Chemical Composition of Atmospheric Air in Nemoral Scots Pine Forests and Submountainous Beech Forests: The Potential Region for the Introduction of Forest Therapy

Studies show that forests are one of the main recreational destinations. This can be explained by their beneficial effects on the health of their visitors, which can be attributed to compounds from the terpene group. The aim of this research was to determine the chemical composition of air in the interiors of Nemoral Scots pine forests and submountainous beech forests, with the determination of compounds of the terpene group. Samples of organic compounds present in the air were collected with the use of Tenax TA sorbent tubes. The process of separation, identification, and determination of the extracted organic compounds was carried out with the use of the gas chromatography technique integrated with a flame ionization detector. Additional identification of the extracted compounds was carried out with the use of GC coupled with mass spectrometry. The most abundant group of compounds was the aliphatic hydrocarbons, both saturated (linear and branched) and unsaturated (terpenes). Carbonyl compounds were also found in the collected samples, but they constituted no more than 10% of all compounds present on the chromatograms. The concentrations of terpenes and terpenoids in the forest atmosphere varied from 10 to 74 µg·m−3, representing on average 33% of the total volatile organic compounds.


Introduction
Research conducted in different parts of the world shows that forests are one of the main recreational destinations for the population of urban agglomerations [1,2]. This is particularly true for the forests closest to the place of residence [3], which for the majority of the population are urban and peri-urban forests. It should be added, however, that in developed countries a considerable interest in forest recreation can also be observed among the rural population [4]. The significant interest in forest recreation observed over the years [3,5,6] can be explained, on the one hand, by the universality and accessibility of forests, and, on the other hand, by their beneficial effects on the health of their visitors. Forests have a positive effect on mental health [7][8][9]. Spending time in a forest has positive effects on concentration and performance [10], stress reduction [11,12], and can even aid in the treatment of depression [13,14]. Recent studies show that snowy forests can also have a positive impact on the mental state of their visitors [15,16]. Furthermore, the beneficial effects of greenery on health can be seen in the treatment of diseases such as obesity, coronary heart disease, hypertension, and hyperlipidemia [17][18][19][20][21]. Increased exposure to greenery has also been shown to reduce the risk of type 2 diabetes mellitus (T2DM) [22,23], and also to lower the risk of metabolic syndrome [24]. It has been demonstrated in laboratory conditions that camphene can be an effective drug in lowering lipid levels [25]. Tsai et al. [26] suggest that the reduced risk of developing T2DM in forest recreationists may be partly due to higher levels of terpenes in the forest air. Previously, Cho et al. [27]  The climate of the region is characterized by indicators such as (1) average annua temperature: 9.7 °C; (2) total annual rain or snow precipitation: 644 mm; (3) mean growin season length: 225-230 days; (4) prevailing wind directions: S-W, W, N-W [45]. Th investigated area is flat, slightly undulating, and characterized by small heigh differences. Elevation ranges from 205 to 235 m above mean sea level. The dominant soil are brown and podzolic soils formed on Quaternary glacial sands [46].
The following criteria were taken into consideration when selecting tree stands fo the study: 3 pine stands with a pine share of at least 70% and 3 beech stands with a beec share of at least 70%; age of stands at least 70 years-older stands are characterized by higher recreational potential [47]; distance between designated stands of at least 1 km. I this way, 6 stands were selected with the use of the Forest Data Bank (BDL https://www.bdl.lasy.gov.pl/portal (accessed on 4 October 2020)), the locations and characteristics of which are given in Table 1. The climate of the region is characterized by indicators such as (1) average annual temperature: 9.7 • C; (2) total annual rain or snow precipitation: 644 mm; (3) mean growing season length: 225-230 days; (4) prevailing wind directions: S-W, W, N-W [45]. The investigated area is flat, slightly undulating, and characterized by small height differences. Elevation ranges from 205 to 235 m above mean sea level. The dominant soils are brown and podzolic soils formed on Quaternary glacial sands [46].
The following criteria were taken into consideration when selecting tree stands for the study: 3 pine stands with a pine share of at least 70% and 3 beech stands with a beech share of at least 70%; age of stands at least 70 years-older stands are characterized by a higher recreational potential [47]; distance between designated stands of at least 1 km. In this way, 6 stands were selected with the use of the Forest Data Bank (BDL; https://www.bdl.lasy. gov.pl/portal (accessed on 4 October 2020)), the locations and characteristics of which are given in Table 1.

Collection of Organic Compound Samples and Screening Measurements of Ozone and Atmospheric Aerosol
While collecting the air samples, the following measurements were made in the field: air temperature and air humidity were measured at a height of 1.5-1.8 m above the ground with an Elemetron PWT-401 thermohygrometer (https://elmetron.com.pl/PWT-401.html (accessed on 1 October 2022); Zabrze, Poland), 3 measurements at 1 min, at 30 min and at 60 min from the start of the stage of collecting compound samples from ambient air.
Apart from measurements made directly in the sampling areas, the following data were additionally obtained from the nearest meteorological station (Rzeszów-Jasionka, Rzeszów, Poland): atmospheric pressure, air temperature, air humidity, and wind speed (www.tutiempo.net (accessed on 1 October 2022)). Data from the station were read for the 3 times at which air sampling started on the 3 consecutive sampling plots, during each day of measurements (9 a.m., 11 a.m., 1 p.m.), and for 2 p.m. on the reference site-meteorological data from the city park of Rzeszów (see Table 2).
The representatives of VOCs, including terpenes, were collected in Tenax TA (60-80 mesh) stainless steel sorption tubes, filled with approx. 200 mg of sorbent tube (O.D. × L 1/4 in. × 3 1/2 in., preconditioned, Merck KGaA, Darmstadt, Germany). Before each sampling campaign, sorption units were cleaned and conditioned for 30 min at 300 • C under the inert gas (nitrogen) flow rate. After the conditioning, as well as the sampling process, the ends of the applied sorption tube were protected by a brass braze nut with a Teflon insert, and then the Tenax TA tube was placed in a tightly sealed polyethylene container-a more suitable solution in the case of sample transportation than glass tubes. During the sampling of analytes from the atmospheric air in the defined forest area (sampling point), the air was passed through the mentioned sampling elements filled with sorption medium employing a portable small-scale aspirator (individual aspirator type AP-8ch, S.I. "TWO-MET", Zgierz, Poland) at a constant flow rate of approx. 150 cm 3 ·min −1 . The sampling time was set to 45 min. Each day in each investigated area, the samples of VOCs present in the atmospheric air were collected on three sorption tubes simultaneously. Each day investigations on the atmospheric air quality in the selected type of forest area were performed between 8:30 a.m. and 1:00 p.m. All samples were collected at the breathing level of an average person-sampling units were installed on a suitable tripod, i.e., 1.5-1.8 m above the ground. After sampling, the stainless steel tubes were secured for transportation to the laboratory and analyzed no longer than one week after sample collection (during this time samples were kept in a refrigerator at a temperature of 2-8 • C). Additionally, as control samples (protecting against potential damage to the sorption tubes containing analyte samples) each time at selected monitoring points, whole atmospheric air samples were collected using 3 L Tedlar bags (two Tedlar bags per sampling point). To prevent the impact of extensive sunlight and temperature exposure Tedlar bags with air samples were kept in a dark environment at room temperature. As supplementary data and to facilitate the interpretation of the obtained research results, at the same time, the concentration of ozone in the atmospheric air in forest areas was monitored. Ozone measurements were performed with the use of a portable personal device-the ozone meter OZ-ONE (Trotec GmbH, Heinsberg, Germany). The device was installed at a height of approx. 1.5-1.8 m above the ground. The measurements were performed in three periods-1 min, 30 min, and 60 min after setting up the monitoring equipment. Additionally, the mentioned portable device has factory-built sensors for measuring air temperature and humidity. The ozone meter OZ-ONE allows the measurement of short-term exposure limits as well as time-weighted averages.

Laboratory Analysis-Analytical Procedure, Extraction/Liberation, and Final Determination Conditions
The separation, identification, and determination of chemical compounds collected on a sorption medium, Tenax TA, was performed according to the procedures described in detail in previous papers, with some technical modifications. In brief, to extract/liberate the analytes adsorbed on a sorption medium, Tenax TA, the thermal extraction supplied with the forced inert gas flow rate was employed-two-stage thermal desorption (TD) technique. Applied TD units were connected to two gas chromatographic (GC) systems: (i) GC equipped with a flame ionization detector (GC-FID), and (ii) GC combined with a mass spectrometer (GC-MS). From each sampling period, two Tenax TA tubes were subjected to a qualitative and quantitative analysis using a TD-GC-FID system, and one Tenax TA tube was subjected to an additional identification of the collected compounds applying the TD-GC-MS unit. General information about the working conditions of employed TD-GC-FID and TD-GC-MS systems for estimating the content level of selected representatives of organic compounds in atmospheric air on investigated forest areas is listed in Table A1.
Additionally, the abovementioned TD-GC-FID system was applied to assess the values of the TVOC parameter, which gives information about the sum of all VOCs eluting between n-hexane and n-hexadecane in the applied GC column (uncalibrated peaks are quantified in terms of toluene equivalents). Furthermore, to obtain more reliable qualitative information (organic compounds identification) for each sample the values of the linear retention index (LRI) of the determined chemical compound were calculated. The LRI value for each of the chemical compounds determined in the investigated sample was assessed based on the rule described in the literature (described by Van den Dool and Kratz [48]) that a chemical compound's retention index is its relative time adjustment between the closest n-alkanes which leave the GC column and reaching the detector immediately before and after an investigated chemical compound [49,50]. The prepared mixture of n-alkanes on the basis of which the values of LRI were estimated contained aliphatic hydrocarbons with a carbon length from C8 (n-octane) to C17 (n-heptadecane). The mixture was analyzed under the same extraction and final determination conditions as real samples. Finally, obtained chromatograms of investigated samples and the n-alkanes mixture were analyzed and the numerical values of LRI were estimated based on Formula (1): As markers of atmospheric air quality in investigated forest areas, the representatives of terpene compounds were selected. The identification and quantitative analysis of collected analytes classified as terpenes were performed with the use of the external standard calibration method (ESTD). The calibration of the TD-GC-FID system and the organic compounds identification process (based on the comparison of retention times and retention indexes) was carried out using a reference standard mixture of 20 terpenes diluted in methanol at content level 2000 µg·mL −1 each (Cannabis Terpene Mix A certified reference material, TraceCERT ® , Merck KGaA, Darmstadt, Germany). For each of the chemical compounds enclosed in mentioned reference standard mixture, five-point calibration curves were prepared in the concentration range from 2 to 200 ng per tube. Each calibration point was repeated three times. It was found that calculated R-square values were above 0.992, which confirms the good detector linear response. A parallel calibration procedure (with a similar concentration range of each analyte per tube) was performed to identify and determine the BTEX compounds. In this case, to carry out the calibration process of TD-GC-FID system reference standard mixture of 13 VOCs (including benzene, toluene, ethylbenzene, and p, m-xylene) diluted in methanol at content level 2000 µg·mL −1 each (EPA VOC Mix 2 certified reference material, TraceCERT ® , Merck KGaA, Darmstadt, Germany) was applied. More detailed information about the calibration of TD-GC-FID was described in former papers [51][52][53]. The limit of detection (LOD) was assessed based on the average value of signal-to-noise ratio (3 × S/N) and was below 0.30 ng.

Statistical Analysis
The Shapiro-Wilk test was used to study the normality of the distribution. The scale in all groups is continuous. Student's t-test was used to compare meteorological indices (temperature and air humidity) measured directly on the sampling plots in the forest and at the nearest meteorological station-Rzeszów-Jasionka. Pearson's correlation coefficient was used to test the correlation between temperature and the total amount of terpenes, and between air humidity and the total amount of terpenes. The Mann-Whitney U test was used to check whether there was a statistically significant difference in the total amount of terpenes measured in Nemoral Scots pine forests and Central European submountainous beech forests and whether there was a statistically significant difference in TVOC levels recorded in Nemoral Scots pine forests and Central European submountainous beech forests.
The differences were deemed statistically significant with α = 0.05. All statistical analyses were performed with the use of Statistica v13.3 software (StatSoft Inc., Tulsa, OK, USA).

Meteorological Conditions over the Sampling Areas
The results obtained for air temperature and humidity measurements in the sampling areas in the forests and the weather station are comparable ( Table 2). The distribution of air temperature and humidity tested with the Shapiro-Wilk test is normal ( Figure 2).
The Shapiro-Wilk test was used to study the normality of the distribution. The scale in all groups is continuous. Student's t-test was used to compare meteorological indices (temperature and air humidity) measured directly on the sampling plots in the forest and at the nearest meteorological station-Rzeszów-Jasionka. Pearson's correlation coefficien was used to test the correlation between temperature and the total amount of terpenes and between air humidity and the total amount of terpenes. The Mann-Whitney U tes was used to check whether there was a statistically significant difference in the tota amount of terpenes measured in Nemoral Scots pine forests and Central European submountainous beech forests and whether there was a statistically significant difference in TVOC levels recorded in Nemoral Scots pine forests and Central European submountainous beech forests.
The differences were deemed statistically significant with α = 0.05. All statistica analyses were performed with the use of Statistica v13.3 software (StatSoft Inc., Tulsa, OK USA).

Meteorological Conditions over the Sampling Areas
The results obtained for air temperature and humidity measurements in the sampling areas in the forests and the weather station are comparable ( Table 2). The distribution of air temperature and humidity tested with the Shapiro-Wilk test is norma ( Figure 2).  Differences between mean temperatures (t = 0.2608, df = 50.00, p = 0.7953; homogeneous variances: F = 1.1170, p = 0.7843) and mean humidity (t = 0.4764, df = 50.00, p = 0.6359; homogeneous variances: F = 1.3930, p = 0.4131) measured with the t-test are statistically insignificant. A negative correlation between humidity and air temperature can be observed-as temperature increases, humidity decreases (r = −0.5065, p = 0.012). The mean ozone concentration measured in the forest sampling plots with the exception of one measurement (15 July, Hu42f, Ozone STEL = 19 µg-m −3 ) was always below the LOQ. In contrast, the mean ozone concentration measured in the Rzeszów city park on both measurement days was 19 µg-m −3 , with a maximum instantaneous concentration of 622 µg-m −3 (Table 2). A study by Peters et al. [54] showed that ozone concentrations recorded during air sampling resulted in a 10-50% decrease in terpene recovery.

Representatives of Volatile Organic Compounds Determined in the Investigated Forest Areas
Ambient air quality is one of the most important factors affecting human health and life quality. A special case of ambient air is forest air. People associate being in a forest with pleasant sensations, both visual and olfactory. The so-called forest scent is usually considered to be a pleasant smell, improving, among other things, our mood and general well-being, which can subsequently result in an improvement of our general health. The olfactory composition of forest air consists mainly of volatile organic compounds from the terpene and terpenoid group, emitted by the vegetation of forest complexes with varying intensity, depending, inter alia, on air temperature and humidity. An exemplary description of the odor (odor quality) associated with the presence of a particular representative of compounds from the terpene group is presented in Table A2.
The aim of the research was to study the composition of air in forest complexes with various stands. The research was carried out in six locations and air samples were collected at the level of the human respiratory zone (about 1.5 m from the surface) in order to better reflect the potential influence of forest air on the human respiratory system.
By assessing the composition of forest air in terms of the content of volatile organic compounds (based on the results of the qualitative analysis of the samples carried out with the use of TD-GC-MS), it can be concluded that the group of compounds most abundantly present in all the tested samples was aliphatic hydrocarbons, both saturated (linear and branched) and unsaturated (terpenes). The collected samples also showed the presence of carbonyl compounds (organic acids-mainly acetic acid, aldehydes, and ketones), but they constituted no more than 10% of all compounds present in the chromatograms and were not quantified in the tests performed. TVOC is a parameter that illustrates the air quality well in terms of VOC content. In the tests performed, this parameter was determined as the total content of organic compounds on the chromatograms between the C6 and C16 n-alkanes.
The group of compounds present in the forest air that received the most attention was terpenes and terpenoids. A detailed analysis of the chemical composition of the forest air samples, based on the obtained results of chromatographic analyses, allowed for the distinction, identification, and quantification of 14 compounds from the group of terpenes and terpenoids. The identification of 14 terpenes and terpenoids was carried out based on (i) the analysis of the mass spectra obtained during the TD-GC-MS analysis and their comparison with the NIST 11 Mass Spectral Database (the identification of the compound was considered satisfactory when the probability of the compound identification after comparing the live spectra with the spectra from the database was higher than 75%); (ii) the system of linear retention indices (LRI) determined using the TD-GC-FID and TD-GC-MS methods and comparing the retention times of compounds present in the samples with the retention times of the reference mixture containing the selected (identified) compounds from the group of terpenes and terpenoids. The researchers also attempted to compare the chemical composition of air in terms of the content of terpenes and terpenoids in forest complexes with different stands (Nemoral Scots pine forests (Hu) and Central European submountainous beech forests (Kl)). Information on the calculated ranges of the LRI parameter for the separated and identified terpenes and terpenoids in all studied samples of atmospheric air, along with a comparison with the literature data, is presented in Table 3.
Moreover, by examining the chemical composition of forest air in terms of the content of VOCs, the content of aromatic hydrocarbons (BTEX) was also determined, i.e., a group of compounds that constitute typical pollutants introduced into the environment as a result of human activity. The summary of the results of the qualitative and quantitative tests obtained for all locations is presented in Table 4.   Analyzing the obtained test results, it can be concluded that the contents of compounds from the group of terpenes and terpenoids in the forest air were varied and ranged between 10 and 74 µg·m −3 (the average value for all locations was 30.2 µg·m −3 ), which was on average approximately 33% of the total volatile organic compounds present in the tested air samples. A greater differentiation of the share of identified terpenes and terpenoids in the TVOCs was observed for the Kłapówka location (range from 9% to 70%) than for the Hucisko location (range from 9% to 59%). The distribution of total concentrations of terpenes and terpenoids for each tested location, in all measurement periods, and their percentage share in TVOCs are presented in Figures 3 and 4. Analyzing the obtained test results, it can be concluded that the contents of compounds from the group of terpenes and terpenoids in the forest air were varied and ranged between 10 and 74 µ g·m −3 (the average value for all locations was 30.2 µ g·m −3 ), which was on average approximately 33% of the total volatile organic compounds present in the tested air samples. A greater differentiation of the share of identified terpenes and terpenoids in the TVOCs was observed for the Kłapówka location (range from 9% to 70%) than for the Hucisko location (range from 9% to 59%). The distribution of total concentrations of terpenes and terpenoids for each tested location, in all measurement periods, and their percentage share in TVOCs are presented in Figures 3 and 4.  The smallest share of the identified terpenes in the total content of volatile organic compounds in all four measurement periods was observed for the forest area Kl 213g (Figure 3). In general, the highest total terpene concentration for all locations was observed in June, with the highest value of 73.7 µg·m −3 for Kl 219d, and the lowest value was observed in September-5.2 µg·m −3 in Hu 27a (Figure 4). In location Kl 219d, the highest values of total terpenes were observed during all measurement campaigns. The distribution of TVOCs and terpenes and terpenoids for the Kl location does not exhibit the characteristics of a normal distribution ( Figure 5). The smallest share of the identified terpenes in the total content of volatile organic compounds in all four measurement periods was observed for the forest area Kl 213g ( Figure 3). In general, the highest total terpene concentration for all locations was observed in June, with the highest value of 73.7 µ g·m −3 for Kl 219d, and the lowest value was observed in September-5.2 µ g·m −3 in Hu 27a (Figure 4). In location Kl 219d, the highest values of total terpenes were observed during all measurement campaigns. The distribution of TVOCs and terpenes and terpenoids for the Kl location does not exhibit the characteristics of a normal distribution ( Figure 5).  The smallest share of the identified terpenes in the total content of volatile organi compounds in all four measurement periods was observed for the forest area Kl 213g ( Figure 3). In general, the highest total terpene concentration for all locations wa observed in June, with the highest value of 73.7 µ g·m −3 for Kl 219d, and the lowest value was observed in September-5.2 µ g·m −3 in Hu 27a (Figure 4). In location Kl 219d, the highest values of total terpenes were observed during all measurement campaigns. The distribution of TVOCs and terpenes and terpenoids for the Kl location does not exhibi the characteristics of a normal distribution ( Figure 5). On the basis of the Mann-Whitney U test, it can be stated that there are no statistically significant differences in the total number of terpenes between the investigated forest types (U = 53, Z = −1.0681, p = 0.2855). Similarly, no statistically significant differences in the TVOCs recorded in these two types of forest can be found (U = 45, Z = −1.5300, p = 0.1260). Our results indicate a large variation in the content of terpenes and terpenoids in the forest air and although a beech forest might produce more terpenes than a pine forest, these differences are not statistically significant. This variability is influenced by dynamically changing weather conditions: temperature, air humidity, wind speed, and direction [57][58][59]. Similarly, Meneguzzo et al. [39] indicate the variability of the concentration of biogenic volatile organic compounds (BVOCs) in time and space. However, contrary to the present study, these authors put forward a cautious conclusion that conifers are more efficient than beeches in terms of the concentration of BVOCs in the forest air. However, in the abovementioned study, the measurements were carried out along a path through various forest types, without specifying the distance between forest types, hence perhaps the BVOCs produced by one species were mixed with that produced by another. The distribution of concentrations of individual compounds from the group of terpenes and terpenoids, for each location independently, for all measurement campaigns, is presented in the figures- Figures A1 and A2.
Detailed analysis of the content of compounds from the group of terpenes and terpenoids in the tested air showed that the highest concentrations of selected terpenes and terpenoids were observed in June, although the shares of individual compounds representing the group were varied. Park et al. [40], conducting year-round research in the Pinus densiflora forest in Korea, found the highest concentration of natural volatile organic compounds in summer, followed by spring, autumn, and winter. Meneguzzo et al. [39], conducting research from August to October in the Italian Apennines in coniferous (spruce, fir) and beech forests, found the highest TVOC concentration at the end of September. Interestingly, studies in the Mediterranean basin on seven tree species showed no clear seasonal trend in the emissions of terpenes and terpenoids, except for Cistus albidus, for which peak values were observed in spring and minimum values in autumn [58]. According to Meneguzzo et al. [39], detailed comparisons of the results of the analysis of the chemical composition of forest air from different forests and different latitudes do not make much sense, with which the authors agree. In the present study, there was no statistically significant relationship found between the total amount of terpenes and terpenoids and air temperature (r = 0.1048, p = 0.626) or between the total amount of terpenes and terpenoids and air humidity (r = 0.2947, p = 0.162).
Additionally, for the Kl location, farnesol (RI 1536) was an important compound from the group of terpenes and terpenoids; its share in the total content of terpenes and terpenoids was in the range of 0.7-48.6%, and the mean value was 12.8% (see Figure A2). A significant difference in the profile of terpene compounds present in the forest air of the studied locations is the absence of limonene in the Hu 27a, Hu 42f, and Hu 59c locations during all measurement campaigns (see Figure A1).
Another group of chemical compounds that was monitored was the BTEX group (benzene, toluene, ethylbenzene, p-, m-xylene). When assessing the concentrations of BTEX compounds in the forest air, it can be concluded that they were also varied. It should be emphasized, however, that the concentrations of individual representatives of the BTEX group in no case exceeded the permissible values for atmospheric air (it is particularly important that the concentration of benzene was below the permissible value of 5 µg·m −3 ; Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on air quality and cleaner air for Europe). The total BTEX content for all locations and all measurement campaigns is shown in Figure 6. be emphasized, however, that the concentrations of individual representatives of the BTEX group in no case exceeded the permissible values for atmospheric air (it is particularly important that the concentration of benzene was below the permissible value of 5 µ g·m −3 ; Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on air quality and cleaner air for Europe). The total BTEX content for all locations and all measurement campaigns is shown in Figure 6. The concentration values of individual compounds from the BTEX group are presented in the aforementioned Table 4. It is worth noting that the compound with the largest share in the total BTEX content was toluene, a compound with a lower toxicity than benzene. This state is not surprising because in the vast majority of cases toluene is recorded in the atmospheric air in open spaces at a much higher level than benzene [60][61][62]. In the case of the area where the research discussed in the present paper was carried out, the level of benzene content in the forest air ranged from <0.014 to 2.712 µ g·m −3 (Table  4). For comparison, the concentration of benzene in the atmospheric air in Berlin was 6.9 µ g·m −3 [63], and in 13 sites in 8 US states it was 0.8-3.6 µ g·m −3 [64]. In the case of atmospheric air in the area of a city considered a recreational and health resort-Sopot, Poland-the concentration of benzene in 2013 and 2014 was 0.53 ± 0.43 µ g·m −3 and 1.07 ± 0.97 µ g·m −3 , respectively [65]. The presence of BTEX chemicals in the atmospheric air is mainly a consequence of various types of human activity, especially the movement of mechanical vehicles powered by internal combustion engines and activities of different types of industrial centers. The presence of these compounds in the monitored forest areas (far away from the main communication routes or industrial centers) may be caused by the movement of air masses and their movement over long distances to the monitored green areas. Low levels of xylene and ethylbenzene in the atmospheric air in forest areas could constitute confirmation of this hypothesis, since they indicate the presence of the so-called old air masses-compounds determined in forest air may come from distant emission sources, e.g., urban agglomerations, industrial plants, and main communication routes [65,66]. The concentration values of individual compounds from the BTEX group are presented in the aforementioned Table 4. It is worth noting that the compound with the largest share in the total BTEX content was toluene, a compound with a lower toxicity than benzene. This state is not surprising because in the vast majority of cases toluene is recorded in the atmospheric air in open spaces at a much higher level than benzene [60][61][62]. In the case of the area where the research discussed in the present paper was carried out, the level of benzene content in the forest air ranged from <0.014 to 2.712 µg·m −3 (Table 4). For comparison, the concentration of benzene in the atmospheric air in Berlin was 6.9 µg·m −3 [63], and in 13 sites in 8 US states it was 0.8-3.6 µg·m −3 [64]. In the case of atmospheric air in the area of a city considered a recreational and health resort-Sopot, Poland-the concentration of benzene in 2013 and 2014 was 0.53 ± 0.43 µg·m −3 and 1.07 ± 0.97 µg·m −3 , respectively [65]. The presence of BTEX chemicals in the atmospheric air is mainly a consequence of various types of human activity, especially the movement of mechanical vehicles powered by internal combustion engines and activities of different types of industrial centers. The presence of these compounds in the monitored forest areas (far away from the main communication routes or industrial centers) may be caused by the movement of air masses and their movement over long distances to the monitored green areas. Low levels of xylene and ethylbenzene in the atmospheric air in forest areas could constitute confirmation of this hypothesis, since they indicate the presence of the so-called old air masses-compounds determined in forest air may come from distant emission sources, e.g., urban agglomerations, industrial plants, and main communication routes [65,66].

Aerosol Concentration in Forest Air
In order to make a more complete characterization of forest air, the aerosol concentration was also assessed during the August measurement campaign. The aerosol content (both the number and the mass concentration of the aerosol) was determined in the particle diameter range from 0.3 to 10 µm. The obtained results, representing the number and the mass concentration of the aerosol, taking into account the distribution of the aerosol particle diameters, are presented separately for Nemoral Scots pine forests (Figure 7a,b) and Central European submountainous beech forests (Figure 8a,b).
In order to make a more complete characterization of forest air, the aerosol concentration was also assessed during the August measurement campaign. The aerosol content (both the number and the mass concentration of the aerosol) was determined in the particle diameter range from 0.3 to 10 µ m. The obtained results, representing the number and the mass concentration of the aerosol, taking into account the distribution of the aerosol particle diameters, are presented separately for Nemoral Scots pine forests (Figure 7a,b) and Central European submountainous beech forests (Figure 8a,b).

Figure 7.
Aerosol number (a) and mass concentrations (b), taking into account the distribution of aerosol particle diameters, for Nemoral Scots pine forests. Figure 7. Taking into account the distribution of aerosol particle diameters, for Nemoral Scots pine forests.
Higher values of concentrations were observed for the Hu forest complexes, the highest value being 85.7 particles/cm 3 (Hu 27a). The highest mass concentration of atmospheric aerosol was observed for the Kl 213g location and was 14.3 µg·m −3 . At Hu locations, the share of particles with smaller diameters (0.3-0.5 µm) was more significant in the total distribution of aerosol particles than for Kl forest complexes, which resulted in a higher value of the number concentration and a lower value of the mass concentration; nevertheless, the measured values of the number and the mass concentration aerosol levels in forest air in all locations are lower than those described in the literature mainly for the urban environment [67,68] and are considered to be "neutral level" (considered the level acceptable for clean air).
The concentration of aerosol in forest air varies and depends, among other things, on the concentration of terpenes and other compounds from the group of reactive organic compounds, tropospheric ozone, and humidity. Moreover, it should be emphasized that the conducted measurements of atmospheric aerosol concentration are illustrative measurements intended to be used for the general characterization of forest air, and not for comparison with the applicable regulations. Terpenes in the atmosphere are reactive compounds that take part in the formation of atmospheric aerosols in the presence of atmospheric oxidants such as hydroxyl radicals, tropospheric ozone, or nitrogen oxides; however, our research on the level of atmospheric aerosol content in forest air is meant to be used to describe the quality of forest air more fully in terms of influencing the human respiratory system. Higher values of concentrations were observed for the Hu forest complexes, the highest value being 85.7 particles/cm 3 (Hu 27a). The highest mass concentration of atmospheric aerosol was observed for the Kl 213g location and was 14.3 µ g·m −3 . At Hu locations, the share of particles with smaller diameters (0.3-0.5 µ m) was more significant in the total distribution of aerosol particles than for Kl forest complexes, which resulted in a higher value of the number concentration and a lower value of the mass concentration; nevertheless, the measured values of the number and the mass concentration aerosol levels in forest air in all locations are lower than those described in the literature mainly for the urban environment [67,68] and are considered to be "neutral level" (considered the level acceptable for clean air).
The concentration of aerosol in forest air varies and depends, among other things, on the concentration of terpenes and other compounds from the group of reactive organic compounds, tropospheric ozone, and humidity. Moreover, it should be emphasized that the conducted measurements of atmospheric aerosol concentration are illustrative measurements intended to be used for the general characterization of forest air, and not for comparison with the applicable regulations. Terpenes in the atmosphere are reactive compounds that take part in the formation of atmospheric aerosols in the presence of atmospheric oxidants such as hydroxyl radicals, tropospheric ozone, or nitrogen oxides; however, our research on the level of atmospheric aerosol content in forest air is meant to

Conclusions
Our results indicate a large variation in the content of terpenes and terpenoids in the forest air (ranging between 10 and 74 µg·m −3 ) and although a beech forest might produce more terpenes than a pine forest, these differences are not statistically significant. A significant difference in the profile of terpene compounds present in the forest air of the studied locations is the absence of limonene in the pine forest during all measurement campaigns. The highest total terpene concentration for all locations was observed in June (73.7 µg·m −3 for Kl 219d), and the lowest value was observed in September (5.2 µg·m −3 in Hu 27a).
The compounds that were present in all locations, during all measurement campaigns, and whose concentrations constituted a significant contribution to the total terpenes content were α-pinene (RI 940), terpinolene (RI 1089), and α-terpineol (RI 1193); additionally, for the Kl location, farnesol (RI 1536).
The air of the studied forest types contains less benzene (<0.014 to 2.712 µg·m −3 ) and aerosol particles (to 14 µg·m −3 ) than urban air.
Considering all aspects related to the conducted research and the obtained research results, it can be concluded that the presented characteristics of forest air, taking into account compounds of both natural (quantitative and qualitative terpenes) and anthro-pogenic origin (found in forest air), will prompt a wide interdisciplinary discussion on the possibility of incorporating "forest treatment" into general public health management programs. The forest atmosphere has a positive effect on building immunity [69,70], which is particularly important in terms of public health (mental and physical) and can be an effective tool supporting healthcare services in the fight not only against diseases of affluence but also with periodically emerging pandemics, e.g., COVID-19. Research [71] proves that monoterpenes, including e.g., α-pinene, easily transfer from the forest air through the respiratory tract into the body and are accumulated in the serum. Thus, we believe that with the appropriate medical knowledge, and with the results published by us, we can effectively implement forest therapy in Nemoral Scots pine forests and Central European submountainous beech forests as a method of supporting the pharmacological treatment of many diseases, especially of the respiratory system.

Acknowledgments:
The financial support for the maintenance of research facilities used in these studies from the Gdańsk University of Technology by the DEC-2/2021/IDUB/V.6/Si grant under the SILICIUM SUPPORTING CORE R&D FACILITIES-"Excellence Initiative-Research University" program is gratefully acknowledged.

Conflicts of Interest:
The authors declare no conflict of interest. Table A1. Thermal desorption (TD) GC-FID and GC-MS system parameters used to assess the content level of selected representatives of organic compounds in atmospheric air in the investigated forest areas.   Figure A1. Distribution of concentrations of identified terpenes and terpenoids in atmospheric air over the Hucisko area for all measurement campaigns.