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Isotopes and atmospheric sulphur
Pergarnon
AtmosphericEncironmenrVol. 29, No. 18, pp. 2553-2556, 1995 Elswier Science Ltd Printed in Great Britain 135%2310/95 $9.50 + 0.00
13522310(95)OOltB-3
SHORT COMMUNICATION ISOTOPES NICOLA
AND ATMOSPHERIC C. MCARDLE
SULPHUR
and PETER
S. LISS
School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K. (First received 2 February
1995 and in final form 19 April 1995)
Abstract-,4 combination of complementary approaches, dealing with the natural and the man-made sulphur sources respectively, is employed to show the power of the isotopic method to distinguish quantitativs:ly between atmospheric sulphur sources. At two remote European sites we found that - 15-20% of the sampled aerosol sulphur was biogenic in origin rising to -30% in spring and summer. Key word index: Atmospheric sulphur, dimethylsulphide, sulphur isotopes.
The sulphur cycle is the major source of acidity in the atmosphere, with the natural component (mainly via marine dimethylsulphide (IDMS)) dominant in remote regions (Wagenbach et al., 1988; Savoie and Prospero, 1989) and man-made emissions of SO, controlling in heavily industrialized areas. In intermediate zones both sources are likely to be significant. Although the potential of stable sulphur isotopes as a means of discriminating between atmospheric sulphur of different sources has been recognised (Grey and Jensen, 1972; Nriagu et al., 1987), the few existing tests of the hypothesis have been inconclusive in quantifying the biogenie component for several reasons including small sample sets, dominance of one source and lack of supporting data (Nriagu et al., 1987; Nriagu et al., 1991; Calhoun et al., 1991; Calhoun and Bates, 1989). The isotopic method relies on the different sources having characteristic isotopic signatures-about + 20960for DMS-derived sulphur (Nriagu et al., 1991; Calhoun and Bates, 1989; Wadleigh, 1989) and O-5%0(Newman and. Forrest, 1991; Newman et al., 1975a, b) for that coming from power plants. If various sources of sulphur have distinctly different isotopic signatures then the following equation can be used to find the strengths of the sources: s,c, = 5 sici, i=0
(1)
where N is the number of components Ci is the concentration of the ith species, C, is the total concentration in the mixture, and di and 6, refer to the corresponding isotopic values (defined in equation (2) below). At the locations in this study we assume tlhat the major sources of sulphur are sea-salt sulphate, anthropogenic sulphur dioxide and marine biogenic sulphur, with other minor sources such as volcanic sulphur not considered. Although the dJ4S of sea-salt sulphate and marine biogenic sulphur are expected to be similar, it is possible to apportion the sea-salt sulphate by other means: 634s
=
(34~/32%rn,,, [ 34~/32%t*ndard
0 Crown Copyright (1995).
_
1
1
x looo
The standard used is a meteoritic iron sulphide, the Canyon Diablo Troilite, and the unit is 960. The isotopic signature of aerosol sulphur was determined for samples collected at two sites, Mace Head, Eire and Ny Alesund, Spitsbergen. Both sites are coastal and in regions where high offshore marine biological activity has been observed (Smith et al., 1991; Malin et al., 1993). The high volume (typically -3000 m3) air samples were collected on Whatman 41 cellulose filters and 3/4 of each available filter were used for the isotopic analyses. Chloride, nitrate, sulphate and methanesulphonate were determined by ion chromatography of an aqueous solution obtained by extracting portions of the filter ultrasonically into Milli-Q water. Sulphur was recovered from the solution by precipitating as BaSO, (after ultraviolet irradiation of the sample) and then converted to SO, by direct thermal decomposition, the isotope ratio of which was measured by mass spectrometry. The isotopic analyses required at least 1 mg of sulphur. An acid digestion method (Yaaqub et al., 1991) was used to prepare a solution for the determination of metals by Inductively Coupled Argon Plasma Optical Emission Spectrometry. Further experimental details are available elsewhere (McArdle, 1993; McArdle and Liss, in preparation). The 634S values for both sites range from - + 5 to + 23%, an upper limit rather higher than most previously reported values for atmospheric sulphur (Grey and Jensen, 1972; Nriagu et al., 1987; Nriagu et al., 1991; Calhoun et al., 1991; Calhoun and Bates, 1989; Wadleigh, 1989). Figure la shows S34S values for both sites plotted against the percentage of sulphate that is sea-salt derived (calculated using chloride concentrations; where both sodium and chloride concentrations were available the difference between the two estimates of % sea-salt sulphate was small). If the sampled sulphur were only of sea-salt and anthropogenic origin one would expect to find the data points distributed on a simple two source mixing curve with end members of + 20% and O-5%0.It can be seen that the data in fact lie in a triangular area indicating an additional source of isotopically heavy sulphur. This sulphur is generally assumed to be derived from the oxidation of DMS. At the latitudes of the two sites one would expect to find a strong seasonality in biological productivity (Malin et al., 1993). Figures lb and c show the data divided into “winter” (October-March) and usummer” (April-September) periods. There is a marked difference in the distribution of the data with season; almost all points
(2) 2553
Short Communication
2554
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8
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70
3
80
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1 1
100
Sulphate
Fig. 1. (a) S3?S values plotted against % sea-salt sulphate for Ny Alesund (0) and Mace Head (o), (b) winter data only, the line is that of linear regression (with 95% confidence limits shown) through the Mace Head data (Ra = 0.94, intercepts at 0 and 100% sea-salt sulphate 5.0 f 0.7 and 19.7 f 3.6% respectively) and (c) summer data only with winter regression line and visually estimated upper boundary. The Ny bilesund filters were selected from archived samples collected by the Norwegian Institute for Air Research. These samples all have low sea-salt sulphate concentrations as the sampler was equipped with a pre-impaction stage that excluded any particles > 2.5 pm. Local pollution events were excluded by the use of a condensation nucleii counter. The Mace Head sampler only operated when air flow was within the marine sector and wind speeds were neither very high nor low. The uncertainty in the % sea-salt sulphate vahres.are - f 7%, and in the a3’S values f 0.3%.
sulphate of a regression line (95% confidence limits shown) of a3*$Son % sea-salt sulphate for winter samples only. At Ny Alesund the mean winter non-sea-salt (NSS) sulphate 634S value was taken as the anthropogenic endmember. A 634S value of + 22%~ was estimated for DMS derived sulphur from the intercept at 0% sea-salt sulphate of the upper boundary line of the data in Fig. lc. This value of + 22% is within the theoretical estimated range for atmospheric sulphur that originated as DMS (Calhoun and Bates, 1989; McArdle, 1993), albeit at the higher end. The 6’4S values derived above were substituted in equation (1) to calculate the proportion and actual concentration of biogenie sulphate in each sample. These concentrations display a strong seasonality with values rising during April and May to a peak in June, with little if any occurring during the winter. However, even in summer the proportion of biogenic sulphur varied widely (from -0 to 100%) from sample to sample. At both sites the maximum concentration found was 26 nmol m- 3 with summer median values of - 4 nmol m- 3 at Mace Head and -2 nmol me3 for Ny Alesund. The values for biogenic sulphate concentration are consistent with other studies, for example in the outer Hebrides (Davison and Hewitt, 1992). At Mace Head over the sampling period from March 1991 to June 1992 -20% of the sampled (NSS) sulphate was biogenic in orgin, in summer this rose to -30%. At Ny Alesund for the whole of 1989 - 15% of the NSS sulphate was biogenic and for the summer -30%. For both sites it must be pointed out that these figures apply to the sampled sulphate only, which is not a continuous record. Additional information was used to corroborate the isotopic findings. Methanesulphonate (MSA) is another oxidation product of DMS in the atmosphere and is generally assumed to be solely from this source. Figure 2 shows MSA concentration plotted against Julian day and reveals a seasonal pattern which echoes that of the biogenic sulphur. Some attempts have been made to use MSA/NSS sulphate ratios to estimate the amount of DMS derived sulphur (Savoie and Prospero, 1989; Galloway et al., 1990) but this method is fraught with difficulties as even in remote regions the ratio is far from constant (Savoie and Prospero, 1989; Prosper0 et al., 1991; Galloway et al., 1990; Ayers et al., 1991). However, if an MSA concentration of 1.5 nmolme3 (May-June mean) is used in conjunction with a range in ratios of 0.2-0.4 plausible for these locations (Galloway et al., 1990; Ayers et al., 1991; Burgermeister and Georgii, 1991) then biogenic sulphur concentrations of 3.8-7.5 nmol me3 can be calculated (summer median 4 nmol mm3 from isotopes).
”
4 CI
0
I J
0 Mace Head
0
0 Ny Alesund 0
F
M
A
M
J
J
A
SO
ND
requiring an additional heavy source occurring during the Fig. 2. Methanesulphonate (MSA) concentrations for Ny summer. Anthropogenic end membe c values of + 5% and + 6960 Alesund (0) and Mace Head (o), determined by ion chromatography with a precision of f 5%, showing a clear were derived for Mace Head and Ny Alesund respectively. seasonal cycle. For the Mace Head data this was the intercept at 0% sea-salt
Short Communication Table 1 shows the correlation matrix obtained from concentration data of other species for the Mace Head samples. The species divide into three distinct groups related to source, Na, Mg, and Cl with a sea-water source are all well correlated, as are the metals, nitrate and sulphate reflecting their continental origin. MSA was inversely associated with the sea-salt species as MSA is highest in the summer whereas sea-salt is highest in the winter. Table 2 shows the effect of dividing the sulphate into three components, sea-salt, anthropogenic and biogenic (using the isotopic ratios). It is interesting to note that the ANTH and BIO parameters are well correlated with the appropriate species. A principal components analysis of the concentration data yielded three major components. The weightings of the various species within each component suggested that Cpt 1 represents the continental contribution, Cpt 2 sea-salt and Cpt 3 biogenic. If the analysis is repeated using the divided sulphate then each part appears in the correct component. Air mass back trajectory data were obtained for all the Ny Alesund samples and up to the end of 1991 for Mace Head samples. At both sites the highest S34S values were associated with predominantly marine air masses and the lowest values occurred where air had obviously been in recent contact with industrial areas. In conjunction with the trajectories, CFC data were used to identify periods when pollution events were being sampled at Mace Head. As expected the CFC data were closely linked with low 634S air masses
2555
that had circulated over Europe before being sampled from the marine sector. We consider that the isotopic data in this study can be used as a basis for future assessments of DMS derived sulphur, although, in order to apply the method to other localities it is necessary to define the anthropogenic end member and its variability for that site. At locations where seasonal variations in biological production are not so pronounced this would be more dificult to achieve. This technique is of most use in environments where DMS is the dominant natural source of sulphur as volcanic and terrestrial biogenic sulphur most likely have @‘S values similar to anthropogenic sulphur (Lein, 1991; McArdle, 1993). Furthermore, sulphur from biogenic H,S tends to be isotopically light, i.e. low bJ4S values (Hitchcock and Black, 1984; Nakai et al., 1991) and so could make apportionment diflicult if combined with isotopically heavy DMS derived sulphur. We conclude that isotopes can be successfully used to apportion atmospheric sulphur between that derived from DMS and man-made sources in the marine environment. The technique gives results without the need to know source strengths and does not depend on atmospheric modelling of their dispersion. It can potentially be used in many different atmospheric contexts, e.g. aerosols, rain, snow, ice, cloud water. In the present study we 6nd that although manmade acidity clearly dominates over much of industrialised Europe, in remote parts of the region the natural sulphur
Table 1. Correlation coefficients for Mace Head data :&ISA MSA Cl Na Mg Ca
1 -- 0.33 -- 0.32 -- 0.32
Cl
Na
- 0.33 - 0.32 1 0.69 0.69 1 0.65 0.99 0.44 0.74
Mg
Ca
- 0.32 0.65 0.99 1 0.77
0.44 0.74 0.77 1
NO3
0.31 0.39 0.38
so4
Mn Fe Zn Pb
NO,
SO4
Mn
Fe
Zn
Pb
1 0.56 0.73 0.56 0.63 0.54
0.31 0.56 1 0.61 0.57 0.66 0.44
0.39 0.73 0.61 1 0.95 0.64 0.67
0.38 0.56 0.57 0.95 1 0.51 0.59
0.63 0.66 0.64 0.51 1 0.73
0.54 0.44 0.67 0.59 0.73 1
Note: Only coefficients that are significant at 95% confidence limit are shown.
Table 2. Coefficients obtained when sulphate divided into DMS derived (BIO), sea-salt (SS) and anthropogenic (ANTH) components MSA MSA BIO Cl Na Mg ss Ca ANTH
-
1 0.58 0.34 0.33 0.32 0.34
BIO
Cl
0.58 1
- 0.34 1 0.68 0.65 1.00 0.42
Na
Mg
- 0.33 - 0.32 0.68 1 0.99 0.68 0.74
0.65 0.99 1 0.63 0.77
SS
Ca
NO3
Mn
Fe
Zn
Pb
0.38 0.74 0.72 1 0.95 0.63 0.66
0.37 0.65 0.55 0.95 1 0.50 0.58
0.83 0.63 0.63 0.50 1 0.74
0.61 0.53 0.66 0.58 0.74 1
- 0.34 1 0.68 0.65 1 0.42
0.42 0.74 0.77 0.42 1
NO3
Mn Fe Zn Pb
ANTH
0.38 0.37
1 0.61 0.74 0.74 0.83 0.61
0.61 1 0.72 0.55 0.63 0.53
Short Communication
2556
cycle still plays a significant role-15-20% of the yearly averaged aerosonal sulphur sampled being biogenic, rising to N 30% in spring and summer. Acknowledgement-This work was funded by the Natural Environment Research Council. We would like to thank the followinn for their assistance; University of Leeds Stable Isotope ilaboratory, particulaily Simon Bottrell; Hal Maring, Gerry Spain, Maureen O’Dowd and University College Galway for help at Mace Head; Norwegian Institute of Air Research, especially Jozef Pacyna; Willy Maenhaut; Peter Simmonds for CFC data; John Merrill and Joyce Harris for back trajectories.
REFERENCES Ayers G. P., Ivey J. P. and Gillet R. W. (1991) Coherence between seasonal cycles of dimethyl sulphide, methanesulphonate and sulphate in marine air. Nature 353, 834-835. Burgermeister S. and Georgii H.-W. (1991) Distribution of methane sulfonate, NSS sulfate and dimethylsulfide over the Atlantic and the North Sea. Atmospheric Environment 25,587-595.
Calhoun J. A. and Bates T. S. (1989) Sulfur isotope ratio tracers of non-sea-salt sulfate in the remote atmosphere. In Biogenic Sulphur in the Environment (edited by Saltzman E. S. and Cooper W. J.), pp. 367-379. ACS Symposium Series 393, Washington. Calhoun J. A., Bates T. S. and Charlson R. J. (1991) Sulfur isotope. measurements of submicrometer sulfate aerosol oarticles over the Pacific Ocean. Geo&s. _ . Res. Lett. 18, i877-1880. Davison B. and Hewitt C. N. (1992) Natural sulphur species from the North Atlantic and their contribution to the United Kingdom sulphur budget. J. geophys. Res. D97, 2475-2488. Galloway J. N., Keene W. C., Pszenny A. A., Whelpdale D. M. Sieverina. H. Merrill J. T. and Boatman J. F. (1990) Sulfur in the W&tern North Atlantic: Results from a summer 1988 ship/aircraft experiment. Global Biogeochem. Cycles 4, 34945 Grey D. C. and Jensen M. L. (1972) Bacteriogenic sulphur in
air pollution. Science 177, 1099-l 100. Hitchcock D. R. and Black M. S. (1984) 34S/34SEvidence of biogenic sulfur oxides in a salt marsh atmosphere. Atmospheric Environment 18, 1-17. Leih A. Y. (1991) Flux of volcanogenic sulphur to the atmosohere and isotonic composition of total sulphur. In Stable Iiotopes: Natural and Akhropogenic Sulphir in the Environment, SCOPE 43 (edited by Krouse H. R. and Grinenko
V. A.), pp. 116-125. Wiley, Chichester. Malin G., Turner S., Liss P., Holligan P. and Harbour D.
(1993) Dimethyl sulphide and dimethylsulphoniopropianate in the North East Atlantic during the summer coccolithophore bloom. Deep-Sea Res. 40, 1487-1508. McArdle N. C. (1993) The use of stable sulphur isotopes to distinguish between natural and anthropogenic sulphur in the atmosphere. Ph.D. thesis, Univ. of East Anglia. Nakai N.; Tsuji Y. and Takeuchi Y. (1991) Sources of atmospheric sulphur compounds based on the sulphur isotopic composition of SO:- in precipitation in Japan, 1960-79. In Stable Isotopes: Natural and Anthropogenic Sulphur in the Environment, SCOPE 43 (edited by Krouse H. R. and Grinenko V. A.), pp. 352-358. Wiley, New York. Newman L. and Forrest J. (1991) Sulphur isotope measurements relevant to power plant emissions in the northeastern United States in Stable Isotopes: Natural and Anthropogenic Sulphur in the Environmknt, SCOPE 43 (edited by Krouse H. R. and Grinenko V. A.), pp. 331-343. Wiley, Chichester. Newman L., Forrest J. and Manowitz B. (1975a) The ap plication of an isotopic ratio technique to a study of the atmospheric oxidation of sulphur dioxide in the plume from a coal-fired power plant. Atmospheric Environment 9, 959-968.
Newman L., Forrest J. and Manowitz B. (1975b) The ap plication of an isotopic ratio technique to a study of the atmopheric oxidation of sulphur dioxide in the plume from an oil-fired power plant. Atmospheric Enoironment 9, 969-977.
Nriagu J. O., Coker R. D. and Barrie L. A. (1991) Origin of sulphur in Canadian Artic haze from isotope measurements. Nature 349, 142-145. Nriagu J. O., Holdway D. A. and Coker R. D. (1987) Biogenic sulfur and the acidity of rainfall in remote areas of Canada. Science 237, 1189-1192. Prosper0 J. M., Savoie D. L., Saltzman E. S. and Larsen R. (1991) Impact of oceanic sources of biogenic sulphur on sulphate aerosol concentrations at Mawson, Antartica. Nature 350, 221-223.
Savoie D. L. and Prosper0 J. M. (1989) Comparison of oceanic and continental sources of non-sea-salt sulfate over the Pacific Ocean. Nature 339, 685-687. Smith W. O., Codispoti L. A., Nelson D. M., Manley T., Buskey E. J., Niebauer H. J. and Cota G. F. (1991) Importance of Phaeocystis blooms in the high-latitude ocean carbon cycle. Nature 352, 514-516. Wadleigh M. A. (1989) Geochemical characterization of coastal precipitation, natural versus anthropogenic sources. Ph.D. thesis, McMaster Univ. Wagenbach D., Gijrlach V., Moser K. and Miinnich K. 0. (1988) Coastal Antartic aerosol: the seasonal pattern of its chemical composition and radionuclide content. Tellus 4OB, 426-436. Yaaqub R., Davies T., Jickells T. and Miller J. (1991) Trace elements in daily collected aerosols at a site in southeast England. Atmospheric Environment ZSA, 985-996.
AtmosphericEncironmenrVol. 29, No. 18, pp. 2553-2556, 1995 Elswier Science Ltd Printed in Great Britain 135%2310/95 $9.50 + 0.00
13522310(95)OOltB-3
SHORT COMMUNICATION ISOTOPES NICOLA
AND ATMOSPHERIC C. MCARDLE
SULPHUR
and PETER
S. LISS
School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K. (First received 2 February
1995 and in final form 19 April 1995)
Abstract-,4 combination of complementary approaches, dealing with the natural and the man-made sulphur sources respectively, is employed to show the power of the isotopic method to distinguish quantitativs:ly between atmospheric sulphur sources. At two remote European sites we found that - 15-20% of the sampled aerosol sulphur was biogenic in origin rising to -30% in spring and summer. Key word index: Atmospheric sulphur, dimethylsulphide, sulphur isotopes.
The sulphur cycle is the major source of acidity in the atmosphere, with the natural component (mainly via marine dimethylsulphide (IDMS)) dominant in remote regions (Wagenbach et al., 1988; Savoie and Prospero, 1989) and man-made emissions of SO, controlling in heavily industrialized areas. In intermediate zones both sources are likely to be significant. Although the potential of stable sulphur isotopes as a means of discriminating between atmospheric sulphur of different sources has been recognised (Grey and Jensen, 1972; Nriagu et al., 1987), the few existing tests of the hypothesis have been inconclusive in quantifying the biogenie component for several reasons including small sample sets, dominance of one source and lack of supporting data (Nriagu et al., 1987; Nriagu et al., 1991; Calhoun et al., 1991; Calhoun and Bates, 1989). The isotopic method relies on the different sources having characteristic isotopic signatures-about + 20960for DMS-derived sulphur (Nriagu et al., 1991; Calhoun and Bates, 1989; Wadleigh, 1989) and O-5%0(Newman and. Forrest, 1991; Newman et al., 1975a, b) for that coming from power plants. If various sources of sulphur have distinctly different isotopic signatures then the following equation can be used to find the strengths of the sources: s,c, = 5 sici, i=0
(1)
where N is the number of components Ci is the concentration of the ith species, C, is the total concentration in the mixture, and di and 6, refer to the corresponding isotopic values (defined in equation (2) below). At the locations in this study we assume tlhat the major sources of sulphur are sea-salt sulphate, anthropogenic sulphur dioxide and marine biogenic sulphur, with other minor sources such as volcanic sulphur not considered. Although the dJ4S of sea-salt sulphate and marine biogenic sulphur are expected to be similar, it is possible to apportion the sea-salt sulphate by other means: 634s
=
(34~/32%rn,,, [ 34~/32%t*ndard
0 Crown Copyright (1995).
_
1
1
x looo
The standard used is a meteoritic iron sulphide, the Canyon Diablo Troilite, and the unit is 960. The isotopic signature of aerosol sulphur was determined for samples collected at two sites, Mace Head, Eire and Ny Alesund, Spitsbergen. Both sites are coastal and in regions where high offshore marine biological activity has been observed (Smith et al., 1991; Malin et al., 1993). The high volume (typically -3000 m3) air samples were collected on Whatman 41 cellulose filters and 3/4 of each available filter were used for the isotopic analyses. Chloride, nitrate, sulphate and methanesulphonate were determined by ion chromatography of an aqueous solution obtained by extracting portions of the filter ultrasonically into Milli-Q water. Sulphur was recovered from the solution by precipitating as BaSO, (after ultraviolet irradiation of the sample) and then converted to SO, by direct thermal decomposition, the isotope ratio of which was measured by mass spectrometry. The isotopic analyses required at least 1 mg of sulphur. An acid digestion method (Yaaqub et al., 1991) was used to prepare a solution for the determination of metals by Inductively Coupled Argon Plasma Optical Emission Spectrometry. Further experimental details are available elsewhere (McArdle, 1993; McArdle and Liss, in preparation). The 634S values for both sites range from - + 5 to + 23%, an upper limit rather higher than most previously reported values for atmospheric sulphur (Grey and Jensen, 1972; Nriagu et al., 1987; Nriagu et al., 1991; Calhoun et al., 1991; Calhoun and Bates, 1989; Wadleigh, 1989). Figure la shows S34S values for both sites plotted against the percentage of sulphate that is sea-salt derived (calculated using chloride concentrations; where both sodium and chloride concentrations were available the difference between the two estimates of % sea-salt sulphate was small). If the sampled sulphur were only of sea-salt and anthropogenic origin one would expect to find the data points distributed on a simple two source mixing curve with end members of + 20% and O-5%0.It can be seen that the data in fact lie in a triangular area indicating an additional source of isotopically heavy sulphur. This sulphur is generally assumed to be derived from the oxidation of DMS. At the latitudes of the two sites one would expect to find a strong seasonality in biological productivity (Malin et al., 1993). Figures lb and c show the data divided into “winter” (October-March) and usummer” (April-September) periods. There is a marked difference in the distribution of the data with season; almost all points
(2) 2553
Short Communication
2554
t.
20 16
??Z
12
0
??
0
: 8 :
4
4 p 8
i
01 0
1
10
’
20
’
30
’
40
a
50
a
60
b
70
*
’
80
90
1
b)
24
i
20. 16s
WINTER
01
’
0
c
10
8
20
0
30
r 40
0
50
’
60
’
70
fi
I
’
80
90
100
_A’ ,A
.
ZO-
16.
I
01 0
SUMMER 1
10
’
20
’
30 %
8
40
’
50
Sea Salt
’
60
a
70
3
80
’
90
1 1
100
Sulphate
Fig. 1. (a) S3?S values plotted against % sea-salt sulphate for Ny Alesund (0) and Mace Head (o), (b) winter data only, the line is that of linear regression (with 95% confidence limits shown) through the Mace Head data (Ra = 0.94, intercepts at 0 and 100% sea-salt sulphate 5.0 f 0.7 and 19.7 f 3.6% respectively) and (c) summer data only with winter regression line and visually estimated upper boundary. The Ny bilesund filters were selected from archived samples collected by the Norwegian Institute for Air Research. These samples all have low sea-salt sulphate concentrations as the sampler was equipped with a pre-impaction stage that excluded any particles > 2.5 pm. Local pollution events were excluded by the use of a condensation nucleii counter. The Mace Head sampler only operated when air flow was within the marine sector and wind speeds were neither very high nor low. The uncertainty in the % sea-salt sulphate vahres.are - f 7%, and in the a3’S values f 0.3%.
sulphate of a regression line (95% confidence limits shown) of a3*$Son % sea-salt sulphate for winter samples only. At Ny Alesund the mean winter non-sea-salt (NSS) sulphate 634S value was taken as the anthropogenic endmember. A 634S value of + 22%~ was estimated for DMS derived sulphur from the intercept at 0% sea-salt sulphate of the upper boundary line of the data in Fig. lc. This value of + 22% is within the theoretical estimated range for atmospheric sulphur that originated as DMS (Calhoun and Bates, 1989; McArdle, 1993), albeit at the higher end. The 6’4S values derived above were substituted in equation (1) to calculate the proportion and actual concentration of biogenie sulphate in each sample. These concentrations display a strong seasonality with values rising during April and May to a peak in June, with little if any occurring during the winter. However, even in summer the proportion of biogenic sulphur varied widely (from -0 to 100%) from sample to sample. At both sites the maximum concentration found was 26 nmol m- 3 with summer median values of - 4 nmol m- 3 at Mace Head and -2 nmol me3 for Ny Alesund. The values for biogenic sulphate concentration are consistent with other studies, for example in the outer Hebrides (Davison and Hewitt, 1992). At Mace Head over the sampling period from March 1991 to June 1992 -20% of the sampled (NSS) sulphate was biogenic in orgin, in summer this rose to -30%. At Ny Alesund for the whole of 1989 - 15% of the NSS sulphate was biogenic and for the summer -30%. For both sites it must be pointed out that these figures apply to the sampled sulphate only, which is not a continuous record. Additional information was used to corroborate the isotopic findings. Methanesulphonate (MSA) is another oxidation product of DMS in the atmosphere and is generally assumed to be solely from this source. Figure 2 shows MSA concentration plotted against Julian day and reveals a seasonal pattern which echoes that of the biogenic sulphur. Some attempts have been made to use MSA/NSS sulphate ratios to estimate the amount of DMS derived sulphur (Savoie and Prospero, 1989; Galloway et al., 1990) but this method is fraught with difficulties as even in remote regions the ratio is far from constant (Savoie and Prospero, 1989; Prosper0 et al., 1991; Galloway et al., 1990; Ayers et al., 1991). However, if an MSA concentration of 1.5 nmolme3 (May-June mean) is used in conjunction with a range in ratios of 0.2-0.4 plausible for these locations (Galloway et al., 1990; Ayers et al., 1991; Burgermeister and Georgii, 1991) then biogenic sulphur concentrations of 3.8-7.5 nmol me3 can be calculated (summer median 4 nmol mm3 from isotopes).
”
4 CI
0
I J
0 Mace Head
0
0 Ny Alesund 0
F
M
A
M
J
J
A
SO
ND
requiring an additional heavy source occurring during the Fig. 2. Methanesulphonate (MSA) concentrations for Ny summer. Anthropogenic end membe c values of + 5% and + 6960 Alesund (0) and Mace Head (o), determined by ion chromatography with a precision of f 5%, showing a clear were derived for Mace Head and Ny Alesund respectively. seasonal cycle. For the Mace Head data this was the intercept at 0% sea-salt
Short Communication Table 1 shows the correlation matrix obtained from concentration data of other species for the Mace Head samples. The species divide into three distinct groups related to source, Na, Mg, and Cl with a sea-water source are all well correlated, as are the metals, nitrate and sulphate reflecting their continental origin. MSA was inversely associated with the sea-salt species as MSA is highest in the summer whereas sea-salt is highest in the winter. Table 2 shows the effect of dividing the sulphate into three components, sea-salt, anthropogenic and biogenic (using the isotopic ratios). It is interesting to note that the ANTH and BIO parameters are well correlated with the appropriate species. A principal components analysis of the concentration data yielded three major components. The weightings of the various species within each component suggested that Cpt 1 represents the continental contribution, Cpt 2 sea-salt and Cpt 3 biogenic. If the analysis is repeated using the divided sulphate then each part appears in the correct component. Air mass back trajectory data were obtained for all the Ny Alesund samples and up to the end of 1991 for Mace Head samples. At both sites the highest S34S values were associated with predominantly marine air masses and the lowest values occurred where air had obviously been in recent contact with industrial areas. In conjunction with the trajectories, CFC data were used to identify periods when pollution events were being sampled at Mace Head. As expected the CFC data were closely linked with low 634S air masses
2555
that had circulated over Europe before being sampled from the marine sector. We consider that the isotopic data in this study can be used as a basis for future assessments of DMS derived sulphur, although, in order to apply the method to other localities it is necessary to define the anthropogenic end member and its variability for that site. At locations where seasonal variations in biological production are not so pronounced this would be more dificult to achieve. This technique is of most use in environments where DMS is the dominant natural source of sulphur as volcanic and terrestrial biogenic sulphur most likely have @‘S values similar to anthropogenic sulphur (Lein, 1991; McArdle, 1993). Furthermore, sulphur from biogenic H,S tends to be isotopically light, i.e. low bJ4S values (Hitchcock and Black, 1984; Nakai et al., 1991) and so could make apportionment diflicult if combined with isotopically heavy DMS derived sulphur. We conclude that isotopes can be successfully used to apportion atmospheric sulphur between that derived from DMS and man-made sources in the marine environment. The technique gives results without the need to know source strengths and does not depend on atmospheric modelling of their dispersion. It can potentially be used in many different atmospheric contexts, e.g. aerosols, rain, snow, ice, cloud water. In the present study we 6nd that although manmade acidity clearly dominates over much of industrialised Europe, in remote parts of the region the natural sulphur
Table 1. Correlation coefficients for Mace Head data :&ISA MSA Cl Na Mg Ca
1 -- 0.33 -- 0.32 -- 0.32
Cl
Na
- 0.33 - 0.32 1 0.69 0.69 1 0.65 0.99 0.44 0.74
Mg
Ca
- 0.32 0.65 0.99 1 0.77
0.44 0.74 0.77 1
NO3
0.31 0.39 0.38
so4
Mn Fe Zn Pb
NO,
SO4
Mn
Fe
Zn
Pb
1 0.56 0.73 0.56 0.63 0.54
0.31 0.56 1 0.61 0.57 0.66 0.44
0.39 0.73 0.61 1 0.95 0.64 0.67
0.38 0.56 0.57 0.95 1 0.51 0.59
0.63 0.66 0.64 0.51 1 0.73
0.54 0.44 0.67 0.59 0.73 1
Note: Only coefficients that are significant at 95% confidence limit are shown.
Table 2. Coefficients obtained when sulphate divided into DMS derived (BIO), sea-salt (SS) and anthropogenic (ANTH) components MSA MSA BIO Cl Na Mg ss Ca ANTH
-
1 0.58 0.34 0.33 0.32 0.34
BIO
Cl
0.58 1
- 0.34 1 0.68 0.65 1.00 0.42
Na
Mg
- 0.33 - 0.32 0.68 1 0.99 0.68 0.74
0.65 0.99 1 0.63 0.77
SS
Ca
NO3
Mn
Fe
Zn
Pb
0.38 0.74 0.72 1 0.95 0.63 0.66
0.37 0.65 0.55 0.95 1 0.50 0.58
0.83 0.63 0.63 0.50 1 0.74
0.61 0.53 0.66 0.58 0.74 1
- 0.34 1 0.68 0.65 1 0.42
0.42 0.74 0.77 0.42 1
NO3
Mn Fe Zn Pb
ANTH
0.38 0.37
1 0.61 0.74 0.74 0.83 0.61
0.61 1 0.72 0.55 0.63 0.53
Short Communication
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cycle still plays a significant role-15-20% of the yearly averaged aerosonal sulphur sampled being biogenic, rising to N 30% in spring and summer. Acknowledgement-This work was funded by the Natural Environment Research Council. We would like to thank the followinn for their assistance; University of Leeds Stable Isotope ilaboratory, particulaily Simon Bottrell; Hal Maring, Gerry Spain, Maureen O’Dowd and University College Galway for help at Mace Head; Norwegian Institute of Air Research, especially Jozef Pacyna; Willy Maenhaut; Peter Simmonds for CFC data; John Merrill and Joyce Harris for back trajectories.
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