Long-Term Temporal Changes of Precipitation Quality in Slovak Mountain Forests

: The paper is focused on the evaluation of long-term changes in the chemical composition of precipitation in the mountain forests of Slovakia. Two stations with long-term measurements of precipitation quality were selected, namely the station of the EMEP (European Monitoring and Evaluation Programme) network Chopok (2008 m a.s.l.) and the station of the ICP Forests (International Co-operative Programme on Assessment and Monitoring of Air Pollution E ﬀ ects on Forests) network Pol’ana-Hukavsk ý gr ú ˇn (850 m a.s.l.). All basic chemical components were analyzed, namely sulfur (S-SO4), nitrogen (N-NH4, N-NO3), and base cations (Ca, Mg, and K) contained in precipitation. The time changes of the individual components were statistically evaluated by the Mann–Kendall test and Kruskal–Wallis test. The results showed signiﬁcant declining trends for almost all components, which can signiﬁcantly a ﬀ ect element cycles in mountain forest ecosystems. The evaluated forty one-year period (1987 to 2018) is characterized by signiﬁcant changes in the precipitation regime in Slovakia and the obtained results indicate possible directions in which the quantity and quality of precipitation in the mountainous areas of Slovakia will develop with ongoing climate change.


Introduction
The Slovak Republic is a party of the UN ECE (United Nations Economic Commission for Europe) Convention on Long-range Transboundary Air Pollution. The Implementing Protocols have been progressively adopted to this Convention which, among other things, has been designated by the parties to the Convention to reduce the anthropogenic emissions of pollutants involved in global environmental problems. The Global Environment Report in Europe [1], published by the European Environment Agency, indicates that emissions of acidifying substances have declined significantly since 1990, mainly in Central and Eastern Europe due to economic restructuring. The reduction in Western Europe is mainly related to changes in fuel use, desulfurization, and denitrification of combustion gases and the introduction of three-way catalytic converters in cars. Due to significant emission reductions 5.8 • C, during the vegetation period (April-September) 11.9 • C. A more detailed climatic characteristic is given by the climate chart ( Figure 2).

Precipitation Quality Data
Source data for this article has been obtained for Chopok public data available in the EMEP database. For Polana, the data obtained from the Polana-Hukavsky Grúň Research and Demonstration Facility (VDO) was established by the Forest Research Institute in Zvolen in the spring of 1991. Since 1992, the basic components of precipitation quality have been monitored. It takes place in the weekly measurement and sampling cycle during the summer and the two-week cycle in the winter period is observed in 80% of cases.
The design of precipitation water sampling and the implementation of chemical analyzes of precipitation were performed in accordance with the EMEP manual [14] for the Chopok station and the ICP Forests manual [15] for the Polana-Hukavsky grun station. Chemical analyzes were performed in accredited laboratories of SHMI (Chopok) and Forest Research Institute in Zvolen (Polana) in accordance with the cited manuals. Both laboratories have been involved in international ring testing programs and related quality programs (quality assurance/quality control).
At the Chopok station, total precipitation was measured with a standard METRA rain gauge with wind protection with a collected area of 500 cm 2 , samples for chemical analyzes were collected into plastic PET containers with a collected area of 500 cm 2 , in a wet-only version for summer period  and bulk version for winter period. At the Polana site, precipitation totals were measured with a  Hellman's rain gauge with a collected area of 200 cm 2 , samples for chemical analyzes were collected  into PET bulk collectors (3 pieces with an individual collected area of 200 cm 2 ). The precipitation quality measurements at Polana site have been changed due to organizational reasons and therefore the statistical analyzes have been performed up to 2014 year.

Data Analysis and Statistical Methods
The sulfates, nitrates, and ammonium have been recalculated according to the atomic weights to sulfur and nitrogen concentrations.
In the evaluation, individual statistical characteristics are evaluated for annual mean values at monitored stations. Annual weighted means of concentrations shall be assessed. For detecting and estimating trends of annual means have been used the non-parametric Mann-Kendall Test. It is the test of a monotonic increasing or decreasing trend. It is standard method when occur missing values and when data are not normally distributed. Sen's method can be used in cases where the trend can be assumed to be linear. It is an estimator used to quantify the magnitude of potential trends. Thus, Sen's slope is used to estimate the percent reduction in the concentration level while the Mann-Kendall test is used to indicate the significance level of the trend [5,6,16,17]. This test has been used for various studies on long-term environmental and economic impacts [18][19][20][21].
Decrease of concentration was calculated from differences of the first three and the last three years averages (depending on availability of measured data) [8]. The rate of decrease of the concentration of the analyzed element in precipitation was expressed on the basis of the regression coefficient "a" of the linear trend from the equation y = a.x + b.
The non-parametric Kruskal-Wallis test was performed by comparing the weighted averages of the stated values from the open areas between the stations Chopok and Pol'ana.

Precipitation
In the monitored period 1978-2018, the annual precipitation total was recorded at the Chopok regional station, which varied in the range of 840-1590 mm, as pointed out in Figure 3. The maximum was recorded in 2014. From the statistical assessment by the Mann-Kendall test and the graphical representation of the values, it is clear that in the long run there is a slight increase in precipitation totals at a rate of 8.8 mm of atmospheric precipitation per year. Total precipitation has been recorded in Pol'ana since 1993. During the analyzed 21-year period, precipitation totals did not show large fluctuations. The maximum total precipitation was recorded in 2010 at the level of 1270 mm. On average, precipitation at a given locality reaches 886 mm per year. Statistical analysis of precipitation total data obtained from this locality did not confirm the significance of the time trend. Using the Kruskal-Wallis test, we proved a statistically significant difference between these two localities in the years from 1993 to 2018 (see Table 1).

pH Values and H + Concentrations
Evaluation of temporal changes of pH values can be quantified only in the case of long-term stable observations performed at the same site. Figure 4 shows changes in pH and H + values during the observed period for Chopok 1978-2018 and Pol'ana 1993-2013. The Mann-Kendall test confirmed that the acidity of atmospheric precipitation in the observed time period shows a significant trend (see Table 1). At both studied localities, the pH value gradually increases slightly. As the pH increases, the concentration of hydrogen ions H + decreases naturally. This trend is mainly due to a decrease in the concentrations of major acidifying ions such as sulfate ions, nitrate ions. The decrease of these ions is evident from the following graphical representations ( Figure 4). The time series and pH trend over a longer period indicate a decrease in acidity not only in Slovakia, but also in Europe [12,18,22]. The pH values correspond well with the pH values according to the EMEP maps (SHMÚ 2015). At the 22 sites within EMEP with long term pH measurements from 1980 to 2009 the average decrease in H+ concentration was 74% [5]. The increase of precipitation pH value in recent years in bulk precipitation and throughfall has been explained by a greater decrease in acidic anion concentrations [23].
The lowest pH in Europe is observed in the Eastern part of the continent which has relatively high sulfate deposition and a low base cation deposition [5]. Sicard et al. [24] published the pH values in precipitation with a significant decreasing trend of −0.025 ± 0.02 unit pH year −1 in France.

Sulfur and Nitrogen
A basic overview of the weighted annual concentrations of acidic elements (S-SO 4 , N-NO 3 , and N-NH 4 ) separately for the station Chopok and Pol'ana is shown in Figure 5. At both monitored stations, sulfate ions dominate in the rainwater. This is confirmed by work [25], that they obtained much higher mean values of sulfur deposition compared to the nitrate-nitrogen deposition-by more than 4-times in case of the open area in mountains in central part of Slovakia. For the main air pollutants, the largest reductions across the EU-28 (in percentage terms) since 1990 have been achieved for SO x emissions (which decreased by 87%), followed by CO (−66%), NMVOCs (−59%), NO x (−54%), and NH 3 (−27%) [1].
Sulfate  Table 2).   [1] between 1990 and 2013, SO x emissions dropped in the Slovakia by 90% and in the EU-28 by 87% [1]. In 1990 the highest sulfur deposition areas were found in the central-east European areas in countries such as Germany, Poland, Czech Republic, and Slovakia. At year 2006 the highest load, although lower than previously, are found in eastern European countries such as Bulgaria, Romania, Serbia, and Bosnia and Herzegovina [4,26].
In France [24] the concentrations of SO 4 2− and nss−SO 4 2− (nss-non see salt) concentrations in precipitation have a significant decreasing trend, −3.0 ± 1.6 and −3.3 ± 0.6% year −1 , respectively, corresponding with the downward trends in SO 2 emissions in France (−3.3% year −1 ). A good correlation (R 2 = 0.84) between SO 2 emissions and nss−SO 4 2− concentrations was obtained. The decreasing trend of NH 4 + was more significant (−5.4 ± 5.2% year −1 ) than that of NO 3 − (−1.3 ± 2.4% year −1 ). In Latvia, the SO 4 -S ion concentrations in bulk precipitation also showed a significant negative linear trend [23]. Nitrates are involved in precipitation acidity to a lesser extent than sulfates [11,27]. The results of the analysis of time changes of nitrates concentrations in precipitation showed a statistically significant trend with decreasing tendency, as is evident from Figure 5 and Table 2. The rate of decrease in N-NO3 concentration in both stations is approximately the same, at Chopk is 0.0116 mg·l −1 per year and at Pol'ana −0.0125 mg·l −1 per year. The precipitation content of nitrates does not change as fast as the sulfate content.
At Chopok, nitrate concentrations decreased by 41.0% between 1985 and 2018 and 45.01% at Pol'ana for 1993-2013. Between 1990 and 2013, NO X emissions dropped in the Slovakia by 65% and in the EU-28 by 54% [1]. The reduction of NO X emissions in Europe from 1990 to 2009 were mainly caused by a change from burning of coal and gas to nuclear power. In Eastern Europe increased NO X emissions from road traffic after 2000. On the other hand, NO X emissions from traffic in Western European decreased, even though fuel consumption increased [5].
Monitoring of Ammonium ion in rainfall water Chopok started in 1994 and one year later at Pol'ana. At Chopok, the annual weighted mean of N-NH 4 concentrations ranged from 0.29 to 1.34 mg·l −1 to the maximum value recorded in 1995. The maximum at Pol'ana was measured in 1996 with a value of 1.02 mg·l −1 and a minimum in the year 2010 0.28 mg·l −1 . A statistically significant trend (p < 0.0001) ( Table 2) with a decreasing trend for Chopok −0.0206 mg·l −1 nitrogen per annum and Polana −0.0311 mg·l −1 is observed from the analysis of time changes of ammonia nitrogen concentrations in precipitation at both stations.

Base Cations (Mg 2+ , Ca 2+ , K + )
A basic overview of the weighted annual concentrations of base cations (Ca, Mg, and K) separately for the station Chopok and Pol'ana is shown in Figure 6. In Chopok, the measurement and determination of the concentration of basic cations in precipitation began in 1992, and a year later the measurement of the concentrations of these cations also began in Pol'ana. At the Chopok station, for all the basic cations mentioned, the maximum value of the weighted annual concentration was recorded in 1995 for Ca 1.83 mg·l −1 , Mg 0.33 mg·l −1 , and K 0.7 mg·l −1 . A comparison of the concentration of basic cations in precipitation between stations shows mostly higher values at Pol'ana compared to EMEP station Chopok. In this case, since basic cations are the main component of terrestrial dust, the difference in the methods of measuring "wet-only" (rain gauges) vs. "bulk" (rainwater collectors) [28] is evident. In addition, basic cations are produced mainly in the production of building materials, as well as from local dust (roads, stone processing, etc.), which are more localized at lower altitudes [22,29].
From 1992 to 2018, there was an obvious decrease in basic cations in precipitation waters. A more significant decrease is at the Chopok station where the weighted annual average concentration of potassium decreased by 80.0%, magnesium by 86.7%, and calcium by 77.5%. In Pol'ana, within the concentration of the monitored basic cations, magnesium decreased the most by 73.6%, although the decrease of potassium by 57.8% and calcium by 55.9% is not negligible either.
Ca cations have the highest concentration from base cations observed in precipitation. The weighted annual means of calcium concentration in precipitation ( Figure 6) at the Chopok station during the observed period varied in the range of 0.09-1.83 mg·l −1 and at Pol'ana 0.318-1.159 mg·l −1 . The studied localities differ statistically significantly from each other ( Table 2). Statistical analysis of the data confirmed a trend that is significantly significant (Table 1). In the case of both stations there is a decreasing tendency of calcium concentrations in precipitation, while at the station Chopok by −0.0328 mg·l −1 per year and at Pol'ana by −0.03 mg·l −1 per year.
A majority of the EMEP sites showed a decreasing trend of calcium in precipitation with an average decrease of 47% from 1980 to 2009 and 26% from 1990 to 2009. In the early 1990s, the closing of many lignite-fired power stations, iron, and steel smelters as well the implementation of effective abatement technologies for sulfur caused a reduction also in the emissions of base cations [5].
Magnesium concentrations are several times lower than calcium and potassium. When comparing the weighted annual averages of magnesium concentrations between the Pol'ana and Chopok stations in Figure 6. See the big differences among them. Precipitation from the Pol'ana station reaches significantly higher magnesium concentrations in almost all monitored years than at Chopok. We confirmed the statistically significant difference (p < 0.0001) between the given localities by  Figure 6, however, it can be seen that the concentrations of K reach lower values than in the first half of the observed period. Using the Kruskal-Wallis test, we proved a statistically significant difference between the Chopok and Pol'ana stations.

Discussion
The long-term changes of chemical composition of atmospheric precipitation depends on many factors, but the decisive factors are changes in air pollutant emissions as well as changes in meteorological processes affecting their transformation into a liquid phase in clouds and precipitation [30,31]. The resulting changes are thus an integrated indicator of complex physico-chemical transformations of pollutants from the emission source to the captured precipitation at a specific location [30,[32][33][34]. The results of many observations, especially in the northern hemisphere, point to significant changes in precipitation chemistry, especially in pH values, concentrations of sulfur, nitrogen, but also basic cations. Most notably, these changes are visible at pH values and sulfate concentrations in precipitation water [2,11,24,27,34,35].
Several studies confirm that the primary cause of the decrease of pollutants in atmospheric precipitation is a decrease in sulfur and nitrogen emissions due to the application of measures under the Convention on Long-Range Transboundary Air Pollution-CLRTAP [1,2,7], although the decrease in emissions does not fully explain all observed pollutant trends [3]. Changes in other factors such as changes in air temperature and precipitation, changes in atmospheric circulation, or the occurrence of extreme weather events must also be taken into account [30,32,33].
Trends in decreasing sulfur and nitrogen concentrations in precipitation over the last 2-3 decades have been recorded mainly in Europe and North America, although the intensity of the decrease has been regionally different [2,4,5,23,27,34]. The most significant changes have been identified in Central and Eastern Europe, mainly due to structural changes in energy and industry sectors [1,2,4,5]. Recently studies have emerged on the possible impact of climate change on the further development of acidic and basic components contained in atmospheric precipitation, in particular the impact of changes in precipitation (decrease or increase in precipitation totals), air temperature and atmospheric circulation changes on long-range transmission, and chemical transformation of pollutants in gas and liquid phases [11,12,32,33]. For this reason, it is therefore necessary to maintain the monitoring of rainwater quality as much as possible, especially in localities with their long-term measurement.
Changes in the chemical composition of precipitation also significantly affect the biogeochemical cycles in forest and aquatic ecosystems and can contribute to changes in habitat diversity and biocenoses as well as changes in biomass production (e.g., changes in nitrogen content in atmospheric precipitation). In areas with sufficient precipitation, long-term changes in the chemical composition of precipitation will gradually be reflected in the chemical regime of surface and groundwater [35][36][37]. The issue of the economic effects of air pollution and precipitation quality, which may take on a different dimension in climate change, must not be forgotten either [38].

Conclusions
The presented work deals with the issue of long-term changes in the quality of precipitation in mountain areas of Slovakia at the station EMEP Chopok and VDO Pol'ana-Hukavský grúň. A 41-year time series of measured data of most of the analyzed elements of precipitation chemistry was available for the Chopok locality. At the Pol'ana station, the concentrations of elements in precipitation are monitored and are available for a 21-year period. In this work we evaluated the long-term development of concentrations of elements in precipitation (S-SO 4 , N-NO 3 , N-NH 4 , Ca, and Mg, K).
The concentration of sulfur in sulfates in precipitation water decreases significantly at both monitoring stations. At Chopok the decrease represents up to 84.5% decrease compared to the first three years of measurement and at Pol'ana 68.5% decrease. Nitrogen concentrations in nitrate and ammonium ions decreased with a significant trend at both monitored mountain stations. In both cases, its deposition decreased by almost 50%. Nitrogen concentrations in both nitrate and ammonium ions decreased at approximately the same rate per year at both stations.
For all basic cations, a decrease in their concentration in atmospheric precipitation is observed, with the exception of the potassium cation at the Pol'ana station, where the trend was not statistically confirmed.