Anal. Chem. 1994,66,983-987
Determination of Reduced Sulfur Compounds in the Atmosphere Using a Cotton Scrubber for Oxidant Removal and Gas Chromatography with Flame Photometric Detection Cecilia Persson and Caroline Leck' Department of Meteorology, University of Stockholm, S- 106 9 1 Stockholm, Sweden A method for simultaneous determination of atmospheric dimethyl sulfide (DMS), carbon disulfide (CSz), and dimethyl disulfide (DMDS) is described. Prior to cryogenic trapping, atmosphericoxidantswere successfully removed by a new highcapacity scrubber system based on 100%cotton wadding in combination with a Nafion drier. Using on-line calibration, the overall accuracy was within &12%for DMS and f15%for C S 2 and DMDS. The precision of the method, for relative humidities of 130% ( T = 22 "C), was better than &3%for DMS and about *5% for C& and DMDS. Ambient air samples have successfully been taken and analyzed during the International Arctic Ocean Expedition (IAOE) in the summer and fall of 1991. The main compound present was DMS in concentrationsranging from 1 (detection limit) to 370 ppt(v). Reduced sulfur compounds and in particular dimethyl sulfide (DMS, CHsSCHs) have gained increasing attention during the past decade for severalenvironmental reasons. DMS plays an important role in the global atmospheric sulfur cycle in that it transports biogenic sulfur from the ocean to the atmosphere. It has been suggested that the emission of DMS and its oxidation to particulate matter [sulfate (Sod2-) and methanesulfonate (MSA-, CHsS03-)] play important roles in the formation of cloud condensation nuclei (CCN) over remote oceanic regions. The possibility for this process to change the earth's radiation budget by affecting cloud albedo' has inspired development of a variety of instrumental techniques for the determination of sulfur compounds, such as DMS, in the atmosphere. In general, all techniques combine a preconcentration stage either by cryogenictrapping2Jor by chemisorption using metal surfaces such as gold4 with some kind of oxidant removal device. Techniques like cryogenic trapping for sampling of atmospheric sulfur compounds can lead to variable and extensive loss of reduced sulfur through oxidation by cotrapping of atmospheric oxidants (OS,NO,, S02, et^.).^^^ For the removal of oxidants, Andreae et al.' reported on the use of a (1) Charlm,R. J.;Lovelock, J. E.;Andrcae, M.O.; Wamn,S.G.Narure(bndon)
1987,326,655-661. (2) k k , C.; BBgander, L. E. Anal. Chem. 1988,60, 168G1683. (3) Saltzman, E.; Cooper, D. J. Afmos. Chem. 1988,6, 191-209. (4) Bamard, W. R.; Andrcae, M.0.; Watkins, W. E.; Bingemer, H.; Gcorgii, H.-W. J. Geophys. Res. 1982,87,8787-8793. (5) Hofmann, U.;Hofmann,R.; KcPselmeier,J.Armos. Emiron. 1992,26A, 24452449. (6) Saltzman, E., Cooper, D., Eds. Biogenic Su!fur in rhe Etwironmen?;ACS SymposiumSerics 393; American Chemical Society: Washington, DC, 1989; pp 33C-351. (7) Andrcae, M. 0.;Ferek, R. 0.;Bermond, F.; Byrd, K.P.; Engstrom, R. T.; Hardin, S.;Houmere, P. D.; LcMarrcc, F.; Raemdonck, H.; Chatfield, R. B. J. Geophys. Res. 1985, 90,12891-12900. 0003-2700/94/03650983$04.50/0 0 1994 American Chemical Society
sodium carbonate coated Anakrom scrubber. Bates et ala8 and Ayers et al.9 referred to oxidant removal by a prefilter impregnated with potassium or sodium hydroxide. Saltzman and Cooper3 used a phosphate-buffered neutral aqueous potassium iodide solution kept in a glass frit bubbler. A modified version of this latter technique was developed by Kittler et a1.lO and consisted of a dry glass-fiber filter soaked in a solutionof KI/glycerol/Vitex, which allowed the reductive state of the filter to be monitored, but had to be stored under absolute air-tight and dark conditions. Kittler et al.1° also reported the results of an intercomparison of the various oxidant scavenging methods mentioned above, whereas he found the KI/glycerol/Vitex filter to be superior to the filter scrubbers using Na2CO3 and KOH/NaOH. Moreover, Saltzman and Cooper3 reported the results of a comparison in ambient air between the carbonate-based Anakrom scrubber and the KOH filter. In that study, the latter scrubber technique revealed rapid losses of efficiency. By using a new high-capacity scrubber system based on 100%cotton wadding in combination with a Nafion drier we have improved on the sampling and analysis method for simultaneous determination of reduced sulfur compounds in the atmosphere. The cotton scrubber was easy to prepare and store, dry, and absolutely harmless. For better convenience in making field measurements, the experimental system was separated into two units, an easily handled sampling apparatus and an analytical apparatus. This offers one the possibility to bring only the sampling apparatus out in the field, store the samples, and perform the analyses at a later date in the laboratory where analytical conditionsare stable. The analyses were made using a gas chromatograph (GC) with a flame photometric detector (FPD). This analytical system was specially designed to work under Arctic conditions and functioned satisfactorily during the International Arctic Ocean Expedition in the summer and fall of 1991.
EXPERIMENTAL SECTION Sampling Apparatus. The sampling apparatus (Figure 1) enabled continuous atmospheric sampling [ 1.1 standard liter per minute (SLPM)] with an integration time of generally 20 min. The air flow was drawn with a pump (Figure la). A counter (Figure IC)registering the exact volume of air sampled was connected to the mass flow controller (Bronkhorst, Hi(8) Bates, T.s.; Johnson J. E.; Quinn P. K.; Goldan P.D.; Kuster W. C.; Covert
D.C.; Hahn C. J. J. Armos. Chem. 1990, 10, 59-81. (9) Aycrs, G. P.; Ivcy, J. P.; Gillett, R. W. Nature (London) l991,349,40M6. (10) Kittler, P.; Swan, H.; Ivey, J. J. Afmos. Emiron. 1992, 26A, 2661-2664. I
AnalyticaiChemistry, Vol. 66, No. 7, April 1, 1994 983
SAMPLE INLET
X
h
r--------
I I
r----
I
o p I
I
--_ L -
e
N 0 inject
1
retention time (min)
15
stop
Flgure 2. Chromatogram of a standard taken from the dilution system containing "close to ambient" air concentrationsof CH3SH S (3 ng), CS2 S , (2.5 ng), DMS S (6 ng), and DMDS S (2.5 ng).
Flgure 1. Schematic diagram of the sampling apparatus: (a) gas pump, (b) mass flow controiler, (c) counter, (d) cryogenic trap, (e) laboratory jack, (1) oxidant scrubber, (9) Naflon drier, (h) Teflon filter, and (i) ten-port Valco Valve. Filled arrows mark the flow of sample air with sulfur compounds.
tech, El-flow) (Figure lb). The reduced sulfur compounds were preconcentrated with a cryogenic technique modified from Leck and BAgander.2 A 4-mm-i.d., 6-mm-o.d., 30-cmlong U-shaped Pyrex glass tubing trap (Figure Id) was loosely packed with glass-fiber wool (Supelco) and fitted with leaktight glass/Teflon connections (reducing unit 6 mm to I/g in.). Teflon tubing (l/g in. 0.d.) was connected by an LDC/ Milton Roy Teflon plug. To ensure maximum recovery, the glass-fiber wool was replaced for every sample. The packed trap was precleaned at 250 OC in N2 for 3 min using an 160cm-long, 11-QKanthal wire wound around the lower part of the trap, cooled to room temperature, and sealed with Teflon end plugs. Both the interior surface of the glass trap and the wool were silanized (dimethylchlorosilane in dry toulene) to avoid adsorption losses to the surface of the glassware. Condensation of oxygen in the trap was avoided by using liquid argon (L-Ar) (-186 "C) as cryogen. For scavenging of oxidants, a cotton wadding scrubber (Figure I f ) was combined with a Nafion drier (PD 625 12,Permapure Products; Toms River, NJ) (Figure lg). The scrubber was made of Teflon (PTFE) tubing (30 mm o.d., 20 mm i.d., 220 mm long), filled with 3 g of commercial 100%cotton, (chemically clean, bleached, and defatted; mean fiber length 16 mm). Dry air for the Nafion drier counterflow was obtained by drawing ambient air (3 L-min-', Rotameter Krohne) through a highcapacity molecular sieve (Merck, 0.5 nm, beads 2 mm, 6 g of H20/ 100 g of molecular sieve). Particles were removed with a 2-bm Gelman Teflon filter, (47 mm diameter, Figure lh). All material in the sampling apparatus was carefully chosen 904
Analytical Chemistty, Vol. 66,No. 7,April 1, 1994
to avoid losses of sulfur by adsorption to surfaces. Tubing was Teflon (FEP; l / g and l / 4 in. o.d., minimal lengths), and all nuts contacting the sample gas were Teflon or stainless steel. The sample inlet tubing was masked with A1 foil to avoid photochemical reactions. For sampling exchange, a ten-port Valco Valve with stainless steel zero volume and FEP Teflon filled rotor, equipped with two sampling channels, and two stand by channels was used (Figure li). Analytical Apparatus. Sample analysis was performed on a Hewlett-Packard 5890 (11) gas chromatograph with flame photometric detection (Hewlett-Packard 19256A FPD) and two serially connected Chromosil 330, Supelco, FEP Teflon columns packed with a modified silica gel (3.20 m X 1/8 in. 0.d.). Under our conditions, detection response was optimal with gasflow rates of 75 mLsmin-' H2, 20 mlamin-l 0 2 , 20 ml-min-l carrier gas (N2), and 120 mlmmin-' N2 make-up gas. The column was preconditioned overnight at 100 OC with a carrier gas flow of 20 ml-min-'. For optimalseparation of CO2, CH3SH, CS2, DMS, and dimethyl disulfide (DMDS) the temperature program was 30 OC for 2 min, then at 5 OC.min-l to 41 OC, holding there for 1 min, and then at 30 'Cemin-' to 100 OC, holding there for 7.8 min. The total analysis time was 15 min. With this temperature program C02 and CH3SH were resolved, avoiding quenching of CH3SH by C02 in the FPD (cf. Figure 2 ) . The GC was equipped with a cryogenic cooling system [CO2(1)] to minimize the oven cooling time. A six-port Valco valve with stainless steel zero volume and FEP Teflon filled rotor was used for sample gas injection. The Nz, H2, and 0 2 gases were of 99.996%, 99.99% and 99.988% quality, respectively, and were purified from trace moisture, CS2 and H2S with a scrubber system containing molecular sieve (Merck, 0.5 nm, beads 2 mm) and Drierite (Hammond, 10-20 mesh), followed by a carbon column (1 m X l / g in. o.d., with trace palladium added to improve the capacity). In addition, N2 was purified from trace oxygen with an Bxytrap (Alltech Associates, Inc.). For a more detailed description of the analytical apparatus, see ref 2. Sampling. As samples can be stored after collection, it was possible to bring only the sampling apparatus into the
field, store the samples, and perform the analyses at the laboratory under stable analytical conditions. When sampling, the cryogenic trap was connected to the sampling apparatus (Figure 1) with Teflon tubing ('/8 in. 0.d.) and an LDC/Milton Roy Teflon plug (fastened with a special locking device). The trap was immersed into L-Ar and precooled for 1 min. During sampling, a motorized laboratory jack (Figure le) slowly lowered the trap into the L-Ar in order to avoid pressure drops caused by blockage from sublimated water. After the desired sampling time, the trap was disconnected and sealed with Teflon end plugs, and it and the dewar were stored in a freezer. The dewar was topped with L-Ar regularly (every 4 h). In tests for losses during sample storage, reduced sulfur concentrations were found not to change over a 2-week period.2 Analyses. For analysis, the cold trap, still in the dewar, was connected to the GC Valco Teflon six-port valve with Teflon connections (LDC/Milton Roy plug), locked with a special locking device, and N2 (-50 ml-min-l) for -2 min. At injection, the valve was first pulled to inject position, and then the PC-controlled E-Lab chromatography data system (OMS Tech, Miami, FL) for data aquisition and the GC temperature program were started. After 15 s the dewar was removed and the trap heated to a final temperature of 90 OC (inside) using a Kanthal wire coil for 45 s with additional heating from a hair drier for the first 15 s, mostly on the upper part of the trap. After a total time of 90 s, the valve was reset to sampling position, finishing the injection. To achieve a maximum desorption efficiency of sulfur compounds while at the same time a minimal release of water vapor, it was necessary to avoid temperatures above 90 OC in the trap, which cause excessive water vapor injection onto the Chromosil330 silica gel column which eventually would gradually block its active sites and alter the response of sulfur. Calibration. Permeation devices for CH3SH, CS2, DMS (low), DMS (high), and DMDS with permeation rates (2' = 30.0 "C) in nanograms of S per minute of 12.0 f 3.5% 11.4 f0.9%,2.1 f7.8%,25.8f0.9%,and11.3f4.4%,respectively, were used. These permeation tubes were both commercial (VICI Metronics, Santa Clara, CA) and homemade. Permeation rates were measured gravimetrically every second week. The permeation tubes were continuously flushed with N2 in a vessel (Figure 3b) held in a water bath (Lauda RMS/ RM6) at 30 f 0.05 OC. For calibration the gases from the permeation device were diluted in nitrogen gas, led through a silanized glass loop (-7 cm3),and injected onto the column with the six-port Valco valve. The sulfur response is a complex function related to S2*in the flame. The log(peakarea) varies linearly with 2 log(su1fur concentration). To maximize the detector dynamic range yet minimize nonlinear response problems, it was necessary to use calibration curves covering the range of an ambient sample. Dilution System. The precision and accuracy of the experimental setup (sampling and analytical apparatus) was examined using gaseous standardsin the 15-350 ppt(v) range, obtained from the permeation devices in combination with a two-stage flow dilution system (Figure 3). The flow in the dilution system was regulated by mass flow controllers (Figure 3a) and needle valves (Figure 3e, Nurpo Co.). For dilution a combination of purified gases, N2 (first stage) and tubed air
EXCESS FLOW
Q
2.4 L/min
EXCESS FLOW
4
1.6 L h i n
Figure 3. Schematic diagram of the dilution system: (a) mass flow controller, (b) permeation system, (c) mixing chamber, (d) rotameter, (e) needle valve, (f) ozone generator, (9) air humidifier, and (h) probe for measurement of relative humidity. Filled arrows mark the flow of sulfur through the system. "" I
I
h
-
0 0 1 2 3 4 5 6 7 8 9 101112
Time (hour) Figure 4. Breakthrough time for the cotton scrubber as a function of ozone concentration. The tests were performed with 3 g of cotton wadding.
(second stage), was used. After the second dilution the flow of 1.1 SLPM was drawn to the sampling apparatus (Figure 1). An ozone generator (Dasibi) (Figure 3f) and air humidifier (Figure 3g) were connected in series downstream of the mass flow controller in the second dilution stage. Relative humidity (Rotronic YA- 1) was monitored in the excess flow line (Figure 3h), which was maintained in order to ensure atmospheric pressure in the system.
RESULTS AND DISCUSSION CottonScrubber. To investigate the efficiencyof the cotton scrubber, we tested the removal of 12,30, and 65 ppb(v) ozone from a dry air stream and the impact of humidity (35% R H at T = 22 "C) on the breakthrough volume of the scrubber. Theresults showed (Figure4) that the 100% ozonescavenging efficiency was not affected by humidity. At ambient ozone concentrations ranging between 20 and 40 ppb(v), the cotton scrubber had a capacity of 430 and 130 L of sampled air, respectively, for a 100% scavenging efficiency. This capacity Analyticel Chemistty, Voi. 66, No. 7, Aprii 1, 1994
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compares well with the reported capacity of the KI/glycerol/ Vitex filters (breakthrough volume of 263 L of sampled air). Even at ambient ozone concentrations of 65 ppb(v), a 22-L sample was found to be well within thecapacity of the scrubber. Rather than attempt to monitor the oxidation state of the cotton wadding, it was renewed prior to application of every sample. To test for losses of CH3SH, CSz, DMS, and DMDS in the cotton scrubber, we sampled "close to ambient" mixing ratios of the sulfur compounds and ozone from the gas dilution system (cf. Figure 3). Tests were performed under both dry and humidified conditions, altering the relative humidity of the sample stream by adding varying amounts of water vapor in the second stage of the dilution system. In samples that combined various ozone levels [0-65 ppb(v)] with dry air, no significant deviation from the control was seen, except for an almost 100% loss of CH3SH. When water vapor (35% R H at T = 22 "C) was added to the matrix of sulfur and ozone, a significant lower yield (-35%) was found for CH3SH, DMS, and DMDS. Carbon disulfide concentrations were unaffected. From these results, it was clear that the oxidant removal system could be improved with a drying device, and a Nafion drier was therefore added to the system at the beginning of the train (cf. Figure lg). Nafion Drier. In a recovery test of the Nafion drier, concentrations of sulfur compounds from the dilution system were determined with the Nafion drier in line, at 0%, 15%, and 30% RH ( T = 22 "C). This relative humidity range was chosen to resemble the absolute water content in an ambient air sample representative of the Arctic region during summer (100% RH at T = 4 OC i= 30% RH at T = 22 "C). DMS and DMDS concentrations after the Nafion drier showed at the most a f 5 % deviation from the control and were independent of humidity. Relative to the control, CH3SH and CS2 showed a total increase up to 25% with humidity. The Nafion drier was further tested for drying efficiency and chemical reproducibility during 24 h of continuous operation, at two relative humidities (15% and 50% R H at T = 22 "C). The relative humidity of the sample stream was measured before and after the Nafion drier. In the first case the relative humidity was reduced from 15% to between 0.6% and 1.0% R H by the Nafion drier, and in the higher case relative humidity was reduced to between 4.3% and 5.3% RH. Table 1 summarizes the results for the tests of chemical reproducibility with the Nafion drier in line. It lists the amount of sulfur measured normalized to the mean value (n = 8-9) over the 24-h period. At 15% RH, DMS, CS2, and DMDS were measured with a precision of about f3%. Sampling at a higher relative humidity showed only a minor decrease in the precision of the DMS and DMDS measurements, but the precision for CS2 and CH3SH dropped to f13% and f12%, respectively. From the tests of the cotton wadding scrubber and the Nafion drier, it would seem that CH3SH is most sensitive to losses due to humidity, particularly at low relative humidity, and was not included in further testing of the experimental system. Experimental System: Precision, Accuracy, and Detection Limit. The precision of replicate measurements through the entire experimental setup (sampling and analytical apparatus) was tested for a combination of humidity (1 5%, 30%, 50% 986
Analytical Chemistry, Vol, 66, No. 7, April 1, 1994
Table 1. Results from Tests for the Fractlons Recovered afler Passage through the Nation Drler durlng 24 h of Continuous Operation at Two DMerent Humldltles' time (h:min) DMS CHaSH cs2 DMDS
0:oo 0:30 14:40 15:lO 1810 1840 23:25 23:55 precision (%)
0.94 0.99 1.02 1.05 1.05 0.98 1.00 0.98 f3.4
0:oo 0:40 0:55 8:05 8:35 2005 2035 23:35 24:OO precision ( % )
1.05 1.11 1.01 1.03 0.92 0.95 1.02 0.92 1.00 15.8
15%RH 0.49
nd
0.80 1.09 1.18 1.13 1.16 1.09 f24 50%RH 0.84 0.98 0.98 0.90 0.98 1.09 1.00 0.98 1.27 f12
1.03 0.98 1.00 0.99 1.06 0.99 0.99 0.96 f2.8
1.03 0.97 1-00 0.99 1.02 0.97 1.02 0.99 f2.4
1.17 1.21 1.04 1.06 0.88 0.87 0.95 0.83 0.99 f13
1.05 1.04 1.05 1.06 0.95 0.94 1.01 0.92 0.97 *5.2
Results are normalized to the mean value of sulfur [DMS S (2.5 ng), CH3SH S (0.5 ng), CS2 S (1.0 ng), and DMDS S (0.9 ng)] determined in each series. Time in hours and minutes from start. nd, not detected. Total sample volume was 22 L. The precision is given within one standard deviation as percentage of the mean value. 0 CS,
+ DMS
+ DMDS
14
:I, ,i,, ,;, 1 6
+
0
0
10
20
30
40
50
60
RH (%) Flgure5. Precision of the experimentalsystem (givenwith one standard deviation as percent of the mean value) for DMS, CS2, and DMDS as a function of relative humidity. All values represent SIXreplicates of 22-L samples taken from the dilution system with ozone concentrations ranging between 0 and 65 ppb(v).
RH) and ozone [O-65 ppb(v)] over a range of sulfur loadings (0.2-9 ng of S). Samples obtained at 15% R H showed a precision of f1.2%, f3.5%, and *4.3% for DMS, CSz, and DMDS, respectively, independent of ozone levels. Neither did any of the corresponding measurements at higher relative humidities show any systematic interferences from ozone. On the other hand it was obvious, as summarized in Figure 5, that the reproducibility for all constituents decreased with increasing sample relative humidity (discussed in text below). In conclusion the overall precision of the experimental system for relative humidities of 130% was found to be independent of ozone levels and better than f3% for DMS and about &5% for CS2 and DMDS. To verify the accuracy of the experimental system, a known addition test for DMS [ 15% R H and 30 ppb(v) ozone] was
S (cf. Figure 6). In the range between 0.6 and 7 ng of DMS S,the overall accuracy of the on-line method was found to be
0
1
2
3
4
5
6
7
8
Flgwe 6. Ratios of calculated (on-line calibration) to known amounts of DMS S as a function of DMS S mass. All values represent two to six repllcates of 221 samples taken from the dilution system.
conducted. DMS at close to ambient air concentration was maintained by the dilution system. The accuracy for DMS (0.4-5 ng of S)was found to be 104% ( n = 8). The accuracy of the experimental system was further assessed for all constituents by comparing the yield of sulfur after the sampling system to the control. The tests were performed in a matrix of 30 ppb(v) ozone at 19% and 50% RH (T = 22 “C). For DMS, the result at 15% R H agreed with the known addition test discussed above. For CS2 and DMDS, the accuracy was 99% and 102%, respectively. From the results at 50%RH it was obvious that the accuracy (1 10% 109% and 120% for DMS, CS2, and DMDS, respectively) for all constituents was affected by the absolute amount of water in the system. The observed dependence of both precision and accuracy on relative humidity may have several causes. As shown earlier, the efficiency of the Nafion drier was lower at higher humidities. Consequently,at higher relative humidity, larger amounts of water will be introduced into both the cotton wadding scrubber and the analytical apparatus, adsorbing on the surfaces of the cellulose fibers, the glass wool in the trap, and the active sites of the column. Except for enhanced losses of sulfur compounds in the cotton scrubber due to the water, the presence of water in the analytical apparatus has been shown to improve the response of the column.2 How each separate part of the system (the cellulose fibers, the glass wool, and the column) contributes to the loss of precision and accuracy during measurements at relatively high humidities can only be established through further tests. The detection limits, defined as twice the level of the peakto-peak instrument noise, are 1 ppt(v) for DMS, CS2, and DMDS in a -60-L sample (140, 140, and 280 pg, respectively). Calibration. Calibration during field measurements was done by injection of standards from the permeation vessel (Figure 3b) directly onto the column (on-line calibration). The advantages of this method is that it is easy and reproducible and especially convenient during long field campaigns; its main drawback is the disparity in treatment between standard and ambient sample. The overall accuracy of the experimental system was determined by comparing results from the on-line calibration with standards exposed to the entire experimental setup [ 15-30% R H at T = 22 OC and 30 ppb(v) ozone]. The ratio of the result from the on-line calibration to the known mass of DMS S is plotted as a function of the mass of DMS
-
M2%. Below 0.6 ngofDMSS,calibration by on-linestandard injections was found to overestimatethe “true value” (-60% for 0.2 ng of DMS S). Therefore,we recommend that ambient samples with DMS S below 0.6 ng should be avoided by prolonging sampling times. In similar tests for CS2 and DMDS, no equivalent overestimateat low mass loadings was found. The overall accuracy for these two compounds was found to be f15%.
FIELD MEASUREMENT The instrumental system was used for atmospheric measurements of DMS, CS2, and DMDS in the Arctic basin. Air was sampled from the 1-pm inlet of the sampling system mounted at 27 as1 on the ice breaker ODEN. The sample line was continuously flushed with 550 SLPM, to make the turnover time in the tube as short as possible. From the sample line a flow of 1.1 SLPM was drawn to the sampling apparatus. Altogether -400 samples were determined during the International Arctic Ocean Expedition summer and fall of 1991. The main sulfur compound present was found to be DMS in concentrations ranging from 1 to 370 ppt(v). Maximum concentrations were observed above open water during the summer months. DMDS was detected at a few ppt(v) during periods of high DMS concentrations. Samples representative of the lower range of DMS concentrationswere typically found during the fall period (beginning of October). CS2 was mainly detected in samples which could be classified as either contaminated from the ship stack or perturbed by regional anthropogenic sources. On the average, sampling occurred at below zero air temperatures at relative humidities equivalent to 15% R H at T = 22 “C. The sampling time was extended when ambient DMS concentrations were below 20 ppt(v) in order to avoid the calibration problems discussed in the previous section. Occasionally, DMS concentrationswere found to be so low that sampling time had to be increased to 60 min in order to get above the detection limit [l ppt(v)]. The described method has a potential for future development. The system could be converted for analyses of COS. COS was detected in every ambient air sample from the Arctic. However, quantification was not possible, due to quenching with carbon dioxide in the FPD. For use in warmer and more humid conditions, the drying efficiency of the Nafion drier has to be improved. This could be achieved by heating the sampling stream prior to the Nafion drier. ACKNOWLEDGMENT We thank H. Allee for her valuable contribution in the development of this analytical method. For invaluable technical assistance, we thank L. Biickelin. We also thank Dr. R. Janson for comments on an early version of the manuscript. Funding was supplied by the Swedish Natural Science Research Council (Contract E-EG/GU 9906-30). Received for revlew May 10, 1993. Accepted January 3, lQ94.@
Abstract published in Aduance ACS Absfmcrs. February 15, 1994.
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