Sulphur in air and soil

S U L P H U R I N A I R A N D S O I L

ROSEMARY PRINCE and F. F. ROSS Central Electricity Generating Board, London, England

(Received 21 October, 1971)

Abstract. Natural S supplies are insufficient for high crop yields. The S brought down by rain is limited, even in industrial areas, and much of it is lost in drainage. Additional supplies are needed either as fertilizers or as S02, which vegetation readily absorbs. The margin between the minimum SOz concentration for full growth and the concentration at which damage occurs is wide enough for distribution via the air, without deleterious effects, of such S as is available from the combustion of fuels.

1. Introduction

On the one hand the SO2 content of the air is at times and in places so high as to be damaging to vegetation and possibly injurious to human health, and there are fears

that increasing emissions from the combustion of fossil fuels will make the situation worse. On the other hand crops of various kinds are dependent upon atmospheric

sources for a large proportion of their S needs and will become more dependent with the trend to high analysis fertilizers. It is the purpose of this paper to analyze the information currently available, with a view to deducing in what manner and to what degree the air should be supplied with SO2 so as to provide maximum benefit

to vegetation with minimum damage or loss of amenity.

2. Sulphur Needs of Vegetation

The S requirements of vegetation first need to be established. For the biosphere as a whole, which is mainly tree forest, Deevey (1970) estimates an average biomass of

200,000kgha -1 with the composition H2900014soC14aoN16Pl.sS. This is 1 kg of S for every 7 kg of N and 142kg S ha -1. Stewart (1967) has found that the N: S ratio of the protein in plant tissue is about 16:1 for wheat, alfalfa, and sugar beet; ratios of 17:1 or greater indicate S deficiency. Cowling and Jones (1970) found that rye grass (Loliurn perenne L.) growing under conditions of plentiful N and S supply developed an N: S ratio of about 10:1 by weight, and this ratio has also been found to be typical of the organic matter in a wide variety of soils. Different crops are known to take up S at different rates. Table I is a compilation from various sources (e.g. Aslander, 1958; Whitehead, 1964) showing the needs of various agricultural

crops. These crops are in general dependent upon nutrients in the top layer of the soil.

For S they are dependent on the sulphate ion. Whatever may be the reserves at lower depths accessible to the roots of trees and some other species, for most agricultural produce soluble sulphate must be available in the upper layers. I t is so readily leached out that it cannot be regarded as available from year to year and supply and demand

Water, Air, and Soil Pollution 1 (1972) 286-302. All Rights Reserved Copyright © 1972 by R. Deidel Publishing Company, Dordreeht-Holland

S U L P H U R I N A I R A N D S O I L 287

q o . .

c¢'l t 'q ,~-

g g g ~

I l l I

¢q

o r-- o o. o.

t 'q

..a

[ -

b0

. n

, ~ o

O

Z

~5 ,.6 ,,,q

¢q ,-..-~ ~D ¢q ¢'xl t-q

¢q ,~- ¢q

¢.i ~ r.i

¢q

O

>

I

O O O O O O

',~ tel O

e~ e¢l t"q

,.6

8

r - ~.

t ' -

o t"q t'~

288 ROSEMARY PRINCE AND F. F. ROSS

must be considered on an annual basis. Some of the S locked up in organic matter can be mineralized and drawn upon, thus giving some insurance against S deficiency over a period of a season or two. Another form in which S can be stored in soil is that of gypsum; this occurs chiefly in arid conditions. In calcareous soils some CaSO, can be co-precipitated with CaCO3. Acid clays may contain sufficient sulphate to attack concrete, but such 'soil' is inhospitable for plant roots. Fertile soils generally require a fresh supply of S for each growing season.

The total S required is that needed by the crops plus that carried away in the drainage. Likens et al. (1967) found by analysis of rain and drainage for a catchment area in New Hampshire that the loss in drainage (9.8 kg S h a - ' ) was almost as much as that brought in by rain (10 kg ha- l ) . Land from which good yields are expected therefore requires annual supplies of 20 to 50 kg h a - ' of S. Unfertilized forests, estimated to need about 20 kg ha -1 for new growth but carrying forward about three-quarters of this as organic matter from dropped leaves and branches, need a net supply of perhaps 15 kg ha -1 allowing for drainage loss.

Sulphur reaches crops by (a) absorption of SOz by the soil, (b) similar absorption through the leaves, (c) dissolving in precipitation, and (d) application of fertilizer or fungicidal dressings. The first three of these are part of a cycle which involves SO 2 in the atmosphere as one stage. This cycle will now be considered quantitatively.

3. Natural Sources of Airborne Sulphur

Sulphur enters the atmosphere from natural sources and in the form of emissions by man. Taking first the natural sources, the two principal modes of entry into the atmosphere are as sea spray and as evolution of H2S. These have been estimated by Robinson and Robbins (1968) as given in Table II. Sea spray is an important factor for coastal areas, and rainfall sampled near the coast contains much excess suphate. But the effect rarely extends as far as 50 km inland and for the major part of the vegetatively productive portion of the Earth's land surface it may be neglected.

Hydrogen sulphide is believed to be rather rapidly oxidized to SO2, and it is considered that little error will result from the assumption that the conversion is rapid enough to enable this to be regarded as a source of SO2. It should be noted that it also arises in coastal areas and from swamp or marshland.

TABLE II Natural supply of S to the atmosphere

kg y-1 (as S)

H2S from natural biological decay on land 68 × 109 Sulphate from sea spray 44 × 109 H2S from natural biological decay in oceans 30 × 109

Total: 142 × 109

S U L P H U R IN AIR AND SOIL 289

Juvenile S from volcanoes, fumaroles, and sulphurous springs is an additional

source. Once the only source of S for the biosphere, its magnitude will be assumed to

be within the limits of error of the estimates in Table II. The total natural S at 140 x 109 kg yr -1 is seen to be about 103 kg km -2 of the

Earth's land surface of 149 x 10 6 km z, or 10 kg ha -1.

4. Anthropogenic Emissions

The total S emitted to the atmosphere by man, mainly as SO2, was estimated for 1956 as 30% of that from natural sources by Junge (1960). Globally a world consump- tion of 2.3 x 1012 kg annually of coal would, with an average S content of 2% and one-tenth retention in ash, result in the emission as SO 2 of 42 x 109 kg of S. The

burning of wood and other solid fuel may add some 5% to this figure. The emission of SO2 from the burning of residual fuel oil has been estimated by Rockett et al. (1970) at 25 x 106 kg of S daily, or 9 x 109 kg yr -~ Other sources such as smelter works may amount to 5 x 109 kg yr -1. The total anthropogenic emissions thus now approach 60 x 109 kg yr -1, or about 40% of the natural emissions.

Whereas the natural emissions are concentrated around coast lines, the anthropogenic emissions are associated with industrialized areas, and can locally exceed the natural supplies. For example, emissions in Britain (0.23 x 106 km 2 but taken as 0.25 x 106 km 2 for arithmetic convenience) have increased f rom 9.1 to 11.4 Mg km -6 since

1950, an increase of 2.3 Mg km -2 in 20yr.* On the same basis the emissions in the U.S.A. (9 x 106 km 2) are now approaching 2 Mg km -2 and are expected to reach

3.3 Mg kin -2 by 1980 if the fossil fuel becomes available and no steps are taken to reduce emissions. In Western Germany (0.25 x 106 km 2) the annual emission of 8 Mg km -1 has been valued at 25 x 106 U.S. dollars in terms of agricultural benefit

(Kuhn and Faller, 1970). Over smaller areas the rates of emission can be much larger, for example 40 Mg

km -2 for two areas of 25 x 103 km 2 in the U.S.A. and 50 to 60 Mg km -2 for an

area of similar size in north-central England. The atmospheric conditions arising f rom emissions are considered later.

5. Manufactured Sulphur

Theworld production of S in all forms is about 40 × l06 Mg yr -~ which is comparable

with the emissions from fuel combustion. More than half of this is mined elemental S, the source of the remainder beingiron pyrites, nonferrous metal sulphides, anhydrite, and S removed from the distillate fractions of oil and f rom natural gas. About 30 x 106 M g y r -1 are converted to H2SO 4. A large proportion of this is used in the manufacture of fertilizers and some of the S remains in the product. A number of pesticides and fungicides applied to crops contain S in one form or another.

A quantitative examination of the whole S cycle can in principle be completed by

* Megagramme, or metric tonne = 103 kg.

290 ROSEMARY PRINCE AND F.F. ROSS

determining the S carried away in rivers that drain the land. From home production and imports Britain uses nearly 1.5 x 106 Mg in industry, and over 106 Mg is converted to H2SO4. This is potentially too large an addition to the 2.8 x 106 Mg of emissions to be neglected. However, most of it is discharged after use into tidal waters. The amount entering any particular river catchment area in fertilizers, in household detergents containing alkyl benzene sulphonate, or from inland industry can be estimated with reasonable accuracy.

6. Maximum Permissible Concentrations in Air

If the atmosphere is to be used, whether as a deliberate policy or by accident of circumstances, to distribute back to the land for today's vegetation the S locked up in fossil fuels, then various questions have to be answered. Concentrations of SO/ will be high near the point of discharge and it must be established what concentrations are tolerable. Over larger areas there must be a relationship between emissions and concentrations, and it will be desirable to establish the relationship. The rate of supply to the land either by absorption or rainfall will depend upon the concentration, so that to achieve adequate supply a certain minimum concentration is necessary.

The concentrations at which atmospheric SO/ will damage plants have been extensively studied, e.g., Stratmann (1961), Zahn (1963) and references therein. Dreisinger and McGovern (1970) found a number of species to be slightly less tolerant during the growing season, especially in humid weather, than the European findings indicate. For these most sensitive species under these conditions the results can be expressed as no visible damage at dosages less than (C-400) t --- 1700 #g h m-3, where C is the actual concentration in #g m-3 and the duration t is expressed in hours. Zahn has derived formulas for the tolerance threshold under intermittent fumigation, showing that with sufficient recovery time a plant may survive undamaged from separated periods of overdose which, if consecutive, would be damaging. The recovery times are typically a few hours. For example, a power station chimney designed to give a normal maximum 3 rain concentration at ground level not exceeding 1400 #g m -3 may in abnormal weather conditions produce twice this dosage. Chimneys inadequately designed for high S coal in the U. S. A. have been observed to produce dosages at ground level of 2800 gg m-3 for about half an hour. This dosage, (2800-400) x 0.5 = 1200, would not be damaging, although if continued for 45 rain some species might suffer if the leaves were damp. The Zahn results indicate that two half-hour periods could be tolerated if separated by a few hours at a much lower concentration.

Kuhn and Faller (1970) report growing sunflower, maize, and tobacco in atmos- pheres with 0, 200, 500, 1000, and 1500 #g m -3 of SO2 for three weeks continuously. All showed higher growth rates at 200 #g m-3 than at zero concentration. Tobacco yields increased up to the highest concentrations tested, although sunflower and maize reacted with a yield decrease to the high concentrations. It is reasonable to conclude that modest SO2 concentrations are generally beneficial to vegetation, which

S U L P H U R I N A I R A N D S O I L 291

during the course of evolution has come to obtain S in just this way, but that concentrations over 400/~g m -3 should be restricted in duration.

If this can be taken as a guide to tolerable maximum dosages, there is no need to consider effects on animals or humans. They appear to be more tolerant than vegetation (A. P. O., 1969; Lawther and Bonnell, 1970).

7. Emissions and Concentrations

From the point of view of the supply of atmospheric S to crops and the soil, the concentrations at leaf level are of overriding significance. The concentrations above this level provide a reserve from which the ground level concentration can be replen- ished as the air moves over a region where absorption exceeds low level emissions. The concentrations produced at ground level are markedly dependent upon manner and height of emission. Because insufficient attention has been given to this aspect in quantitaive studies elsewhere, useful data are available mainly from Britain.

From about 1950 onwards the importance of emission height began to be recog- nized and applied in practice. The chimney height for all large installation~ is now prescribed by certain recommendations of the Department of the Environment, and for over 12 yr every new source above a certain size has had to comply. Figure 1 shows the emissions of SO 2 at low, medium, and high levels since 1950. In the absence of detailed data about individual chimneys it has been assumed that transport, domestic, and commercial emissions, and in addition gases equivalent to those from

12-

~IO-

Z

z~

d B -

o.

~n

z 4-

F-

2 ~

0

[950

J

J H [ G H - L E V E L E M I S S I O N S

M E D I U M - L E V E L EMISS IONS

z

8 GAUGES ~)

i - too

I u

I u

RAI NFALL SULPHUR I I I O

I I I [960 [965 1969

LOW-LEVEL ~ ( E M I S S I O N S

GAUGES

I 1955

Fig. 1. Sulphur in air and rain in the U. K.


one-third of the coal burned in industry generally, should be classed as low level. Most power station and oil refinery chimneys can be classed as high, together with a small proportion of industrial chimneys; to allow for old power stations still in service, and the changing pattern of industry, we have taken 75~ of power stations and 10~o of industry in 1950, increasing these proportions to 95~o and 20~, respectively, today. On this basis the low level emissions have decreased from 2.2 to 1.33 x 106 Mg of SO2 annually, the high level emissions have increased from 0.91 to 2.89 x 106 Mg, and the inbetween medium level emissions have been within 1.49 _+ 0.06 x l06 Mg. The scale on the left of Figure 1 is in terms of S emission per unit of land area (Mg km-2). The line marked 'rainfall sulphur' (Section 9) applies this same scale in terms of deposition. The dotted lines show concentrations of SO2 (in terms of S, #g m - 3, right hand scale) in the air at ground level. The upper dotted line is the average of eight urban gauges in continuous service at the same sites. The lower dotted line has been constructed (Craxford et al., 1970) by taking all the gauges in service (200 to 500) for pairs of consecutive years, weighting the average according to density of population, and thus obtaining the true average of the changes in breathing air from year to year. The slope of the first line shows close correlation with the low level emissions. The slope of the second shows some influence of medium level emissions, but it has been going down faster than the sum of low and medium level emissions together. There is no apparent influence of high level emissions.

It is possible that a true average of the ground level concentrations over the country as a whole and thus available to vegetation would show some correlation with total emissions. Insufficient gauges are available at sites too remote for easy servicing, but such gauges as are in operation show 5 to 40/~g m-3 of SO 2. Data do not exist to enable a possible 20~o increase in average to be confirmed or denied. The values are such, however, that an increase considerably greater than this would have been, and would still be, beneficial to vegetation.

In north-central England an area of about 90 km radius centered at latitude 53015 ' would probably include half the total emissions, or 1.4 x 106 Mg of S, thus exceeding the estimates of about 10 6 Mg in similar-sized regions around Chicago and in the New York/New Jersey area. Three hundred km to the west of this center is Ireland where gauges show averages below t0/~g m -3 of SO2. Seventy km north (and thus within the circle) an observation site near the city of Leeds has shown 10~ of the monthly averages below 25/~g m -3 and 10~ above 95/~g m -3. Three hundred thirty km north the Eskdalemuir observation site has produced corresponding figures of 5 and 32/ag m -3 of SO 2. It is apparent that at ground level SO 2 does not travel far.

8. Absorption of Sulphur Dioxide

The rate of absorption of SO 2 from the air by leaves and soil is not well established. Surface moisture is a significant factor. Spedding (1969) has found a coefficient of above 1 cm s- 1 for wet leaves, taking the total (both sides) leaf area as eight times the area of ground occupied. If this were applied to a growing season of 107 s it would amount

SULPHUR IN AIR AND SOIL 293

to 100mgm -2 of SO 2 or 0.5kg ha -~ of S for each 1 ktgm -3 of SO 2. An average of

40 #g m -3 thus would provide 20 kg ha -1 yr -1 of S directly to the plants. If the

contribution from rain minus drainage loss is effectively zero (Section 9), this

would be marginally sufficient for a wide range of crops. Martin and Barber (1971) found that a hedge absorbed SO2 at a greater rate than this on the basis of occupied ground area.

A bare soil surface will also absorb SO 2. The rate has been estimated (Alway et al., 1937) at 22~ of that by a lead peroxide surface. This leads to an approximate estimate of the rate of absorption by soil in kg S ha -~ at 0.55 times the concentration of SO 2 in #g m - 3. This is closely similar to the estimate of that absorbed by leaves during a limited growth period. The two mechanisms will probably not operate simultaneously, because when leaves are present very little SO2 will reach the ground beneath them.

An average of about 40 #g m -a is thus desirable, and this is not attained even in Britain where the productive land is associated with the highest density of SO2 emissions in the world.

9. Sulphur in Precipitation

The increase over 20 yr in Britain's emissions of SO 2 has been accompanied by a significant increase in the average height of emission and by a fall in the concentrations at breathing level in and near urban areas. One might therefore expect a significant increase in concentration at cloud level and a consequent increase in rain sulphate; this has not occurred. Some data are given in Table III and the average for these three inland sites is plotted on Figure 1. The reason may be that there is an effective limit to the solubility of SO 2 in rain. Most of the solubility is due to ionization of H2SO3, supplemented by oxidation to sulphate, which may occur rapidly in solution especially if any metals with catalytic effect for this reaction be present. From the known solubility of SO2 in water at moderate pressures and the equations:

SO 2 (gas) ~- SO2 (dissolved)

H 2 0 + SO2 (dissolved) ~ H2SO 3

[H +] [HSO~] = 1.74 × 10 -2 (at 25°C)

[ H 2 5 0 3 ]

2 H 2 S O 3 -4- 0 2 = 2 H 2 S O 4 (slow, b u t irreversible)

[H +] [SOIl - 1.2 × 10 -2 (at 25°C)

[so;1 [H +] + [M +] = [HSO~] + 2 [SO4]

and neglecting second order relationships such as:

[H +] [SO;] _ 10- ~ [HSO~]

[H +] x [OH' ] = i0 -~4

294 R O S E M A R Y P R I N C E A N D F. F . R O S S

t'¢) ¢e3 h'3

'~" ("q kO ¢.i ~q c5

o o ~ ¢ q

~ ,,@ oo

c'q ',~- t 'q ('q v..-~ v..-~

S U L P H U R IN A IR A N D SOIL 295

one can derive theoretical estimates for the partial pressure of SOz in equilibrium with cloud drops in which a proportion of the S is oxidized to sulphate and which also

contain certain amounts of alkaline cations. Figures 2, 3, and 4 have been derived on

this basis, assuming the clouds to be at 25 °C and at normal atmospheric pressure. Axes for zero°C and actual pressure at 1000 m or more altitude would be slightly different, but the curve shapes would be similar. Figure 2 is for zero oxidation, showing a major effect of 2 and 5 x 10 -5 equivalents per liter of alkaline cations such as NH4, K, Ca, etc. Figures 3 and 4 are, respectively, for the cases of 75 and 90~ oxidation to sulphate. Contours for pH are indicated on the figures. The left hand ordinates are in terms of mole 1-1 of sulphite or sulphate, whereas the right hand ordinates are calibrated in Mg S km -z for 1000 mm of rainfall.

The horizontal scale shows the concentration of SO2 remaining in gaseous form when equilibrium has been established. As an indication of the amount dissolved at this stage it may be assumed that the cloud does not contain more than 1 g m-3 of water droplets. At 5 x 10 -5 mole 1 -~ these would contain the equivalent of 3.2 g of SO2. Even with 90~ oxidation and a comparable amount of alkaline cations this is only a tenth of the SO2 remaining in gaseous form. Rain formed in a region of low concentration may take up further quantities of S on falling through lower layers of the atmosphere where the concentration of SO2 is higher. The graphs illustrate that with a high degree of oxidation to sulphate and some neutralization by N H 3 and alkaline dust particles SO2 can be taken out by rain to very low residual values. At

IO-

7 o x

L

0 E 4 -

3-

2-

I [6

i - 3 . 2 0

I I I-2.88 I

D2.56 I E I E

ALKALINE C T l O ~ / / i O0 1 . 9 2 %

/ / / / , / / / , / I / / / / " / - F.W- " ' ' ^ ~

/ / / I I / __~" P I .ok.; eL.

.~.~/ .~.o/%%../q/_.'~m.:/%',- . - ~ " . '1 =~ ~t -- .'6 s'/ -' /I~" .v / / _ e. o I W / vX~'/~ -i q l / / i~ i " h l . 2 8

,'KAL,N .." ..-" CATIONS / " ~ "" // . ~ " ./_~/ I 2x,O- ..-'" . / ' t ko.96 ==

EQ/L I I i i i i l i i I 1 i ' 2 ! " I ~ t " / 1 " I . I i , . ~ I

. / / / / . / 1 "~ 0 6 4 / " / . / t S ~ ALKAL IN ITY r- . 1 1 . t ~. i ~ ~ _ _ . C . . : . , ~ ' - ~ I

[ 0 . 3 2 2'0 s'o do -7o 9'016o i6o

yglm s of S02 Fig. 2. SO2 solubility in rain: No oxidation.

296

IO-

9 -

8 -

7 - ? g x .~6-

~ 5 - (3.

~ 4 -

3

2

I 16

= 7 - b x ~ 6 - a-.

~s- "6 s 4 -

3 -

2 -

I I 16

ROSEMARY PRINCE AND F. F. ROSS

r3.2o I I F2.88 I

1-2.ss I P I E ~fF, T M

o

, . . / d ~'/ ^x" / ALKALINE CATIONS ~ / , > , I ~ ' ~ 2 q / ' / ~ '~ /~1 "

ALKALINE CATIONS ~.,......~""~/~ d~.~> / / X I ~ P " . / ~ / j i " " ~9" [ ~ ~ . ' ~ - " / s x ' ° - s ~ . - 7 ./~"~-/¢Y _.~.~'~.-'..~j-~ ,~8 ,'~ ,.2a ~,

~+~"" ~>" . Y > ~ . .'_S-'~ ' 7 . / / ,,~,~,,,,.-,~ ./ ~ NO ALKALINITY o ~ 0.96

• /, I/, 11 -I -

I ~ / / ~ !0.64

I io ~o do s'o ~o ' I o.~2 7o ~o ~o i&o 16o

~g/m 3 of SO:,

Fig. 3. SO2 solubili ty in ra in: 75%00 oxidation.

ra,2o

: .88

ALKALINE CATIONS 5xIO "s LKALINE CATIONS x IO "s

NO ALKALINITY

20 310

Fig. 4.

/o ~o ~o )o ~og~IGo Jlg/m 3 of SO 2

SO2 solubility in rain: 90 % oxidation.

%.5° I E I E I-2.24 I 0 i 2 i - i .92 ,,-. , ~ I [-I.60 I

I-- 1.2B -5

I 8 I-0.96 f: I o I I-0.64 I I

IO.32 160


c loud level, however, SO 2 can be expected to survive many cycles o f condensa t ion

and to t ravel large distances i f it remains unoxid ized in the gaseous state, and the

S content o f ra in is unl ikely to exceed a ra te of 100 kg km -2 per 25 ram. The average

will be less than this and bo th the 1 to 1.2 M g k m -2 prec ip i ta ted annual ly in southern

Sweden and the 1.5 to 2 M g k m -2 for 750 to 1000 m m of rain in Bri tain seem reason-

able. Higher figures r epor ted f rom other par ts o f the wor ld (e. g., 3 M g k m -2 for 40

in. o f rainfal l in I n d i a n a ; Ber t r amson et al., 1950) are difficult to explain, but may

conceivably be due to larger quanti t ies of alkali or more comple te oxidat ion .

Bear ing in mind tha t the higher the rainfal l the greater the run-of f with consequent

loss o f sulphate in drainage, it is seen that rain alone cannot be a sufficient source of

S for high-yie lding land.

10. Sulphur Loss in Drainage

To make a S ba lance for a ca tchment a rea and thus es t imate the to ta l absorp t ion o f

SO2 by leaves and soil requires a knowledge of: (a) fertil izers appl ied, (b) household

S p roduc ts d ischarged in drains, (c) any significant indust r ia l discharges including

mine water, (d) crops, dai ry products , and meat removed f rom the area, and (e) a t rue

assessment o f the sulphate in the river in f lood and in drought . Moreover , the sul-

pha te reserve in the l and is sufficient to affect the run-off in any par t i cu la r year, and

da t a for a number o f consecutive years are required. So far it has not been poss ible

TABLE IV

Sulphur in land drainage. Thames and Lea catchment areas

Year Total natural flow Rainfall in in. SO'4 (ppm) Tons S km -2 in 106 imp. gal. concentration

T L T L T L T L

1953/54 341 601 25257 26.3 21.4 49 104 2.5 3.8 1954/55 666703 30344 31.6 26.0 50 110 5.0 4.8 1955/56 419942 26249 23.l 22.2 49 106 3.1 4.0 1956/57 472538 29197 28.3 25.5 57 106 4.0 4.4 1957/58 498098 26082 27.6 23.5 58 - 4.3 - 1958/59 741582 48112 32.5 30.3 58 122 6.4 8.4 1959/60 519751 26601 26.7 22.6 54 104 4.2 4.0 1960/61 905526 51 684 36.2 31.0 58 112 7.8 8.3 1961/62 522211 32078 26.9 24.3 50 94 3.9 4.3 1962/63 437095 26744 25.2 22.9 61 101 4.0 3.9 1963/64 555620 30603 27.7 24.3 60 108 5.0 4.7 1964/65 364492 22808 21.8 19.0 60 108 3.3 3.5 1965/66 560235 32398 30.0 27.8 64 98 5.4 4.5 1966/67 747923 40623 33.3 29.0 64 109 7.1 6.3 1967/68 680129 41208 30.2 26.0 64 104 6.5 6.1 1968/69 863798 49817 35.8 31.6 62 96 8.0 6.8

Areas Thames (T) 3855 mi 2 = 9960 km 2 Lea (L) 400 mi ~ = 1040 km 2


to complete such an inventory for any catchment area in Britain. However, the London Metropolitan Water Board has average sulphate contents for the rivers Thames and Lea going back over several years. The catchment areas and water flows are known. The information is summarized in Table IV.

Significantly lower and higher figures are indicated for other parts of Britain. The data available from River Authorities and other Water Companies are less complete but range from 2 to 3 Mg km -2 in nonindustrial areas to over 20 Mg km -2 for agricultural land close to heavily populated areas. This high figure can arise through normal dressings of the traditional fertilizers together with rain and SO2 absorption from air containing an average of 100/~g m -3. It does not seem to affect soil fertility adversely. It is interesting to note in this context the report that in the U.S.A. in recent years crop yields have failed to show response to added N (Commoner, 1971). After two decades of yields rising in step with the increasing application of N fertilizer, the last 5 yr have shown no further increase despite continuance upward of the N dose. With the steady conversion to high analysis fertilizers, the first explanation to to be tested is S shortage.

A broad view of the river drainage data so far available for Britain indicates that perhaps half the emitted S reaches the land and the rivers. At this rate of absorption comparatively small areas of the surrounding seas would take out most of the remainder.

11. Future Trends

There are four major factors to be considered in the assessment of a balance between air pollution by SO 2 and the supply of S to the land. Firstly there are large areas of the world where, as would be expected from the readiness with which sulphate is leached out of soil, S is deficient; such areas include at least parts of central Africa, the Gulf Atlantic States and much of Midwestern and Western U.S.A. (Coleman, 1966), central Ireland (Tierney, 1969), and even small areas of Britain. Secondly, the rising cost of labor is leading farmers to prefer the new high analysis fertilizers which contain little sulphate. Thirdly, there are areas of the world where SO z pollution is excessive by any standard. Fourthly, if the fossil fuels can be obtained, a generally increasing output of SO z from combustion plant is forecast.

The course to be steered can best be deduced from a consideration of the U.S.A. where all four factors are fully relevant. In assessing what emissions are desirable, allowance must be made for loss over the coast. This will probably be mainly from the east coast and may amount to 0.15 x 106 Mg of S per 100 km of smoothed coastline annually, say 4 x 106 Mg. The land area of 9 x 106 km z requires between 2 and 5 Mg km -z, and will not be harmed by up to at least 10 Mgkm -2 if excessive atmospheric dosages can be avoided. The minimum is thus 22 x 106 Mg S yr -1, the desirable is 40, and, bearing in mind the difficulties of uniform distribution so that it is inevitable for some areas to receive more than others, a total emission of 50 x 106 Mg of S or a hundred million tons of SO2 annually is a reasonable aim.

The present output is about 36 x 106 Mg of SO2, and this may rise to 60 in 1980


and to 80 in 1990. At the same time the atmosphere of certain cities is excessively polluted by SO2 and fears have been expressed that an increased total of emission will makes things worse; this is not the case. The problem is entirely one of distribution

Figure 1 has shown that by attention to manner of emission Britain has achieved an increased output of SO2 and a falling urban ground level concentration. The increase is greater than the entire U.S.A. output on a Mg km -2 basis. The average concentrations are now low enough to constitute no problem, and so it is instructive to look at the diminution of peak concentrations. We have constructed Figure 5 by taking 14 major cities with over 200 000 inhabitants together with 6 London Boroughs, taking the gauge in each which generally gave the highest readings throughout the period, and finally totalling for each year the city-days with 24 hr averages exceeding 500, 1000, 1500, 2000, and 3000/~g m-3. Even this does not reveal the full extent of the improvement. In the earlier years several gauges would show over 1000 yg m -3 on the same day but in later years only one.

The problem for the U.S.A. therefore is not one of total emission, which will

45

Fig. 5.

3s- r

i i

oc i

a: I

2s s l - I I

8 i w 20 4

z I ,- I \ OVER Iooo --

21- \ " I \ OVER,SOO \ \ ,,X \ OVER 2ooo \ , ,

I "'~-.. ~ : - - . . . . " , . ~ . OVER 1500 ~'x~.

1962-3 m 1963-4 I 1964-5 m 1965-6 m 1966-7 I 1967-8 11968-9 I YEARS

Average number of days per year for which high SO2 concentrations have been recorded in the fourteen largest British cities and six London boroughs.


remain inadequate for two or three decades, but of distribution. Qualitatively the measures required to solve this problem are known. They comprise essentially a strict limit on the S content of fuel used in small appliances in cities, and the larger the city the lower the limit. Combined with this, the larger installations require chimneys appropriate to their output of hot gas and S content, and generally require to be sited so that their gases will disperse over the land that needs the S. This matter of chimney heights has now been well quantified for conditions in Britain (Clarke et al., 1970) and there are no reasons to suppose that for the majority of sites in the U. S. A. any modification of the formulas would be called for (Frankenberg et al., 1970).

12. Further Research

The three major questions where more precise quantification is required relate to urban pollution, absorption by land and water surfaces, and long distance drift. The urban pollution problem was well explored at a symposium in North Carolina in 1969 (Stern, 1970), but the various investigators achieved only limited success in predicting local concentrations from the inventory of emissions. From observations in Boston, Mahoney et al. (1970) deduced that the best correlation could be obtained with emission and meteorological data if an overall city average is taken; individual gauges are too much affected by very local sources. The meaningfulness of a city average can be greatly enhanced by means of the probability charts illustrated in the Summary by Stern (1970). One general criticism of the investigations is that insufficient attention was paid to height of emission. It is possible that this may be an even more important factor in the urban situation than in open country. The gases from the Crawford power station in Chicago, regularly predicted to reach ground level, consistently failed to affect a gauge over which they passed (Roberts et al., 1970).

It would probably be rewarding to investigate whether any comparatively simple relationships exist between (a) low level emissions per unit area, (b) gross dimensional factors such as city size, shape, and topography, and (c) annual city average concentrations. It would also be worthwhile to determine how useful a correlation can be established between the average concentration and the high hourly or daily readings at different sites within a city under different meteorological conditions. The results might prove to be reliable enough to justify local legislation that would place a limit on low level S emissions with the object of avoiding undesirably high concentrations.

On the question of absorption and the life of SO2 in the atmosphere, the investigations in Connecticut are of interest (Hilst, 1970). Concentrations calculated on the assumption that SO2 had a half-life of 3 hr were much higher than observed. To obtain reasonable agreement it was necessary either to assume a 30 min half-life or to disregard 40% of the emissions. The author rightly supposed that the inventory of emissions could not be high by that amount, any error being more likely to be in the other sense. He did not, however, discuss the possibility that at least 40% of the emissions might properly be classed as 'high'. There is, on the other hand, some


support from Germany for the short half-life (Weber, 1970); simultaneous measure- ments of CO 2 and SO2 at a distance from a source of both showed the latter to have

a half-life of between 10 and 60 min. This question of the disappearance of SO z from the air is particularly relevant to

long distance drift. It is clear from observations in Ireland (Tierney, 1969) that the contribution of SO 2 from Britain either in rain or in the air is so small as to be undetectable by existing instruments, yet the wind does from time to time blow from the east. An approximately threefold increase in S in precipitation in Greenland has been established by comparing the deposition of S against Se which does not seem to be transported (Weiss et al., 1971). For an annual snowfall equivalent to 300 mm of rain, the S supply would appear to have increased in a century, and mainly during the last two decades, from 8 to 25 kg km -2, that is, to a present level equal to about a sixtieth of that brought down by rain in Britain.

Apart from absorption by the ground or by the sea, SO 2 can also suffer conversion at higher altitudes to HzSO 4. There is evidence that this reaction may be promoted photochemically at a rate which would give the S O / a half-life of 2 to 3 days (Cox and Penkett, 1970). Although SO 2 is sparingly soluble in rain clouds, HzSO 4 might be expected to be efficiently absorbed in rain, and, having traveled several hundred km from the source of emission, it may be insufficiently accompanied by alkaline material. It is when rain containing incompletely neutralized H2SO e falls on land already short of lime that difficulties arise. In Europe there is such an area in southern Sweden. By contrast the rain in East Germany is alkaline from the combustion of much lignite, but in the west the change over from burning wood and coal to burning oil has reduced the alkalinity available. The 1.2 Mg km -2 of S would, of couse, be more than welcome in Sweden if it were fully accompanied by NH3, Ca, K, and other cations. But the excess H + ions are currently estimated to be equivalent to 5 kg ha -1 of CaCO3 annually. Although the loss from British soils is between 400 and 1000 kg ha -1 yr - I and dressings of 2 tons per acre are commonly applied at 5 to 7 yr intervals, liming the timber land and the catchment areas of the lakes in Sweden is considered to have biological disadvantages (Bolin, 1970). The preferred solution is for Western Europe to reduce total S emissions to half the present total. Otherwise a 10% reduction in timber yield by the end of the century is feared. The cost of reducing S emissions by half, and of replacing the S in fertilizers in European agriculture, has not been calculated. Nor is it certain that starving the timber land of S as well as minerals will enable yields to be maintained. This is obviously a field in which further research is needed.

Acknowledgments

The authors thank the Meteorological Office, Warren Spring Laboratory, Levington Research Station, The Sulphur Institute, River Authorities, The Water Research Association, and Water Companies in the U.K. for help in their investigations, as well as their colleagues in the Central Electricity Generating Board with whose permission this paper is published.


References

Alway, F. J., Marsh, A. W., and Methley, W. J. : 1937, Soil Sci. Soc. Amer. Proc. 2, 229. Aslander, A. : 1958, in Ruhland (ed.), Encyclopaedia of Plant Physiology 4, 1002. A. P. O. : 1969, Air Quality Criteria for Sulfur Oxides, U. S. Dept. of H. E. W., NAPCA Publication

AP-50. Bertramson, B. R., Fried, M., and Tisdale, S. L. : 1950, Soil Sci. 70, 27. Bolin, B. (Chairman): 1970, Sweden's Case Study for the United Nations Conference on the Human

Environment. Roy. Min. for Foreign Affairs, Roy. Min. Agr., Stockholm. Clarke, A. J., Lucas, D. H., and Ross, F. F. : 1970, Paper, Second Int. Clean Air Congress, Washing-

ton, D. C. Coleman, R. : 1966, Soil Sei. 101,230. Commoner, B. : 1971, Address to the Chemical Society Annual Meeting. Cowling, D. W. and Jones, L. H. P. : 1970, Soil Sci. 110, 346. Cox, R. A. and Penkett, S. A.: 1970, Nature 229, 486. Graxford, S. R., Weatherley, M-L. L. P., and Gooriah, B. D.: 1970, Paper, Second Int. Clean Air

Congress, Washington, D. C. Deevey, E. S.: 1970, Sci. Amer. 223, 148. Dreisinger, M. F. and McGovern. P. C.: 1970 Paper, Conference on Impact of Air Pollution on

Vegetation (Air Pollution Control Association), Toronto. r Frankenberg, T. T., Singer, I. A., and Smith, M. E. : 1970, Paper, Second Int. Clean Air Congress,

Washington, D. C. Hitst, G. R.: 1970, Proc. Symposium on Multiple-Source Urban Diffusion Models, U.S. EPA Pubn.

AP-86. Junge, C. E. : 1960, J. Geophys. Res. 65, 227. Kuhn, H. and Faller, N.: 1970, Paper S.15/70, Int. Symposium on Sulphur in Agriculture (The

Sulphur Institute). Lawther, P. J. and Bonnell, J. A. : 1970, Paper, Second Int. Clean Air Congress, Washington, D.C. Likens, G. E., Bormann, F. H., Johnson, N. M., and Pierce, R. S. 1967: Ecol. 48, 772. Mahoney, J. R., Maddaus, W. O., and Goodrich, J, C.: 1970, Proc. Symposium on Multiple-Source

Urban Diffusion Models, U.S. EPA Pubn. AP-86. Martin, A. and Barber, F. R. : 1971, Atmos. Env. 5, 345. Roberts, J. J., Croke, E. J., and Kennedy, A. S. : Paper, Proc. Symposium on Multiple-Source Urban

Diffusion Models, U.S. EPA Pubn. AP-86. Robinson, E. and Robbins, R. C.: 1968, Sources, Abundance and Fate of Gaseous Atmospheric

Pollutants, Prepared for A. P. 1. (Stanford Research Institute). Rockett, K. W., Sterba, M. J., and Watkins, C. H.: 1970, Paper, Seminar on Desulphurisation of

Fuels and Flue Gases, Geneva, United Nations ST/ECE/AIR POLL/1. Spedding, D. J. : 1969, Nature 224, 1229. Stern, A. C.: 1970, Proc. Symposium Multiple-Source Urban Diffusion Models, U.S. EPA Pubn.

AP-86. Stewart, B. A.: 1967, in Jacks (ed.), Soil Chem. Fert. Trans., Int. Soc. Science, 131. Stratmann, H. : 1961, Staub 21, 61. Tierney, S. L. : 1969, Paper, Symposium on Sulphur in Agriculture, Eire (The Agricultural Institute,

Dublin). Weber, E. : 1970, Paper CP 25-F, Second Int. Clean Air Congress, Washington, D. C. Weiss, H. V., Koide, M., and Goldberg E. D. : 1971, Science 172, 261. Whitehead, D. C." 1964, Soils Fert. 127, 1. Zahn, R. : 1963, Staub 23, 343.

Documents

Sulphur in air and soil