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Natt: All values are expressed in 10" t

Natt: All values are expressed in 10" t

The tropospheric burden of sulfur in reduced compounds is very speculative. Nevertheless, for H2S the following figure is proposed. Let us assume that the global H2S concentration in surface air over the ocean is 0.05 ng m "3 STP, while it is 0.1 /ig m - 3 STP above the continents (see above). From these data a global concentration of0.065 /ig m ~3 STP can be calculated by averaging according to the relative area of continents and oceans. Based on vertical profiles published by Jaeschke et al. (1978), it is speculated with caution that the hydrogen sulfide scale height28 is around 1000 m. This leads to a global H2S—S burden of 0.03 x 106 t. However, this mass does not include the quantity of sulfur in other reduced species. The burden of all reduced sulfur, however, can hardly be more than 0.1 x 106 t.

16.5 Atmospheric cycle of sulfur

On the basis of the foregoing discussion and further considerations the schematic atmospheric sulfur budget represented in Fig. 19 is proposed. The terms in the cycle were determined as follows. We accepted the figures of Friend (1973) for the strength of anthropogenic sources and for the sulfur production rate by volcanos (65 x 1061 yr ~1 and 2 x 1061 yr ~respectively). On the other hand, we assumed, in accordance

-Natural sources and sinks "] expressed

----Anthropogenic sources and sinksy ,n ig6 • yr-l

—s Chemical transformation J

I Atmospheric reservoirs in 10® t

Fig. 19

Cycle of sulfur compounds in the atmosphere

-Natural sources and sinks "] expressed

----Anthropogenic sources and sinksy ,n ig6 • yr-l

—s Chemical transformation J

I Atmospheric reservoirs in 10® t

Fig. 19

Cycle of sulfur compounds in the atmosphere

28 The scale height, H, is a hypothetical altitude defined as

H = J c(z)dz/c(0), o where dz) is the concentration at level z, while c(0) is the same parameter for surface level air. The integral gives the column concentration of the substance considered. The weakness of this approach to calculating global burdens is the linear dependence of the burden on the surface level concentration, which often is not well established.

with the careful study of Granat et al. (1976), that the sum of the total wet deposition and the dry deposition of sea salt particles is 106 x 106 t yr"1 (for further details see the original paper). The dry deposition of sulfate particles formed by gaseous reactions was neglected since their deposition velocity is estimated to be no more than 0.1 eras"' (Garland, 1978). The source intensity of sea salt-sulfur was taken from data of Eriksson (1960). The natural biological sulfur release was calculated by balancing the cycle. It is proposed with caution that 23 x 106 t yr"1 from the value obtained is provided by H 2S—S. This latter figure was calculated by using the above 0.03 x 1061 H2S — S burden and a global residence time of half a day (e.g. Jaeschke et al., 1978).

The dry deposition of S02 was calculated from a 1 ng m"3 mean continental concentration (expressed in sulfur) and a 0.1 ¡ig m"3 mean oceanic surface air concentration, which were multiplied by respective deposition velocities of 1.0 and 0.7 cm s "1. These latter figures are provided by observational findings showing that soil and vegetation are important S02 absorbers (Garland, 1978; see Chapter 5). On the other hand it was demonstrated, among others by Beilke and Lamb (1974), that the ocean surface provides a sulfur dioxide sink. Thus, it is not excluded that the same oxidation processes that occur in cloud water (see Chapter 5) also occur in the ocean surface layer, where the relatively high pH (8.1+0.2) does not limit the reaction rate. In this way 48 x 106 t yr"1 S02—S global dry deposition was obtained.

The wash-out of reduced sulfur compounds was neglected owing to the small solubility in water of these species. Furthermore it was assumed that the dry deposition of reduced compounds is negligible as compared to the strength of chemical sinks. Thus, Judeikis and Wren (1977) demonstrated by laboratory studies that the dry deposition velocity of DMS and H2S on selected soil samples ranges from 0.015 to 0.28 cm s "1. They mention, however, that these values are likely to be upper limits due to a possible reversible physical adsorption.

The term characterizing the transformation of H2S to S02 is determined by the source strength of 23 x 1061 yr ~1 calculated above. The chemical conversion of S02 is estimated by assuming a global overall reaction rate of 1% h"1 including processes in cloud water. In this way we obtained a value of 39 x 106 t yr " 1 which was normalized by requiring the S02—S reservoir to be balanced. The direct sulfate formation rate from reduced sulfur gases was obtained by balancing the budget of reduced S and S04—S. The accuracy of these terms is not better than within a factor of 2.

For completeness the stratospheric inventory is also plotted in Fig. 19. The value of stratospheric sulfur burden is based on data for 1971-1973 as reported by Karol (1977). The term representing the sedimentation of sulfate particles from the stratosphere to the troposphere is estimated by assuming a 1 year stratospheric residence time (Junge, 1963). Moreover, the arrow going from the tropospheric reduced S reservoir to the S04—S one in the stratosphere represents the possibility, proposed by Crutzen (1976), that COS may be a source of stratospheric S02.

It follows from data given in Fig. 19 that the overall residence time of sulfur compounds in the atmosphere is x =

154 x10 t yr"1 3

which is equivalent to about 3 days. The corresponding figures for S02—S and S04—S are 2 days and 3 days, respectively. Further, the global lifetime of reduced sulfur compounds in the atmosphere is less than 1 day.

We have to emphasize here that the majority of the sulfate-sulfur in the tropospheric reservoir in not sea salt. Friend (1973) estimated that the atmospheric sea-salt burden is around 0.1 x 106 t. By subtracting this value from the sulfate-sulfur loading given in Fig. 19 and considering only the strength of chemical sources (62 x 106 t yr"1), a residence time of more than 4 days is obtained for the "excess" sulfate.

3.6.6 Atmospheric sulfur cycle over continents

Kellogg et al. (1972) estimated that 90 % of the anthropogenic sulfur is emitted into the atmosphere over the Northern Hemisphere. The sulfur emission due to human activity is 17 x 106 t yr"1 over the U.S.A. (Urone 1976, based on data of 1970), while the corresponding figure for the whole of Europe is estimated to be 25 x 1061 yr"1 (Semb, 1978, data for 1973). The combination of these values with the strength of the global anthropogenic emission (65xl06t yr"1) leads to the conclusion that 65 % of all man-made sulfur emission is provided by the U.S.A. and the European countries. It is obvious from this high proportion that the strength of anthropogenic sources may be much greater over some areas of the world than is the intensity of biological production. The aim of this book is to study the pollution of the global atmosphere. In spite of this fact, it seems meaningful to discuss the problem of continental pollution at least for one pollutant. S02 is particularly suitable for this purpose since some information is available in the field.

The sulfur budget over the U.S.A. was studied by Junge (1960). On the basis of data obtained by a precipitation chemistry network operated during 1955-1956 he was able to calculate the sulfur deposition of this country (3.5 x 106 t yr" ■). Junge speculated that 90 % of this deposition comes from continental sources. In 1957 the anthropogenic source strength was 8.6 x 106t yr"1 over the United States. By neglecting the effect of natural production it is easy to show that only 40 % of the sulfur emission is deposited. Since at these latitudes in the troposphere westerly winds prevail, it is tempting to explain the S02 concentration maximum over the Atlantic Ocean (see Fig. 18) by this sulfur export. We have to recognize, however, that Junge (1960) neglected the dry deposition of S02 which may be comparable with the wet deposition. Furthermore, the S04" distribution over the Atlantic Ocean does not clearly support this assumption.

An atmospheric sulfur inventory for the whole European continent has been recently constructed by E. Meszaros et al. (1978). These authors show on the basis of the comparison of anthropogenic sulfur emission (Semb, 1978) and sulfur advection from the Atlantic that the sulfur gained by advection is small. 70-85 % of the sulfur emitted and imported is removed over the continent equally by dry (mostly in form of S02) and wet deposition. Meszaros and his associates have estimated the dry deposition of SOz by using an average European S02—S concentration calculated from data in Table 13 (3.2 /xg m~3) and a dry deposition velocity of 1 cm s"1 (Garland, 1978). The value of wet deposition was based on precipitation chemistry measurements. It follows from this quantitative calculation that Europe contributes 15-30 % of its sulfur emission to the tropospheric sulfur cycle of other areas.

The regional sulfur budget for the northwestern part of the European continent was studied in several papers. Thus, Rodhe (1972b) investigated the sulfur budget of the two regions shown in Fig. 20. Over the region labelled II the anthropogenic sulfur emission is nearly 10 x 106 t yr-1, which means that about 15-16 % of the total man-made sulfur (see Table 12) is emitted into the air over this region, what

Fig. 20

The two regions used for sulfur budget studies over N. W. Europe (Rodhe, 1972b). (By courtesy of Tellus)

Fig. 20

The two regions used for sulfur budget studies over N. W. Europe (Rodhe, 1972b). (By courtesy of Tellus)

comprises 1 % of the surface of the Earth. Rodhe speculated that the natural source intensity over this area (estimated on the basis of Friend's data in Table 12) is at least one order of magnitude less than the strength of anthropogenic sulfur production. Table 15 tabulates the results. The wet deposition values given were estimated on the basis of data on the chemical composition of precipitation. The wet deposition due to anthropogenic sources was determined by subtracting the negligible natural sulfur deposition from these figures. Rodhe assumed, as seems reasonable, that the sum of dry and wet deposition is at most twice the wet removal alone. In this way he calculated that over region I 20-40 % of the sulfur emitted is removed from the air. The corresponding figure for region II is estimated to be between 40 and 75 %. Thus 25-60 % of the emission of region II is exported into the air of other regions. This result indicates that sulfur emission cannot be considered as a purely national problem. Well organized international projects are needed (see e.g. Ottar, 1978) to study the long-range transport of pollutants across national boundaries.

Table 15

Atmospheric sulfur inventory over two regions of N. W. Europe (see Fig. 20) according to Rodhe (1972b)

Anthropogenic ^ , Wet deposition Total Deposition in »„

Region emission deposition minus natural anthropogenic or anthropogenic wet dep. deposition emission

Note; Values tabulated are expressed in 10" t S/yr

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