STABLE ISOTOPE ANALYSIS FOR CHARACTERIZATION OF … · POLLUTANTS AT HIGH ELEVATION ALPINE SITES ... tants to and their deposition into the high-alpine areas is a major concern because

-Author to whom correspondence should be addressed.

Atmospheric Environment Vol. 32, No. 23, pp. 4075—4085, 1998( 1998 Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain1352—2310/98 $19.00#0.00PII: S1352–2310(97)00405–6

STABLE ISOTOPE ANALYSIS FOR CHARACTERIZATION OFPOLLUTANTS AT HIGH ELEVATION ALPINE SITES

FRIEDRICH PICHLMAYER,*- WOLFGANG SCHO® NER,‡ PETRA SEIBERT,°WILLIBALD STICHLER,± and DIETMAR WAGENBACHE

*O® sterreichisches Forschungszentrum Seibersdorf, A-2444 Seibersdorf, Austria; ‡ Zentralanstalt fur Me-teorologie und Geodynamik, A-1190 Wien, Austria; ° Institut fur Meteorologie und Physik, Universitat furBodenkultur, A-1180 Wien, Austria; ±GSF, Institut fur Hydrologie, D-85758-Oberschlei{heim, Germany;

and E Institut fur Umweltphysik, Universitat Heidelberg, D-69120, Germany

(First received 30 April 1996 and in final form 21 August 1997. Published September 1998)

Abstract—Variations in the abundance of stable isotope of sulfur, nitrogen and carbon in sulfate, nitrateand organic carbon, respectively, were studied in snow and air samples from high-alpine sites. Theirsuitability as a tool for characterizing the source regions which contribute to the pollution of thehigh-alpine areas was investigated.

The contributions of different sources to a specific receptor site are generally unknown and depend onemission patterns and meteorological conditions. Measured isotope ratios reflect the actual superpositionof the distinct source types under the assumption that no major isotope fractionation occurs duringtransport and deposition and that post-deposition isotope effects can be excluded. In order to study therelationship between source region and isotopic pattern, daily high-volume samplings of the atmosphericaerosol at Sonnblick (Austrian Alps, 3106 m asl) were combined with twice-daily backtrajectories andevaluated statistically. In addition, vertical snow profiles taken in pits at various locations in the Alps wereanalyzed. For their interpretation, a so-called snow calendar was constructed which relates specific snowstrata to the dates of the respective precipitation events. Furthermore, the isotope ratios of hydrogen andoxygen in the snow samples were used to support the meteorological information.

Main findings of the investigations are regional patterns of d34S in airborne sulfates, the seasonality ofnitrogen isotope composition in nitrates, and a pronounced isotopic difference in nitrates and sulfates ofpre-industrial and modern origin, respectively.

This study was part of EUROTRAC subproject ALPTRAC. ( 1998 Published by Elsevier Science Ltd.All rights reserved

Key word index: Sulfur, nitrogen, carbon, isotopes, d34S, d15N, d13C, aerosols, snow, source regions, backtrajectories, Alps, pre-industrial, air pollution.

INTRODUCTION

Investigations of the atmospheric transport of pollu-tants to and their deposition into the high-alpineareas is a major concern because of the ecologicalsensitivity of these regions. In addition to the knowl-edge of concentrations and loads of the major ions(NO

3, NH

4, SO

4) in the snow cover, information

about the contribution of different emission regionsand the importance of anthropogenic versus naturalinput is desirable.

The application of stable isotope ratio analysis forsource apportionment studies is based on the fact thatthe isotopic composition of an element within a chem-ical compound is not constant but depends on the

origin of this substance and the chemical reactions ithas undergone. The reason for isotope fractionationsis, in short, that the vibrational zero-point energy ofa molecule is a function of the nuclide masses involvedwhich leads to isotope effects in the course of kineticand isotope exchange reactions. Kinetic reactionsgenerally discriminate the heavier isotope. Equilib-rium reactions favor the heavier isotope in the chem-ical species bound stronger (Urey, 1947; Bigeleisen,1965).

Isotopic ratios can be used as source tracers if thevarious sources are isotopically different from eachother and provided that changes in isotopic composi-tion during pollution transport and transformationare small. Fortunately, the latter criterion is met, e.g.in the case of sulfur oxidation in the atmosphere(Hitchcock and Black, 1983; Krouse, 1987).

This work concentrates on sulfur, nitrogen andcarbon in the pollutants. In addition, hydrogen and

4075

oxygen isotope ratios were determined in the snowwater to distinguish different precipitation events andtheir associated meteorological conditions.

The stable isotope distribution is expressed in theconventional d notation which is the relative differ-ence between the isotope ratios of sample and a stan-dard, e.g. for sulfur:

d34S"A34S/32S

4!.1-%34S/32S

45!/$!3$

!1B]1000 (&).

The internationally agreed standards used are: CDT(canyon diablo troilite) for sulfur, N

2!*3for nitrogen,

PDB (Pee Dee Belemnite) for carbon and SMOW(standard mean ocean water) for oxygen and hydro-gen.

Sulfur isotopes

The sulfur cycle is significantly influenced by anthro-pogenic sulfur emissions. Sulfate particles are hydro-philic and thus efficient cloud condensation nuclei.Furthermore, they represent a major acidifying com-ponent in precipitation. Sulfates have a residence timein the atmosphere of several days (Bondietti andPapastefanou, 1993) and can therefore be transportedsome thousand kilometres, in contrast to sulfur diox-ide which has a significantly shorter life time. Sulfatesin the atmosphere can originate from natural sources(sea spray, volcanic emissions, soil erosion, andoxidation of biogenic compounds) or anthropogenicsources (oxidation of SO

2or primary sulfates). Anthro-

pogenic sulfates exhibit a wide range of d34S values,depending on regionally varying sources [combus-tion, fossil fuel refining, gypsum processing, ore smelt-ing (Nielsen, 1974; Krouse and Grinenko, 1991)].The d34S value of sea spray is approximately 20&,the average of all volcanic emissions is around 5&while most of the biogenic emissions are isotopicallylighter.

Nitrogen isotopes

Under tropospheric conditions, gaseous nitric acidand particulate nitrate are produced within a fewdays from nitrogen oxides whose main sources arestationary combustion processes, automotive exhaust,soil emissions, biomass burning, lightning, strato-spheric input and oxidation of ammonia (Heaton,1987). Nitrates contribute to the acidification ofprecipitation while ammonium is a major neutraliza-tion agent.

Nitrogen oxides originating from ammonia exhibitlower d15N values (!10&) than those from com-bustion processes (power plants 5&, automotiveexhaust(0&). Isotope fractionation of nitrogencompounds during long-range transport may alsoplay a role (Freyer et al., 1993). The principal anthro-pogenic NO

xsource is fossil fuel combustion; 85% of

these emissions take place in the latitude belt 30° to60°N. Organic nitrogen compounds (amino acids,

amides, proteins) are found as aerosol constituents,too.

Carbon isotopes

The isotopic composition of atmospheric carbondioxide is mainly determined by the atmosphere—ocean exchange but is also influenced by photo-synthesis and human activities. Due to equilibriumisotope fractionation, CO

2is depleted in 13C as com-

pared to the marine carbonates. The present meand13C value of CO

2is close to !7.8& relative to the

PDB standard. Compared to the atmospheric CO2,

the d value in vegetation biomass is shifted due tokinetic isotope fractionation (preferential 12C uptake)during photosynthesis by about !18& for C

3plants

and !4& for C4

plants, respectively (Winkler andSchmidt, 1980). Fossil fuel burning produces CO

2with d13C of approximately !27& (coal !25&,natural gas !40&, petroleum !30&) (Tans, 1981).As carbonaceous aerosols reflect the isotopic signa-ture of their precursors (e.g., biogenic and fossil fuelhydrocarbons), their d13C value can be used as anindicator for anthropogenic input (Gaffney et al.,1984).

Hydrogen and oxygen isotopes

The stable isotope content of hydrogen and oxygenin precipitation can be related to the local surfacetemperature (Dansgaard et al., 1973). In addition, the2H- and 18O contents are influenced by temperatureand relative humidity at the source areas of the watervapor and the history of air mass.

Theoretical considerations show that 2H and 18Ocontent in precipitation can be assumed to be linearlycorrelated (Merlivat and Jouzel, 1979). The slopeof this regression and the deuterium excess d"d2H!8d18O are linked to the meteorological condi-tions. The deuterium excess depends primarily on themean relative humidity of the air masses at the sourcearea. Generally, the seasonal variation of thedeuterium excess in precipitation is in anti-phase withthe d2H- or d18O values. In the special case of thealpine snow cover, the deuterium excess gives an addi-tional tool to distinguish between air masses, forexample, originating in the Mediterranean area (dvalue around 20&) or from the Atlantic Sea (6—14&).High deuterium excess values can also be caused byevaporation from large fresh water bodies. In thiscase, the d values can be used to distinguish betweendifferent source areas. d18O values of precipitationoriginating in the Mediterranean area are in therange of !15 to !20&, depending on the evapor-ation temperature. Lower d18O values, in the orderof !30&, in combination with high excess valuesindicate origin areas in the far Northeast ofEurope.

The stable isotope ratios of hydrogen and oxygen inthe snow cover can therefore supplement the meteoro-logical information of the trajectories.

4076 F. PICHLMAYER et al.

EXPERIMENTAL

Isotopic analyses were performed in snow profiles for thesnow accumulated between the previous October and Mayin the years 1991—1994. The snow was sampled in 10 cmdepth intervals at Goldbergkees (Sonnblick, Austria) whileat the other glacier fields (Careser, Laaser Ferner, Gries-ferner, Hintereisferner, Jungfraujoch, Zugspitze) bulk sam-ples were collected which roughly represent the autumn,winter and spring deposition periods. These data allow theinvestigation of seasonal variations. In addition, totals overthe whole depth are presented as indicators of the geographi-cal variation of the isotopic signature. Care was taken toavoid sample contamination (plastic gloves, breath masks).The snow samples, collected by ourselves or delivered by theother ALPTRAC groups, were transferred and stored inprecleaned polyethylene bottles in a frozen or at least chilledstate, to minimize bacterial and chemical reactions, and wereprocessed in the laboratory as soon as possible. The samp-ling sites are described in detail in Kromp-Kolb et al. (1993).

To provide pre-industrial Alpine ice for a variety of ana-lyses requiring large sample amount high-volume ice blockswere recovered by the Institut fur Umweltphysik from a spe-cifically suited position at the ablation zone of Gornerglet-scher (Swiss Alps). Sampling site is the still non-temperatedtongue of Grenzgletscher draining high elevated cold areasof the Monte Rosa Massif (Haeberli, 1975). The pre-indus-trial origin of the ice (i.e. older than 100 years) was previouslyconfirmed by low-level tritium and 210Pb analyses (Wagen-bach, unpublished) as well as by the methane concentrationmeasured in the air bubbles (Wahlen, Schmitt, personalcommunications). Apart from some thin, blue ice horizonsthere is no evidence of previous melting process which wouldhave led to an elution of the ice matrix.

At the Heidelberg laboratory sub-samples were cut fromindividual ice blocks (in order to exclude melt layers) whichwere then decontaminated by partial surface melting andpre-concentrated by sub-boiling evaporation in closedquartz and PFA-containers. The ice sample amounts usedfor d15N and d34S analyses from five different ice blocksranged between 4 and 6.5 kg, respectively.

Aerosol collection was accomplished with a high-volumeair sampler (Sierra Andersen UV11H PM10) using PTFEfilters (Zefluor, pore size 1 km) to minimize SO

2sampling

artifacts (Anlauf et al., 1986). Samples were taken for 12 or24 h during a number of episodes in the years 1992—1995 atthe Sonnblick observatory situated on a mountain peak inthe Austrian Alps (12.96°E, 47.05°E, 3106 m asl; see Kromp-Kolb et al. (1993) for a detailed site description). The filtersheets were sealed in polyethylene bags and stored under lowtemperature conditions until they were extracted from thefilters by ultrasonically agitated, deionized water. Sulfur di-oxide was collected by means of a home-made large-areaglass filter impinger device, capable of sucking 20 m3 of airper hour through an aqueous H

2O

2solution, which proved

to be superior to impregnated cellulose filter techniques(Forrest and Newman, 1973).

Both snow water and filter solutions were treated inthe same way. After pH measurement, the principal ionicconstituents sulfate, nitrate, chloride and ammonium weredetermined by ion chromotography (Dionex 2010 i) and aphotometric method, respectively. Then, the slightly acidicsolutions were divided into three parts for the S/N/C-isotopeanalyses. While no pH adjustment was needed for sulfurmeasurements, the ammonium was expelled as NH

3from

the solution for the d15N determination by addition ofNaOH (pH'8). For the DOC analysis, the carbonates wereremoved by adding HCl (pH 1). The solutions were concen-trated by vacuum distillation and subsequently evaporatedgently to dryness in the tin capsules used for subsequenton-line combustion isotope mass spectrometry (Pichlmayerand Blochberger, 1988).

In total, approximately 500 samples were processed andanalysed, mostly in duplicate or triplicate, depending on theactual impurity content of the sample. About 20 kg of S, N,and C, respectively, was the minimum quantity for reliableisotope analysis. Anticipating typical background pollutionconcentrations in air (Puxbaum et al., 1993) and precipita-tion (Puxbaum et al., 1991), about one liter of snow waterwas required per sample. For aerosol collection, filter samp-ling times of at least 12 h with an air throughput of30 m3h~1 were needed.

The quality of the preparation step was checked withsynthetic solution series and blank procedures. No artifactswere observed and recovery rates of 100$3% were deducedfrom the comparison of ion chromatography and elementalanalysis.

Investigations of postdepositional changes in the isotopicratios and their local variability in two adjacent snow pitsare reported elsewhere (Schoner et al., 1993; Pichlmayeret al., 1993). The analytical precision of the d values for C, N,and S is 0.1—0.3&.

Oxygen and hydrogen isotope measurements of the snowwater were performed at the International Atomic EnergyAgency (Vienna) with an isotope ratio mass spectrometerapplying CO

2-equilibration and a H

2-preparation device,

respectively.

¹rajectories

For the interpretation of the measurements, two differenttypes of back trajectories from Sonnblick were used. Isobarictrajectories were available for each day, arriving at 00 and 12UTC, with a length of 72 h. Three-dimensional (3D) trajecto-ries were available for the year 1992 only, arriving at 00, 06,12, and 18 UTC and with a length of 96 h. Where feasible, the3D trajectories were used because they are closer to the realmotion of air parcels. The isobaric trajectories were com-puted for the levels 850, 700, and 500 hPa, corresponding toheights above sea level of approximately 1500, 3000, and5500 m. According to the height of Sonnblick, the 700 hPatrajectories are of primary relevance; the other heights areincluded in some plots as additional information, becausethe real air parcels need not remain on the 700 hPa levelduring the transport (Seibert et al., 1998).

RESULTS AND DISCUSSION

The chemical and isotopic data of the Sonnblicksnow pit profile from May 1993 are shown in Fig. 1.The concentrations of NO

3and organic carbon are

related to the SO4

content in order to obtain addi-tional information about the regional influence(Treloar, 1993) Concentration variations in the majorions of one order of magnitude are found betweensingle snow layers as a consequence of the air pol-lution situation during the different precipitationperiods.

Efforts were made to identify the major pollutionevents, as they contribute a high share to the totalionic load in the yearly snow accumulation. Dating ofthe polluted snow layers was done by means of thesnow calendar method (Schoner et al., 1993). It wasshown for the sampling site Sonnblick that each layerof the snow cover could be assigned to the respectiveprecipitation event as careful stratigraphic descrip-tions and continuous meteorological measurementswere available.

Stable isotope analysis 4077

Fig. 1. Sulfate, nitrate and organic carbon concentration-, and S/N/C/O isotope ratio-, as well asdeuterium excess (d)- depth profiles in the 1992/1993 snow pack at the sampling site Goldberg Glacier,Sonnblick, Austria. Isobaric back trajectories for the marked events are shown in the attached figures; the700 hPa (850/500 hPa) trajectory is indicated by a continuous (dashed/dotted) line with markers every 12 h.

On 4 March 1993 trajectories indicate the advec-tion of polar air masses from the North, leading dir-ectly over the ‘‘black triangle’’, the region around thetriple point of the borders of Poland, Czech Republic,and Germany which is surrounded by strong sulfursources. This explains the elevated sulfate content ofthe corresponding snow layer. The correct dating ofthis deposition event is supported by the extremelynegative d18O value measured in this sample, indicat-ing cold continental air masses. The d34S value re-flects the isotopic signature of anthropogenic sulfatefrom that region.

On 25 March 1993, the 700 hPa trajectory movedover the Po Valley while the 850 hPa trajectory(whose air parcels might in reality have been lifted inthe Alps to higher levels and influenced the snow fallat Sonnblick) passed over Southern Germany andBelgium. Both paths would be able to explain the highnitrate/sulfate ratio, typical for West-European sour-ces. The sulfur isotope information, however, pointstowards contributions from Italy, which are charac-terised by low d34S values (Pichlmayer et al., 1993).

The last trajectory (28 March 1993) shows againairflow from the North, passing over the former GDR.Emissions from this region are likely to be the reasonfor the elevated sulfate concentrations; the moderated34S value (around 3&) also indicated that the sulfateis not from Polish sources.

As an example for the seasonal variations of theionic concentrations and isotopic signatures, analyti-cal results of a snow profile from Jungfraujoch(7.98°E, 46.55°N, 3400 m, Switzerland), sampled infour depth sections on 12 May 1994, are given inTable 1. The trace substances in the winter snow(213—314 cm) are depleted in 34S and 13C, suggestingpre-dominance of anthropogenic input. A similar sea-sonality of the sulfur isotopes has been found in Arctichaze (Nriagu et al., 1991). Nitrate in the winter snowpack is less depleted in 15N compared to spring snow,probably due to the lack of biogenic nitrates; seasonaldifferences in photochemistry could also be a reason(Freyer, 1978). Considerable amounts of organic ni-trogen compounds appear to be present in severalalpine snow and aerosol samples, as indicated by acomparison of the nitrate concentration determinedby ion chromatography and the total nitrogen con-tent detected by elemental analysis. In these samples(which are not included in the data set), positive d15Nvalues were found.

The geographic pattern of the stable isotopes ofS/N/C at different snow pack sampling sites over themain alpine ridge (Fig. 2) shows weak contrasts. Inthe 1994 data set, the following tendency is obviousfor the North/South-transect: d34S (North) (d34S(South), d13C (North)(d13C (South), possibly be-cause of enhanced (local) anthropogenic input at the


Table 1. Concentrations of major ions and stable isotope ratios (d) of sulfate, nitrate and organic carbon in four profilesections of the annual snow accumulation, sampled 12 May 1994 at Jungfraujoch, Switzerland

Distance from Concentration Stable isotope compositionsurface (cm)

Cl~ SO2~4

NO~3

NH`4

Corg d34SSO4

d15NNO3

d13Corg(keq l~1) (keq l~1) (keq l~1) (keq l~1) (mg l~1) (&) (&) (&)

0—155 0.7 4.6 5.7 6 0.28 4.2 !4.0 !24.8155—213 1.4 8.1 7.1 3 0.49 5.1 !2.7 !24.6213—314 5.6 2.9 3.6 2 0.17 3.3 !0.3 !26.1314—454 1.2 2.5 2.8 1 0.27 3.0 !1.8 !25.4

Fig. 2. Mean stable isotope ratios (d34S, d15N, d13C) of SO4, NO

3and C

03'in the snow accumulations

(1990/1991"91 to 1993/1994"94). Sampling sites: J"Jungfraujoch, Z"Zugspitze, G"Griesferner,H"Hintereisferner, L"Laaser Ferner/Careser, S"Sonnblick. Arrow points to the pre-industrial ice

sampling site.

Zugspitze and contributions of C4

plant material tothe aerosol in the South. The d15N is lower in thecentral Alpine region. The East—West transect is char-acterised by d15N (East)(d15N (West), with a localminimum in the Tyrolean Alps. The respective d34Sand d13C variations are not significant however.

The pre-industrial ice samples stand out by signifi-cantly higher d15N and d34S values compared torecent firn (Table 2). As depicted from Alpine ice corestudies at the upper most part of the Grenzgletscher(Colle Gnifetti, 4450 m asl) these isotopic shifts areaccompanied by a concurrent decrease of the meannitrate and sulfate levels between modern and pre-1900 samples by roughly a factor of 4 and 6, respec-tively (Wagenbach and Preunkert, 1996). The iceblock samples show typical d18O values of around!18&, hence by 4& lower than the average value at

the Colle Gnifetti drill sites where only a minor part ofthe clean winter precipitation is preserved. Their ex-tremely low ionic contents may thus be explained bya substantial winter snow contribution (note that elu-tion effects can be excluded in this context due to thequite normal total gas content of the ice material).

Taking the d15N values of the ice block as represen-tative for pre-industrial precipitation in the Alps anddisregarding stratospheric NO

xwould suggest that

the continental source mix of nitrate precursor (i.e.NO soil exhalation, bio-mass burning and lightning)is characterised by a positive d value close to 5&similar, the combination of continental sulfate sources(i.e. soil dust, biogenic sulfur gases, bio mass burningand non-eruptive volcanic emission) appears to beassociated with a typical d34S value of 11& (marinesulfate may be neglected at high Alpine sites).


Table 2. Concentrations of major ions and stable isotope ratios (d) ofsulfate and nitrate in pre-industrial ice samples from Monte Rosa, Switzer-

land

Sample Concentration Stable isotope composition

Cl~ SO2~4

NO~3

d34SSO4

d15NNO3

(keq l~1) (keq l~1) (keq l~1) (&) (&)

1 0.50 0.19 0.23 — #2.62 0.39 0.35 0.24 11.3 —3 0.22 0.38 0.28 11.6 —4 0.24 0.25 0.10 — #6.35 0.25 0.15 0.13 — #5.4

Fig. 3. Sulfate concentrations and sulfur isotope ratios of SO2

and airborne sulfate at Sonnblick Observ-atory. 3D trajectories for the designated times are shown in the attached figures. The trajectories arrive at

the height of Sonnblick (3100 m asl or about 700 hPa).

Freyer et al. (1996) measured in ice core samplestaken at Summit (Central Greenland) also a shift ind15N-nitrate from modern values between #5& and!5& to around 15& in some pre-industrial samples.Part of this much larger isotopic shift in Greenlandmay, however, be due to long-term diffusional nitratelosses favored by the generally acidic snow propertiesand the relatively low-snow accumulation rate at thissite (Wolff, 1995).

Although snow layers are excellent archives forretrospective investigation of the pollutant input aswell as its seasonal and geographic distribution, theyare not well suited for the study of different sourceregions because of uncertainties in the dating of snowlayers and the limited time resolution of the snowpack samples. Therefore, attempts were also made toinvestigate the isotopic composition of aerosols col-lected by high-volume sampling.

The results of SO4

and SO2

sampling in the period12 May—16 May 1992, are reported in Fig. 3. The d34Svalues of both species differ only slightly (Grey andJensen, 1972). In the beginning of this episode (13 May00 UTC), the air came from the West and showedmaritime character (low sulfate concentration, rela-tively high d34S value). Later (14 May 12 UTC), theair passed over the black triangle region, and still later(16 May 00 UTC) over Slovakia and Poland. Thisdevelopment was connected with an increase of thesulfate concentration and a decrease of the d34S value,which in this case corresponds well to the one found inPolish coal (Pichlmayer and Blochberger, 1991). Forthis case, 3D trajectories were available, and theyshow that air arriving at Sonnblick came from levelsbelow 3000 m (depicted as bold line) only for the lastdates, which show also the highest sulfate concentra-tion. The absolute values of the sulfate concentration


Fig. 4. Airborne concentrations (dotted lines) and S/N/C-isotope ratios of sulfate, nitrate and organiccarbon (full lines), sampled 29 April—2 May 1994 at Sonnblick Observatory. Isobaric back trajectories forthe marked events are shown in the attached figures; the 700 hPa (850/500 hPa) trajectory is indicated by

a continuous (dashed/dotted) line with markers every 12 h.

Fig. 5. Organic carbon content versus SO4-concentration in air (°) and snow (*), respectively.


Fig. 6. d34S distribution of sulfates in air (°) and snow (*),respectively, related to their concentrations.

are not very high throughout the period, which is inaccordance with the fact that trajectories indicate nodirect transport of boundary layer air; instead, an-thropogenic pollutants are entrained in this airflow bycloud venting.

A further example of aerosol sampling is given inFig. 4, including isotopic results for sulfur, nitrogenand carbon. The first day of this episode, 29 April1994, showed an extraordinarily high d34S value(9.4&) in combination with high sulfate concentra-tions. The air was coming from the Northeast (Po-land, Belarus) at this time. 24 h later the concentrationhad dropped (probably due to precipitation scaveng-ing) but d34S was still high (6.8&), with a similar pathof the air masses. On 1 May 12 UTC, the meteorologi-cal situation had changed to a westerly flow, witha tendency towards stagnation and probably stronginfluence of local sources, especially from SouthernGermany. The d34S value had returned to about 3&.

A high correlation was found between the concen-trations of sulfate and organic carbon in the airborneaerosol as well as in the snow cover (Fig. 5) witha correlation coefficient of 0.77 (aerosol) and 0.78(snow cover). For aerosol samples, it increases to 0.90if two outliers are removed. This indicates that bothspecies have the same origin.

Figure 6 shows the relation between d34S and sul-fate concentrations in airborne aerosol and snowsamples. Low sulfate concentrations ((2 kg m~3

and 0.5 mg l~1, respectively) occur within the wholeobserved range of d34S (0.5—10&). However, highsulfate concentrations ('6 kg m~3 and 1.5 mg l~1,respectively) were only found with d34S values be-

tween 3 and 5& (with the exception of one outlier);the pattern is more distinct for aerosol samples. Thus,this value seems to be characteristic for those anthro-pogenic emissions which typically cause high concen-trations of sulfate at Sonnblick.

Figure 7 shows the (isobaric) trajectories for allsampling intervals with low (1—2&) and high ('5&)aerosol d34S values, respectively. The low values wereassociated with air from the Mediterranean regionwhile the trajectories of air with high d34S valuescame either from the Atlantic Ocean or from North-eastern Europe.

Results from a more quantitative evaluation of therelationship between aerosol d34S values and air massorigin are shown in Fig. 8. The plotted distributionwas obtained by imposing a 150 km grid on the do-main and calculating for each grid element the aver-age of the d34S values observed at the arrival of thetrajectories crossing this element, weighted with theresidence time in the element. The resulting rawvalues were smoothed within the statistical confidenceintervals (Seibert et al., 1994, but note that data werenot logarithmized here). It was found that there aretwo areas with elevated d34S, namely for air comingfrom the Atlantic Ocean and from Poland; the aver-age values in both of the maxima are between 5 and6&. With respect to the Atlantic, the results indicatethat pure maritime air, with d34S values between 15and 20& (Calhoun et al., 1991; Mc Ardle et al., 1995),is not usually found at a continental site like Sonn-blick. Even air masses which may be called maritimein a meteorological sense obviously take up enoughanthropogenically polluted air on their path from thecoast to the observation site to mask the maritimecontribution. Some influence of transatlantic trans-ports, with anthropogenic impact from sources inAmerica, also cannot be excluded. In the southernparts of Europe, a gradient with lower d34S valuestowards the South was found. A possible explanationmight be a different pattern of industrial emissions,especially as this gradient is found mainly in winter,where biological activity is low compared to anthro-pogenic contributions.

CONCLUSIONS

The results of this work contribute to an under-standing of the major pollution and regions that influ-ence the alpine sites. Snow packs are certainly veryuseful for revealing seasonal and geographical trendsin the isotopic signatures of sulfur, nitrogen, and car-bon in air pollutants. Ice samples from glaciers ashistorical archives were used to elucidate the isotopicchanges from the preindustrial to the industrial emis-sion regime.

Their mean d values (d15N"4.8&, d34S"11.5&)are much higher than in recent firn samples (cf. Table1 and Fig. 2). As a consequence of industrialization,the contamination levels have increased roughly by


Fig. 7. Plot of all isobaric trajectories (700 hPa) arriving during aerosol sampling intervals with d34Sbetween 1 and 2& (a) and more than 5& (b).

a factor of 10; and a significant shift towards thelighter isotopes in sulfate and nitrate has occurred,indicating a corresponding difference in the isotopiccomposition of natural and anthropogenic emissions.

Selected snow layers could be related to depositionevents and thus linked with trajectories indicating theorigin of air masses during these events. However,snow strata are not that appropriate for source attri-bution studies due to the dating uncertainties and thelimited time resolution.

High-volume air sampling, on the other hand, pro-vides information with comparatively high temporalresolution. It thus can easily be combined with trajec-tory analyses to obtain regional isotopic signatures.However, due to infrastructural limitations at thehigh-alpine site Sonnblick, such measurements couldbe made only during a few campaigns. This approachshowed that low d34S values are associated with airmasses from the Mediterranean while air masses withhigh d34S are either of maritime origin or have been


Fig. 8. Mean d34S-values at Sonnblick associated with back trajectories passing through grid elements(indicated on the axes).

impacted by emissions in Poland. For the rest ofEurope, no significant patterns in the d34S signalcould be found. It should be noted that even theso-called maritime air masses, i.e. those which havepassed over the Atlantic while not spending muchtime over the continent, have d34S values far belowthose found at coastal stations, indicating that sea-saltor dimethylsulfide derived sulfate makes no relevantcontribution at Sonnblick.

The number of reliable d15N measurements in aero-sol samples was too small to allow the investigation ofregional patterns. Analyses of d13C values indicatemore negative values in air of southerly origin thanwith northerly origin; however, the British Isles seemto be also a source of carbon with somewhat morenegative d13C values (possibly because of the in-creased utilisation of oil and natural gas) compared toNorthern and Central Europe.

Acknowledgements—The authors would like to thank U.Nickus, A. Novo, G. Rossi, M. Schwikowski and V. Trock-ner for providing snow samples from their glacier sites, aswell as T. Wallner and the staff of the Sonnblick Observatoryfor their help during their aerosol sampling campaigns. Ourspecial thanks go to K. Blochberger for his skillful fieldassistance and for running the analyses at the ResearchCentre Seibersdorf. We are indebted to K. Radunsky and M.

Staudinger for placing equipment and infrastructure of theSonnblick Observatory at our disposal and to ZAMG foraccess to the wind fields for trajectory calculations. Thiswork was funded by the Fonds zur Forderung der Wissen-schaftlichen Forschung (grants P7811-GEO and P7809-GEO) and by the Austrian Ministry of Science, Research andCulture (GZ 30.513-IV/8/94).

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