specifically reduce various types of sulphur compounds (Hallberg et al., 1976), predominantly sulphate-reducing bacteria, such as Sporovibrio desulphuricans, present in fine-grained muds (Hitchcock, 1976a; Natusch and Slatt, 1978). The quantities of volatile sulphur compounds emitted from biogenic sources are almost impossible to measure or calculate directly, mainly due to lack of quantitative knowledge concerning their reactions in the atmosphere. Hydrogen sulphide reacts only slowly with oxygen in the absence of catalysts but is oxidized photochemic~ly (Cox and Sandalls, 1974; Eggleton and COX, 1978). There are differing views as to its rate of interaction with ozone, which may diffuse down into the troposphere (lower atmosphere). The latter reaction may be quite rapid [especially in the presence of aerosols (Cadle and Ledford, 1966)] or take place only at a negligible rate under normal atmospheric conditions (Hales et of., 1969, 1974; Slatt ei at., 1978). Estimates of the lifetime of hydrogen sulphide near the Earth’s surface range from 2 h (Cadle and Ledford, 1966 ; Levy, 1974) to nearly 4d (Friend, 1973). Furthermore, marked changes may occur in the concentrations of sulphur compounds between their initial production and eventual emergence into the atmosphere. In some cases, these compounds may be “trapped” in an involatile form by metal-containing

1265

compounds (Jarvie et al., 1975; Rowland et al., 1977; Craig and Bartlett, 1978). Initially it was assumed that the predominant sulphur compound emitted biogenically was hydrogen sulphide, formed in soil on land as well as in mud on the sea-bed. It is easily detected in swamps, marshes, tidal flats and other places, such as continental shelves, where access of oxygen is restricted (Rasmussen, 1974; Natusch and Slatt, 1978); and under these conditions its concentration often exceeds its odour detection threshhold (0.5 ppb) (Hitchcock, 1976a). However, hydrogen sulphide cannot usually be detected in open sea-water, presumably because the oxidizing environment is too strong for it to survive as such (&tlund and Alexander, 1963; Penkett, 1972; Friend, 1973). Most of the hydrogen sulphide emitted into the atmosphere from muddy tropical lakes is also probably washed out again by rain, so that there is little or no net contribution to theatmospheric sulphur budget at great distances from the source (Brinkmann and Santos, 1974). In deep parts of the ocean where oxygen cannot penetrate, much of the sulphur may be present as sulphide rather than as sulphate (Nissenbaum and Kaplan, 1976). Certain reduced organic sulphur compounds now appear to fill the role originally assigned to hydrogen sulphide (Lovelock et af., 1972; Rasmussen, 1974). Since many microorganisms, especially marine algae, produce dimethyl sulphide (Challenger, 195l), attempts have been made to detect it in the sea, where it has been shown to be present at a concentration of lo-” pg cme3, although it could not be detected in the adjacent air (Lovelock et al., 1972). M~sur~ents in air above a pond have, however, shown a considerably higher concentration of dimethyl sulphide than of hydrogen sulphide (Rasmussen, 1974). Similarly, carbon disulphide has been found in parts of the sea, in lower concentrations than dimethyl sulphide (Lovelock, 1974). Reduced organic sulphur compounds may thus be the principal volatile sulphurcontaining species emitted into the atmosphere from the Oceans (Loveiock et al., 1972; Rasmussen, 1974). Dimethyl sulphide is much less readily oxidized than hydrogen sulphide (Cadle et al., 1974; Cadle, 1976), although it may sulfer some photochemical oxidation (Cox and Sandalls, 1974; Eggleton and Cox, 1978). Its production would thus be consistent with the very low concentrations, in the atmosphere above remote parts of the ocean, of sulphur dioxide (Nguyen et al., 1974; Prahm et al., 1976), which would be formed quite readily, if hydrogen sulphide were the predominant volatile suiphur compound emitted. Recent calculations of the maximum amounts of dimethyl sulphide emitted by marine algae (and from decaying vegetation and soils on land) suggest, however, that this compound can be responsible for the emission of only a small fraction of the atmospheric sulphur generated by biogenic sources (Hitchcock, 1975), and therefore the principai sufphur com~und emitted from the oceans is probably hydrogen sulphide. Nevertheless, recent coastal studies (Bonsang et

1266

(‘ F. (‘1 I I is and M. M. HIKSWII K

ui., 1976) support the view (Lovelock rt al., 1972; Rasmussen, 1974) that organic sulphur compounds are very important and indeed dimethyl sulphide may account for the emission of ca 27 Tg S a- 1 from the oceans (Nguyen et cd.,1978). It is widely believed that hydrogen sulphide is rapidly oxidized in the oceans (&tlund and Alexander, 1963; Friend, 1973 ; Rasmussen, 1974; O’Brien and Birkner, 1977), but there is some evidence that it is quite resistant to oxidation, particularly under acid conditions (Chen and Morris, 1972). The oxidation of hydrogen sulphide is, however, strongly catalysed by aerosol particles (Cadle and Ledford, 1966), so that this compound can, in the atmosphere, be rapidly converted directly into sulphate. The rate of bacterial reduction of sulphate to hydrogen sulphide is, like all microbiological metabolic reactions, strongly dependent on temperature (Hitchcock, 1976b). Thus levels of atmospheric sulphate, part of which is derived from hydrogen sulphide, exhibit in non-urban areas a pronounced seasonal variation, reaching well-defined maxima during the summer months when, in urban areas, sulphur dioxide emissions from fuel combustion are smallest (Hitchcock, 1976b). In non-urban areas, sulphur dioxide levels attain maximum values in midwinter (Seaton et al., 1978). Even in areas with large man-made sources of atmospheric sulphur, the biogenie contribution may therefore exceed the industrial one in summer (Grey and Jensen, 1972). In contrast, the background level of sulphate is only slightly affected by localized emissions of sulphur dioxide (Altshuller, 1973, 1976; Georgii, 1978). Both hydrogen sulphide and organic sulphides are also formed as a result of biological processes on land, although the amounts of hydrogen sutphide liberated from dry soils may be small (Swaby and Fedel, 1973, 1977). Summertime emissions of sulphur compounds from soils and wetlands have been shown to be an important source, being partly derived from pollutant sulphur deposited in preceding years (Nriagu and Coker, 1978). Bacteria, such as E. coli, which act on sulphur-containing aminoacids such as cysteine, also produce considerable amounts of hydrogen sulphide (Schlegel, 1974), which are thus found near refuse heaps containing large proportions of animal matter (Nicol et al., 1970). The rates of emission of atmospheric sutphur, both by man and from volcanoes and sea-spray, are known at least approximately, and the rates of removal of sulphur from the atmosphere can be determined directly. Thus the biogenic emissions of volatile sulphur compounds can be calculated by difference. Owing to the widely differing estimates of the rates of removal of sulphur from the atmosphere, both by precipitation and by dry deposition, there are correspondingly large variations in estimates of biogenic emissions ofsuiphur as shown in Table 1. Most of these estimates have been based on the assumption of considerable biogenic emissions from land, but it is believed that such emissions are almost negligible

(Bolin and Charlson, 1976; Hallberg, 1976; Granat, 1976) in view of calculations of the maximum possible amounts of dimethyl sulphide which can be generated into the atmosphere (Hitchcock, 1975). Total biogenic land emissions are, however, probably considerably higher than 5 Tg S a- ‘, since hydrogen sulphide, carbon disulphide (Lovelock. 1974, carbonyl sulphide (Aneja et al., i979a, b) and other, hitherto undetected, sulphur compounds (Lovelock, 1978) may make a substantial contribution. Early analyses of the sulphur cycle (Junge, 1960; 1963a: Eriksson, 1963) included large transfers of sulphur from oceans to land, whereas the estimates based on very low biogenic emissions from land (Bolin and Charlson, 1976 ; Hallberg, 1976 ; Granat, 1976) involve almost no overall transfer between land and sea. Net transfer from land to sea has however been found experimentally (Junge, 1963b) and the same is true in most later estimates (Table 1). Estimates of removal processes in which the rate of dry deposition of atmospheric sulphur is taken as twice the precjpitation rate on the basis ofmeasurementsmade near major source areas (Garland, 1977) differ considerably from all others. Estimates of emissions can be made, from the areas of the world’s non-polar regions, of the land areas subject to tropical climates, the areas covered by nontropical forests, non-tropical areas under cultivation and desert wasteland areas (McHale, 1972) (Table 2). The tropical areas will emit considerable amounts of reduced sulphur compounds, due to the high temperatures and degree of humidity, which lead to abundant water-logged soils. Dry soils with crops which fertilisers received sulphur emit have 200-300 kg km- 2 S a _ ’ as hydrogen sulphide (Siman and Jansson, 1976). If other fertilisers are added, to encourage microbial growth, this value can increase significantly (Bloomfield, 1969). No measurements have been made of emissions of other reduced sulphur compounds. Since 31 y/i of all fertilisers are used in Europe, the emission factor should be highest for Europe’s cultivated areas Progressively lower emission factors have been used for North America, USSR and Oceania and other cultivated areas. The emissions of hydrogen sulphide from tropical areas are estimated at a value which corresponds to slightly more than twice the average emission factor for cultivated areas. Since most non-tropical forest areas are found in the Northern Hemisphere, a fairly high emission factor can also be used for them, although, of course, this is much lower than that for cultivated land. The total emission of sulphur compounds should be higher than the value of 43 Tg S a- *, a part of which may be organic, obtained from these calculations (Table 2), because about 5 Tg S a- 1 more are probably emitted in organic form. An estimate of 48 Tg S a ._1for biogenic land emissions thus seems appropriate. Oceanic emissions of dimethyl sulphide are 27 Tg S a-r (Nguyen et al.. 1978). Hydrogen sulphide emissions from the oceans are probably lower, per unit area, than from land and, since the major part of the

129

159 143

106 112 ii3

315 36.5

212

184 217

143 149 144 266 200

Junge (1963a) Eriksson (1963) Robinson and Robbins (1972) Kellogg et al. (1972) Friend (1973) Bolin and Charlson (1976) Hallberg (1976) Granat (1976) Garland ( 1977)f Davey (1978% 38 154 77

25 14

83

175 165

10

3 3 3

1 2 44 44 44 44 44

44 44

44

45 45

65 65 65 70 60

50 65

70

40 40

31 37 32 152 86

89 106

98

2M 280

28 34 27 46 26

18 48

30

160 170

3 3 5 106 60

71 58

68

70 110

-1 -7 +2 f 84 + 60

+ 60 + 33

+ 64

- 95 - 65

Net transfer from land to sea (Tga-‘)t

*Where subtotals are given only for the combined effects of precipitation and dry deposition, 20 ‘Aof the sulphur is assumed to be removed by dry deposition, by analogy with nuclear fall-out (Junge, 1963b; Friend, 1973). tThe values are calculated by adding biogenic emissions from land areas to those from volcanoes and due to man and subtracting biogenic emissions from sea and those due to sea-spray. $The distribution between land and sea has been calculated on the basis of the percentages used by Robinson and Robbins (1972) in their estimates, from which the data of Garland (1977) originate. $Somewhat similar estimates are given in an earlier paper by Davey (1973).

140 200

Total

Authors

Table 1. Estimates of natural emissions of atmospheric sulphur -Emission processes (Tga-*) Removai processes Biogenic emissions (Tga-‘) -SeaManSubFrom From Dry Precipitation deposition * Volcanoes total sea land made spray

.,_._...-.._... ~.~

Table 7. World areas (non-polar land) (lOh kmL)* -. . ~. _ Desert Tropical Cultivation Forest hot

World 52.9 (560) 34.5 (260) 22.9 (160) Africa 23.0 4.0 (ZOO) 1.3 America (N) 0.5 9.0 (300) 7.0 America (Latin J 17.0 3.0 (200, 0.1 Asia (S) I I.3 1.0 (200) 0.5 Asia (E) 0.5 3.0 (200) 2.3 Europe tt 3.0 (350) 1.5 Oceania 0.6 5.5 (250) 0.X USSR 0 6.0 (750) 9.4 H,S emissions+ 30 9 4 -.-- .- ..- ~-~~~~~ ._ ..__ -_. __-._ ___~ _ ___ *Figures in brackets represent estimates of emissions of H,S in kg krne2 a- ‘. tThese values are in Tg S a _ 1

Earth’s surface is covered by the oceans (59 and 83 “/, of the Northern and Southern Hemispheres, respectively), an emission of ca 23 Tg S a- * (as H2S) seems reasonable. The total oceanic biogenic emission then becomes 50 Tg S a- 1and the overall biogenic emission is thus 98TgSa-‘. In summary, however, it is clear that many more experimental measurements need to be made before such estimates of biogenic emissions become generally accepted.

4. MAN-MADE

SOURCES OE ATMOSPHERIC SULPHUR

4.1. General The total sulphur emitted by man into the atmosphere, and indeed also the percentage effect of his activities, has been increasing markedly throughout the past lo0 y (Table 3). The main industrial sources of atmospheric sulphur remain the combustion of coal and petroleum, petroleum refining and the smelting of non-ferrous ores, although their relative weightings have been changing considerably.

Desert cold

6.7 2.0 0. I 0 3.0 0 (1 1.6 0 0

1X.9 0 4.9 Cf.5 0 6.0 0.5 0 7.0 0

4.2. Coal The most abundant source of atmospheric sulphur is still the burning of coal and its by-products. Unfortunately, the almost infinite variety of coals makes it difficult to quantify exactly the sulphur content of emissions from this source. Nevertheless, there are precise data regarding the amounts both of total hard coal and of lignite (or brown coal) consumed per year, and of coal being used for coking and other secondary metallurgical processes. The amount of coal used in the cement industry can also easily be estimated (Rohrman and Ludwig, 1965; UN, 1969a) and this does not produce any sulphur emissions. Furthermore, the average sulphur content of that fraction of coal in the United States, which has not been used for coking or for cement manufacture, has been shown to be 2.41 ‘;b for industrial and public utility combustion and 2.52 “/, for domestic and smallindustry combustion (Rohrman and Ludwig, 1965). The former figure should probably be used for the overall calculations, since, owing to the lower ef-

Table 3. The historical increase in sulphur dioxide emissions (TgSOZ a _ ’ I*

_.

Coal*

1910

12.2

42.09 (90.5)

51.3 (85.5)

(0.6)

0.70 (1.5)

3.12 (5.1)

0.26 (2.1)

3.71 (8.0)

4.91 (8.2)

7.38 (8.9)

12.28 (9.4)

12.90 (X.71

(1.1)

I.408 (1.7)

2.21~ (1.7)

2.68‘ (1.8)

59.99

83.08

(97.4) Petroleum+ Non-ferrous or&

0.07

1930

0.66:

Other Total

. _._..

1X80

12.53

46.50

1960

1965

95.7 (73.6)

io2.0 (68.5)

8.30

19.90

(10.0)

(15.3)

X.50 (19.1)

1950 66.0 (79.4)

* The figures in brackets represent percentages of the total. t These vafues are taken from Robinson and Robbins (1972). :UN (1949, 1959). &IN (1949, 1953, 19.59). !iUN (1964, 1970). “UN (1969a).

130.09

148.88

Atmospheric sulphur : natural and man-made sources

1269

Table 4. Production of fossil fuels (A) Coal production* (Tg S a-‘)

Hard coal production Coal cokedt Coal added to stocks Handling losses: World cement production Coal used for cements Coal combustion Lignite production I/

(B) Petroleum productiono

1965

1970

1974

1975

1976

2014.65 449.91 + 14.97 8.99 433.0 58.49 1482.29 789.17

2159.79 466.45 +0.51 9.33 567.0 74.59 1606.91 855.14

2236.54 461.27 - 16.41 9.34 696.0 94.01 1682.33 887.36

2353.69 458.94 + 18.20 9.18 694.0 93.78 1773.58 922.59

2413.69 473.11 + 4.50 9.46 729.0 98.51 1828.11 930.86

1.71 3.34 24.60 2.96 0.15 4.28 22.84 34.66 1.07 2.97 0.07 0.06 1.29 2747.7%

.1.73 3.68 24.11 3.08 0.16 4.11 23.26 34.66 1.08 2.77 0.08 0.04 1.23 3672.8@

(% of total) --

Liquefied petroleum gas Naphtha Motor spirit Kerosene White spirit Jet fuel Distillate fuel oils Residual fuel oils Lub~cating oils Bitumen (asphalt) Paraffin wax Road oil Petroleum coke Total productionj.:

1.77 26.98** 3.54tt 3.87 20.94 36.56 1.53 3.22 0.10 0.14 1.37 1512.0

1.80 3.34 23.23 3.66tf 4.19 21.74 36.37 1.22 3.08 0.09 0.12 1.17 2272.0

1.94 3.65 22.91 2.92 0.12 3.97 22.97 36.05 1.19 2.96 0.09 0.08 1.15 2882.6 #

* UN (1969a, 1973, 1976, 1977a. 1977c, 1978a, 1978b). tcomprises briquettes and coke. $Estimated at 2 y0 of coking coal. 5 Based on 7.4 tons of cement per ton of coal (Rohrman and Ludwig, 1965; UN, 1969a). /IComprises brown coal and lignite. QJN (1969a. 1973, 1977a, 1978a). **Includes naphtha. itIncludes

white spirit.

c+:Tga-‘. RIPIS (19761978).

ficiency of domestic users, slightly more of the sulphur will remain in the ashes. The metallurgical industry has rather cleaner fuel (0.9 wt 7; S) and retains, on average, about 70% of the sulphur contained in it. Seams of coking coal of low sulphur content are however becoming exhausted, so that their average S level is now probably higher than the value given. From coal production data [Table 4(A)] and the appropriate emission factors (see Section 4.6), the total sulphur emission from coal for 1976 can be calculated as 123.8 Tg SO2 (61.918 S). 4.3. Petroleum The next most important source of atmospheric sulphur is petroleum products. The proportion of sulphur generated from petroleum is still increasing. Although the rate of growth of petroleum consump tion has been higher than that of coal, the amount of sulphur emitted by all activities of the petroleum industry has increased less rapidly than the total con-

sumption of petroleum. Thus, even though the crude material contains, overall, progressively more sulphur, emission factors are lower and, in fact, the amount of sulphur emitted per ton of petroleum consumed has decreased after reaching a maximum in 1965 (Hirschler, 1980). On the other hand, the sulphur content of the different petroleum products contributing to emissions was considerably lower in 1974 than, for example, in 1965. The fraction of crude petroleum converted into distillate fuel oil is gradually increasing at the expense of the fraction in residual fuel oils, which is considerably richer in sulphur. Recovery has also been more effective. In 1965, the average refinery emitted 50 tons of sulphur dioxide per 100,000 barrels (Robinson and Robbins, 1972) C7.3 barrels to the ton (IPIS, 1977)]. In recent years the size of refineries has been increasing [average size in 1971: 42,000 barrelsd-* and most new refineries tending to be ca 150,000 barrels d- l, as opposed to 10,00+20,000 barrels d- ’ in 1965 (Considine, 1977)], and de-

C. F. CULLISand M. M.

1270

HIRSCHLEK

Table 5. Sulphur content of petroleum products (wt 7; S) 4.4. Non-ferrous ores -~.-~-~ -.---- -.--~~__._ __. .I __.__.__~..___.. The next source of sulphur emissions, in quantiMotor spirit tative terms, is the smelting of non-ferrous ores. Here, Premium grade (USA)* 0.026 Regular grade (USA)* 0.040 copper is the main contributor, and lead and zinc are UK? 0.040 the other, less important, ones. The emission of Average 0.036 sulphur compounds from smelting has been steadily Kerosene* 0.320 (85qb retention) declining in comparison with that associated with Jet fuel* 0.048 ~troieum products, but is gaining ground relative to Distillate fuel oil* City buses 0.12 (loo/, of total) coal. The corresponding emission factors have been Lorries 0.24 (357; of total) calculated from several previous estimates (Jones, Railway engines 0.32 (25% of total) 1972; Katz, 1973 ; Robinson and Robbins, 1972,1975). Ships 0.45 (5 % of total) The factor for copper takes into account the very Domestic 0.10 (25 ‘?/,of total) different values for smelted and refined copper. The Average 0.22 (100%) Residual fuel oil* 1x0: emission factors have been assumed to become proPetroleum coke 2.25 (70 7” retention) gressively smaller in 1970 and 197441976. ~~ ..._ _- ._..-.-.._ _ ._ The total productions of copper, lead and zinc are *Considine (1977). shown in Table 6 and the total sulphur emissions from tweatherley (1977). $Values for the EEC may be as high as 3.00 but 1.80 smelting operations in 1976, calculated using the appears to be the world-wide average. appropriate emission factors (see Section 4.6), are

sulphu~za~on processes have become more efficient. Thus, although the maximum practicable retention of sulphur has not yet been achieved [1.37 kg SO2 emitted per metric ton (lo3 kg) of petroleum refined (calculated from data given by Russell, 1973)], standards like those introduced by Spain of 2.56 kg SOZ ton- ’ [for existing refineries of at least 20,000 barrels d-’ (Stem, 1977)] must have been surpassed. The sulphur contents of the petroleum products responsible for emissions are shown in Table 5. Other petroleum products, such as naphtha and liquefied petroleum gas, are freed of sulphur before use. Residual fuel oils, of course, constitute the main source of suiphur emissions. The total amount of petroleum produced and the percentage of each of the products derived from it are shown in Table 4(B). Using the appropriate emission factors (see Section 4.6), the total sulphur emission from petroleum products in 1974 was 58.3 Tg SO2 (2Y.l Tg S).

21.4Tg SO2 (10.7TgS), of which 18.8Tg SOZ (9.4 Tg S) are associated with the production ofcopper. 4.5. Other sources The only other significant contribution is from the manufacture of sulphuric acid. Here, the average retention encountered is between 95 and 98% corresponding to emissions of 35 and 13 kg of sulphur dioxide, respectively, per ton of 100% H2S04 (Sittig, 1975 ; Seinfeld, 1975). The average value used is 24 kg, which yields a total annual emission of 2.5 Tg SOZ (1.3TgS) for 1976. All the sources mentioned yield essentially sulphur dioxide (although this compound is always accompanied by sulphur trioxide and sulphuric acid mist in small proportions). The other, minor, contributors are : (a) The conversion of pulp into paper, emitting mainly hydrogen sulphide; 65 Y{of the production is based on Kraft methods and about 20% on sulphite

Table 6. Other production* (Tg S a _ ’ )

Copper (smelted) Copper (refined) Lead Zinc Sulphuric acid? Mechanical pulp Chemical pulp Newsprint Other paper Total paper/pulp Sulphur Byproduct Pyrites Total

__-____-

1965

1970

1974

1975

1976 7.9

5.1

6.4

7.7

6.2 2.6 3.9 67.58 21.70 57.84 16.98 80.48 177.10

7.6 3.3 4.8 85.98 26.30 77.92 21.38 106.10 231.71

8.7 3.4 5.3 104.85 26.72 90.99 23.12 128.08 268.92

7.3 8.3 3.2 4.8 101.18 23.00 79.82 20.96 113.98 237.16

8.7 3.4 5.1 105.74 24.27 87.84 22.21 128.71 240.06

26.29

6.02 10.01 16.03

14.87 10.34 25.2 I

15.22 10.62 25.84

13.80 10.12 23.92

__.__.__.___~__~

_

__-.-.-

---..-

*UN (1969a, 1973, 1977a, 1978a). tin terms of lOOo/, H,SO, and including the sulphuric acid equivalent of “oleum”.

---

1271

Atmospheric sulphur : natural and man-made sources Table 7. Emission factors (kg SOz per 10’ kg

Hard coal Lignite Coal coke Petroleum refining Motor spirit Kerosene Jet fuel Distillate fuel oils Residual fuel oils Petroleum coke Copper smelting Copper refining Lead Zinc Sulphuric acidf Pulp/paper Sulphur

produced)

1965’

1970

1974-1976

48.2 35.6 5.4 3.65 0.90 2.40

48.2 35.6 5.4 3.0 0.72 0.96 0.96 4.47 36.0 13.5 2OoOt 400 520 260 24 2 2

48.2 35.6 5.4 2.0 0.72 0.96 0.96 4.47 36.0 13.5 2OOOt 350 470 200 24 2 2

7.00 40.0 2000 450 570 330 24 2 2

*Robinson and Robbins (1972). t Estimated from values of Jones (1972), Katz (1973), and Robinson and Robbins (1972, 1975). @XIterms of 105% H,SO,.

methods (with 90% recovery) (Seinfeid, 1975 ; Sittig, 1975 ; Natusch and Slatt, 1978). (b) The incineration of refuse, which has for some time remained more or less constant at an estimated 0.65Tg SOz a-‘. (c) The production of sulphur, with 99.9 % recovery (Sittig, 1975). Of these last sources, onfy the pr~uction of pulp is causing slightly increased emissions, but these are likely to be controlled more effectively. Neither the incineration of refuse nor the production of sulphur are generating increased amounts of atmospheric sulphur and these will gradually disappear as important sources in the total emissions spectrum. The total for the three sources, in 1976, was 1.2 Tg SO2 (0.6 Tg S). Another source, formerly considered very important, is the coal refuse banks, where mines and large industrial plants dispose of the undesirable material mined together with coal, including waste, rock, shale, slate and pyritic excess. These used to be impro~rIy disposed of and fr~uent~y ignited spontaneously. In 1962,400 of the approx 500 banks in the United States were burning simultaneously (Hall, 1962). However, with an average combustion life of 20 y, most of them

Table 8. Man-made sulphur emissions: sources (Tg SO, a-‘)* 1965

1970

1974

1975

1976

71.4 (47.9) 28.1 (18.9) 2.4 (1.6) 102.0 (68.5)

77.5 (45.0) 30.5 (17.7) 2.5 (1.5) 110.4 (64.1)

81.1 (43.3) 31.6 (16.9) 2.5 (1.3) 115.2 (61.5)

85.5 (45.4) 32.8 (17.4) 2.5 (1.3) 120.8 (64.2)

88.1 (42.5) 33.1 (16.0) 2.6 (1.2) 123.8 (59.7)

$oaJ

Hard coal Lignite Coal coke Subtotal Petroleum Refining Motor spirit Kerosene Jet fuel Distillate fuel oil Residual fuel oil Petroleum coke Subtotal Non-ferrous ores Copper Lead Zinc Subtotal Others HSOa Pulp/paper Refuse Sulphur Subtotal Total --

-

5.7 0.3 0.2

(3.8) (0.2) (0.13)

28.5 (19.1)

6.8 (4.0) 0.4 (0.2) 0.1 (0.05) 0.05 (0.03) 2.2 (1.3) 29.8 (17.3) 0.4 (0.2) 39.6 (23.0)

5.8 (3.1) 0.5 (0.3) 0.1 (0.04) 0.1 (0.06) 3.0 (1.6) 37.4 (20.0) 0.4 (0.2) 47.2 (25.2)

5.5 (2.9) 0.5 (0.3) 0.1 (0.04) 0.1 (0.06) 2.8 (1.5) 34.3 (18.2) 0.5 (0.3) 43.8 (23.3)

7.3 0.6 0.1 0.1 3.8 45.8 0.6 58.3

12.9 (8.7) 1.5 (1.0) 1.3 (0.9) 15.7 (10.5)

15.8 (9.2) 1.7 (1.0) 1.2 (0.7) 18.8 (10.9)

18.4 (9.8) 1.6 (0.9) 1.1 (0.6) 21.1 (11.3)

17.5 (9.3) 1.5 (0.8) 1.0 (0.5) 20.0 (10.6)

18.8 (9.1) 1.6 (0.7) 1.0 (0.5) 21.4 (10.3)

2.0 (1.3) 20.3 (13.6)

1.6 0.4 0.65 0.05 2.7

(1.1) (0.2) (0.4) (0.04) (1.8)

14s.9t

2.1 (1.2) 0.5 (0.3) 0.65 (0.4) 0.03 (0.02) 3.2 (1.9) 172.2f

2.5 (1.3) 0.5 (0.3) 0.65 (0.3) 0.05 (0.03) 3.7 (2.0) 187.3

2.4 0.5 0.65 0.05 3.6

(1.3) (0.3) (0.3) (0.03) (1.9)

188.2

(3.5) (0.3) (0.04) (0.06) (1.8) (22.1) (0.3) (28.1)

2.5 (1.2) 0.5 (0.2) 0.65 (0.3) 0.05 (0.02) 3.7 (1.8) 207.2

*The figures in brackets represent percentages of the total. *This is a previous value for coal, petroleum and non-ferrous ores (Robinson and Robbins, 1972), to which emissions from other sources have been added. $This may be compared with the vahre of 166TgSOI predicted for 1970 (Peterson and Junge, 1971).

I272

(-‘ I-‘. C‘r I Table 9. Production

1950 1976 Total ‘;b increase Yearly “/, increase

(Tg S a

and M. M. HIKSC,,I I I<

I I\

’)

Hard coal

Lignite

Petroleum

1434.72 2413.69

418.58 930.X6

53x.47 3672.X0

6X

122

2.03

standards demanded by the US Clean Air Act of 1970 start to come into effect (they should have been met by 1975 but a clause allowed postponement until 1977). It will probably be possible for data for the United States. at any rate, to be based on lower emission factors. Table 8 shows the total emissions calculated on the basis of the figures in Table 7, together with the percentages of the total emissions correspondmg to each of the sources. Coal’s contribution has decreased from over 97 :‘,, in 1880 (Table 3) to under 60”,, m 1976 (Table 8). In contrast, the contribution of petroleum over the same period has increased from less than I to 28 “/. Non-ferrous ores have been responsible for everincreasing percentages of man-made atmospheric sulphur, due chiefly to increased emissions from copper smelting. The growth of petroleum production as compared with that of coal has been phenomenal in the years since 1950 (Table 9). The serious oil crisis of the early 1970s put a temporary halt to the growth so that, in 1975, there was actually a decrease in petroleum production. This was partly compensated for by an increase in the coal produced (Table 10) bur still resulted in an overall fall in the amount of energy used worldwide. The dramatic increase, in 1976. III pctroleum production more than made up for the earlier loss. Increases in sulphur emissions show the came pattern as the production figures (Table 11 ), There is now growing pressure for industrialized countries to limit the increases in sulphur emissions by the conservation of energy and the increased installation of sulphur removal technologies (OECD, 1978). Thus, sulphur emissions in the United Kingdom have not increased over the last 10y (Weatherley. 1979).

582

3.18

7.66

have become depleted, and their successors are now being properly dealt with. The only other large combustion source, natural gas, is a very clean fuel, since the sulphur content does not, usually, rise above Sppm (Medici, 1974) and almost all of it is removed before use. A large amount of natural gas, associated with petroleum, is burnt at the oil wells in some Middle Eastern countries. This gas may be quite acid (containing several per cent S by weight) but it does not contribute significantly to overall emissions. The total productions of the materials indicated (Table 6) give a combined total emission from all these sources of 3.7 Tg SO, for 1976 (1.9Tg S). 4.6. Conclusions Table 7 shows the emission factors employed in this work, together with the corresponding values used in a previous estimate of global man-made sulphur emissions (Robinson and Robbins, 1972). The use of such factors is clearly not the ideal method for calculating emissions but it is the only one available for making estimates over large areas. The errors involved in some of the emission factors may be quite large but the overall trend is unmistakeable and could not be seen unless detailed figures are given. When the stringent

Table

10. Production

(9; a

‘)

Lignite

Hard

coal

1950-1960 1960-1965 ----.---~ 1965-1970 1970-1974

-ii}

Lg6--

1974-1975 197551976

5.24 2.55

Table

increases

Petroleum

g]

--~~$$l:

3.97 0.90

11. Man-made

sulphur

emission

Lignite

Hard coal

7.24

- 4.68 33.61

increases

(“, a- ‘)

Petroleum

Overall

1950-1960 1960-1965 1965-1970 1970-1974 1974-1975 1975-1976 1950-1976*

5.43 3.04

3.97 0.90 122J

~72 8%

* y0 increase

over total period.

- 7.20 33.11 602

0.48 10.07 149

Region

3296f 36323 389011 3967n 4044”

Area (km’) 9,363,353 1,768,410

Population (x 106)

196.9**

120.4

Emission (kg SO, per capita) .~._. 45.2 47.4 48.2 47.4 51.2 .-.-_ 1965 1970 1974 1975 1976

1000 1157 1258 1264 1392

292 338 367 369 406

Heicklen (1976) Whelpdale (1978b) Semb (1978) Semb (1978) Semb (1978) Semb (1978) Semb (1978) Semb (1978) Semb (1978) Semb (1978) Davey (1978)

1966 1972 1973 1973 1973 1973 1973 1973 1973 1973 1976

3050 11,423

145.0 167.7 7333 32.705 14,488 5941 59X 3681 23,194 15,790 183

References

Year

Emission (kgS0, per kmz)

Emission (kg SO2 per capita)

{B) Regional emissions

Year

Emission [kg SO, per km2 (land mass)]t

Emission (kgS0, per km’)*

78.9 223.1 2.4 x 10” EUIOpe 103.4 9.7 30,515 Belgium 124.5 5.0 43,069 Denmark 65.3 49.5 543,998 France 47.1 3.9 307,988 Norway 24.2 6.2 41,288 Switzerland 103.4 54.2 241,705 United Kingdom 64.2 61.2 248,764 West Germany 102.3 13.6:: 7.69 x lo6 Australia --~ *Global area: 510 x lo6 km2. tGlobal land mass area: 148.9 x lo6 km*. SUN (1969a). VJN (1973). l/UN (1977a). VJN (1978a). **UN (1969b). ‘i?Comprises all states in the US included in area shown in Fig. 2. ‘IfUN (1977b).

United States North-eastern United Statestt North-western

-_

Population ( x 106)

(A) Global emissions

Table 12. Man-made sulphur emissions: estimates per capita and per unit area

C. F. CULLISand M. M.

1274

L-__!L&-____._+ 90" Fig. 2. Estimated annual man-made emissions of sulphur dioxide in north-eastern North America. Values shown are in TgSO, (after Whelpdale, 1978b).

HIRSWLER

Data for emissions can also be estimated on both a per capita and a per unit area basis (Table 12). These can be compared with the corresponding estimates for the total global land mass area and with estimates for other areas. Figure 2 shows estimates of annual manmade emissions of sulphur dioxide in north-eastern North America. The hemispheric contributions of man-made sulphur emissions into the atmosphere are shown in Table 13. The Northern Hemisphere generates 94:‘, of these, in consequence of the fact that almost all industrialized societies are located in this half of the globe, which also contains a larger proportion of land area. Only the emissions due to the smelting of nonferrous ores are at all comparable in both hemispheres, since about one-third of the emissions from copper are generated in the Southern Hemisphere. In fact, copper is responsible for over 50 y>,of the whole of the sulphur emissions in the Southern Hemisphere, while it accounts, at its highest, for only loo/; of the overall spectrum of emissions in 1974. If the two big sources of sulphur emissions are compared, coal is evidently least

Table 13. Hemispheric man-made sulphur emissions (TgSO, a- I)* 1970

Coal Hard coal Lignite Coke Subtotal Petroleum Refining Motor spirit Kerosene Jet fuel Distillate fuel oil Residual fuel oil Petroleum coke Subtotal Non-ferrous ores Copper smelting Copper refining Lead Zinc Subtotal Other --Sulphuric acid Sulphur Pulp/paper Refuse Subtotal Total

1974

Northern

Southern

Northern Southern .._~~_~~~._ _._ . . ~_~~ _~~.....--

76.0 29.5 2.5 108.1

(98) (97) (98) (98)

1.4 (2) 0.93 (3) 0.05 (2) 2.4 (2)

79.6 30.6 2.5 112.6

(94) (95) (93) (97)

0.39 0.02 0.005 0.002

(6) (5) (7) (3)

5.4 0.45 0.08 0.11

2.1 (95)

0.10

(5)

28.4 (96)

1.3

0.35 (98) 37.8 (95)

6.4 0.36 0.08 0.04

8.4 (65) 2.4 1.5 1.1 13.4

2.0 0.03 0.45 0.62 3.1

(98) (97) (98) (98)

1.5 (6) 1.0 (3) 0.05 (2) 2.6 (2)

(94) (95) (92) (96)

0.35 0.02 0.006 0.005

(6) (5) (8) (4)

2.8 (95)

0.14

(5j

(4)

35.6 (95)

1.8

(5)

0.01 (2) 1.x (5)

0.44 (98) 44.9 (95)

0.01 2.3

(2) (5)

4.4

10.3 (67)

5.1

(33)

2.4 1.4 0.95 15.0

(79) (86) (90) (71)

0.65 0.22 0.11 6.1

(21) (14) (1OJ (29)

2.5 0.04 0.52 0.62 3.6

(97) (98) (96) (95) (97)

0.07 0.001 0.02 0.03 0.12

(3) (2) (4) (5) (3)

(35)

(79) (87) (90) (71)

0.63 (21) 0.23 (13) 0.13 (10) 5.4 (29)

(97) (96) (97) (95) (97)

0.06 0.001 0.02 0.03 0.11

(3) (4) (3) (5) (3)

9.1

(6)

162.3 (94)

176.2 (94)

*The figures in brackets represent the percentages in each hemisphere

11.1

(6)

Atmospheric sulphur : natural and man-made sources

1275

Table 14. Present estimates of emissions of atmospheric sulphur (Tg S a- *)’ 1970 Northern and Southern Hemispheres

1976 Northern and Southern Hemispheres

Natural Volcanoes Sea-spray Subtotal non-biogenic Biogenic (land) Biogenic (oceans) Subtotal biogenic Total natural

5 44 49 48 50 98 147

5 44 49 48 50 98 147

Man-made Total

86 233

104 251

59 37

71 41

Sources

Man-made (% of natural) Man-made (% of total)

1976 Northern Hemisphere

3 19 22 32 22 54 76

(60) (43) (45) (67) (44) (55) (52)

98 (94) 174 (69) 129 56

1976 Southern Hemisphere

2 25 27 16 28 44 71

(40) (57) (55) (33) (56) (45) (48)

6 (6) 77 (31) 9 8

*The figures in brackets represent the percentages in each hemisphere.

used in the south, since it accounts for only 2 % of the emissions due to coal, while emissions from petroleum in the Southern Hemisphere are about 5% of the total. The sulphur emissions for 1976 show virtually the same hemispheric distribution as for the earlier years (Table 13) but, if anything, there is an overall tendency for the difference between the Northern and Southern Hemispheres to widen. Stratospheric transport of sulphur has caused important contributions of manmade sulphate to polar areas in the Northern Hemisphere (Greenland) (Koide and Goldberg, 1971; Weiss et al., 1975; Nyberg, 1977; Delmas, 1979) but there appears to be no effect in the Southern Hemisphere (Antarctica) due to the short residence time of sulphur in the stratosphere (Nguyen et al., 1974; Delmas and Boutron, 1978). In summary then, man’s global emission of atmospheric sulphur has increased from 148.9Tg SO2 (74.4 Tg S) in 1965 to 172.2 Tg SO, (86.1 Tg S) in 1970 and to 207.2 Tg SO1 (103.6 Tg S) in 1976.

total sulphur

5. GENERAL DISCUSSION

Reasonable estimates for the current rates of generation of atmospheric sulphur from volcanoes and seaspray are 5 Tg S a-r and 44Tg Sal r, respectively (Section 3.1). Similarly, detailed consideration of man’s activities (Section 4) suggests that ca 104TgSa-’ were emitted into the atmosphere in 1976. Previous estimates for biogenic emissions (Se+ tion 3.2) differ very widely. The earlier estimates (Junge, 1960, 1963a; Eriksson, 1963) are probably considerably too high, whereas those of Bolin and Charlson (1976), Hallberg (1976) and Granat (1976) are almost certainly too low, particularly in view of the latest figures for the emission of dimethyl sulphide (Nguyen et al., 1978). Other estimates of annual amounts of sulphur removed (Robinson and Robbins,

1972 ; Kellogg et al., 1973 ; Friend, 1973 ; Davey, 1978) have all been based on figures for man’s contribution, ranging from 50-70TgSa-‘, which, in the light of more recent data, are now recognized to be rather low. If the latest figure for man’s activities of 104 Tg S a- ’ is accepted as correct (Section 4), the total amount of sulphur removed annually from the atmosphere should increase considerably, although perhaps not quite as much as Garland (1977) has estimated on the assumption of very high deposition rates. The value for total biogenic emissions of 98 Tg S a- ’ used here is in fair agreement with the estimates of Robinson and Robbins (1972), Kellogg et al., (1972), Friend (1973) and Davey (1978). It corresponds to deposition figures of 233 for 1970 and 25 1 Tg S a- ’ for 1976. Table 14 summarizes the estimates of atmosph~ic sulphur emissions for 1970, 1974 and 1976, the separate hemispheric contributions being included for the latest year. While man’s contribution in the Northern Hemisphere is already greater than that of Nature, his activities in the Southern Hemisphere are unlikely to produce, in the foreseeable future, emissions comparable with those from natural sources. The total industrial emission of sulphur for 1956 (80 Tg SOZ or 40 Tg S) was estimated to be only 3 1% of that derived from natural sources (Junge, 1960) but the present data indicate that man’s contribution to the sulphur budget had, by 1976, reached almost 210TgS02 a-i (or 105TgSa-‘), which is over twothirds of the total natural emissions (Table 14). Man’s contribution is increasing at a rate of 2.2 % a- ‘, so that it will, ifthe present rates ofgrowth are maintained and the emission factors are not significantly reduced, equal and then overtake Nature’s well before the end of the present century. Acknowledgements - The authors thank Dr. T. R. A. Davey and Dr. M. F. R. Mulcahy for helpful discussion. Certain material from this paper was presented at the Society of Chemical Industry International Symposium on Sulphur Emissions and the Environment, London, 810 May 1979.

1216

C. F. CC,I I.IS and M. M. HIKS( t11.1I< REFERENCES

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