trends in atmospheric mercury concentrations at the summit of the wank mountain, southern germany

Pergamon Atimphm En~~rronmenr Vol. 32, No. 5. pp. X45-853. 1998 C’ 1998 Elsewer Science Ltd All rwhts reserved

Prmted m &eat Bntain 1352-2310/9X $19.00 + 0.00 PII: S1352-2310(97)00131-3

TRENDS IN ATMOSPHERIC MERCURY CONCENTRATIONS AT THE SUMMIT OF THE WANK MOUNTAIN,

SOUTHERN GERMANY

F. SLEMR and H. E. SCHEEL Fraunhofer Institute for Atmospheric Environmental Research, Kreuzeckbahnstr. 19,

D-82467 Garmisch-Partenkirchen, Germany

(Firsr received 20 October 1996 and in final,form 24 February 1997. Published March 1998)

Abstract---Total gaseous mercury (TGM) has been monitored at the summit of the Wank mountain (1780 m a.s.1.) in the Bavarian Alps since March 1990. A statistical analysis of the data set until May 1996 consisting of 1670 individual TGM measurements shows a linear decrease of 0.169 k 0.009 ng Hg m-’ yr I, i.e. about 7% per year. The seasonal variation shows maximum TGM concentrations in March and minima in October-December. The frequency of occurrence of extremely high TGM concentrations and the amplitude of the seasonal variation decreased over the observation time. The observed decrease of the TGM concentration is in agreement with measurements in Scandinavia, indicating that the measurements at Wank are representative for the region of central and northern Europe. The decrease in TGM concentration of 23.3% between 1990 and 1994 was consistent with decreases of 20.4 and 21.2%, respectively, observed by us over the northern and the southern Atlantic Ocean. This and the observation of a decreasing trend in mercury wet deposition in the U.S.A. indicate the global significance of the TGM trends observed in Europe. Several causes may add up, but this 45% change in TGM concentrations observed over the period of 6 years cannot be plausibly explained without a substantial decrease of anthropogenic mercury emissions on both regional and global scales. Such decrease, however, is difficult to reconcile with most of the current anthropogenic emission inventories. This points to possible gaps in our understanding of the anthropogenic emission processes. c> 1998 Elsevier Science Ltd. All rights reserved.

Kry word index: Mercury; atmospheric concentrations; trend: emissions; sources.

INTRODUCTION

Medium- and long-range transport of atmospheric mercury, its deposition. biomethylation and the en- richment of highly toxic methylmercury compounds in the aquatic nutrition chain pose a serious environmental problem even in remote areas of Canada and Scandinavia (e.g. Lindqvist, 1991; Watras and Huckabee, 1994; Porcella et al., 1995). Considerable efforts are, therefore, devoted to curbing the use of mercury and by this to reducing its emissions into the atmosphere (e.g. OECD, 1994). These control efforts are counteracted by the increasing burning of coal (until the end of the 1980s) and waste, which represent two major anthropogenic sources of mercury (Nriagu and Pacyna, 1988), and by the increasing use of mercury in small-scale gold mining (e.g. Barbosa et al., 19951, the impact of which is not well known on a global scale. A quantitative assessment of the impact of these changes in emissions on the atmospheric mercury cycle has so far been unsuccessful due to large uncertainties in the global inventories of natural and anthropogenic release of mercury and due to gaps in our understanding of the mechanisms involved in the atmospheric cycle of mercury.

Long-term monitoring has provided unequivocal evidence of trends in the atmospheric concentrations of CO,, CH,, N,O, halocarbons, etc. (e.g. WMO, 1985) which reflect changes in sources and sinks of these species. The establishment of trends for these species and their temporal changes is facilitated by the low concentration variability of these species which is inversely related to their residence time in the atmosphere (Junge, 1974). In the case of elemental mercury the atmospheric residence time is about 1 yr (Slemr et al., 1985; Lindqvist and Rodhe, 1985). Therefore, long-term monitoring of atmospheric mercury concentrations may provide direct evidence of temporal trends and by this of the efficiency of control measures on a regional and global scale. Unfortunately, only few data sets for atmospheric mercury are long and homogeneous enough to support trend estimates. To our knowledge, two data sets presently cover a period of at least 5 yr and consist of measurements made by the same laboratory at the same sampling sites: one from Sweden by Iverfeldt et a/. (1995) and our monitoring at the summit of Wank mountain (Slemr et al., 1995). Both data sets suggest a decreasing trend in atmospheric mercury concentration since 1990 and possibly a few years before. Measurements

845

846 F. SLEMR and H. E. SCHEEL

of atmospheric mercury over the Atlantic Ocean in the years 1977-1980, 1990, and 1994 suggest a global increase between 1977 and 1990 and a decrease after 1990 (Slemr and Langer, 1992; Slemr et ul., 1995; Slemr, 1996). In this paper we report on a detailed statistical trend analysis of the data from the Wank which are based on measurements until the end of May 1996. The analysis indicates a nearly linear decrease of atmospheric mercury concentration since March 1990, a shift in the shape of the frequency distribution of TGM concentrations, and a decrease of the amplitude of seasonal variations over the period from March 1990 to May 1996.

EXPERIMENTAL

A measurement station at the summit of the Wank mountain (1780 m a.s.1.) in southern Germany was chosen for the quasi continuous monitoring of total gaseous mercury (TGM). The site is located at the northern rim of the Bavarian limestone Alps, about 1.1 km above Garmisch- Partenkirchen. Beginning in 1972, air chemical monitoring has been performed at the Wank. Since 1982 the station has been part of the WMO Background Air Pollution Monitor- ing Network (BAPMoN). The local meteorology and its influence on air chemical measurements has been extensively studied (Reiter et al., 1985 and references therein). In winter, an inversion layer lies below the station for most of the time, thus blocking the transport of local pollution to the station. In summer, the station is influenced by the local mountain valley wind system, which transports air from the valley and the prealpine region to the station during the day, and air from higher altitudes during the night. No significant diurnal variation in TGM concentrations (i.e. larger than the precision of the measurement of 5.8%) has been observed on summer days with pronounced convective activity during daytime. The absence of significant diurnal variations indicates that the influence of nearby sources including those of short-lived Hg’+ is small. This is in agreement with Hg’ + measurements at rural sites of Scandinavia which mostly represented less than 5% of the TGM concentrations (Brosset, 1987). Concurrent monitoring of meteorological parameters and of O,, SO,, NO, NO,, and CO, concentrations facilitates the interpretation of the mercury data.

Ambient air for mercury measurements is sampled from a glass manifold which provides air for all monitoring instru- ments. Mercury in the sampled air is collected on gold or silver coated quartz wool collectors (Slemr et al., 1985) and analyzed by atomic absorption (Slemr et al., 1979) or atomic fluorescence spectroscopy (Slemr et al., 1995; Slemr, 1996). Occasional direct sampling on the roof of the station ensured the integrity of samples drawn through the sampling manifold with respect to mercury concentrations. In randomly distributed tests with 2 collectors in series, the collection efficiency was always found to be higher than 95%. There- fore, most of the measurements were made with single collectors only. Usually about 700 / of air were sampled at a flow rate of 6-7 / min- ‘. Air samples were taken manually until August 1982 using a Desaga GS 312 sampler which re- stricted the TGM measurements to days when personnel was present at the station. A programmable sequential sampler (Zambelli, model Explorer Gas) has been in use since Sep- tember 1992. The TGM monitoring at the Wank summit was focused on determining slow variations in the TGM concentrations such as seasonal and year-to-year variations. Consequently, the sampling frequency was usually only one or two samples a day. For most of the time the sampler was programmed to sample air every 17 h for 2 h periods. Thus,

the TGM measurements were distributed over the whole day to avoid any bias due to possible but so far undetected diurnal variation. The sample flow rate was checked against dry gas meters every 446 months which in turn were an- nually calibrated using a standard gas meter.

The collectors were found to collect elemental mercury. methyl mercury chloride (MMC). and mercury II chloride (MC) as well as about 70% of the particle mass (Slemr er al., 1979, 1981). At a sampling flow rate of 10 (.:min the gold collectors collected qu&&tively also dimethyl mercury (DMM) whereas the collection efficiencv of DMM on silver collec;ors was nearly zero. During *almost all our measurements at rural and background sites no signili- cant differences were observed between mercury measurements with gold and silver collectors indicating that the DMM concentration is smaller than the measurement precision. This is consistent with mostly non-detectable concentrations of DMM in remote areas reported by us and others (e.g. Slemr et (II., 1985; Schroeder, 1994) and with the short atmospheric lifetime of DMM due to its fast reaction with OH radicals (Niki et al., 1983). Conse- quently, the data are reported without distinguishing between the collectors although most of the measurements at the Wank summit were made with silver coated wool collectors. In our previous articles we used the term “total gaseous mercury” (TGM) for our measurements irrespective of whether they were made by gold or silver collectors. In view of the particulate contributions being only 1% or less (Slemr et al., 1985; Lamborg et al.. 1995). the term TGM is iustified. The precision of the measurements of + 5.8% was deter- mined from the differences of duplicate samples (Slemr et al., 1985). Good agreement of our TGM measurements and measurements by other groups was found during an international field experiment at Mace Head in Ireland (Ebinghaus et al., 1996).

The data set was screened for maximum and mimmum values. Maximum values were sometimes produced by a collector previously used for other measurements at heavily polluted sites. In this case and in the absence of concurrent enhanced CO, NO, or SO, concentrations, the extremely high TGM values were considered to be due to a memory artefact and were rejected. Extremely low values were mostly produced by collectors with reduced release efficiency as described above. These collectors were replaced and all corresponding TGM values were removed from the data set. With this procedure, less than 5% of the TGM measurements were rejected each year.

As indicated in Table 1, summarizing all TGM measurements between March 1990 and May 1996, only about 20% of the days of a year were covered by measurements with manual sampling in 1990 and 1991. The data coverage was increased to almost 40% by automatic sampling in subsequent years with exception of the year 1993. Major interruptions of the TGM monitoring were in October. November, and December 1990 and 1994 when the instru- ments were used for the cruises with the research ship Polar- stern, and from February to August 1993. For statistical analyses daily means were calculated from the original data for days with several measurements. Individual TGM concentrations were taken for days with only one measurement. To fill in the gaps. a harmonic fit (Young and Benner, 1991) was applied to this data set providing TGM daily values also for the days without measurements. The original data set (daily values with gaps) and the fitted curve are shown in Fig. I. Trends were estimated by linear regression applied on both the daily data and on monthly means derived from both the uninterpolated and interpolated daily values. The fitting curve obtained from harmonic regression on the monthly means was further analyzed with respect to seasonal variations and long-term behaviour of TGM concentrations during the different seasons. There was no significant difference between the results of statistical analyses based on both data sets and. consequently, only the results of the analysis

Trends in atmospheric mercury concentrations x47

Table 1. Summary of the measurements of total gaseous mercury (TGM) at the summit of the Wank mountain (1780m a.s.1.)

TGM concentration (ng Hg me3)

Year Range Median Mean Std. Dev. No. of samples

1990 1.76-8.00 2.86 2.973 0.957 300 1991b 1.6996.71 2.68 2.771 0.648 264 1992’ 1.6778.51 2.20 2.331 0.611 334 1993d 1.31-3.30 2.18 2.161 0.428 117 1994’ 1.61-5.00 2.20 2.281 0.428 276 1995’ 1.55-3.79 2.03 2.095 0.381 228 1996g 1.38-3.77 1.93 1.822 0.342 151

VKh (%)

31.7 22.7 25.5 18.9 17.9 17.2 17.9

No. of days

63 75

125 83

121 173 113

Data coverage: a March 30 - October 7, manual sampling, analyses by AAS or AFS b January 12 - December 23, manual sampling, analyses by AAS or AFS ‘January 6 - December 31, manual sampling until August, automatic sampling afterwards, analyses by

AFS only d no measurements between February 1 and August 30, analyses by AFS ‘January 1 to September 27, analyses by AFS f February 5 to June 18 and September 29 to December 17, analyses by AFS g January 1 to May 31, analyses by AFS hvariation coefficient after correction for the reproducibility of the analytical method; VKZ = VK’

(observed) - VK’(analytical), where VK (analytical) = 5.8% [Slemr et al., 19851.

6 t

5- _ + f + 1

+ r * ;;i 4- E t * B

:+* + ;

= 3-

r” 2-

l- *

01 90 91 92 93 94 95 96

YEAR

Fig. 1. Daily means of TGM concentrations at the Wank summit between March 1990 and May 1996 together with a smoothing curve. Number of the year on horizontal axis

applies to January.

based on uninterpolated daily values are presented, unless mentioned otherwise.

RESULTS

The daily values in Fig. 1 show a short-term variability on the scale of a day up to a few weeks. This variability was found to be related to different air masses (Slemr, 1996) and will not be considered here. Much of the short-term variability is filtered out by the smoothed curve also shown in Fig. 1. The data show a distinct decreasing trend of the TGM concentrations with time and pronounced seasonal variations. Extremely high TGM concentrations observed in the period between 1990 and 1992 disappeared

almost completely in later years. This gave rise to a decreasing amplitude of the seasonal variation. These four features, which are apparent in the daily data, were substantiated by the statistical analyses presented below.

In Fig. 2 the results of linear regression for the daily TGM concentrations are shown. The slope of the regression line corresponds to a decrease rate of - 0.169 _t 0.009 ngHgme3 yr-‘. The negative trend is significant at a higher than 99% confidence level. When using monthly means instead, a decrease rate of - 0.194 + 0.026 ngHgme3 yr-’ was obtained, which is still significant at the 99% confidence level. Statist- ical parameters indicated that a description of the trend by a higher order polynomial is not supported

6 ‘I+’ ” ” 1, ” ” 1

90 91 92 93 94 95 96

YEAR

Fig. 2. Daily means of TGM concentrations and results of linear regression. The dotted lines correspond to the 95%-confidence interval, while the dashed lines indicate the

respective prediction limits.


12 - 1991-92 ;

0 1 2 3 4 5 6

Hg [n&31 14

2

0 0 1 2 3 4 5 6

Hg WW Fig. 3. Frequency distribution of the TGM concentration

for the years 1991/1992 and 1994/1995.

by the data. Related to the average TGM concentration of 2.42 ngHgme3 over the whole monitoring period, the above decrease rates correspond to an average decrease rate of TGM concentration by - 6.98 + 0.37% yr-’ based on the daily values and

of - 8.03 + 1.08% yr-r when considering the monthly means. Related to the same average TGM concentration, this decrease rate corresponds to a decrease of 43.1% based on the daily values and 49.4% when considering the monthly means over the whole monitoring period of 74 months.

To illustrate the change of the concentration distribution, the data from 1991 and 1992 were lumped together and compared with the lumped 1994 and 1995 data. The corresponding 1991/1992 and 1994/1995 frequency distributions are shown in Fig. 3. Both distributions are positively skewed due to few high TGM values. The comparison shows that the 1994/1995 frequency distribution is narrower than the 1991/1992 one and that the TGM concentrations at the high end became lower and less numerous with time. A MannWhitney test showed that the medians for 1991/1992 and 1994/1995 are different at the 95% confidence level. The narrowing of the observed concentration range with time is further illustrated by Fig. 4, which compares 12-month running averages of monthly means with the respective 25th, 75th, and 95th percentiles.

The average seasonal component of the time series as derived from harmonic regression on the monthly means is shown in Fig. 5. The TGM concentrations increase from a minimum in December and January to maximum values in February, March, and April. From April on, the TGM concentrations decrease slowly towards the minimum. The peak-to-peak amplitude of the seasonal variation is 0.75 ngHg m -’ corresponding to 31.0% when referred to the average TGM concentration of 2.42 ng Hgm-” derived from all data. The largest standard deviation of the monthly means is observed in March which is due to the frequent occurrence of heavily polluted air masses arriving at the site during this part of the year (Slemr, 1996). As mentioned above, the frequency of

3.6

3.2 i

3 E 2.6 & S. 0, 2.4 I

1.6 t

.__. . .

I - I ’ I ’ 1”. ’ s ‘* ’ 90 91 92 93 94 95 96

YEAR

- ____.. . I.,...

-,: ._.. “Y, P 95 I& _. ,._. P 75

. .._. --._ . . --__ “..p25 --.

I

,: \

~.....__,. ,>

i_,.-..,.. ._._ :: ..: :

*_

. .

Fig. 4. 12-month running average of the monthly means and the corresponding 25th, 75th, and 95th percentiles. The line represents the least squares fit of the means, with a slope of - 0.192 ng Hgme3 yr-‘.

Trends in atmospheric mercury concentrations 849

I I I I I I I ! I I I I JAN FEB MARAPR MAY JUN JUL AUG SEP OCTNOV DEC

Fig. 5. Average seasonal component derived from the curve of harmonic regression performed on the monthly means.

the elevated TGM concentrations diminished with time and the maximum TGM concentrations became smaller. Since almost all highest TGM concentrations were observed in February, March and April, the amplitude of the seasonal variation decreased during the measurement period. This is also reflected by seasonally averaged monthly mean TGM concentrations which are shown in Fig. 6. Referred to the annual average TGM concentrations, the maximum difference between the seasonal values amounted to 23.8% in 1991 and 33.6% in 1992 but only 17.7% in 1994 and 16.7% in 1995.

DISCUSSION

Trend observations made at Wank are supported by TGM measurements made in Sweden (Iverfeldt et al., 1995) and in Norway (Pacyna and Berg, 1996).

Iverfeldt et al. (1995) lumped the TGM measurements for the 1985-1989 period and compared them with the lumped data for the 1990-1992 period. Since our measurements only started in 1990, a quantitative comparison is not possible. But the trend observed at the Wank summit is in reasonable agreement with the decrease of TGM average concentrations in Sweden from 3.2 + 1.4 ngHgmm3 (n = 123) for the period of 1985-1989 to 2.7 + 1.0 ngHgmw3 (n = 404) for 199CL1992. Iverfeldt et al. (1995) also report a narrowing of the distribution of TGM concentrations which resulted from the disappearance of extremely high TGM values. They ascribed the decreasing trend and the distribution change to lower emissions of mercury from the European continent. The TGM measurements in Norway reported by Pacyna and Berg (1996) started in 1992 and their annual TGM means follow very closely the course of our annual TGM means. In absolute terms, their annual TGM means are con- sistently smaller by about lo%, possibly due to the latitudinal gradient as indicated by comparison of our measurements at Wank (Slemr, 1996) with measurements at Alert, Northwest Territories, Canada (Schroeder and Schneeberger, 1996).

The average decrease in TGM concentration at Wank amounting to 23.3% between 1990 and 1994 is in excellent agreement with TGM measurements over the Atlantic Ocean which decreased by 20.4% in the northern and 21.2% in the southern hemispheres for the same period (Slemr et al., 1995). This agreement suggests that the trend observed at Wank may be representative for hemispheric and global trends in TGM concentrations. It also suggests that regional cycles of short-lived mercury compounds such as

Hg ‘+ do not substantially influence the trends observed at Wank. These contentions are further qualitatively supported by recently reported substantial declines in wet mercury deposition in the Upper Midwest, U.S.A., as inferred from the analyses of lake

3.2 -

2.4 -

I I I 1 1 I I ’

90 91 92 93 94 95 90

YEAR

Fig. 6. Seasonally averaged concentrations as calculated from the values of harmonic regression on the monthly means, including data from an extrapolation of the fitting curve until the end of 1996. The dots

show annual averages calculated from the regression curve.


sediment cores (Engstrom and Swain, 1996). The widespread trend observation and the magnitude of the decrease observed since 1990 can only be explained if mercury emissions decreased in other areas of the world as well, in addition to the reduction of European sources.

Our measurements of TGM over the Atlantic Ocean during the period 1977 ~ 1980, 1990 and 1994 and the Wank measurements since 1990 suggest that the decline in TGM concentration is a recent phenomenon. From the 1977-1990 measurements over the Atlantic Ocean an average annual TGM increase rate of + 1.46% in the northern and + 1.17% in the southern hemispheres was derived (Slemr and Langer, 1992). This is substantially smaller than the annual increase rate of + 10 f 8% in the northern and + 8 f 3% in the southern hemispheres suggested by the 1977-1980 data alone (Slemr et al., 1985). The slowdown of the average annual increase rates between 1977 and 1990 as well as the linear decrease observed since 1990 indicates that the TGM concentration must have reached a maximum somewhere between 1980 and 1990. Unfortunately, the available data base is not sufficient to determine when the maximum was reached and the decrease started.

The reversal of the TGM trend between 1980 and 1990 and an average annual decrease of almost 7% since 1990 is an intriguing phenomenon pointing to substantial inconsistencies in our understanding of the global budgets of atmospheric mercury. As dis- cussed recently, a change in sink strength due to a change in oxidation capacity of the atmosphere (e.g. Crutzen and Zimmermann, 1991; Crutzen, 1994) and/or a change in natural emissions of TGM due to e.g. climate change can explain only a small fraction of the observed trends in TGM concentrations (Slemr and Langer, 1992; Slemr et al., 1995; Slemr, 1996). In addition, changes in oxidation capacity and/or of natural emissions cannot explain the reversal of the trend in TGM concentration between 1980 and 1990. Consequently, the assessment and the search for a possible explanation has to focus on anthropogenic mercury emissions.

Natural emissions are assumed to contribute by about 4-4 to all emissions (e.g. Lindqvist, 1991). Assuming that the natural emissions and the sink processes have not changed substantially in the past, the 45% decrease in TGM concentrations between 1990 and 1996 implies, as a first approximation, that the anthropogenic emissions must have decreased by 56-68%. The change of the frequency distribution of TGM concentrations with the substantial reduction of the occurrence of high TGM concentrations (5-8 ngHgm_~ 3), which are found in the plumes of power plants and urban regions, suggests that the emissions in Europe must have decreased substantially. However, the decrease by 56-68% is not substantiated by reported emission inventories.

Anthropogenic mercury emissions are assumed to originate from two major sources: coal burning

contributing some 65% of total anthropogenic emissions and waste incineration contributing some 25% (Nriagu and Pacyna, 1988; Lindqvist, 1991). Emis- sions from waste incineration probably decreased substantially since 1989 due to the efforts by the OECD countries to curb the use of mercury (OECD, 1994). Worldwide annual mercury production decreased from 5.92 x lo9 g in 1989 and 5.51 x lo9 g in 1990 to 3.65 x lo9 g in 1991; in the U.S. the annual mercury consumption decreased from 1.21 x lo9 g in 1989 to 0.72 x lo9 g in 1990 (OECD, 1994). The mercury content of general purpose batteries has been reduced from 0.15% in 1989 and 0.10% in 1990 to 0.025% in 1991 (all in weight %, OECD, 1994). Extremely high emissions in eastern Germany of 0.33 x lo9 g yr-’ were reported by Petersen et al. (1995) for 1987 and 1988, and these have been substantially reduced after the German reunification in 1990.

All these changes certainly resulted in a substantial reduction of mercury emissions from waste incineration and mercury use. However, the data available to us are not sufficient to quantify the reduction of mercury emissions from these sources. But even if these emissions were eliminated completely, another 30% reduction in anthropogenic emissions has to be ac- counted for. Neglecting small mercury emissions from non-ferrous metal production and wood combustion and assuming that 65% of anthropogenic emissions are due to coal burning (Nriagu and Pacyna, 1988), mercury emissions from coal burning must then have been reduced by about 46%. Most emission inventories are based on emission factors with an inherent assumption of a proportionality between the mercury emissions and coal consumption. However, the 46% reduction in mercury emissions from coal burning is not supported by the worldwide coal consumption data. Worldwide coal consumption increased at an annual rate of 2.1% between 1965 and 1987 (Bundesanstalt fur Geowissenschaften und Rohstoffe, 1989). The maximum worldwide coal consumption was reached in 1989 and it has remained almost constant since then (Lenssen, 1996). The disagreement between these data and the observed linear decrease in TGM concentrations since 1990 suggest that the assumption of a proportionality between coal consumption and mercury emissions is not valid. It may also indicate that mercury emissions from coal burning in relation to other sources are smaller than hitherto assumed.

Mercury emissions from coal combustion are assumed to be mostly due to industry and residential heating whereas electrical utilities are assumed to contribute only about 20% (Nriagu and Pacyna, 1988). Several developments may decouple mercury emissions from the assumed proportionality to coal consumption. A number of large electrical utilities, mostly without flue gas cleaning, in eastern Germany were closed and many utilities in the countries of the former Eastern block were equipped with flue gas

Trends in atmospheric mercury concentrations X51

cleaning which substantially reduces mercury emissions (Lindqvist, 1991). Another substantial reduction can be expected from the replacement of coal by oil and gas in residential heating, especially in the countries of the former Eastern block, and the resulting shift of coal consumption to large-scale facilities with flue gas cleaning. The effect of this shift, however, may be partly offset by the fact that more Hg2’ is emitted due to lower burning temperatures of residential heating (e.g. Galbreath and Zygarlicke, 1996), and this form of mercury tends to be deposited near the source. Shifts between the coal producing regions with smaller amount of coal from mercuriferous belt and/ or change in coal use technology, such as increasing application of coal washing, may have also contrib- uted to the decoupling of mercury emissions from the proportionality to coal consumption. However, the unknown impact of the changed combustion technology and numerous other factors presently do not allow us to interpret the observed decline in TGM concentrations. However, a 46% worldwide reduction of mercury emission from coal burning alone within the last 6 years seems unlikely to us in light of the flat coal consumption since 1989.

The above discussion shows that it is difficult to reconcile the observed trends in TGM concentration and their reversal between 1980 and 1990 with the current mercury emission inventories. The reconcili- ation would be easier if the contribution of mercury emission due to mercury use and waste incineration were much higher than hitherto assumed. As dis- cussed by Slemr (1996), changes in sinks and natural sources can explain only a small part of the observed trends. The observed magnitude of the trends and their changes also suggest that the contribution of the natural emissions might be smaller than hitherto assumed.

The seasonal variation of TGM concentrations observed at the Wank summit is similar to the seasonal variation derived from TGM measurements in Sweden (Brosset, 1982, 1987). A different seasonal variation of TGM concentrations with a summer maximum was reported by Lindberg et al. (1992) for a rural site in Tennessee between April 1988 and March 1989. Their TGM concentrations were substantially higher in comparison with ours and the Swedish data, suggesting an influence of nearby TGM outgassing from soil and/or vegetation and a depend- ence of mercury outgassing from soil on the air and soil temperatures. These two different types of seasonal variation reflect two major driving forces: predominantly source modulation with ambient tem- perature in the case described by Lindberg et al. (1992) and predominantly sink modulation by oxidant concentration in the case observed by us and by Brosset (1982, 1987). A small seasonal variation in mercury emissions from coal combustion with a maximum in winter and an amplitude of about 18% (Rotty, 1987) adds to the latter driving force. The hypothesis of the predominatly sink-modulated type of seasonal

variation in TGM concentration suggested by our observations and observations in Sweden (Brosset, 1982, 1987) is supported by several reports of a seasonal variation of wet mercury deposition with a pronounced summer maximum and winter minimum at sites in Sweden and in North America (Jensen and Iverfeldt, 1994; Sorensen et al., 1994; Hoyer ut al.. 1995; Burke et al., 1995).

CONCLUSIONS

A statistical analysis of the daily TGM concentrations measured at the Wank summit shows a linear decrease of 0.169 f 0.009 ng HgmF3 yr~ i, i.e. about 7% per year. This trend observation is supported by TGM measurements in Scandinavia, our TGM measurements over the Atlantic Ocean and by in- direct determination of mercury deposition in North America from analyses of lake sediments. The observation of similar trends at different sites in the northern and southern hemispheres suggest that the trend observed at Wank may be representative not only for Europe but also for the world. Opposite (increasing) trends in TGM concentrations were observed by us over the Atlantic Ocean between 1977 and 1980 and they became smaller between 1980 and 1990. Taken together, these observations suggest that the reversal of the trend in TGM concentrations occurred between 1980 and 1990.

The lesser occurrence of the extremely high TGM concentration in more recent years indicates that the anthropogenic emissions decreased at least on a regional scale. However, the magnitude of the decrease of TGM concentrations observed since 1990 cannot be explained solely on an European scale and is difficult to reconcile with the current mercury emission inventories. The explanation of the observed trend would require a greater contribution of the anthropo- genie relative to natural emissions and a greater contribution of emission due to mercury use and waste incineration relative to emission from coal burning than hitherto assumed. This and the flat coal consumption since 1989 indicate that the frequent assumption of proportionality between mercury emissions and coal consumption was not fulfilled at least during the last 6 years and possibly even earlier.

The seasonal variation shows maximum TGM con-

centrations in March and minimum concentrations in October-December. The magnitude of TGM maxima and the frequency of their occurrence, mostly observed in February, March, and April, decreased with time. These shifts in the frequency distributions of TGM concentrations have resulted in the statistically derived decrease of the amplitude of the seasonal variation.

Acknowledgements-We thank many of our students and coworkers for their help, without which the TGM monitoring would have been impossible to sustain over the period of


more than six years. Special thanks are due to E. Langer and M. Kern, who did a large part of the analyses, and to F. Boswald, B. Kroecker, J. Conrad and S. Gltick, who took the air samples. We also thank V. Mohnen for a critical reading of the manuscript. This work has been supported by the Deutsche Forschungsgemeinschaft.

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