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Recent trends in chemical composition of bulk precipitation at Estonian monitoring stations 1994–2001
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Atmospheric Environment 38 (2004) 7009–7019 www.elsevier.com/locate/atmosenv
Recent trends in chemical composition of bulk precipitation at Estonian monitoring stations 1994–2001 K. Treiera, K. Pajustea,b,, J. Freya a
Institute of Geography, University of Tartu, Vanemuise 46, 51014, Estonia Estonian Environmental Research Centre, Marja 4D, 10617 Tallinn Estonia
b
Received 8 September 2003; received in revised form 20 April 2004; accepted 5 May 2004
Abstract 2+ , Mg2+, K+, Na+) in Monthly and annual means of main anions (SO2 4 , NO3 , Cl ) and summed base cations (Ca bulk precipitation were studied at 10 stations during an 8-year monitoring period. The data showed statistically significant decreasing trends in most cases. Average declines of mean annual volume-weighted concentrations for both anions and cations were about two-fold. Despite the decrease, the loads of S and cations are still relatively high in Estonia (about 4–14 kg S ha1 and 0.6–1.2 keq ha1, respectively) compared with the loads in Finland and Sweden. Estimated linear decline trends followed the same pattern as annually combusted oil shale from Estonian power plants and emissions of SO2 and fly ash. Recent trends in chemical composition of bulk precipitation at the monitoring stations reflected economic changes in Estonia as well as transboundary fluxes from neighbouring countries. r 2004 Elsevier Ltd. All rights reserved.
Keywords: Atmospheric deposition; Oil shale emissions; Precipitation chemistry; Monitoring; Concentrations; Loads; Economic change
1. Introduction Estonia is a small country (area 45 227 km2, population about 1.5 million) that was occupied by the Soviet Union until 1991. Restoration of independent statehood initiated rapid economic change in 1990s—decreased use of fertilizers, increased areas of abandoned agricultural land, a 44% growth in the number of automobiles and a decrease in energy production. These changes have had an impact on the air pollution situation. Currently, air pollution problems in Estonia are still mostly due to oil shale combustion (Saare et al., 2001).
Corresponding author. Estonian Environmental Research Centre, Marja 4D, 10617 Tallinn, Estonia. Fax: +372 611 2901. E-mail address: [email protected] (K. Pajuste).
In Estonia 90% of electricity generation and one fourth of heat production is based on oil shale (Statistical Yearbook of Estonia, 2003). Estonian oil shale is rather unique, its reserves are the largest commercially exploited deposits in the world (Dyni, 2002). Estonian oil shale as a fuel is characterised by high ash share (45–50%), moderate moisture (11–13%) and sulphur (S) content (1.4–1.8%), and low heating value (net heating value of moist fuel 8.5–9 MJ kg1). One of the peculiarities of Estonian oil shale is chlorine (0.75%) combined with organic matter. The estimated annual emission of HCl in 1991 was about 15 000 t (Ots, 1992). As Estonian oil shale is rich in mineral matter (the basic mineral components of the carbonate part are calcite and dolomite), the amount of ash formed by the combustion of oil shale in Estonian power plants is
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.05.061
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where bigger cities and heavier polluters like power plants, oil shale mines, chemical industries and the cement factory of Kunda are situated. The Estonian network of precipitation monitoring (10 stations) was started in 1994. The first stations were situated mainly in seaside areas, starting from Vilsandi, Estonia’s westernmost island, continuing along Estonia’s northern coast up to the industrial area in the North East. As background stations Vilsandi, Lahemaa and Saareja¨rve are part of the international monitoring network, the data collected there are forwarded, within the framework of the Convention on Long-Range Transboundary Air Pollution (CLTRAP), to the databases of EMEP (Cooperative Programme for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe) and the International Cooperative Programme on Integrated Monitoring. Since 1999 new monitoring stations in South-Estonia have been added to the local precipitation network. As a result, for the first time in history it is possible to estimate the level of air pollution on most of Estonia’s territory on the basis of a uniform methodology. The deposition data measured before 1994 are unfortunately not available, since data concerning air pollution were confidential in the Soviet Union. After regaining independence in 1991, environmental monitoring underwent reorganisation, new methods and equipment were introduced, and since 1994 there exists a comparable database for precipitation compounds. The
enormous. Due to rather high chimneys (150 and 300 m), the exhaust gases are distributed over a large territory even under conditions of still air (Paat, 2002). The atmospheric emissions of 4 oil shale-fired plants in North-Eastern Estonia have become a cause of concern in Estonia as well as in the neighbouring countries, Russia and Finland (Sofiev et al., 2003). Oil shale power plants have comparatively high emission of sulphur dioxide (SO2) in spite of the binding capacity of SO2 by ash when the gas passes through the boilers. Approximately 15–20% of the total sulphur in the oil shale goes into the stack as SO2 (Kallaste et al., 1999). The power plants utilised about 26 Mt of oil shale per annum in the 1980s, producing about 200 000 t of fly ash and 250 000 t SO2 according to expert estimations (Liblik et al., 2000). Economic changes with decreasing industrial production and collapse of export market for electricity have reduced oil-shale based energy production substantially (Fig. 1). In the year 2000 the amount of oil shale consumed was 9.2 Mt, and the corresponding emissions were 59 500 t of fly ash and 79 900 t of SO2 (Estonia’s Third National Communication Under the United Nation’s Framework Convention on Climate Change, 2001). The drop in emissions is a result of economic changes, transition to EU regulations of air emissions, and increase in pollution fines controlled by the government (Kimmel et al., 2002). In spite of the reduced emissions, till now the most polluted region is Northern Estonia
SO2 from manufacturing industry SO2 from energy and transformation industry total emission of SO2
0
0
oil shale megatons
5
2001
50
2000
10
1999
100
1998
15
1997
150
1996
20
1995
200
1994
25
1993
250
1992
30
1991
300
1990
35
1985
350
1980
primary energy PJ
SO2, fly ash kilotons
excavated oil shale primary energy fly ash from power plants
Fig. 1. Annually excavated oil shale (Mt), primary energy supply (PJ) and emissions of SO2 and fly ash (kt) from power plants (Statistical Yearbook of Estonia, 2003).
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aim of this paper is to assess the changes and trends of acidic anion and base cation concentrations in the available data of bulk precipitation during the period 1994–2001.
2. Material and methods In Estonia the average annual amount of precipitation is 530–730 mm and the average annual air temperature is 5.3 1C. The warmest month is July, August has the largest amount of monthly rainfall and February is the coldest and driest. Yearly sum of precipitation has slightly increased during the assessment period, which is mainly due to relatively warmer winters that are rich in precipitation. The difference between the average precipitation amounts of 1994–1997 compared to the period of 1998–2001 is about 100 mm. Precipitation was collected by bulk collectors (20 cm in diameter), placed in an open area at a height of 120 cm. Samples were collected on a 24-h basis but they may still contain a certain amount of dry deposition. Collected samples were stored in refrigerators and mixed in proportion to the total sample volume for monthly samples before analysis, as described in the EMEP manual (EMEP, 1996). Ca2+, Mg2+, K+, Na+, SO2 4 , NO concentrations in water samples were 3 and Cl determined by ion chromatography (EN–ISO 14911 and EN–ISO 10304). NH+ 4 was analysed by spectrophotometry. Acidity (pH) and electric conductivity of samples were also measured. Samples were analysed at internationally accredited laboratories of the Estonian Environmental Research Centre in Tallinn and the Environmental Studies Laboratory in Tartu. Bulk concentrations reported in the tables are precipitation-weighted averages, which are not corrected for sea salts. Although some stations are situated by the sea, sea salt influence is not considered relevant here as the assumption is that the proportion of sea salt contribution has not changed over the assessment period. Deposition was calculated by multiplying annual bulk concentrations by measured precipitation amounts at the same stations. Dry deposition was not measured separately nor included in the deposition estimates. Thematic deposition maps were produced with MapInfo Professional 6.0 and with Surfer 7. The Spearman’s nonparametric correlation analysis was used for the comparison of monthly mean concentrations between stations and between different components. The nonparametric Mann–Kendall test was used for detecting trends in the time series of precipitation-weighted annual means of precipitation concentrations, and the slope of a linear trend was estimated with the nonparametric Sen’s method (Salmi et al., 2002).
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3. Results and discussion 3.1. Acidic anions Data concerning the major ions are shown in Table 2. At all stations the prevailing anion in bulk precipitation was SO2 4 , which formed more than 50% of anionic composition on an equivalent basis (Vilsandi was an exception with only 39%), the anionic proportion of Cl and NO varied between 21–39% and 16–25%, 3 respectively, on an equivalent basis. Sea salt correction (ratio of sodium to sulphate in seawater) shows that anthropogenic sulphur in precipitation constitutes a major fraction (95%) of sulphate in most stations. In two westward stations (Vilsandi, Nigula) the marine fraction of sulphate reached 10% on average. Data for the 8-year study period show linear decreasing trends in sulphate and chloride concentrations at nearly all the stations (Fig. 2, Table 1). The negative linear trend for sulphate was statistically significant at all stations (SO2 slope estimates, i.e. 4 change per year varied from –5.7 mg l1 at Kunda to –0.24 mg l1 at Vilsandi). At most stations (except Harku) the decline of chloride was significant as well, slope estimates varied from –1.3 mg l1 at Kunda to –0.089 mg l1 at Tooma. Monthly concentrations of both anions were positively correlated (at po0.05) with one another, the highest correlations were found in NE stations (r=0.69–0.78). Although chloride is usually of marine origin, in our case both SO2 4 and Cl anions are the main acidic components of flue gases of oil shale. Linear decline trends of SO2 4 and Cl annual concentrations followed the same pattern as annual SO2 emission and annually combusted oil shale amounts. The decrease of sulphate concentration by 1.5–2.7 times and that of chloride by 1.3–3.3 times followed the decline pattern of annually combusted oil shale (1.3 times during 1994–2000, 119 847 and 92 489 TJ yr1 accordingly, Fig. 1). The fact that monthly concentrations of both anions were positively correlated between stations suggests that the power plants located in North–Eastern Estonia are probably an important pollution source having an effect on the deposition content of a significant area of the country (Figs. 3 and 4). In the last years the monthly concentrations of SO2 4 and Cl have reached the lowest values throughout the measuring period (Table 2) coinciding with lowest emissions from the power plants during the study period. The turning point of the trend in SO2– 4 time series occurred at most stations in 1997. Spearman’s nonparametric correlation between stations showed the highest coeficents in NE stations (r=0.7–0.8). Southern stations may be more influenced by pollution transport from neighbouring areas, but still the correlations between NE and S stations were significant (r=0.4–0.6). Over the
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1 Fig. 2. Annual volume-weighted mean concentrations of SO2 4 , Cl , NO3 and summed cations (meq l ). Note the difference in scales.
last 3 years the annual mean deposition of total sulphate has ranged between 4 and 14 kg ha1 yr1 (Fig. 3). In order to protect sensitive natural ecosystems, a long-
1 yr1 term goal of SO2 4 deposition should be 6 kg ha (Hettelingh et al., 1993). For Estonia it means an annual mean SO2 concentration of o1.5 mg l1. In order to 4
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Table 1 Significant decreasing trends (1994–2001) of annual mean concentrations in precipitation, estimated by Mann–Kendall nonparametric test Location
SO2 4
Cl
Ca2þ
K+
Mg2+
Kunda (NE) Jo˜hvi (NE) Saka (NE) Harku (N) Lahemaa (N) Tiirikoja (E) Tooma (E) Saareja¨rve (E) Nigula (W) Vilsandi (W)
*** ** * * ** ** * * * **
* ** *
***
*** *
*** **
*
** ** **
* ***
*
**
** * ** * * **
** **
* * *
Na+
Sum of cations
** *
*** * * ** *** * *
Significance level ***0.001; **0.01; *0.05.
achieve this level, further reduction of SO2 emission is necessary. The Estonian Environmental Strategy has set a goal of decreasing the emission of SO2 to the level of 50 000 t from the current level by the year 2005 (Eesti Keskkonnastrateegia, 1997). The deposition rates of sulphur do not result from local emissions only but are also influenced by longrange transported oxidised sulphur compounds. According to calculations based on the Unified EMEP model (Tarrason et al., 2003), 19 countries and international sea shipping are responsible for 90% of imported sulphur deposition in 1994–2001. The average decline of total SO2 emissions from all emission areas contributing to the long-range transport to Estonia has been about 40%. Reduction in domestic SO2 emissions has been the same. Earlier model estimates (EMEP/MSC-W Report, 1998) of source–receptor relationships of SO2 show that foreign contribution dominates over local sources in many countries. In Estonia 19% of the total annual sulphur deposition in 1998 and 12% in 2000 came from domestic sources. In neighbouring countries, Lithuania, Latvia, Finland and Sweden, the domestic share to sulphur deposition was 8.5%, 4.8%, 5.4% and 7.1%, correspondingly, in 2000 (Tarrason et al., 2003). The data of our study suggest that local contribution to total deposition of oxidised sulphur may be bigger than estimated by EMEP models. Comparing S load at the remote Vilsandi Island with N, NE and E stations suggests that at least till 1997 about 14 territory of Estonia (see Tallinn-Jo˜geva-Tartu line on the map) was mostly influenced by locally originating oxidised sulphur (SO2 4 ) (Fig. 3 and Table 3). In 1994–2001 there was no statistically significant declining trend in air SO2 concentrations at the background stations. It could be suggested that a rapid decline in SO2 concentration in Estonia occurred before the beginning of the study period during the first half of 1990s. During the last 3 years (1999–2001), average
SO2–S concentration has been 0.4 mg m3 at the remote island of Vilsandi and 0.7 mg m3 at the northerly located Lahemaa background station. The directions of air masses with higher SO2 concentrations are related to the location of the measurement stations. At the western border of Estonia (Vilsandi station) higher concentrations are measured from southern and south western air mass transport directions. At the Lahemaa station, high concentrations are observable from sectors between NE and SE where big power plants are located (Pajuste et al., 2003). Throughout the study period NO 3 was the most stabile anion, its annual mean concentration shows a statistically significant decreasing trend at Lahemaa station only. The mean annual concentration varied between 0.3–0.6 mg l1 at different stations and the NO3–N load was, on average, 2 kg ha1 at Tiirikoja, Harku, Vilsandi, Lahemaa, Tooma and Saareja¨rve and 3 kg ha1 at Jo˜hvi, Kunda, Nigula and Saka.
3.2. Base cations Cations originate from natural sources such as sea salt (Mg and Na) and soil dust (Ca, Mg, K and Na), as well as from oil-shale fly ash (Ca, Mg, K and Na). The base cations are important nutrients for forests (RuohoAirola et al., 2003; Frey et al., 2003), but in case of high deposition, long-term cumulative loads may damage the ecosystems of ombrotrophic raised bogs (Karofield, 1996). The deposition of base cations neutralises the acidic deposition at a majority of the stations (only at Vilsandi and Lahemaa the average annual mean pH values of the study period were acidic 4.7 and 4.9, respectively). The annual mean pH of precipitation did not change in most cases during the monitoring period. The only statistically significant decreasing trend was estimated at
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Fig. 3. Calculated annual bulk deposition of sulphur (kg S ha1) in 1996 (A) and 2001 (B).
Kunda, where annual mean pH of precipitation dropped from 7.5 to 6.7. The decreasing trends of annual mean concentrations of summed base cations (the change per year varied from 0.44 meq l1 at Kunda to 0.017 meq l1 at Lahemaa) were statistically significant at 7 stations out of 10, (Table 1). At Vilsandi, Tiirikoja and Harku the decreasing trend of summed base cations was not statistically significant. At most stations, the correlations of summed cations were higher with the NE stations. Correlation between
summed cations and suphate was stronger at the NE stations (r=0.75–0.85) and weaker at the E stations (r=0.47–0.49). Fig. 6 presents the relation between annual average sulphate concentrations and summed cations at the monitoring stations during the study period. Average ratio of sulphate and summed cations is between 1.5–3.0, except for near the Kunda cement factory where the ratio is around 5. The deposition of summed base cation in bulk precipitation decreased about 2 times at NE stations
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Fig. 4. Calculated annual bulk deposition of chloride (kg ha1) in 1996 (A) and 2001 (B).
(at Kunda 3.9 times) and about 1.5 times at all others stations during the study period. Base cation declines are in good accordance with emission data of solid particles from stationary sources that decreased 2.9 times (161 500 t in 1994 and 56 400 t in 2001). There was a 1.9-fold decrease of fly ash emission from the two biggest power plants (85 600 t in 1994 and 45 000 t in 2001) during the monitoring period. In addition, in 1997 new particle emission controls were installed at the cement factory in Kunda. As a result, emission of solid particles decreased 8 times in 1997–2001.
As has been shown by model calculations (Sofiev et al., 2003), about 30% of annual local alkaline emissions are deposited to Estonia and the Gulf of Finland. The high-deposition area, affected by local sources, is about 200 200 km2. Thereby the influence of Estonian emissions is reflected in Finnish monitoring stations (Kulmala et al., 1998; Ruoho-Airola et al., 2003, 2004). The time series of annual mean concentrations of SO4–S, Ca2+ Mg2+ and K+ at a Finnish south eastern station shows comparable significant decreasing trends during 1980–2000.
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Table 2 Volume-weighted mean concentrations (mg ll, except H+meq ll) and standard deviation (in parenthesis) from monthly data for the period of 1994–1997 and 1998–2001 Location
Study period
H+
SO2 4
Cl
Kunda (NE)
1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001
0.05 (0.04) 0.15 (0.08) 0.63 (1.5) 0.65 (0.5) 0.7 (16) 4.2 (6) 1.7 (3.6) 2.3 (4.1) 11.2 (11) 15.8 (14) 1.8 (4.1) 1.3 (2.3) 5.4 (15) 7.9 (11) 3.3 (9) 7.2 (24) 1.1 (1.1) 4.0 (4.3) 30 (32) 20 (15)
28.9 4.6 10.0 6.7 7.9 5.5 7.4 2.4 2.8 1.4 5.2 2.9 3.1 1.8 4.7 1.7 3.2 2.1 2.9 1.8
8.5 1.6 7.8 2.4 2.3 1.1 2.1 1.3 1.3 0.5 1.8 1.3 0.9 0.6 1.2 0.5 2.2 1.4 3.1 1.4
Jo˜hvi (NE) Saka (NE) Harku (N) Lahemaa (N) Tiirikoja (E) Tooma (E) Saareja¨rve (E) Nigula (W) Vilsandi (W)
(26.5) (3.4) (7.3) (5.5) (5.4) (7.0) (1.5) (1.3) (1.2) (0.5) (2.8) (2.0) (1.8) (1.1) (3.5) (0.7) (1.7) (0.9) (6.7) (0.7)
(9.1) (1.1) (7.9) (1.4) (2.5) (1.0) (1.6) (1.4) (1.5) (0.3) (1.0) (0.8) (0.9) (0.4) (0.8) (0.3) (1.2) (0.7) (2.9) (1.4)
NO 3
Ca2þ
Na2+
Mg2+
K+
3.1 2.4 3.1 2.4 3.2 1.5 1.8 1.1 3.4 1.5 1.9 1.6 1.7 1.0 1.8 1.2 2.1 2.3 3.4 1.5
33.5 (15.5) 7.4 (4.7) 3.6 (3.3) 2.7 (1.9) 3.4 (3.4) 1.9 (2.7) 3.0 (2.5) 2.2 (2.6) 1.3 (1.4) 0.6 (0.2) 2.3 (2.4) 1.1(0.9) 1.3 (1.2) 0.6 (0.7) 2.1 (1.6) 1.4 (0.6) 1.6 (1.2) 0.9 (0.5) 0.6 (0.4) 0.4 (0.2)
3.8 0.8 4.8 1.3 1.2 0.4 1.0 0.6 0.4 0.2 1.0 0.7 0.4 0.2 0.9 0.5 1.4 0.7 1.2 0.7
3.4 (1.98) 0.57 (0.25) 1.25 (1.37) 0.29 (0.09) 0.4 (0.1) 0.23 (0.09) 1.1 (1.68) 0.18 (0.03) 0.2 (0.21) 0.06 (0.02) 1.55 (0.38) 0.14 (0.2) 0.58 (0.67) 0.14 (0.06) 0.61 (0.48) 0.28 (0.02) 0.18 (0.02) 0.1 (0.03) 0.48 (0.64) 0.11 (0.03)
10.4 (6.56) 0.78 (0.29) 1.56 (0.31) 0.76 (0.29) 1.1 (0.64) 0.56 (0.17) 0.41 (0.24) 0.13 (0.06) 0.21 (0.08) 0.1 (0.04) 0.85 (0.33) 0.65 (0.06) 0.31 (0.11) 0.1 (0.02) 0.49 (0.16) 0.48 (0.14) 0.73 (0.04) 0.45 (0.76) 0.42 (0.27) 0.24 (0.14)
(4.2) (2.7) (2.9) (2.8) (3.4) (1.7) (1.7) (1.6) (1.0) (0.8) (1.6) (1.6) (1.3) (1.0) (3.3) (1.4) (2.4) (1.9) (2.1) (1.1)
(4.7) (0.6) (6.3) (0.8) (1.7) (0.5) (0.8) (0.8) (0.3) (0.2) (0.5) (0.5) (0.3) (0.2) (1.2) (0.3) (0.8) (0.4) (1.0) (0.7)
Table 3 Mean annual precipitation amounts (mm), deposition of major ions (kg ha1), summed cations (keq ha1) and H+ (k eq ha1) for periods 1994–1997 and 1998–2001 Location
Study period
Prec. mm
H+ eq ha1
SO4–S
Cl
NO3–N
Ca
Mg
Na
K
Summed cations
Kunda (NE)
1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001
421 578 686 798 337 516 679 712 569 691 584 678 626 716 497 719 622 690 358 530
0.2 0.9 4.3 5.2 2.2 21.7 11.4 16.4 60 109 10.4 8.8 34.1 56.6 16 52 6.6 27.7 112 106
43.17 8.82 22.75 17.81 9.03 9.11 16.13 5.78 4.13 3.15 10.31 6.67 6.28 4.34 7.52 4.13 6.74 5.50 3.63 3.19
35.75 9.30 51.43 18.95 7.84 5.79 13.53 9.18 4.56 3.90 10.70 8.78 5.64 4.02 5.72 4.00 13.61 9.16 12.29 7.25
2.99 3.20 4.76 3.97 2.54 1.65 2.52 1.71 1.51 1.61 2.46 2.44 2.25 1.70 2.08 1.96 2.94 3.69 1.38 1.97
145.3 42.08 24.22 21.08 11.53 9.29 18.99 15.45 5.72 4.95 13.42 7.52 8.01 4.43 10.17 10.32 9.77 5.68 2.14 2.05
15.03 3.30 8.13 2.27 1.35 1.26 6.45 1.28 0.83 0.40 9.38 0.89 3.34 0.93 2.61 2.04 1.09 0.72 1.99 0.65
16.20 4.58 31.51 10.13 3.97 2.11 6.80 4.59 4.99 4.58 5.68 4.87 2.68 1.47 3.99 3.74 8.91 5.05 4.40 3.50
46.81 4.00 10.50 6.16 3.59 2.97 2.74 0.91 0.95 0.81 5.00 4.39 1.86 0.73 2.29 3.21 4.52 3.10 1.50 1.43
10.4 2.7 3.5 1.8 1.0 0.7 1.8 1.1 0.5 0.4 1.8 0.8 2.2 0.8 1.0 0.9 1.1 0.2 0.5 0.3
Jo˜hvi (NE) Saka (NE) Harku (N) Lahemaa (N) Tiirikoja (E) Tooma (E) Saareja¨rve (E) Nigula (W) Vilsandi (W)
Fig. 5 shows the loads of base cations in 2001. Lowest loads reached the level of 0.4 keq ha1 in the west, and the highest ones fluctuated around 1–1.6 keq ha1 at NE stations. Base cation loads at
all stations are currently very high compared with Finland (highest loads around 0.2 keq ha1) and Sweden (Ruoho-Airola et al., 2003; Lo¨vblad et al., 2000).
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Fig. 5. Deposition of summed cations (keq ha1) in 2001.
sulphate concentration (meq/l)
0.4
0.3
Harku Kunda Jõhvi Tiirikoja Tooma Lääne-Nigula Saka Vilsandi Lahemaa Saare
0.2
0.1
0.0 0.0
0.1
0.2
0.3 0.4 0.5 0.6 sum of base cations (meq/l)
0.7
0.8
Fig. 6. Correlation between annual average sulphate concentration and sum of cations.
Ca2+ was the most prevailing base cation in precipitation, but in comparison with summed cations, the correlation of Ca with sulphate was weaker (0.3–0.7) (Fig. 6). The share of Ca2+ in cation equivalents varied from 44% at Nigula to 76% at Kunda. An exception was Vilsandi where the share of Na+ was larger (40%) than that of Ca2+ (32%). At Vilsandi Island Na+ originates mainly from seawater. The average sea salt fraction of deposited calcium was 1–4%, while it was 7% at Vilsandi.
Although annual mean concentrations of calcium decreased by 1.4–4.7 times during the study period, the measured loads remained rather high (Table 3), and the trend was statistically significant only at 3 out of 10 stations. This could indicate that there is an additional calcium source nearby, causing greater variation in the cation concentration of precipitation. Monthly mean concentration of Ca2+ varied greatly, especially in summer, which is probably due to a large number of gravel roads in rural Estonia.
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Remaining base cations: Na+, K+ and Mg2+ formed on average 25%, 12% and 9% of summed cations on an equivalent basis, respectively. Annual mean concentrations of Na+, K+ and Mg2+ at all stations decreased, and the trends were statistically significant in 57% of the cases (Table 1). Sodium is regarded as a sea salt element, but in our case it is emitted in fly ash along with magnesium and potassium. It is difficult to distinguish between Na that is of marine origin and fly ash Na. A significant correlation (at po0.05) was found between + monthly concentrations of SO2 4 , Cl and Na . The most variable base cation was Mg2+. The contribution of sea salt to the deposition levels of Mg in different stations was 19–77%. Annual mean concentration of Mg2+ decreased 4.3 times on average (at Kunda 5.9 times). Annual deposition of Mg2+ decreased 3.5 times between 1994–1997 and 1998–2001 (Tables 1 and 3). Extremely high values of standard deviation characterised monthly mean concentrations of Mg2+ at almost all the stations for the period 1994–1997, the only exeption being Nigula. Annual mean concentration and deposition of K+ decreased 1.7 and 1.6 times, respectively, on average (Kunda was not taken into account as the concentration levels there were 10 times higher than the national average). The sea salt contribution to K deposition was low (1–14%). As a rule, the highest monthly mean concentrations of Mg2+, Ca2+and SO2 in time series were measured 4 from January till March (i.e. in the heating season). At all stations the greatest decrease in the high monthly mean concentrations occurred since 1997 or 1998. Comparing two different regions, North and South Estonia with acid deposition critical loads, it can be said that the NE Estonia is characterised by increased base cation deposition buffering fully the acidic deposition. In Southern Estonia the actual base cation deposition corresponds to the natural background level and the deposition of acidifying compounds may reach or exceed the critical loads in some more sensitive areas (Oja, 2000).
4. Conclusions The data of monthly and annual values of main ions 2+ (SO2 , Mg2+, K+ and Na+) in bulk 4 , Cl , Ca precipitation at 10 stations during an 8-year monitoring period show in most cases a statistically significant decreasing trend. On average, there was a two-fold decline in concentrations of both anions (SO2 4 and Cl ) and cations (Ca2+, Mg2+, Na+ and K+). Even though precipitation amounts have slightly increased, the total deposition of the above-mentioned ions has decreased. NO 3 showed no trends. Due to reduced alkaline deposition, H+ concentration of precipitation near
Kunda cement factory showed a significant increasing trend, which has lead to the normalisation of pH. During the study period the quantities of emitted pollutants, SO2 and fly ash have decreased by 40% and 65%, respectively, mostly due to a decline in annually combusted oil shale amounts, i.e. due to a decrease in the production of electric energy in the power plants. The use of new purification equipment in the cement factory at Kunda has especially influenced ion concentrations in the bulk precipitation collected at the Kunda station. In order to achieve the objectives of the energy policy, which include a further reduction in air pollution in the whole area, considerable technological innovation is necessary before electricity production can be increased.
Acknowledgements Current research was supported by Estonian Ministry of Environment in the framework of the national environmental monitoring program.
References Dyni, R., 2002. Origin and resources of some world oil shale deposits. In: Seepo˜ld, M. (Ed.), Abstracts of Symposium on Oil Shale, Tallinn, 18th to 19th November 2002, 7. Tallinn. Eesti Keskkonnastrateegia (Estonian Environmental Strategy), 1997. Riigi Teataja 1(26), 390. EMEP, 1996. EMEP manual for sampling and chemical analysis. EMEP/CCC-Report 1/95, NILU, Kjeller, Norway. EMEP/MSC-W Report, 1998. Transboundary Acidifying Air Pollution in Europe. Part 2: Numerical Addendum, Oslo. Estonia’s Third National Communication under the United Nation’s Framework Convention on Climate Change, 2001. Ministry of Environment, Environmental Information Center, Institute of Ecology, Tallinn Pedagogical University, Tallinn, p. 79. Frey, J., Frey, T., Pajuste, K., 2003. Input–output analysis of macroelements in ICP-IM catchment area, Estonia. Landscape and Urban Planning 1020, 1–7. Hettelingh, J.-P., Downing, R.J., Smet, P.A.M., 1993. Maps of critical loads, critical sulphur deposition, and exceedances. Calculation and Mapping of Critical Loads in Europe: Status Report 1993. Coordination Center for Effects, The Netherlands, pp. 5–20. Kallaste, T., Liik, O., Ots, A., 1999. Possible Energy Sector Trends in Estonia. SEI, Tallinn, 240p. Karofield, E., 1996. The effects of alkaline fly ash precipitation on the Sphagnum mosses in Niinesaare bog, NE Estonia. Suo 47, 105–114. Kimmel, V., Tammet, H., Truuts, T., 2002. Variation of atmospheric air pollution under conditions of rapid economic change-Estonia 1994–1999. Atmospheric Environment 36 (25), 4133–4144.
ARTICLE IN PRESS K. Treier et al. / Atmospheric Environment 38 (2004) 7009–7019 Kulmala, A., Leinonen, L., Ruoho-Airola, T., Salmi, T., Walden, J., 1998. Air quality trends in Finland. Finnish Meteorological Institute, Helsinki, 91p. Liblik, V., Pensa, M., Kundel, H., 2000. Temporal changes in atmospheric air pollution in industrial areas of Ida- and La¨a¨ne Virumaa counties. In: Mandre, M. (Ed.), Air Pollution and Forests in Industrial Areas of North-East Estonia. Forestry studies XXXIII (summary in English) Estonian Academic Publishers, Tallinn. Lo¨vblad, G., Persson, C., Roos, E., 2000. Deposition of base cations in Sweden. Report 5119. Swedish Environmental Protection Agency, Trelleborg, 59p. Oja, T., 2000. Critical loads in Estonia. In: M. Holmberg (Ed.), Uncertainty of Critical Loads in the Baltic Countries. TemaNord 2000:544, Nordic Council of Ministers, Copenhagen. Ots, A., 1992. Formation of air-polluting compounds while burning oil shale. Oil Shale 9 (1), 63–75. Paat, A., 2002. About the mineral composition of Estonian oil shale ash. Oil Shale 19 (3), 321–333. Pajuste, K., Kimmel, V., Kohv, N., Truuts, T., 2003. Assessment of the Estonian EMEP data. In: Bartnicki, J., Lo¨vblad, G. (Eds.), EMEP Assessment Part II National Contributions. Web: http://www.emep.int/assessment/estonia.pdf Ruoho-Airola, T., Alaviippola, B., Salminen, K., Varjoranta, R., 2003. An investigation of base cation deposition in Finland. Boreal Environment Research 8, 83–95.
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Ruoho-Airola, T., Anttila, P., Salmi, T., 2004. Airborne sulphur and nitrogen in Finland-trends and exposure in relation to air transport sector. Journal of Environmental Monitoring published on the web 20th November 2003. Saare, L., Talkop, R., Roots, O., 2001. Air pollution effects on terrestrial ecosystems in Estonia. Water, Air, and Soil Pollution 130, 1181–1186. Salmi, T., Ma¨a¨tta¨, A., Anttila, P., Ruoho-Airola, T., Amell, T., 2002. Detecting trends of annual values of atmospheric pollutants by Mann–Kendall test and Sen’s slope estimates—the Exel template application MAKESENS. Publication on Air Quality No. 31. Finnish Meterological Institute, Helsinki, p. 35. Sofiev, M., Kaasik, M., Hongisto, M., 2003. Model simulations of the alkaline dust distribution from Estonian sources over the Baltic Sea basin. Water, Air and Soil Pollution 146, 211–223. Statistical Yearbook of Estonia, 2003. Statistical Office of Estonia, Tallinn, p. 121. Tarrason, L., Jonson, J.E., Fagerli, H., Benedictov, P., Wind, P., Simpson, D., Klein, H., 2003. Transboundary acidification and eutrophication and ground level ozone in Europe, part III, source–receptor relationships. EMEP Status Report 1/2003. Norwegian Meteorological Institute, Oslo.
Atmospheric Environment 38 (2004) 7009–7019 www.elsevier.com/locate/atmosenv
Recent trends in chemical composition of bulk precipitation at Estonian monitoring stations 1994–2001 K. Treiera, K. Pajustea,b,, J. Freya a
Institute of Geography, University of Tartu, Vanemuise 46, 51014, Estonia Estonian Environmental Research Centre, Marja 4D, 10617 Tallinn Estonia
b
Received 8 September 2003; received in revised form 20 April 2004; accepted 5 May 2004
Abstract 2+ , Mg2+, K+, Na+) in Monthly and annual means of main anions (SO2 4 , NO3 , Cl ) and summed base cations (Ca bulk precipitation were studied at 10 stations during an 8-year monitoring period. The data showed statistically significant decreasing trends in most cases. Average declines of mean annual volume-weighted concentrations for both anions and cations were about two-fold. Despite the decrease, the loads of S and cations are still relatively high in Estonia (about 4–14 kg S ha1 and 0.6–1.2 keq ha1, respectively) compared with the loads in Finland and Sweden. Estimated linear decline trends followed the same pattern as annually combusted oil shale from Estonian power plants and emissions of SO2 and fly ash. Recent trends in chemical composition of bulk precipitation at the monitoring stations reflected economic changes in Estonia as well as transboundary fluxes from neighbouring countries. r 2004 Elsevier Ltd. All rights reserved.
Keywords: Atmospheric deposition; Oil shale emissions; Precipitation chemistry; Monitoring; Concentrations; Loads; Economic change
1. Introduction Estonia is a small country (area 45 227 km2, population about 1.5 million) that was occupied by the Soviet Union until 1991. Restoration of independent statehood initiated rapid economic change in 1990s—decreased use of fertilizers, increased areas of abandoned agricultural land, a 44% growth in the number of automobiles and a decrease in energy production. These changes have had an impact on the air pollution situation. Currently, air pollution problems in Estonia are still mostly due to oil shale combustion (Saare et al., 2001).
Corresponding author. Estonian Environmental Research Centre, Marja 4D, 10617 Tallinn, Estonia. Fax: +372 611 2901. E-mail address: [email protected] (K. Pajuste).
In Estonia 90% of electricity generation and one fourth of heat production is based on oil shale (Statistical Yearbook of Estonia, 2003). Estonian oil shale is rather unique, its reserves are the largest commercially exploited deposits in the world (Dyni, 2002). Estonian oil shale as a fuel is characterised by high ash share (45–50%), moderate moisture (11–13%) and sulphur (S) content (1.4–1.8%), and low heating value (net heating value of moist fuel 8.5–9 MJ kg1). One of the peculiarities of Estonian oil shale is chlorine (0.75%) combined with organic matter. The estimated annual emission of HCl in 1991 was about 15 000 t (Ots, 1992). As Estonian oil shale is rich in mineral matter (the basic mineral components of the carbonate part are calcite and dolomite), the amount of ash formed by the combustion of oil shale in Estonian power plants is
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.05.061
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where bigger cities and heavier polluters like power plants, oil shale mines, chemical industries and the cement factory of Kunda are situated. The Estonian network of precipitation monitoring (10 stations) was started in 1994. The first stations were situated mainly in seaside areas, starting from Vilsandi, Estonia’s westernmost island, continuing along Estonia’s northern coast up to the industrial area in the North East. As background stations Vilsandi, Lahemaa and Saareja¨rve are part of the international monitoring network, the data collected there are forwarded, within the framework of the Convention on Long-Range Transboundary Air Pollution (CLTRAP), to the databases of EMEP (Cooperative Programme for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe) and the International Cooperative Programme on Integrated Monitoring. Since 1999 new monitoring stations in South-Estonia have been added to the local precipitation network. As a result, for the first time in history it is possible to estimate the level of air pollution on most of Estonia’s territory on the basis of a uniform methodology. The deposition data measured before 1994 are unfortunately not available, since data concerning air pollution were confidential in the Soviet Union. After regaining independence in 1991, environmental monitoring underwent reorganisation, new methods and equipment were introduced, and since 1994 there exists a comparable database for precipitation compounds. The
enormous. Due to rather high chimneys (150 and 300 m), the exhaust gases are distributed over a large territory even under conditions of still air (Paat, 2002). The atmospheric emissions of 4 oil shale-fired plants in North-Eastern Estonia have become a cause of concern in Estonia as well as in the neighbouring countries, Russia and Finland (Sofiev et al., 2003). Oil shale power plants have comparatively high emission of sulphur dioxide (SO2) in spite of the binding capacity of SO2 by ash when the gas passes through the boilers. Approximately 15–20% of the total sulphur in the oil shale goes into the stack as SO2 (Kallaste et al., 1999). The power plants utilised about 26 Mt of oil shale per annum in the 1980s, producing about 200 000 t of fly ash and 250 000 t SO2 according to expert estimations (Liblik et al., 2000). Economic changes with decreasing industrial production and collapse of export market for electricity have reduced oil-shale based energy production substantially (Fig. 1). In the year 2000 the amount of oil shale consumed was 9.2 Mt, and the corresponding emissions were 59 500 t of fly ash and 79 900 t of SO2 (Estonia’s Third National Communication Under the United Nation’s Framework Convention on Climate Change, 2001). The drop in emissions is a result of economic changes, transition to EU regulations of air emissions, and increase in pollution fines controlled by the government (Kimmel et al., 2002). In spite of the reduced emissions, till now the most polluted region is Northern Estonia
SO2 from manufacturing industry SO2 from energy and transformation industry total emission of SO2
0
0
oil shale megatons
5
2001
50
2000
10
1999
100
1998
15
1997
150
1996
20
1995
200
1994
25
1993
250
1992
30
1991
300
1990
35
1985
350
1980
primary energy PJ
SO2, fly ash kilotons
excavated oil shale primary energy fly ash from power plants
Fig. 1. Annually excavated oil shale (Mt), primary energy supply (PJ) and emissions of SO2 and fly ash (kt) from power plants (Statistical Yearbook of Estonia, 2003).
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aim of this paper is to assess the changes and trends of acidic anion and base cation concentrations in the available data of bulk precipitation during the period 1994–2001.
2. Material and methods In Estonia the average annual amount of precipitation is 530–730 mm and the average annual air temperature is 5.3 1C. The warmest month is July, August has the largest amount of monthly rainfall and February is the coldest and driest. Yearly sum of precipitation has slightly increased during the assessment period, which is mainly due to relatively warmer winters that are rich in precipitation. The difference between the average precipitation amounts of 1994–1997 compared to the period of 1998–2001 is about 100 mm. Precipitation was collected by bulk collectors (20 cm in diameter), placed in an open area at a height of 120 cm. Samples were collected on a 24-h basis but they may still contain a certain amount of dry deposition. Collected samples were stored in refrigerators and mixed in proportion to the total sample volume for monthly samples before analysis, as described in the EMEP manual (EMEP, 1996). Ca2+, Mg2+, K+, Na+, SO2 4 , NO concentrations in water samples were 3 and Cl determined by ion chromatography (EN–ISO 14911 and EN–ISO 10304). NH+ 4 was analysed by spectrophotometry. Acidity (pH) and electric conductivity of samples were also measured. Samples were analysed at internationally accredited laboratories of the Estonian Environmental Research Centre in Tallinn and the Environmental Studies Laboratory in Tartu. Bulk concentrations reported in the tables are precipitation-weighted averages, which are not corrected for sea salts. Although some stations are situated by the sea, sea salt influence is not considered relevant here as the assumption is that the proportion of sea salt contribution has not changed over the assessment period. Deposition was calculated by multiplying annual bulk concentrations by measured precipitation amounts at the same stations. Dry deposition was not measured separately nor included in the deposition estimates. Thematic deposition maps were produced with MapInfo Professional 6.0 and with Surfer 7. The Spearman’s nonparametric correlation analysis was used for the comparison of monthly mean concentrations between stations and between different components. The nonparametric Mann–Kendall test was used for detecting trends in the time series of precipitation-weighted annual means of precipitation concentrations, and the slope of a linear trend was estimated with the nonparametric Sen’s method (Salmi et al., 2002).
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3. Results and discussion 3.1. Acidic anions Data concerning the major ions are shown in Table 2. At all stations the prevailing anion in bulk precipitation was SO2 4 , which formed more than 50% of anionic composition on an equivalent basis (Vilsandi was an exception with only 39%), the anionic proportion of Cl and NO varied between 21–39% and 16–25%, 3 respectively, on an equivalent basis. Sea salt correction (ratio of sodium to sulphate in seawater) shows that anthropogenic sulphur in precipitation constitutes a major fraction (95%) of sulphate in most stations. In two westward stations (Vilsandi, Nigula) the marine fraction of sulphate reached 10% on average. Data for the 8-year study period show linear decreasing trends in sulphate and chloride concentrations at nearly all the stations (Fig. 2, Table 1). The negative linear trend for sulphate was statistically significant at all stations (SO2 slope estimates, i.e. 4 change per year varied from –5.7 mg l1 at Kunda to –0.24 mg l1 at Vilsandi). At most stations (except Harku) the decline of chloride was significant as well, slope estimates varied from –1.3 mg l1 at Kunda to –0.089 mg l1 at Tooma. Monthly concentrations of both anions were positively correlated (at po0.05) with one another, the highest correlations were found in NE stations (r=0.69–0.78). Although chloride is usually of marine origin, in our case both SO2 4 and Cl anions are the main acidic components of flue gases of oil shale. Linear decline trends of SO2 4 and Cl annual concentrations followed the same pattern as annual SO2 emission and annually combusted oil shale amounts. The decrease of sulphate concentration by 1.5–2.7 times and that of chloride by 1.3–3.3 times followed the decline pattern of annually combusted oil shale (1.3 times during 1994–2000, 119 847 and 92 489 TJ yr1 accordingly, Fig. 1). The fact that monthly concentrations of both anions were positively correlated between stations suggests that the power plants located in North–Eastern Estonia are probably an important pollution source having an effect on the deposition content of a significant area of the country (Figs. 3 and 4). In the last years the monthly concentrations of SO2 4 and Cl have reached the lowest values throughout the measuring period (Table 2) coinciding with lowest emissions from the power plants during the study period. The turning point of the trend in SO2– 4 time series occurred at most stations in 1997. Spearman’s nonparametric correlation between stations showed the highest coeficents in NE stations (r=0.7–0.8). Southern stations may be more influenced by pollution transport from neighbouring areas, but still the correlations between NE and S stations were significant (r=0.4–0.6). Over the
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1 Fig. 2. Annual volume-weighted mean concentrations of SO2 4 , Cl , NO3 and summed cations (meq l ). Note the difference in scales.
last 3 years the annual mean deposition of total sulphate has ranged between 4 and 14 kg ha1 yr1 (Fig. 3). In order to protect sensitive natural ecosystems, a long-
1 yr1 term goal of SO2 4 deposition should be 6 kg ha (Hettelingh et al., 1993). For Estonia it means an annual mean SO2 concentration of o1.5 mg l1. In order to 4
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Table 1 Significant decreasing trends (1994–2001) of annual mean concentrations in precipitation, estimated by Mann–Kendall nonparametric test Location
SO2 4
Cl
Ca2þ
K+
Mg2+
Kunda (NE) Jo˜hvi (NE) Saka (NE) Harku (N) Lahemaa (N) Tiirikoja (E) Tooma (E) Saareja¨rve (E) Nigula (W) Vilsandi (W)
*** ** * * ** ** * * * **
* ** *
***
*** *
*** **
*
** ** **
* ***
*
**
** * ** * * **
** **
* * *
Na+
Sum of cations
** *
*** * * ** *** * *
Significance level ***0.001; **0.01; *0.05.
achieve this level, further reduction of SO2 emission is necessary. The Estonian Environmental Strategy has set a goal of decreasing the emission of SO2 to the level of 50 000 t from the current level by the year 2005 (Eesti Keskkonnastrateegia, 1997). The deposition rates of sulphur do not result from local emissions only but are also influenced by longrange transported oxidised sulphur compounds. According to calculations based on the Unified EMEP model (Tarrason et al., 2003), 19 countries and international sea shipping are responsible for 90% of imported sulphur deposition in 1994–2001. The average decline of total SO2 emissions from all emission areas contributing to the long-range transport to Estonia has been about 40%. Reduction in domestic SO2 emissions has been the same. Earlier model estimates (EMEP/MSC-W Report, 1998) of source–receptor relationships of SO2 show that foreign contribution dominates over local sources in many countries. In Estonia 19% of the total annual sulphur deposition in 1998 and 12% in 2000 came from domestic sources. In neighbouring countries, Lithuania, Latvia, Finland and Sweden, the domestic share to sulphur deposition was 8.5%, 4.8%, 5.4% and 7.1%, correspondingly, in 2000 (Tarrason et al., 2003). The data of our study suggest that local contribution to total deposition of oxidised sulphur may be bigger than estimated by EMEP models. Comparing S load at the remote Vilsandi Island with N, NE and E stations suggests that at least till 1997 about 14 territory of Estonia (see Tallinn-Jo˜geva-Tartu line on the map) was mostly influenced by locally originating oxidised sulphur (SO2 4 ) (Fig. 3 and Table 3). In 1994–2001 there was no statistically significant declining trend in air SO2 concentrations at the background stations. It could be suggested that a rapid decline in SO2 concentration in Estonia occurred before the beginning of the study period during the first half of 1990s. During the last 3 years (1999–2001), average
SO2–S concentration has been 0.4 mg m3 at the remote island of Vilsandi and 0.7 mg m3 at the northerly located Lahemaa background station. The directions of air masses with higher SO2 concentrations are related to the location of the measurement stations. At the western border of Estonia (Vilsandi station) higher concentrations are measured from southern and south western air mass transport directions. At the Lahemaa station, high concentrations are observable from sectors between NE and SE where big power plants are located (Pajuste et al., 2003). Throughout the study period NO 3 was the most stabile anion, its annual mean concentration shows a statistically significant decreasing trend at Lahemaa station only. The mean annual concentration varied between 0.3–0.6 mg l1 at different stations and the NO3–N load was, on average, 2 kg ha1 at Tiirikoja, Harku, Vilsandi, Lahemaa, Tooma and Saareja¨rve and 3 kg ha1 at Jo˜hvi, Kunda, Nigula and Saka.
3.2. Base cations Cations originate from natural sources such as sea salt (Mg and Na) and soil dust (Ca, Mg, K and Na), as well as from oil-shale fly ash (Ca, Mg, K and Na). The base cations are important nutrients for forests (RuohoAirola et al., 2003; Frey et al., 2003), but in case of high deposition, long-term cumulative loads may damage the ecosystems of ombrotrophic raised bogs (Karofield, 1996). The deposition of base cations neutralises the acidic deposition at a majority of the stations (only at Vilsandi and Lahemaa the average annual mean pH values of the study period were acidic 4.7 and 4.9, respectively). The annual mean pH of precipitation did not change in most cases during the monitoring period. The only statistically significant decreasing trend was estimated at
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Fig. 3. Calculated annual bulk deposition of sulphur (kg S ha1) in 1996 (A) and 2001 (B).
Kunda, where annual mean pH of precipitation dropped from 7.5 to 6.7. The decreasing trends of annual mean concentrations of summed base cations (the change per year varied from 0.44 meq l1 at Kunda to 0.017 meq l1 at Lahemaa) were statistically significant at 7 stations out of 10, (Table 1). At Vilsandi, Tiirikoja and Harku the decreasing trend of summed base cations was not statistically significant. At most stations, the correlations of summed cations were higher with the NE stations. Correlation between
summed cations and suphate was stronger at the NE stations (r=0.75–0.85) and weaker at the E stations (r=0.47–0.49). Fig. 6 presents the relation between annual average sulphate concentrations and summed cations at the monitoring stations during the study period. Average ratio of sulphate and summed cations is between 1.5–3.0, except for near the Kunda cement factory where the ratio is around 5. The deposition of summed base cation in bulk precipitation decreased about 2 times at NE stations
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Fig. 4. Calculated annual bulk deposition of chloride (kg ha1) in 1996 (A) and 2001 (B).
(at Kunda 3.9 times) and about 1.5 times at all others stations during the study period. Base cation declines are in good accordance with emission data of solid particles from stationary sources that decreased 2.9 times (161 500 t in 1994 and 56 400 t in 2001). There was a 1.9-fold decrease of fly ash emission from the two biggest power plants (85 600 t in 1994 and 45 000 t in 2001) during the monitoring period. In addition, in 1997 new particle emission controls were installed at the cement factory in Kunda. As a result, emission of solid particles decreased 8 times in 1997–2001.
As has been shown by model calculations (Sofiev et al., 2003), about 30% of annual local alkaline emissions are deposited to Estonia and the Gulf of Finland. The high-deposition area, affected by local sources, is about 200 200 km2. Thereby the influence of Estonian emissions is reflected in Finnish monitoring stations (Kulmala et al., 1998; Ruoho-Airola et al., 2003, 2004). The time series of annual mean concentrations of SO4–S, Ca2+ Mg2+ and K+ at a Finnish south eastern station shows comparable significant decreasing trends during 1980–2000.
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Table 2 Volume-weighted mean concentrations (mg ll, except H+meq ll) and standard deviation (in parenthesis) from monthly data for the period of 1994–1997 and 1998–2001 Location
Study period
H+
SO2 4
Cl
Kunda (NE)
1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001
0.05 (0.04) 0.15 (0.08) 0.63 (1.5) 0.65 (0.5) 0.7 (16) 4.2 (6) 1.7 (3.6) 2.3 (4.1) 11.2 (11) 15.8 (14) 1.8 (4.1) 1.3 (2.3) 5.4 (15) 7.9 (11) 3.3 (9) 7.2 (24) 1.1 (1.1) 4.0 (4.3) 30 (32) 20 (15)
28.9 4.6 10.0 6.7 7.9 5.5 7.4 2.4 2.8 1.4 5.2 2.9 3.1 1.8 4.7 1.7 3.2 2.1 2.9 1.8
8.5 1.6 7.8 2.4 2.3 1.1 2.1 1.3 1.3 0.5 1.8 1.3 0.9 0.6 1.2 0.5 2.2 1.4 3.1 1.4
Jo˜hvi (NE) Saka (NE) Harku (N) Lahemaa (N) Tiirikoja (E) Tooma (E) Saareja¨rve (E) Nigula (W) Vilsandi (W)
(26.5) (3.4) (7.3) (5.5) (5.4) (7.0) (1.5) (1.3) (1.2) (0.5) (2.8) (2.0) (1.8) (1.1) (3.5) (0.7) (1.7) (0.9) (6.7) (0.7)
(9.1) (1.1) (7.9) (1.4) (2.5) (1.0) (1.6) (1.4) (1.5) (0.3) (1.0) (0.8) (0.9) (0.4) (0.8) (0.3) (1.2) (0.7) (2.9) (1.4)
NO 3
Ca2þ
Na2+
Mg2+
K+
3.1 2.4 3.1 2.4 3.2 1.5 1.8 1.1 3.4 1.5 1.9 1.6 1.7 1.0 1.8 1.2 2.1 2.3 3.4 1.5
33.5 (15.5) 7.4 (4.7) 3.6 (3.3) 2.7 (1.9) 3.4 (3.4) 1.9 (2.7) 3.0 (2.5) 2.2 (2.6) 1.3 (1.4) 0.6 (0.2) 2.3 (2.4) 1.1(0.9) 1.3 (1.2) 0.6 (0.7) 2.1 (1.6) 1.4 (0.6) 1.6 (1.2) 0.9 (0.5) 0.6 (0.4) 0.4 (0.2)
3.8 0.8 4.8 1.3 1.2 0.4 1.0 0.6 0.4 0.2 1.0 0.7 0.4 0.2 0.9 0.5 1.4 0.7 1.2 0.7
3.4 (1.98) 0.57 (0.25) 1.25 (1.37) 0.29 (0.09) 0.4 (0.1) 0.23 (0.09) 1.1 (1.68) 0.18 (0.03) 0.2 (0.21) 0.06 (0.02) 1.55 (0.38) 0.14 (0.2) 0.58 (0.67) 0.14 (0.06) 0.61 (0.48) 0.28 (0.02) 0.18 (0.02) 0.1 (0.03) 0.48 (0.64) 0.11 (0.03)
10.4 (6.56) 0.78 (0.29) 1.56 (0.31) 0.76 (0.29) 1.1 (0.64) 0.56 (0.17) 0.41 (0.24) 0.13 (0.06) 0.21 (0.08) 0.1 (0.04) 0.85 (0.33) 0.65 (0.06) 0.31 (0.11) 0.1 (0.02) 0.49 (0.16) 0.48 (0.14) 0.73 (0.04) 0.45 (0.76) 0.42 (0.27) 0.24 (0.14)
(4.2) (2.7) (2.9) (2.8) (3.4) (1.7) (1.7) (1.6) (1.0) (0.8) (1.6) (1.6) (1.3) (1.0) (3.3) (1.4) (2.4) (1.9) (2.1) (1.1)
(4.7) (0.6) (6.3) (0.8) (1.7) (0.5) (0.8) (0.8) (0.3) (0.2) (0.5) (0.5) (0.3) (0.2) (1.2) (0.3) (0.8) (0.4) (1.0) (0.7)
Table 3 Mean annual precipitation amounts (mm), deposition of major ions (kg ha1), summed cations (keq ha1) and H+ (k eq ha1) for periods 1994–1997 and 1998–2001 Location
Study period
Prec. mm
H+ eq ha1
SO4–S
Cl
NO3–N
Ca
Mg
Na
K
Summed cations
Kunda (NE)
1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001 1994–1997 1998–2001
421 578 686 798 337 516 679 712 569 691 584 678 626 716 497 719 622 690 358 530
0.2 0.9 4.3 5.2 2.2 21.7 11.4 16.4 60 109 10.4 8.8 34.1 56.6 16 52 6.6 27.7 112 106
43.17 8.82 22.75 17.81 9.03 9.11 16.13 5.78 4.13 3.15 10.31 6.67 6.28 4.34 7.52 4.13 6.74 5.50 3.63 3.19
35.75 9.30 51.43 18.95 7.84 5.79 13.53 9.18 4.56 3.90 10.70 8.78 5.64 4.02 5.72 4.00 13.61 9.16 12.29 7.25
2.99 3.20 4.76 3.97 2.54 1.65 2.52 1.71 1.51 1.61 2.46 2.44 2.25 1.70 2.08 1.96 2.94 3.69 1.38 1.97
145.3 42.08 24.22 21.08 11.53 9.29 18.99 15.45 5.72 4.95 13.42 7.52 8.01 4.43 10.17 10.32 9.77 5.68 2.14 2.05
15.03 3.30 8.13 2.27 1.35 1.26 6.45 1.28 0.83 0.40 9.38 0.89 3.34 0.93 2.61 2.04 1.09 0.72 1.99 0.65
16.20 4.58 31.51 10.13 3.97 2.11 6.80 4.59 4.99 4.58 5.68 4.87 2.68 1.47 3.99 3.74 8.91 5.05 4.40 3.50
46.81 4.00 10.50 6.16 3.59 2.97 2.74 0.91 0.95 0.81 5.00 4.39 1.86 0.73 2.29 3.21 4.52 3.10 1.50 1.43
10.4 2.7 3.5 1.8 1.0 0.7 1.8 1.1 0.5 0.4 1.8 0.8 2.2 0.8 1.0 0.9 1.1 0.2 0.5 0.3
Jo˜hvi (NE) Saka (NE) Harku (N) Lahemaa (N) Tiirikoja (E) Tooma (E) Saareja¨rve (E) Nigula (W) Vilsandi (W)
Fig. 5 shows the loads of base cations in 2001. Lowest loads reached the level of 0.4 keq ha1 in the west, and the highest ones fluctuated around 1–1.6 keq ha1 at NE stations. Base cation loads at
all stations are currently very high compared with Finland (highest loads around 0.2 keq ha1) and Sweden (Ruoho-Airola et al., 2003; Lo¨vblad et al., 2000).
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Fig. 5. Deposition of summed cations (keq ha1) in 2001.
sulphate concentration (meq/l)
0.4
0.3
Harku Kunda Jõhvi Tiirikoja Tooma Lääne-Nigula Saka Vilsandi Lahemaa Saare
0.2
0.1
0.0 0.0
0.1
0.2
0.3 0.4 0.5 0.6 sum of base cations (meq/l)
0.7
0.8
Fig. 6. Correlation between annual average sulphate concentration and sum of cations.
Ca2+ was the most prevailing base cation in precipitation, but in comparison with summed cations, the correlation of Ca with sulphate was weaker (0.3–0.7) (Fig. 6). The share of Ca2+ in cation equivalents varied from 44% at Nigula to 76% at Kunda. An exception was Vilsandi where the share of Na+ was larger (40%) than that of Ca2+ (32%). At Vilsandi Island Na+ originates mainly from seawater. The average sea salt fraction of deposited calcium was 1–4%, while it was 7% at Vilsandi.
Although annual mean concentrations of calcium decreased by 1.4–4.7 times during the study period, the measured loads remained rather high (Table 3), and the trend was statistically significant only at 3 out of 10 stations. This could indicate that there is an additional calcium source nearby, causing greater variation in the cation concentration of precipitation. Monthly mean concentration of Ca2+ varied greatly, especially in summer, which is probably due to a large number of gravel roads in rural Estonia.
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Remaining base cations: Na+, K+ and Mg2+ formed on average 25%, 12% and 9% of summed cations on an equivalent basis, respectively. Annual mean concentrations of Na+, K+ and Mg2+ at all stations decreased, and the trends were statistically significant in 57% of the cases (Table 1). Sodium is regarded as a sea salt element, but in our case it is emitted in fly ash along with magnesium and potassium. It is difficult to distinguish between Na that is of marine origin and fly ash Na. A significant correlation (at po0.05) was found between + monthly concentrations of SO2 4 , Cl and Na . The most variable base cation was Mg2+. The contribution of sea salt to the deposition levels of Mg in different stations was 19–77%. Annual mean concentration of Mg2+ decreased 4.3 times on average (at Kunda 5.9 times). Annual deposition of Mg2+ decreased 3.5 times between 1994–1997 and 1998–2001 (Tables 1 and 3). Extremely high values of standard deviation characterised monthly mean concentrations of Mg2+ at almost all the stations for the period 1994–1997, the only exeption being Nigula. Annual mean concentration and deposition of K+ decreased 1.7 and 1.6 times, respectively, on average (Kunda was not taken into account as the concentration levels there were 10 times higher than the national average). The sea salt contribution to K deposition was low (1–14%). As a rule, the highest monthly mean concentrations of Mg2+, Ca2+and SO2 in time series were measured 4 from January till March (i.e. in the heating season). At all stations the greatest decrease in the high monthly mean concentrations occurred since 1997 or 1998. Comparing two different regions, North and South Estonia with acid deposition critical loads, it can be said that the NE Estonia is characterised by increased base cation deposition buffering fully the acidic deposition. In Southern Estonia the actual base cation deposition corresponds to the natural background level and the deposition of acidifying compounds may reach or exceed the critical loads in some more sensitive areas (Oja, 2000).
4. Conclusions The data of monthly and annual values of main ions 2+ (SO2 , Mg2+, K+ and Na+) in bulk 4 , Cl , Ca precipitation at 10 stations during an 8-year monitoring period show in most cases a statistically significant decreasing trend. On average, there was a two-fold decline in concentrations of both anions (SO2 4 and Cl ) and cations (Ca2+, Mg2+, Na+ and K+). Even though precipitation amounts have slightly increased, the total deposition of the above-mentioned ions has decreased. NO 3 showed no trends. Due to reduced alkaline deposition, H+ concentration of precipitation near
Kunda cement factory showed a significant increasing trend, which has lead to the normalisation of pH. During the study period the quantities of emitted pollutants, SO2 and fly ash have decreased by 40% and 65%, respectively, mostly due to a decline in annually combusted oil shale amounts, i.e. due to a decrease in the production of electric energy in the power plants. The use of new purification equipment in the cement factory at Kunda has especially influenced ion concentrations in the bulk precipitation collected at the Kunda station. In order to achieve the objectives of the energy policy, which include a further reduction in air pollution in the whole area, considerable technological innovation is necessary before electricity production can be increased.
Acknowledgements Current research was supported by Estonian Ministry of Environment in the framework of the national environmental monitoring program.
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