Atmospheric Enrironment Vol. 27A. N o 13, pp. 2069 2073, 1993.
0004 6981,,'93 $6.00+0.00 ( ' 1993 Pergamon Press Ltd
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S E A S O N A L V A R I A T I O N O F M E T H A N E S U L F O N I C ACID IN P R E C I P I T A T I O N AT A M S T E R D A M I S L A N D IN THE SOUTHERN INDIAN OCEAN N. MmALOPOULOS, J.-P. PUTAUD a n d B. C. NGUVEN Centre des Faibles Radioactivit6s, Laboratoire Mixte CNRS-CEA, Avenue de la Terrasse, 91198 Gif sur Yvette Cedex, France (First received 5 Auyust 1992 and in finalform 12 February 1993) Abstract A 29-month record of methanesulfonate (MSA) concentration in 103 rainwater samples has been performed at Amsterdam Island in the southern Indian Ocean. Rain water MSA concentrations range from 0.008 t o 1.150/~eq • - 1 with a mean value of 0.187 __+0.054/~eq ~- ~. A strong seasonal variation in rain water MSA concentration was found with a minimum in winter and a maximum in summer, similar to that observed for atmospheric DMS concentrations measured during the same period. The annual average MSA wet deposition during the studied period was 0.51/aeq m - 2 d - 1 which represents roughly 20% of the annual average DMS flux. Key word index: Methanesulfonic acid, MSA, dimethyl sulfide, DMS, marine atmosphere, rain water.
I. INTRODUCTION
Georgii, 1991). Therefore, the work of Ayers and coworkers consists of the only quantification of a n n u a l wet deposition of M S A in the southern mid-latitudes. This paper presents data on M S A c o n c e n t r a t i o n in rain water at A m s t e r d a m Island in conjunction with a t m o s p h e r i c D M S m e a s u r e m e n t s during a 2 9 - m o n t h period (January 1989 M a y 1991) performed to assess whether there is a link between the seasonal variation of MSA a n d a t m o s p h e r i c DMS.
Recent studies of the global sulfur cycle strongly suggest a link between biogenic dimethyl sulfide (DMS), its oxidation products (sulfur dioxide a n d methanesulfonic acid), cloud c o n d e n s a t i o n nuclei (CCN) a n d climate (Shaw, 1983; C h a r l s o n et al., 1987). The distribution of D M S a n d its oxidation products have been extensively studied b o t h in l a b o r a t o r y a n d field experiments (see for example H a t a k e y a m a a n d Akimoto, 1983; Barnes et al., 1988; Andreae et al., 1988; Bates et al., 1990). G a s - p h a s e oxidation of D M S seems to be the exclusive source of methanesulfonic acid (MSA) in the atmosphere. However, there is not sufficient evidence to exclude fully formation of M S A
2. EXPERIMENTAL
in the liquid, aerosol or particle phase. Since M S A is a relatively stable c o m p o u n d with physical a n d chemical properties similar to that of sulfuric acid (produced by SO 2 oxidation), it has been proposed that M S A could be used as a tracer to define the role of D M S as a source of aerosol sulfur in maritime air (Ayers et al., 1986). However, this would need a perfect knowledge of the M S A / D M S relationships in various areas of the world. The first time-series of the seasonal variation of aerosol M S A in the Pacific a n d Atlantic Oceans (Saltzman et al., 1983, 1986) a n d at Cape Grim, T a s m a n i a (Ayers et al., 1986) show that in the extratropical areas, MSA c o n c e n t r a t i o n varies seasonally with a s u m m e r m a x i m u m and a winter minimum. A detailed time-series of rainwater M S A data (1983-1987) at Cape G r i m confirmed that there was a seasonal M S A signal (Ayers a n d Ivey, 1990). Relatively n u m e r o u s studies have been confined to the m e a s u r e m e n t s of seasonal variation of aerosol M S A in the marine a t m o s p h e r e (see also the latter works of Watts et al., 1987, 1990, a n d Biirgermeister a n d
Amsterdam Island (3T50'S-77°30'E) is a small island (9 km by 5 km) located in the prevailing westerlies of the southern Indian Ocean, 5000 km from both the African and Australian continents and 3400 km from Madagascar. More details about its flora, geology and topography and about the meteorological conditions prevailing in this area are described by Gaudry et al. (1983) and Polian et al. (1986). Rainwater samples were collected at a small hilly elevation approximately 300 m south of the base by using precleaned polyethylene funnels and bottles raised at about 2 m above the ground. The funnels and the bottles were rinsed with deionized water prior to exposure, placed in position before the onset of rain and were recovered soon after. No funnel was exposed for more than 24 h. Each sample was transferred to a clean 250 ml polyethylene bottle, treated with l ml of chloroform and stored in the dark at 4°C until shipment (every 3~, months) to France for chemical analysis. The analyses were performed in the CFR laboratory by ion chromatography (Dionex 2000i) using a 200-gl injection loop, an AG4A anion guard column, an AS4A anion separator column and an AMMS1 anion fiber suppresser column (all Dionex). To avoid interference with the chloride which is present in concentrations roughly three orders of magnitude higher than MSA and is eluted near the MSA, DIONEX ONGUARD Ag cartridges which retain only the chloride anions, were used in front of the injection loop. A gradient mode was used during the elution with the following programme:O-6min, 5 % o f e l u a n t a n d 9 5 % o f w a t e r ; 6 15min,
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N. MIHALOPOULOSet al.
5-100% eluant; where the eluant was a mixture of 2.4 mM NaHCO 3 and 3 mM N%CO 3. Working standards were prepared by using MSA salts provided by Fluka ( > 99% purity) and Milli-Q water ( R - 18.2 M f~cm-z). The precision of the MSA determinations was 5%. DMS sampling was performed on a daily basis at 6.30 a.m. using 6 ( electropolished stainless steel canisters. DMS analysis was performed at Amsterdam Island within less than 1 h after collection by a gas chromatograph fitted with a flame photometric detector. The detection limit is 1 ng DMS and the precision about 10%. For more details see Nguyen et al. (1990).
3. RESULTSAND DISCUSSION 3.1. A n n u a l variation o f M S A MSA concentrations in rain water (n = 103) collected from January 1989 to May 1991 are plotted in Fig. la. They range from 0.008 to 1 . 1 5 0 # e q E - l w i t h a volume-weighted mean (VWM) of 0.187 +0.054/~eq( -~ MSA values present a distinct seasonal variation with a maximum in summer (December-February) and minimum in winter (June-August). Indeed, VWM of MSA were 0.26, 0.52 and 0.18/~ eq ~ - 1 during the summer months in 1989, 1990 and 1991, respectively, and 0.070 and
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0.054/~ eq •- 1 during the winter months in 1989 and 1990, respectively. The DMS sea-air flux around this island presents a strong seasonal cycle with a peak in summer and minimum in winter. This seasonality in the oceanic DMS flux is reflected by the seasonal variations of DMS atmospheric concentrations (Nguyen et al., 1990, 1992). Figure lb shows the daily variation of the atmospheric DMS concentrations measured at Amsterdam Island from January 1989 to May 1991). Putaud et al. (1992) studied the diurnal variation of DMS at Amsterdam Island and demonstrated that the atmospheric DMS concentrations measured at 6.30a.m. represent quite well the daily average of DMS concentrations. The atmospheric DMS concentrations show daily variability, up to a factor of 5, which is mainly due to the changes in physical parameters such as wind speed and insolation. However, seasonal variations with maxima in summer (9.1, 7.4 and 7.7nmolm -3 for 1989, 1990 and 1991, respectively) and minima in winter (1.0 and 0.9 for 1989 and 1990, respectively) are clearly distinguishable. Thus, the seasonality of the MSA concentrations in rain water presents similarities with that of atmospheric and oceanic DMS simultaneously measured at Amsterdam Island. These covariations between atmospheric DMS and MSA throughout 1989-1991 confirm that MSA is an oxidation product of DMS (Figs la and lb). Although atmospheric DMS and MSA follow similar patterns, the correlation between their monthly mean values is poor (R2=0.35) and becomes significant at 95% probability level (R 2 =0.63) only when two values (open circles) are not taken into account (Fig. 2). These two values correspond to the months of January 1989 and 1991 and present a deficit in the MSA concentration relative to the measured atmospheric DMS. It is to be noted that the DMS maximum, mainly occurring between the end of December and the beginning of January, is quite short-lived. On the other hand, the frequency of precipitations and thus of MSA sampling is higher during the winter than the
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Fig. 1. (a) MSA concentrations in rain water ( n = 103) from January 1989 to May 1991 and (b) daily variation of atmospheric DMS concentrations at Amsterdam Island during 1989-1991.
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Fig. 2. Relation between monthly mean atmospheric DMS concentrations and monthly VWM of MSA.
MSA variation in precipitation summer months. For instance, during December 1989 only one significant rain event occurred (on 18 December) and the following rains were on 18 and 31 January, respectively. In addition, during the summer in 1990 and 1991, we had 2 weeks in the middle of December and 12 days in the middle of January without any rain. Thus, as the D M S maxima do not systematically occur in a certain time period and disappear very fast, even a small shift between D M S and MSA sampling time could lead to great differences in M S A / D M S ratio. By considering only the atmospheric D M S data in January 1989 and 1991 during the 5 days preceding and following rain water sampling, the mean atmospheric D M S concentration is lowered by a factor of about 1(~15%. However, there still remains a deficit in the MSA concentration relative to the measured atmospheric D M S concentration to fit with the correlation. Because the lifetime o f M S A in clouds is higher than that of atmospheric DMS, atmospheric circulation patterns could be responsible for a part of the variability in MSA concentrations in rain water at Amsterdam Island. Indeed Moodyetal.(1991)studiedthe precipitation composition and its variability at Amsterdam Island during the period 1982 1987 and used the technique of cluster analysis to identify variability related to the day-to-day variations in meteorology, They found that during the warm season, the highest VWM concentrations for non-sea-salt sulfate (nssSO 2 , another D M S oxidation productl were associated with transport from the north and northeast sector and occurred together with relatively high precipitation amounts. On the other hand, low V W M concentrations of nss-SO 4 from the southwest coincide with low volume events. The analysis of the meteorological conditions during the precipitation events of January 1989 and 1991 revealed that only 30% of the collected rain samples were associated with transport from the north and northeast. On the contrary, all samples collected in January 1990 were associated with transport from this sector. The ana-
2071
lysis of the 23 samples collected during January and February 1989-1991 revealed that up to 60% of the MSA deposited at Amsterdam Island was associated with the northeast flow pattern, in the agreement with the conclusions of Moody et al. (1991). However, as mentioned by Moody et al. (1991), defining transport patterns only on the basis of isobaric wind speed and direction ignores other important factors like the upwind removal by precipitation en route to Amsterdam Island, the venting through cumulus convection and the transformation rates related to both gas- and aqueous-phase chemistry. Thus, as there is a lack of D M S data from the region around Amsterdam Island, it is difficult to conclude whether the higher MSA fluxes associated with transport from the northeast are due exclusively to higher D M S emission from this region. Measurements of oceanic and atmospheric D M S concentrations in the region around Amsterdam Island and generally in the southern Indian Ocean are clearly needed in order to verify our assumption. 3.2 Wet deposition of MSA The monthly VWM of the MSA concentration of the 103 precipitation samples, the monthly rainfall amount, the fraction collected and sampled and the monthly mean number of precipitation samples over the studied period are presented in Table I. Two observations can be made. First, the rainfall is equally distributed, except during December. Second, more than 60°/,, of the total rainfall amount at Amsterdam Island was sampled. The mean rainfall amount of the period between 1989 and 1990 of 107.4 cm is very close to the mean value of l l 2 cm calculated from published records spanning the decade of 1971~1980 (Galloway and Gaudry,1984i. The wet deposition of MSA is calculated for each month based on the monthly VWM MSA concentration and the rainfall amount. ]-he values range from 0.12 to 1.41 /~eq m 2d ~ and present also a seasonal variation similar to that of the VWM of MSA with higher values during summer
Table 1. Rainwater MSA data from samples collected from January 1989 to May 1991
Month Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.
Monthly mean number of precipitation samples
Monthly mean height of total precipitation (mm}*
Monthly mean height of collected precipitation (mm)
4 4 3 4 4.5 3 4.5 3.5 4 4 3.5 2
114.5 62.4 106.4 95.0 93.3 99.2 82.2 76.8 77.3 101.3 79.2 35.7
73.5 42.1 70.2 61.5 71.5 43.0 52.6 34.5 46.8 43.6 57.7 20.8
* Based on local meteorological data. AE(A) 27;13-0
MSA 0teq / ~) monthly volume MSA flux MSA/DMS weighted mean (imq m 2d ~) flux ratio 0.382 0.372 0.195 0.136 0.079 0.073 0.045 0.069 0.167 0.138 0.265 0.347
1.41 0.83 0.67 0.43 0.24 0.24 0.12 0.17 0.43 0.45 0.70 0.40
0.28 0.22 0.28 0.39 0.11 0.15 0.10 0.09 0.31 0.23 0.28 0.09
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N. MIHALOPOULOSet al.
months and minimum during winter. During the studied period, the mean wet deposition of MSA was 0.51/~eq m - 2 d - 1 which is a factor of 2 higher than that measured at Cape Grim, using a similar method for MSA determination in rain water (0.26 p e q m - 2 d - 1 ; Ayers et al., 1991). This factor results from both higher mean rainfall amount (about 50%) and from higher mean VWM of MSA at Amsterdam Island (0.187_0.054peqf-1; this work; measurements from 1989 to 1991) than at Cape Grim (0.12 _+0.03/~eqf-t; measurements from 1983 to 1987; Ayers and Ivey, 1990). It is to be noted that from 1990 to 1991, the mean atmospheric DMS concentrations at Amsterdam Island (Nguyen et al., 1992, this work) were 30-40% greater than those of Cape Grim (Ayers et al., 1991). These differences could reflect the spatial and temporal variability of the DMS fluxes, From the seawater DMS concentration at Amsterdam Island, the wind speed-dependent exchange coefficient (Kw) and a gas exchange model (Liss and Merlivat, 1986), Nguyen et al. (1990, 1992) have calculated an annual average DMS flux of 2.2/t mol m - 2 d - 1 for the 1989-1990 period. It should be noted that the exchange coefficient (Kw) is highly uncertain and that different approaches in calculating K w can lead to differences as large as a factor of 1.8 (Erickson et al., 1990). The DMS flux is therefore calculated with an uncertainty of a factor at least 2. The annual average wet deposition of MSA of 0.51/~ mol m-2 d-1 calculated here is about 20% the annual average DMS flux. It is worthwhile noting that we have to be very careful when we examine the MSA/DMS ratio as the frequency of the sampling is not the same. DMS in sea water was measured roughly every 3 days and MSA in rain water was collected four times a month. This could be crucial during summer when DMS presents its highest variability and can explain the lowest ratio observed during December when the fewest rainwater samples were collected. Table 1 shows the variability of the ratio of the monthly MSA wet deposition vs the oceanic DMS flux. The MSA/DMS ratio was significantly lower during winter months despite the scatter in the data. This observation agrees well with the conclusions of Putaud et al. (1992). These authors, by using a photochemical box model and the simultaneously measured atmospheric concentrations of SO2 and DMS at Amsterdam Island during tbe 1989-1990period, have found that about 70% of the DMS is oxidized to SO 2, the remaining part is transformed in MSA, DMSO and DMSO 2. In addition, Putaud et al. (1992) have found that the best fit between the measured and computed SO 2 concentrations was obtained assuming a seasonal dependence in the branching ratio of DMS oxidation with higher MSA yield in spring and summer (up to 0.3 0.4) and lower in winter (up to 0.1) in agreement with our observations. Note also that Ayers et al. (1991) have observed the same seasonal
pattern in the MSA/nss-SO2- concentration at Cape Grim. However, it is difficult to explain the reasons of the seasonality in the MSA/DMS flux ratio as it is contrary to the explanation most commonly advanced. Indeed, from laboratory experiments Hynes et al. (1986) have found a negative temperature dependence in the ratio of the two pathways (abstraction/addition) of the DMS oxidation by OH radicals. Based on the observations of Hynes et al. (1986), Berresbeim (1987) assumed that the addition and the abstraction pathways lead exclusively to the formation of MSA and SO2, respectively, and suggested that the lower temperatures favour MSA production. Following Berresheim's calculations, we expect a MSA/DMS ratio of 40% and 32% during the cold and warm seasons, respectively, which is contrary to our observations as well with those of Ayers et al. (1991). However, up to now there is no indication from laboratory experiments to support that the addition and the abstraction pathways lead exclusively to the formation of MSA and SO2, respectively. The most recent work (Yin et al., 1990) has shown that MSA can result from both channels. More information about the factors controlling the MSA production from DMS oxidation are clearly needed to explain the seasonality in the MSA/DMS flux ratio.
4. CONCLUSION MSA concentrations have been determined in rainwater samples (n= 103) at Amsterdam Island from January 1989 to May 1991. A very strong seasonal variation in rainwater MSA concentration and wet deposition is found with a summer maximum and a winter minimum, thus presenting close similarities to that observed for atmospheric DMS concentrations measured at the same point during the same period. The 29 months of rainwater MSA data show an annual average wet deposition rate of 0.51 /~eqm-2d -~, which accounts for about 20% of the annual average oceanic DMS flux calculated for this area. This figure has to be viewed with a lot of caution as an absolute value, since the frequency of the DMS and MSA samplings was not the same and since the DMS flux is given with an uncertainty of a factor of 2. However, itis worthwhile notingthat in relative terms the MSA/DMS ratio presents a seasonality with higher values in summer probably reflecting a higher yield of MSA from DMS oxidation during summer. Additional information about the factors controlling the MSA production from DMS oxidation as well as concurrent measurements of size-distributed aerosol of MSA, MSA in rain water, DMS and SO2 over an extended time period would certainly be very interesting and rewarding, and may go towards solving some of the uncertainties in the seasonality of the MSA/DMS flux ratio.
MSA variation in precipitation Acknowled,qements We thank P. Lahoz and L. Gallet for MSA measurements, J. C. Duplessy, G. Lambert, M. Kanakidou, A. McTaggart and an unknown reviewer for helpful comments. This work was supported by the Territoire des Terres Australes et Antarctiques Frangaises, the Centre National de la Recherche Scientifique and Commissariat 5, l'Energie Atomique. This is CFR contribution No. 1353.
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