**2. Air pollution**

6]. Irrespective of their sources in the soil, accumulation of heavy metals can degrade soil quality and reduce crop yield and the quality of agricultural products and thus negatively

Forest ecosystems present one of the main parts of biosphere. They affect the composition and the quality of atmosphere and also shape climate conditions both on regional and on global scales [8, 9]. The forest stands were endangered from the harmful effect of air and soil pollu‐ tants [10–13]. Global change involves simultaneous and rapid alterations in several key environmental parameters that control the dynamics of forests [14]. Climate change and air pollution affect forests by changes in soil processes, tree growth, species composition and distribution, increased plant susceptibility to biotic and abiotic stress factors, increased fire danger, decreased water resources and recreation value [9, 15, 16]. The physical and ecological conditions of forest ecosystems have been influenced mainly by the deposition of atmospheric pollutants and by changing climatic conditions with a series of warm and dry periods. Apart from the weather conditions, heavy metals were shown to be one of the primary causes of tree damages. The knowledge of the heavy metal accumulation in soil, the origin of these metals and their possible interactions with soil properties are priority objectives in the environmental monitoring [17, 18]. The surface soil layer is of particular interest in the forest ecosystem monitoring due to its role as a stable adsorbent of the deposited atmospheric substances. The behaviour of heavy metals in soils and their impact on the living organisms have been described in details in the literature. The main effects of their increased concentration are connected with inhibited microbial activity, delayed litter decomposition processes, changes

impact the health of human, animals and the ecosystem [7].

128 Soil Contamination - Current Consequences and Further Solutions

in nutrient availability and increased accumulation from the plants [19–24].

period when the growth process is intensive [33].

[25, 34].

The movement of air masses from urban and industrial regions results in frequent episodes of high levels of ozone in forests. Being a major phytotoxic atmospheric pollutant in most European countries, ozone is a significant cause of reduction in growth of tree vegetation [25– 27]. It has been shown that the indirect forcing of climate change through ozone effects on the land carbon exchange could be an important factor and can induce a positive feedback for global warming [28]. High concentrations of ozone occur not only in areas with large sources of pollution but also in suburban and rural sites, located away from major sources of emissions [29, 30]. Elevated concentrations of ambient ozone are also of great concern for our country because ozone is turned to be the most important air pollutant in both relatively clear forest areas in Bulgaria [31–33]. At the suburban and remote mountain sites forest trees were subject to the impact of elevated ozone concentrations at especially the beginning of the vegetation

The major contributor to forest degradation was also sulphur dioxide, a gaseous substance with direct and powerful phytotoxic and acidifying effects. Nitric oxides affect woody plants directly by entering through the stomata and indirectly through soil acidification and envi‐ ronmental eutrophication. Drought stress predisposes trees to the negative effect of pollutants

National parks and other protected areas despite their special management regimes are subjected to air pollution. Air pollution impact was reported by the National Parks Conser‐ vation Association of the USA. The analyses showed that national parks have significant air

#### **2.1. Emissions of certain air pollutants and tendencies**

The main sources of emission of air pollutants on the territory of Bulgaria (sulphur dioxide, nitrogen oxides, particulate matter) are the thermal power stations (TPS), operating on solid fuels and fuel oil, road transport and household sources [40]. In 2013, the annual emissions of sulphur dioxide were 193.97 kt/year. The thermal power stations were the main sources of sulphur dioxide—72% (see **Figure 1**). The annual emission of nitrogen oxides was 123.54 kt. The thermal power stations and road transport had the biggest share—62% of the total amount, equally divided between the two sectors (**Figure 2**).

**Figure 1.** Main sources of sulphur dioxide in Bulgaria, 2013.

One of the pollutants, causing the most serious problems regarding air quality in the major Bulgarian cities, is particulate matter (PM10). The total amount of PM10 in 2013 was 42.44 kt.

The main source of particulate matter emissions is domestic heating—59% of PM10 (see **Figure 3**) and 82% of PM2, 5.

The emissions of the main pollutants tend to decrease for the period 2009–2013. This trend is most clearly observed for sulphur dioxide, resulting from the construction of desulphurization installations to the major thermal power stations (TPS), operating on coal. The increased emissions in 2011 were the result of burned larger quantities lignite coal throughout the year (see **Figure 4**).

**Figure 2.** Main sources of nitrogen dioxide in Bulgaria, 2013.

**Figure 3.** Distribution of particulate matter emissions by sources.

The share of the emission sources changed over the years—in 2009, sulphur dioxide from the TPS was 93.9% of the total amount, reported in Bulgaria; the main source of nitrogen oxides was the road transport—49% of the total emissions in the country.

The condition of the air in Bulgaria is controlled by the National Air Quality Monitoring System. Three of the air quality monitoring units equipped with automatic measuring stations (AMS), monitor the air condition in forest territories. These are the stations for intensive monitoring (IM), located in the regions of Yundola, Vitinya and Staro Oryahovo. The obser‐ vations are carried out in relation to the implementation of the International Cooperative Programme "Forests". The aim is to trace the transfer of pollutants and their impact on the different components of the forest ecosystems. The concentrations of the following pollutants are measured—sulphur dioxide, nitrogen oxide, nitrogen dioxide and ozone. The ML®9850 sulphur dioxide analyser is an ultraviolet fluorescence spectrophotometer for continuously measuring of SO2 concentrations. The ML®9841A nitrogen oxides analyser works on the basis of gas‐phase chemiluminescence detection to perform continuous analysis of nitric oxide (NO), total oxides of nitrogen (NOx), and nitrogen dioxide (NO2). Non‐dispersive ultraviolet photometer serves as the basis for the ML®9812 Ozone Analyser. The atmospheric depositions in the open and under the forest canopies are also measured—quantity, acidity, concentration of acidic and basic ions and heavy metals [41].

The main source of particulate matter emissions is domestic heating—59% of PM10 (see

The emissions of the main pollutants tend to decrease for the period 2009–2013. This trend is most clearly observed for sulphur dioxide, resulting from the construction of desulphurization installations to the major thermal power stations (TPS), operating on coal. The increased emissions in 2011 were the result of burned larger quantities lignite coal throughout the year

**Figure 3**) and 82% of PM2, 5.

130 Soil Contamination - Current Consequences and Further Solutions

**Figure 2.** Main sources of nitrogen dioxide in Bulgaria, 2013.

**Figure 3.** Distribution of particulate matter emissions by sources.

was the road transport—49% of the total emissions in the country.

The share of the emission sources changed over the years—in 2009, sulphur dioxide from the TPS was 93.9% of the total amount, reported in Bulgaria; the main source of nitrogen oxides

(see **Figure 4**).

**Figure 4.** Tendencies in emissions of sulphur dioxide, nitrogen dioxide and PM10 (2009–2013).

#### **2.2. Atmospheric pollutants in the intensive monitoring stations**

The average annual concentrations of sulphur dioxide varied from 3.97 to 17.4 μg m−3 for the region of St. Oryahovo, from 3.62 to 18.5 μg m−3 for Vitinya and from 2.09 to 12.9 μg m−3 in Yundola (see **Figure 5**). The highest values for St. Oryahovo and Vitinya stations were deter‐ mined in 2008 and for Yundola in 2009. In the period 2008–2011, there was a significant decrease of the annual concentrations from 4.5 to 6 times, followed by a gradual increase until 2015.

The trends regarding the average annual values of sulphur dioxide were almost the same for the three stations, regardless of the considerable distance between them, which indicates that nearly identical regional values occurred as a result of the transfer. The measured concentra‐ tions did not exceed the limit value (LV) for vegetation protection [42].

**Figure 5.** Average annual concentrations of sulphur dioxide (LV–limit value).

The annual mean values for nitrogen oxides varied in a wider range—from 5.03 to 20.6 μg m−3 for the region of St. Oryahovo, from 7.87 to 51.6 μg m−3 for Vitinya and from 3.37 to 19.8 μg m−3 for Yundola (see **Figure 6**). Higher values were measured during the first 2 years of the period 2008–2015; the lowest values were registered in 2011 for St. Oryahovo and Vitinya, and in 2014, for Yundola.

**Figure 6.** Average annual concentrations of nitrogen oxides (LV–limit value).

During the period 2008–2011, the tendency was similar to that of sulphur dioxide and was characterized by decreased concentrations; after that period, the values continued to decrease with insignificant fluctuations for St. Oryahovo and Yundola. Regarding the region of Vitinya, the values gradually increased till 2015. The measured concentrations exceeded the LV for vegetation protection only in 2008 for the region of Vitinya [42].

The AOT40 index (index of accumulated ozone exposure over a threshold of 40 ppb (80 μg m−3), calculated for the period from May to July, was used to assess the ozone impact on forest ecosystems. The data, presented on **Figure 7**, indicate that ozone is almost constant stress factor for the forests in the region of Yundola, where the target value for protection of vegetation was exceeded for the prevailing part of the period 2008–2015, with a maximum value in 2015—about 2 times above the target value [42].

**Figure 7.** Index of accumulated ozone exposure over a threshold of 40 ppb (80 μg m−3) (AOT40). TV—target value for protection of vegetation.

No exceedances of AOT40 were registered for the region of St. Oryahovo; for the region of Vitinya, the AOT40 was exceeded in two years—2011 and 2015 [42].

#### **2.3. Atmospheric pollutants in industrial regions**

**Figure 5.** Average annual concentrations of sulphur dioxide (LV–limit value).

132 Soil Contamination - Current Consequences and Further Solutions

**Figure 6.** Average annual concentrations of nitrogen oxides (LV–limit value).

for Yundola.

The annual mean values for nitrogen oxides varied in a wider range—from 5.03 to 20.6 μg m−3 for the region of St. Oryahovo, from 7.87 to 51.6 μg m−3 for Vitinya and from 3.37 to 19.8 μg m−3 for Yundola (see **Figure 6**). Higher values were measured during the first 2 years of the period 2008–2015; the lowest values were registered in 2011 for St. Oryahovo and Vitinya, and in 2014,

During the period 2008–2011, the tendency was similar to that of sulphur dioxide and was characterized by decreased concentrations; after that period, the values continued to decrease with insignificant fluctuations for St. Oryahovo and Yundola. Regarding the region of Vitinya, The study was made in Devnya region—a big industrial zone in the Eastern Bulgaria. Forest vegetation consisted of 20‐year‐old plantations of *Celtis australis* L. and *Fraxinus americana* L. grown at 500 m from the sources of intensive air pollution and near a highroad with heavy traffic. Even‐aged control stands were grown as plantations in relatively unpolluted region about 15,000 m far from the chemical plants. The air pollutants, emitted from Devnya industrial region, included sulphur dioxide, nitrogen oxides, CO, HF, NH3, Cl2, HCl, CaO, CaCO3, high levels of silicon, solid and liquid aerosols, organic compounds, particulate matter of dust and soot, Al and heavy metals. The great part of nitrogen oxides and sulphur dioxide are dissolved as nitric and sulphuric acids, which causes acid rains on the region. The monitoring of air pollution in the industrial region was made continuously by automatic station.

Monitoring data for sulphur dioxide during 2004 showed a wide variation of 1‐h means between 1.3 and 210 μg m−3. There were many short time events of high sulphur dioxide concentrations mainly during the winter period. The maximal 24‐h values of sulphur dioxide were between 10.5 and 39.3 μg m−3. Within the six‐month growth period of trees (April— September), the month values for sulphur dioxide were between 4.8 and 17 μg m−3 (**Figure 8**).

**Figure 8.** Month values for SO2 in Devnya industrial region during the growing period of 2004.

Maximal 24‐h means of NO2 for 2004 were between 10 and 30 ppb. The all of 4‐h means for NO2 were below 80 μg m−3 for the entire period of monitoring. Month average concentration of nitrogen dioxide during the growth period of 2004 varied between 20.5 and 55 μg m−3 (**Figure 9**).

**Figure 9.** Month values for NO2 in Devnya industrial region during the growing period of 2004.

The maximal 24‐h means of ozone concentrations within six‐month growth period of 2004 varied between 55 and 83 μg m−3. The highest values of the maximal 24‐h means for ozone concentrations were observed in July and August (**Figure 10**).

were between 10.5 and 39.3 μg m−3. Within the six‐month growth period of trees (April— September), the month values for sulphur dioxide were between 4.8 and 17 μg m−3 (**Figure 8**).

134 Soil Contamination - Current Consequences and Further Solutions

**Figure 8.** Month values for SO2 in Devnya industrial region during the growing period of 2004.

**Figure 9.** Month values for NO2 in Devnya industrial region during the growing period of 2004.

concentrations were observed in July and August (**Figure 10**).

The maximal 24‐h means of ozone concentrations within six‐month growth period of 2004 varied between 55 and 83 μg m−3. The highest values of the maximal 24‐h means for ozone

(**Figure 9**).

Maximal 24‐h means of NO2 for 2004 were between 10 and 30 ppb. The all of 4‐h means for NO2 were below 80 μg m−3 for the entire period of monitoring. Month average concentration of nitrogen dioxide during the growth period of 2004 varied between 20.5 and 55 μg m−3

**Figure 10.** Maximal 24‐h means of ozone concentrations in Devnya industrial region during the growing period of 2004.

The average and maximal 1‐h concentrations of ozone were 52.2 and 103.5 μg m−3, respectively. Over the growing season of 2004, the daily means of ozone concentrations were only during a few days below 50 μg m−3. The target value of the index AOT40 for protection of vegetation [42] was permanently exceeded during the 5‐year period of monitoring (**Figure 11**). In 2003, the index AOT40 was 3 times above the target value.

**Figure 11.** Index of accumulated ozone exposure over a threshold of 40ppb (80 μg m−3) (AOT40) during the five‐year period (1999–2003).

On the basis of the data processing for the concentrations of SO2, NOx and O3 in the air in Devnya region, we can draw the conclusion that the most remarkable air pollution is with ozone. Therefore, a negative effect on the forest ecosystems during the growth period should be expected mainly for the ozone. This pollutant is turned to be the most important ecological risk factor for woody plant in the region during the period of their high physiological activity. In regions with low NOx concentration, ozone formation is dependent entirely on NOx (NOx sensitive regions) [43]. In contrast to the threshold value for accumulated ozone dose (10,000 μg m−3) concerned the six‐month growing period of trees, some studies showed that a possible effect of ozone occurs only at very high AOT40 (>70,000 μg m−3) [44].

#### **2.4. Atmospheric depositions in the intensive monitoring stations**

The amount of depositions for the period 2008–2015 is presented on **Figure 12**, which shows significant variation over the years. The average acidity of depositions for the respective period varied from pH 5.06 to pH 6.75 for the region of St. Oryahovo, from pH 5.05 to pH 5.5 for Vitinya and from 5.42 to 5.89 for the region of Yundola (see **Figure 13**).

**Figure 12.** Amount of atmospheric depositions in the open. St. O—stationar Staro Oriahovo, V—stationar Vitinia, Yu stationar Yundola.

**Figure 13.** Acidity of atmospheric depositions in the open. St. O—stationar Staro Oriahovo, V—stationar Vitinia, Yu stationar Yundola.

From the presented data, it can be concluded that during the respective period, the depositions in the region of Vitinya were within the scope of "acid rain"—pH < 5.5. Regarding the other two regions, the acidic depositions were observed only in certain years—2011, 2012 and 2015 for St. Oryahovo, and in 2014, for Yundola.

be expected mainly for the ozone. This pollutant is turned to be the most important ecological risk factor for woody plant in the region during the period of their high physiological activity. In regions with low NOx concentration, ozone formation is dependent entirely on NOx (NOx sensitive regions) [43]. In contrast to the threshold value for accumulated ozone dose (10,000 μg m−3) concerned the six‐month growing period of trees, some studies showed that a possible

The amount of depositions for the period 2008–2015 is presented on **Figure 12**, which shows significant variation over the years. The average acidity of depositions for the respective period varied from pH 5.06 to pH 6.75 for the region of St. Oryahovo, from pH 5.05 to pH 5.5 for

**Figure 12.** Amount of atmospheric depositions in the open. St. O—stationar Staro Oriahovo, V—stationar Vitinia, Yu—

**Figure 13.** Acidity of atmospheric depositions in the open. St. O—stationar Staro Oriahovo, V—stationar Vitinia, Yu—

effect of ozone occurs only at very high AOT40 (>70,000 μg m−3) [44].

136 Soil Contamination - Current Consequences and Further Solutions

**2.4. Atmospheric depositions in the intensive monitoring stations**

Vitinya and from 5.42 to 5.89 for the region of Yundola (see **Figure 13**).

stationar Yundola.

stationar Yundola.

The amount of sulphate sulphur varied within the range from 0.35 to 4.3 kg ha−1 annually for the region of St. Oryahovo, from 1.91 kg to 7.78 kg ha−1 annually for Vitinya and from 1.57 to 10.53 kg ha−1 annually for Yundola (see **Figure 14**).

**Figure 14.** Intake of sulphate sulphur with the deposition in the open. St.O—stationar Staro Oriahovo, V—stationar Vitinia, Yu—stationar Yundola.

The relatively low concentration of sulphur dioxide in the region of Yundola did not correlate with the high sulphur levels in the depositions. The amount of nitrogen depositions in the region of Yundola was also higher—from 3.01 to 10.46 kg ha−1 annually (see **Figure 15**).

**Figure 15.** Intake of nitrogen (ammonium and nitrate) with the depositions in the open. St.O—stationar Staro Oriaho‐ vo, V—stationar Vitinia, Yu—stationar Yundola.
