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Sulfur Profiles of the Twentieth Century in Peat Bogs of the Swiss Midlands Measured by ICP-OES and by IC

The growth rates of ombrotrophic peat bogs are controlled solely by the atmospheric input of nutrients. Such systems serve as archives for the emission of anthropogenic pollutants close to urban centers. Goal of the present investigation was to unravel the history of the deposition of sulfur during the twentieth century. In two peat bogs of the Swiss Midlands peat cores were collected and dated using the lead isotope 210Pb.After a wet digestion of the sample material in pressure bombs by microwave excitation the resulting solutions were measured for their S concentrations by ICP-OES or by IC. The S concentrations vary between 1-5 g/kg of dry sample mass. The comparison with the estimated total sulfur emissions for Switzerland from the BUWAL shows an excellent correlation with two maximal values around the years 1960 and 1980, respectively. The fact that the same deposited amount of pollutant is determined in the two peat bogs situated 50 km apart indicates that the measured signals are not controlled by local sources.


Introduction
Systems which conserve consecutive deposition of information are very convenient for the investigation of any variations of emissions in history. Shotyk et al. [1] demonstrated that peat bogs recorded lead emissions over the last 2000 years and Sagesser [2] [4].
Peat cores were collected in Diidingen (570 m a.s.l., close to Fribourg) and in Burgaschi (470 m a.s.l., close to Herzogenbuchsee). The two locations lie 50 km apart. Every ombrotrophic peat bog shows a local structure of hollows and hummocks with different growth behavior. Therefore, by comparing the results of the two sites, material of the same structural type was always considered.
It is well known that reliable sulfur measurements for organic material are quite difficult [5]. It is essential that all sulfur is transformed to the same species. For measurement by ICP-OES (inductively coupled plasma optical emission spectroscopy) or IC (ion chromatography) sulfate is the ideal form, which is often obtained by a wet digestion [6-8].

Experimental Procedures
The collected peat core material was frozen, cut into slices of 1 em, freezedried and powdered in a ball mill. The powdered material was then ready for chemical characterization.

Wet Digestion of Organic Material
Transformation of sulfur in organic material into the sulfate was performed in Teflon® pressure bombs in the presence of nitric acid and perhydrol under microwave excitation. The complete transformation of S-bearing material into the corresponding amount of sulfate was tested by the digestion of methionine and cysteine and the inorganic salts Na2S04, Na2S03 or Na2S, In early experiments no constant values were obtained due to losses. Sulfate recovery became total for organic material when some MgO was present during the digestion. The following procedure works very well for the transformation of all types of S-bearing material into the equivalent amount of sulfate: 100 mg sample material is placed into a 100 ml pressure bomb

Sulfur Profile in Dated Peat Bogs
The dating of the peat core of Diidingen using 210Pb gave a maximal age of80 years.
All the measured parameters are given in Table 1 for the evaluation of the age of the individual peat layers. The gamma measurement for the individual sample was continued until the counting error was less than 10%. This gave then for the old layer a deviation of ± 8 years. This fact has to be taken in account in the discussion of the S profiles.
In Fig. 1 the results are presented for the S concentrations in the two peat bogs

Verification of the Experimental Procedure
Sulfur-containing compounds were measured by ICP-OES with and without wet digestion in a closed system by microwave excitation. The two methods gave very different results. Only the measurement of S after the digestion to sulfate gave a recovery of 100% for a solution ofNa2S, Tests with S-bearing amino acids by wet digestion showed large losses and poor reproducibility. For methionine the S recovery amounted to 36% ± 20% when measured by ICP-OES. In the case of IC measurements the recovery was even lower, 6% ± 22%, because the excess nitric acid from the digestion had to be removed by evaporation before the measurement with the ion chromatographic technique. The losses of S during the digestion could be stopped by the addition of excess amounts of MgO with respect to the expected amounts of SOi- [6].
From each I cm peat layer three individual fractions were analyzed for sulfur. The average of the resulting three values shows an error of 4%.

3.37
The investigated peat cores were collected using a stainless steel auger (either of the Eijkelkamp Agrisearch type, diam- The recovered cores had a length of 40-60 cm representing material grown in the time span of about 1920 to the present. The material was sliced and freeze-dried. The actual age of the individual slice within the core was dated using 210Pb. This isotope has a half-life of 22.3 years and is a daughter of the noble gas 222Rn (T I / 2 = 3.6 d) which itself originates from the decay of the long-lived radioactive mother 238U and emanates steadily from ground into the atmosphere. In earlier investigations it was demonstrated that the deposition of 210Pb into natural archives (ice, lake sediments, peat bogs) is quite constant [9]. So, the decrease of its activity with time can be used to date the investigated material. The activity of 210Pb is evaluated by measuring its gamma line of 46.5 keY in a well-tube Ge detector coupled to a multi-channel analyzing system.

Sulfur Measurement by IC
Ion chromatographic measurements (IC) were performed on a Dionex DX 120 system using the following separation columns: pre-column IonPac AG 12A and an anion chromatographic column 10nPac AS 12A. The conductivity of the eluent was reduced by a self-regenerating suppressor. As eluent a solution of 2.7 mM Na2C03, 0.3 mM NaHC03 was used. The system was calibrated using sulfate standard solutions. The retention time for sulfate in the samples and in the standards was 10.2 ± 0.2 min. The evaluation of the chromatogram was made by applying the Peaknet software ofDionex. Since ion chromatographic measurements are sensitive to the total concentrations of ions in a solution, the acid from the wet digestion had to be eliminated. White residue, which remained after the digestion of peat material (silicates), was filtered off through 0.5 !-tm Millipore membranes. The resulting solution was evaporated to dryness. The residue was taken up in water and evaporated again (twice). Finally, the residue was dissolved in 50 ml of H 2 0 and 2 mlused for the measurement by Ie.

Sulfur Measurement byICP-OES
After wet digestion of the material to be analyzed, the silicate residue was removed as for the IC measurements. The filtered solution was diluted to 100 g total weight. This resulted in an acid concentration of 3% in HN0 3 which is optimal for the direct measurement by inductively coupled plasma optical emission spectroscopy (ICP-OES). The measurements were made on a Varian Liberty 150 AX Turbo ICP-OES instrument.
For the measurement 2 mI/min of the solution to be analyzed were nebulized by a Cetac U5000AT ultrasonic nebulizer. The aerosol thus formed was further dried by a heating device, followed by a cooling section for the recondensation and removal of some water vapor.
Sulfur is measured at the emission lines 182.036 nm and 180.734 nm. Since the optical path is not evacuated in the from Di.idingen and Burgaschi. Excellent correlations are found for the two sites despite the fact they are located 50 km apart. The values for both locations for the last 25 years are given in Table 2. This indicates that the measured signal is not controlled by local emissions but records the pollution situation for the Swiss Midlands. The average sulfur concentrations in peat samples from the 1940s are 1.5 g/kg dry mass. This value increases in the following years to 2.5 g/ kg dry mass and reaches, after another  distinct increase, a value of more than 4 g/kg dry mass in the early 1960s. After some decrease at the beginning of the 1970s a maximal concentration of 4.8 g S/kg dry mass is measured for 1980. In the following ten years the measured S concentrations decrease to the lowest values of the last centuries of Ig/dry mass. An excellent correlation is observed when comparing these results with the total sulfur emissions estimated by the BUW AL (Fig. 2) for Switzerland for the same period [10].  [10]. CHIMIA 2001, 55, No. 12 The emISSIOns data reflect the increased consumption of fossil energy after the second world war with high values for the S02 emissions in the early 1960s. The decrease at the beginning of the 1970s is due to the oil crisis in this time span, which is followed by the maximal emission in the early 1980s. Measures were introduced for a reduction of S02 emissions by substituting fuels with high S concentrations with ones with a lower S content. This was achieved by the replacement of coal and 'heavy' heating oil by 'extra-light' quality and by natural gas [10]. In addition measures were enforced to limit the emissions of S02 of distinct emittents such as cell wool production plants or refineries. All these steps led to a reduction of the S02 emissions in Switzerland to about 30000 t/year after a maximal value of about 100000 t/year in the time span ]965-1980. Similar measures were introduced in the neighboring countries with the same effects as in Switzerland. It is also interesting to compare the estimated emission signature of S02 for Switzerland with the one in France, Germany and Italy (Table 3) [] 1]. There is no surprise that the best correlation is found with the emission produced in this country. This is an interesting finding since the pattern of measured sulfur concentrations in ice core samples from the Monte Rosa in Switzerland resemble more the emissions in France. This is due to the fact that the depositionof pollutantsin high-alpineregions are more dominated by long-range transport: the most dominant wind direction for our country is the one from France [12].
Finally it is evident that environmental archives not only record the pollution situation but also react quickly and demonstrate improvements in air quality due to limitation of emissions by legislation measures. tion of the organic material in Teflon pressure bombs and microwave excitations. It could be shown that peat bogs are suitablearchives to record emissionsof S02' For the peat bog of Diidingen, two maximal sulfur concentrations are found: for the early 1960s and the 1980s. This measured trend follows closely the total emissions of S02 for Switzerland estimated by the public administration.