Peculiarities of Particulate Matter Absorption by Urban Tree Species in the Major Cities of Armenia

Abstract

Air pollution, including particulate matter (PM), impacts public health in urban areas. Vegetation acts as a natural filter, removing environmental pollution by absorbing large quantities of toxic substances on the foliage. Ambient air pollution problems are real in Armenia’s cities. This article presents the results of a study based on field sampling in July 2022 undertaken in urban parks and streets in the Armenian cities of Yerevan, Gyumri, and Vanadzor. The three cities have different climates and geographic conditions. The main research goal was a comparative study of the accumulation of PM by urban greenery. The most widespread tree species were selected for the study in each city: in Yerevan, Platanus orientalis and Quercus robur; in Gyumri, Fraxinus excelsior and Tilia caucasica; and in Vanadzor, Aesculus hippocastanum and Acer pseudoplatanus. The ecological status of trees was assessed through visual observation. Tree species with high PM uptake potential were identified and selected for inclusion in urban greening systems (Platanus orientalis, Fraxinus excelsior, and Quercus robur in Yerevan; Tilia caucasica, Sorbus persica, Fraxinus excelsior, and Populus alba in Grumri; Acer pseudoplatanus, Fraxinus excelsior, Aesculus hippocastanum, and Thuja occidentalis in Vanadzor.). High PM accumulation was found on the leaves of tree species in all of the investigated cities, with the largest amount recorded in Yerevan. In these cities, PM levels were higher in street plantations than in parks. All studied tree species have a high potential for PM absorption, demonstrating strong phytofilter properties. Therefore, they can be effectively used in their typical climatic zones and included in street plantings, gardens, and parks. These results can help urban planners and policymakers make informed decisions about urban greening initiatives to improve air quality and overall wellbeing.

1. Introduction

1.1. Particulate Matter (PM) in Ambient Air

Air pollution is considered the main environmental risk to human health and is a growing concern [1,2,3]. Particulate matter (PM) is one of the air pollutants most harmful to human health and the environment [4,5]. The primary sources of PM emissions include power stations, vehicle exhaust, factories, industrial processes, construction sites, wildfires, wood burning, gravel pits, agricultural activities, and dusty roads. PM can remain airborne for a long time and travel over long distances [1,6]). PM often contains various toxic compounds, including polycyclic aromatic hydrocarbons (PAHs) and heavy metals, making it even more hazardous ([7]). The degree of PM’s hazard to human health depends on its size, with the smallest particles (≤0.1 μm) being the most dangerous because they can reach the lungs and even penetrate the bloodstream [8,9]. According to the WHO, in 2019, 99% of the world’s population lived in areas where the WHO air quality guidelines were not met, with annual average PM2.5 concentrations exceeding 10 μg/m³ [10].

Air pollution has been a significant health and environmental issue across urban areas of Armenia due to high population density, rapid urbanization, vehicular emissions, and industrial and construction activities. Consequently, high levels of atmospheric pollution, including PM, gases, and heavy metals, have been detected in Armenia’s major cities, exceeding permissible standards [11,12,13,14,15]. According to WHO data, about 1 in 6 deaths in Armenia from stroke and ischemic heart disease are attributed to air pollution ([5,16,17]). The study of trace elements in PM revealed that maximum values for Cu, Pb, and Zn are observed in the cities of Yerevan and Gyumri [14]. According to data from the Hydrometerology and monitoring center of Ministru of Environment of RA, the concentration of total PM in the air often exceeds the maximum acceptable concentration (MAC) in Yerevan, Gyumri, and Vanadzor. These MAC values are, respectively, 147 µg/m³ (Yerevan), 174 µg/m³ (Gyumri), and 154 µg/m³ (Vanadzor) on a yearly average basis [18]. The main sources of atmospheric air pollution in Yerevan are industry, vehicles, and construction, while in Gyumri and Vanadzor, they are primarily industry and urban development.

In recent years, according to data from the “Hydrometeorology and Monitoring Center” SNCO of the Ministry of Environment, the concentration of PM in the atmospheric air in the cities of Yerevan, Gyumri, and Vanadzor has been observed to increase.

As evidenced by the information provided, the cities mentioned serve as ideal case studies for urban air quality management. Each of these cities faces challenges related to pollution across various components of the environment.

1.2. PM Removal from Ambient Air

As urbanization expands globally, cities face growing environmental challenges such as pollution, rising temperatures, and ecosystem degradation. Phytotechnologies—tree-based solutions—offer a sustainable way to address these issues. Trees provide a range of benefits, including improving air quality, reducing the urban heat island effect, managing stormwater, and supporting biodiversity. By integrating trees into urban landscapes, cities can enhance ecological resilience and improve residents’ quality of life. This introduction explores the role of trees in urban sustainability through phytotechnologies, emphasizing their multifunctional impact on the urban environment.

Plants serve as effective indicators of environmental pollution [19,20] and act as natural phytofilters, capable of absorbing large quantities of toxic substances from the environment, including PM [21,22,23]. The mechanisms of pollutant capture and detoxification in plant metabolism rely on gas exchange between plant tissues and the environment. The size, density, porosity, and foliage characteristics of plants are among the factors affecting PM removal from the air [9,24].

The dynamics of PM accumulation largely depend on the specificities of tree species, bioclimatic conditions, and the period of plant vegetation [25]. Acer, Fraxinus, Pinus, Prunus, Populus, Quercus, Ulmus, Tilia, Platanus, and Betula tree taxa are the main species capturing PM depending on the climate, seasons, foliar retention [26,27,28], deposition velocities (cm s⁻¹) [29,30,31], daily/yearly retention efficiency (mg m⁻² d⁻¹) [32], number density (particles mm⁻²) [8,33,34], and saturation isothermal remanent magnetization (SIRM) (µA) [27,35,36,37,38,39]. Several studies have demonstrated that trees in the urban sites removed 772 tons of PM10 for one year [1]. In similar studies in Chicago (USA), urban trees, which occupy 11% of the city area, removed about 234 tons of PM10 (Nowak 1994). In the whole USA, urban trees and shrubs remove about 215 kilotons of PM10 every year [40]. This is a more important role of vegetation, which has a direct positive effect on human health. Therefore, it is very important to select plant species with high phytofiltration properties for urban planting [41].

1.3. Urban Greenery and Biofiltration in Armenia

Presently, the state of green infrastructure in Armenian cities does not meet the contemporary standards for ecological development of urban spaces [42], leading to the emergence of unmanageable environmental situations. Parks, public gardens, roadside trees, and other green spaces are integral elements of urban planning, with specific guidelines in urban plans regulating the relationship between green and built spaces [42], thereby enhancing the quality of the environment. For instance, due to energy shortages and economic blockades during the 1990s in the Republic of Armenia, green spaces in almost all cities were reduced. In Yerevan, for example, green spaces decreased from 908.3 hectares to 883 hectares, in Gyumri from 528.8 hectares to 297.5 hectares, and in Vanadzor from 191.1 hectares to 136.2 hectares [43,44,45,46,47]. The reduction in green spaces subsequently resulted in a significant deterioration of the environment, as confirmed by numerous scientific studies conducted by Armenian researchers [11,12,14,48,49,50]

The available data on PM absorption by tree leaves in Yerevan show that the Platanus orientalis L. tree exhibited a slightly higher absorption capacity of PM₁₀₀ compared to the Fraxinus excelsior L. tree. However, Fraxinus excelsior was found to be more than 1.5 times more efficient at depositing PM₁₀ and PM_2.5 fractions than the Platanus orientalis L. tree. Additionally, high contents of Al, Si, Ca, Fe, Se, Mo, Ba, and Pb were identified in the chemical composition of PM₁₀, PM_2.5, and PM₁₀₀ fractions [15]. The mean concentration of Hg in leaf-deposited total PM and street dust were 0.57 and 0.26, respectively, in Vanadzor City (near the Vanadzor Chemical Plant) [48]

The main objective of this work was to investigate and compare the accumulation of PM on the leaves of park and street trees under varying environmental and climatic conditions. Six of the most common tree species were selected in the studied cities of Yerevan, Gyumri, and Vanadzor to document the accumulation of PM during the summer season of July 2022. These species include Fraxinus excelsior, Tilia caucasica, Aesculus hippocastanum, Acer pseudoplatanus, Quercus robur, and Platanus orientalis. This study takes a comprehensive approach to assessing the ecological status of trees. By combining visual observation, botanical identification, and measurement of PM capture, it provides a thorough understanding of how trees interact with their environment. Moreover, comparing PM capture data across park and street areas as well as different cities with varying climate conditions offers valuable insights into the effectiveness of different tree species in mitigating air pollution. This holistic comparison enables urban planners and policymakers to make informed decisions about urban greening initiatives tailored to specific environmental contexts, ultimately contributing to improved air quality and overall wellbeing in urban areas.

2. Material and Methods

2.1. Research Sites

The studies were conducted in three large cities of the Republic of Armenia: Yerevan, Gyumri, and Vanadzor for the period 2021–2023 (Figure 1).

Yerevan, the capital city of the Republic of Armenia, is situated at a latitude of 40°10′40″ N and longitude of 44°30′45″ E, covering an area of 223 km² (Figure 1). The climate is continental, characterized by a wide temperature range. Summers typically see temperatures ranging from +22 to +26 °C, while winters can drop to between −20 and −30 °C. The city encompasses semi-desert, arid steppe, and steppe landscapes and is situated at elevations ranging from 850 to 1420 m above sea level. The mean annual precipitation is 277 mm [51]. Yerevan faces significant water scarcity, particularly during the summer months. The city experiences a water deficit, with up to 1000–1200 mm of water evaporating annually, resulting in a humidity index of 0.25–0.30. As a result, greening efforts in Yerevan rely heavily on artificial irrigation [17]. Despite covering only 0.7% of the total area of Armenia, Yerevan is home to 36.6% of the country’s population. Furthermore, the city hosts 41.2% of the country’s industrial capacity [52].

Gyumri, the capital of Shirak Province, is the second-largest city in the Republic of Armenia, following the capital Yerevan. As of 1 January 2022, Gyumri had a population of 112,301 inhabitants. The city is situated at a latitude of 40°47′0″ N and longitude of 43°50′0″ E, covering an area of 44.41 km². Gyumri is located in the central part of the Shirak Plateau, at an elevation of 1550 m above sea level [51]. The climate in Gyumri is characterized as humid continental, with warm summers and cold winters. The warmest month of the year is August, with a mean temperature of +19 °C, while the coldest month is January, with a mean temperature of −9 °C. The city receives an annual mean precipitation level of 500 mm.

Vanadzor is situated in the north of Armenia at a latitude of 40°48′40.53″ N and a longitude of 44°29′6.26″ W, covering an area of 26.6 km². The city is located at an altitude of 1350 m above sea level [51]. The climate in Vanadzor is moderate, characterized by cool summers and cold winters. Average annual temperatures range between −4.2 °C in winter and +24°C in summer. The city receives an annual mean precipitation level of 600 mm.

Gyumri and Vanadzor cities have more favorable humid continental climates than Yerevan.

2.2. Urban Trees Inventory and Mapping Methodology

The ecological and dendrological state of the plants was estimated based on an inventory of trees growing in main parks and street locations in Yerevan, Gyumri, and Vanadzor by the team of Dendrological Laboratory at the Institute of Botany after A.L. Takhtajyan. Based on well-known methods, the dendrological collections behavior, ecological adaptation and assessment of decorativeness (climatic analogs, genus complexes, the ecological adaptation of woody plants, their evaluation of decorativeness, etc.), and methodological guidelines have been studied [53,54,55,56]. The following assessment approach was applied:

• Broad-leaved species: 0–25 points—low decoration, 26–30 points—average decoration, 31–35 points—high decoration, and 36 and more points—very high decoration.
• Coniferous species: 0–10 points—low decoration, 11–15 points—average decoration, 16–20 points—high decoration, and 21 and more points—very high decoration.

In this scoring system, the assessment was made according to the characteristics of the species’ decoration: crown—1–5 points, duration of flowering (day)—1–5 points, the abundance of bloom—1–5 points, size and color of flowers—1–5 points, fruits or cones attractiveness, and color—1–5 points, aroma—1–5 points, autumn leaf color—1–5 points, and duration of leaf presence—1–5 points [50,57,58,59,60].

Based on well-established methods [57,58], we conducted an assessment of the ecological status of plants. Prior to the assessment, we selected a set of visible stress indicators for each specimen, including damage to the assimilation apparatus, the number of dried branches, crown and trunk deformation, and other signs of stress. The condition of the trees was evaluated using a 5-point scale. First category—Excellent: the tree is in optimal health with no visible signs of stress or damage. The tree is thriving and has a healthy appearance (lush green leaves, no damage to the trunk or crown, no signs of disease or pest infestation, and no deformations). Second category—Good: the tree is generally healthy but may show slight signs of stress. It is growing well, but there could be minor issues that do not significantly affect overall health (minor leaf damage or slight crown thinning, but it still exhibits a strong and vigorous growth pattern). Third category—Poor: the tree is showing clear signs of stress and declining health. There might be a noticeable decrease in growth, and the tree could have visible damage or disease (significant leaf loss, crown damage, visible trunk or branch damage, signs of pest infestation, and possibly some deformation of the branches or trunk). Fourth category—Extremely Poor: the tree is in very poor condition, with major visible signs of stress and severe damage. Its survival is at risk, and it may require immediate attention (extensive leaf loss, major trunk or branch deformation, large areas of disease or damage, and possibly parts of the tree dying. The tree’s overall vitality is critically low). Fifth category—Dead: the tree has died and is no longer living. It no longer contributes to the ecosystem and is beyond recovery (the tree is completely leafless, and its trunk and branches may be brittle, dry, or fallen. There is no sign of life, and the tree is no longer performing any biological functions).

Remote sensing and field observation data were combined to produce comprehensive and credible information about problematic sites in Yerevan’s green spaces. Different cartographic materials were developed on green area distribution for 12 administrative districts of Yerevan, green zones per capita, etc., by using GIS packages (ArcGIS 10.5 package). Snapshots of the World Imagery server with a resolution of 0.3 m to 0.03 m were used.

2.3. Plant Material and Sampling for Quantitative Assessment of PM Deposition on Foliage

The study object was tree species commonly grown along busy urban streets and parks in Armenia were selected for this study: Platanus orientalus, Querqus robur, Fraxinus excelsior, Tilia cordata, Aesculus hippocastanum, and Acer pseudoplatanus. Tree leaf sampling was performed based on scientific literature guidance [61,62,63]. All plants had already been growing in the selected locations for several years and were in good condition, healthy, and free from pests.

In the city of Yerevan, sampling was conducted from the Platanus orientalis and Quercus robur tree species, which are the most prevalent in the city’s green plantations. The sampling sites included Mankakan Park (Children’s Park), Mashtots Avenue, and Beirut Street. These areas are all situated in the center of Yerevan. Mankakan Park is located between 4 quite traffic-heavy streets, one of which is Beirut Street, and the other is Agatangeghos Street, which, according to the data of the State Monitoring Center, is one of the most polluted points in terms of air pollution level. Mashtots Avenue is considered one of the most congested areas of the center administrative district (Figure 2).

In the city of Gyumri, sampling was conducted from the Fraxinus excelsior and Tilia cordata tree species. The sampling sites included Central Park “Maxim Gorki” and the streets of Rijkov and Pushkin (Figure 3). All of the listed streets are in the central part of the city and are exposed to all negative anthropogenic factors.

In the city of Vanadzor, sampling was conducted from the Aesculus hippocastanum and Acer pseudoplatanus tree species. The sampling sites included the park after Charles Aznavour (Park of Chemical Factory Workers) and the streets of Tigran-Mec and Abovyan (Figure 4). These areas are in the central part of the city, and it is worth noting that Vanadzor was an industrial city in the past, although, currently, some industrial enterprises are still working at full capacity.

Plant material was harvested in the second half of July 2022 from the parks and streets in 4 replicates (a replicate was a single tree of each species grown in the park or close to the street). Leaves were randomly collected from four directions (east, south, west, and north) at a height of 1.5–2.0 m above the ground (the height of a human face). Each sample was composed of 10–15 tree leaves and had an area of between 300 and 500 cm². Then, harvest samples were placed in a paper bag and stored (22 °C, 70% RH) for about 1 week until analysis. The samples were sent to Life Science University (Laboratory of Basic Research in Horticulture, Faculty of Horticulture, Biotechnology and Landscape Architecture) in Poland for the determination of different PM particles in the leaves.

2.4. Quantitative Assessment of PM and Leaf Wax Content

The content of particulate matter (PM) was examined following the method outlined by [26]. Two categories of PM were considered: (i) water-washable PM from leaf surfaces (SPM) and (ii) PM retained in leaf wax (WPM). The plant material was first washed with 250 mL distilled water for 60 s and thereafter with 150 mL chloroform for 40 s. Fractional division for both categories was performed sequentially. The washing solutions were first sieved through a metal sieve (retention 100 μm, Haver & Boecker, Oelde, Germany) and filtered through a 10 μm paper filter (Whatman, Maidstone, UK, Type 91) and then a 2.5 μm paper filter (Whatman, UK, Type 42). The filtration was carried out using a 47 mm glass filter funnel with a stopper support assembly (PALL Corp., Port Washington, NY, USA) connected to a vacuum pump. Three fractions of PM were thus collected: (i) 10–100 (large), (ii) 2.5–10 (coarse), and (iii) 0.2–2.5 μm (fine). The sum of all PM fractions was designated as the total PM. The filters were dried for 45 min at 60 °C, stabilized in the weighing room for 45 min, and weighed (balance and deioniser gate from HAUG, St. Gallen, Switzerland) before and after filtration. The number of waxes dissolved in chloroform was assayed for every plant sample in the pre-weighed Image Analysis System (Skye Instruments Ltd., Llandrindod Wells, UK, and Skye-Leaf software 1.21), allowing the amount of PM and waxes to be always expressed in this work as μg cm⁻².

Data were subjected to analysis of one-factorial analysis of variance using Statgraphics Plus 4.1 (Statpoint Technologies Inc., Warrenton, VA, USA). Differences between means of combinations were evaluated by post hoc Tukey’s honestly significant difference (HSD) test. Means were considered to be significantly different at p < 0.05. The data are means ± SD. The correlations between the concentrations were calculated using Pearson’s correlation coefficient in Microsoft Excel.

3. Results and Discussion

3.1. Assessment of the Ecological Status of Plants

The obtained results indicate that the green areas of Yerevan encompass approximately 160 types of trees and shrubs, whereas in Gyumri, there are around 60 species, and in Vanadzor, approximately 30 species are observed. Among the most widespread trees and shrubs in Yerevan are both native and non-native species, including Fraxinus excelsior, Platanus orientalis, Platanus acerifolia, Quercus robur, Ulmus pumila, Acer negundo, Robinia pseudoacacia L., Acer campestre, Acer platanoides, Berberis vulgaris, Betula litwinowii, Carpinus betulus, Cerasus mahaleb, Clematis orientalis, Corylus avellana, Cotinus coggygria, Fraxinus excelsior, Padus racemosa, Philadelphus caucasicus, etc. Currently, the basic assortment of green plantations of Gyumri consists of the following species: Fraxinus excelsior, Pinus hamate, Acer campestre, Acer platanoides, Grossularia reclinata, etc. In the green plantations of Vanadzor City, there are currently 30 species belonging to 12 families and 19 genera. The main tree species include Fraxinus excelsior, Acer pseudoplatanus, Acer campestre, Aesculus hippocastanum, Tilia caucasica, Populus gracilis, Thuja occidentalis “Columnaris”, Biota orientalis, Picea abies, and others. Additionally, shrubs such as Ligustrum vulgare, Buxus sempervirens, and Spiraea x vanhouttei are also present in these green areas (

Table 1. Total PM in the atmospheric air in the cities of Yerevan, Gyumri, and Vanadzor, 2022 [18].

Total PM	Mean Concentration, μg/m3	Monthly Concentration, July	Monthly Concentration, August	Maximum Acceptable Concentration (MAC) Average Daily, μg/m3
Yerevan	147	190	180	150
Gyumri	174	250	240
Vanadzor	154	150	170

and

Table 2. Total PM in the atmospheric air in the cities of Yerevan, Gyumri, and Vanadzor, 2018–2022 [18].

	Total PM	2018	2019	2020	2021	2022	2022 Monthly Concentration, July	2022 Monthly Concentration, August	Maximum Acceptable Concentration (MAC) Average Daily, μg/m3
Yerevan	Mean concentration, μg/m3	110	128	117	172	147	190	180	150
	Sample quantity	1711	1729	1542	1755	1803	- ***	- ***
Gyumri	Mean concentration, μg/m3	90	84	51	91	174	250	240
	Sample quantity	295	301	339	332	346	- ***	- ***
Vanadzor	Mean concentration, μg/m3	195	165	180	165	154	150	170
	Sample quantity	205	210	270	266	280	- ***	- ***

- *** Data on sample numbers are missing.

).

As can be seen from

Table 3. The taxonomic composition of the main trees and shrubs used in the cities of Armenia (Yerevan, Gyumri, and Vanadzor).

Family	Yerevan		Gyumri		Vanadzor
	Genera	Species	Genera	Species	Genera	Species
Aceraceae	1	8	1	4	1	5
Anacardiaceae	1	1	1	1	-	-
Berberidaceae	2	3	-	-	1	2
Betulaceae	3	6	-	-	-	-
Bignoniaceae	2	3	-	-	-	-
Buxaceae	1	2	-	-	1	1
Caprifoliaceae	3	7	2	3	-	-
Celastraceae	1	2	-	-	-	-
Cornaceae	1	3	-	-	-	-
Cupressaceae	3	10	2	3	2	3
Elaeagnaceae	2	2	-	-	-	-
Fabaceae	9	13	2	3	-	-
Fagaceae	1	5	1	1	1	1
Ginkgoaceae	1	1	-	-	-	-
Grossulariaceae	1	3	2	3	-	-
Hydrangeaceae	2	4	1	1	-	-
Hippocastanaceae	1	1	1	1	1	1
Juglandaceae	1	2	1	1	-	-
Moraceae	4	6	-	-	-	-
Oleaceae	4	10	3	7	2	2
Pinaceae	3	6	2	6	3	5
Platanaceae	1	3	-	-	-	-
Rhamnaceae	2	3	-	-	-	-
Rosaceae	16	27	8	10	3	4
Salicaceae	2	10	2	11	2	2
Sapindaceae	1	1	-	-	-	-
Tamaricaceae	1	1	-	-	-	-
Tiliaceae	1	2	1	1	1	2
Ulmaceae	1	5	1	4	1	2
Vitaceae	2	2	-	-	-	-
Total	74	152	31	60	19	30

, Yerevan stands out with the most biodiversity of trees: 74 genera and 152 species, the main reason for which is the inclusion of more species of trees during the implementation of greening works in the city. The studies of the green areas of the three cities showed that 10 of the 30 families of tree species are found in the three studied cities despite different climatic conditions, which indicates that the mentioned plants have the most flexible ecological properties: Aceraceae, Cupressaceae, Oleaceae, Pinaceae, Rosaceae, Salicaceae, and Ulmaceae families are especially distinguished by species biodiversity. It should be noted that Fraxinus excelsior and Robinia pseudoacacia from the mentioned families are quite resistant to high levels of atmospheric pollution and can be effective phytofilters [64,65].

The summarized data reveals a diverse taxonomic composition in the studied cities, attributable to their distinct climate conditions. Plant species include both broadleaf and evergreen tree species, as well as shrubs. It is crucial to consider not only the climatic conditions but also the degree of pollution in these areas. Some tree species were found in the three studied cities, showing stability to both climate and high levels of environmental pollution, which is important to consider when planning landscaping works. Understanding these factors can inform decisions regarding urban greening initiatives and contribute to the development of effective strategies for improving air quality and overall environmental health in these cities.

The ecological assessment conducted through visual observation enabled the evaluation of the ecological status of plants in the green plantations of parks and streets in Yerevan, Gyumri, and Vanadzor cities. The parks and street trees under study were assessed using a 5-point grading scale, allowing for a comprehensive understanding of their ecological health and condition (Figure 5).

A general assessment of all tree species in the green spaces allows us to judge the general condition of the streets and parks. Previous studies indicate that if the total quantity of plants in poor condition (the 3rd group) does not exceed 15%, then the condition of plantations is assessed as excellent; up to 30% is good, 31–50% is poor, and over 50% is extremely poor [12,57,58] The results obtained in this work indicate that all parks and streets trees in the studied cities received positive evaluations. Specifically, the second and third categories combined accounted for more than 40% of the total number of plants assessed. This suggests a generally favorable ecological status of the green plantations in these cities. Yerevan and its neighborhood are also known for scarce water resources, frequent heatwaves, and continuous droughts due to the invasion of dry and sandy air from the Arabic deserts [66]. Therefore, greening is impossible without artificial irrigation in Yerevan [17]. Considering this circumstance, the city government has taken stock of the condition of the irrigation system over the last 4 years, restoring and expanding the irrigation network year after year, which in turn positively affected the relatively good condition of the trees. However, some trees have been detected with insects and fungi, which is a big problem not only for Armenia. Some results of studies by European researchers showed that the most important problems related to the trees of parks and streets are the leaves being attacked by insects and fungi as well as dry and broken branches of the crown. Particularly, insects and fungal damage were observed more often in Platanus orientalis and acerifolia [67]. The dry and broken branches of the crown in the trees of the species Celtis australis, Albizia julibrissin, Liquidambar orientalis, and Cupressus arizonica and the clone Populus X euramericana cv. ‘I-45/51’ cause bark damage Celtis australis and Albizia julibrissin and are also a big problem in those trees whose crown touches the crown of the other trees (Cupressus arizonica and Platanus orientalis).

Our investigation showed that in the cities of Gyumri and Vanadzor, prevalent trees are deeply pruned and diseased, often infested with insects. These conditions are attributed to years of incorrect agronomic measures or their absence altogether, resulting in the drying up of many trees. However, in relatively favorable climatic conditions, newly planted trees appear to be in good condition. In contrast, in Yerevan, the number of dry trees is gradually decreasing. This positive trend is attributed to the city government’s efforts to replace old and dry trees with new ones that possess both aesthetic value and vitality.

In certain areas, especially those near industrial sites, numerous trees exhibit visible morphological injuries. These observations confirm the presence of unfavorable environmental conditions, particularly in terms of air pollution. The industrial activities in these areas likely contribute to the degradation of air quality, leading to adverse effects on the health and vitality of the surrounding vegetation (Figure 6).

The most tolerant tree species to the condition of the studied areas were Platanus orientalis, Fraxinus excelsior, and Quercus robur in Yerevan; Tilia caucasica, Sorbus persica, Fraxinus excelsior, and Populus alba in Grumri; and Acer pseudoplatanus, Fraxinus excelsior, Aesculus hippocastanum, and Thuja occidentalis in Vanadzor. It was mentioned in the previous investigations about Yerevan parks and streets that the basic assortment of park and square trees was the following: Acer negundo L. (19%), Ulmus pumila L. (13%), Robinia pseudoacacia L. (10%), Fraxinus excelsior L. (9%), and Platanus orientalis L. (8%) in the total amount of trees. Moreover, introduced tree species were planted in recent years (Koelreuteria paniculata Laxm.), which showed a fairly high resistance to the polluted environment [12].

Summarizing the green plantations of the studied cities, it can be noted that the species composition has not undergone significant changes over time, except for the newly planted trees, which have been planted especially in recent years in the city of Yerevan, which is the reason for such a variety of trees in Yerevan (74 genera, 152 species). All parks and street trees in the studied cities were assessed as satisfactory, although some streets had old and dry trees, sometimes infected with insects and fungi, caused by improper tree care. Otherwise, the trees in all cities were in relatively good condition, especially in the city of Yerevan, where the irrigation system is already fully operational, which is a very important factor for cities with a dry climate.

3.2. Quantitative Assessment of PM and Leaf Wax Content on Plant Foliage

Total PM accumulation on leaves among six tree species in the studied cities showed a significant difference for different species and locations, i.e., in the streets and parks. The largest amounts of PM were recorded in Yerevan city, and the accumulation of PM in street trees was significant: Querqus robur (31.67 μg/sm²) and Platanus orientalis (28.18 μg/sm²) (Figure 7). The high PM concentrations in the case of Yerevan relate to the intensive construction carried out in the city, as well as the presence of 28 mines in the city. Previous studies for Yerevan, Gyumri, and Vanadzor showed higher than normal levels of heavy metals, chlorine, and nitrogen in tree leaves and soil, which were accompanied by changes in the stages of vegetation: early leaf fall, as well as changes in the appearance of foliage, the presence of dry branches, etc. [12,15,49,65]. However, Gyumri and Vanadzor cities also have high amounts of PM, especially on the streets (Figure 7). The obtained data were confirmed with the data from the Center of Hydrometeorology and Monitoring of the Ministry of Environment of RA, where, in the air basin during summer time, the maximum acceptable concentration (MAC) total PMs’ has been, respectively, 180–190 μg/m³ for Yerevan, 150–170 μg/sm² in Vanadzor, and 240–250 μg/sm² in Gyumri.

As for the PM absorption potential of the tree species, it is known that the ability to accumulate PM by plants depends on leaf structure (fluffy leaf, roughness, leaf area, and the amount of wax on the leaves), so tree species with a larger leaf surface and non-smoothness have a greater potential for PM absorption [68,69]. Leaf density also may influence a plant’s ability to accumulate PM. Our investigation confirms that the highest PM was detected in the leaves of Quercus robur (31.67 μg/sm²) (in Yerevan), Platanus orientalis (28.18 μg/sm²) (in Yerevan), Aesculus hypocastanum (23.08 μg/sm²) (in Vanadzor), and Fraxinus excelsior (22.65 μg/sm²) (in Gyumri), which have leaf hair and large leaf areas and as well as street plantings.

In general, significant accumulations of PM were found in four species of the three studied cities (Quercus robur, Platanus orientalis, Aesculus hypocastanus, and Fraxinus excelsior). The highest amounts of PM particles were recorded in the city of Yerevan, which has a dry continental climate limited amount of precipitation, especially in summer. To reduce environmental pollution for urban greening, it is very important to choose tree species that are tolerant to environmental pollution and can show stability in accumulating such large amounts of dust particles. For this purpose, we elaborated the tree species and locations according to their PM particle absorption potential.

• Streets: Quercus robur (31.67 µg/cm²), Platanus orientalis (28.18 µg/cm²), Aesculus hypocastanus (23.08 µg/cm²), Fraxinus excelsior (22.65 µg/cm²), Acer pseudoplatanus (21.24 µg/cm²), and Tilia caucasica (14.75 µg/cm²).
• Parks: Quercus robur (18.36 µg/cm²), Aesculus hypocastanus (17.28 µg/cm²), Platanus orientalis (16.45 µg/cm²), Acer pseudoplatanus (13.96 µg/cm²), Fraxinus excelsior (12.46 µg/cm²), and Tilia caucasica (10.58 µg/cm²).

Taking into consideration the aforementioned circumstance, as well as previous studies in which significant amounts of various toxic substances were also confirmed in these tree species, it is recommended to separate the sections of the street sidewalks with a barrier layer when greening, thereby ensuring the health safety of pedestrians.

As plants can accumulate PM both on foliage surfaces (sPM) and in waxes (wPM), and, according to the percentage ratio, the shares of sPM and wPM in the total amount of PM in the leaf surface of plants are almost equal, although, in some species, the share of sPM slightly prevails over wPM, reaching 61% (Vanadzor city, Aesculus hypocastanus) (

Table 4. The contents of the total PM and its various particles in the leaves of trees in cities of Armenia.

City	Tree Species	Location	sPM μm	sPM % of Total PM	wPM μm	wPM% of Total PM	Total PM μm
Yerevan	Quercus robur	Park	8.1513	49.5	8.3022	50.4	16.4535
		Street	14.4565	51.2	13.7291	48.7	28.1856
	Platanus orientalis	Park	9.7750	53.2	8.5909	46.8	18.3659
		Street	17.1462	54.1	14.5207	45.9	31.6669
Gymri	Fraxinus excelsior	Park	6.3411	50.9	6.1156	49.1	12.4566
		Street	12.0416	53.1	10.6093	46.8	22.6509
	Tilia	Park	5.5326	52.2	5.0511	47.7	10.5837
		Street	7.9549	53.9	6.7907	46.1	14.7457
Vanadzor	Aesculus hypocastanus	Park	10.6564	61.7	6.6244	38.3	17.2809
		Street	14.2845	61.9	8.8019	38.1	23.0864
	Acer	Park	8.1644	58.4	5.8005	41.5	13.9649
		Street	11.5886	54.6	9.6544	45.4	21.2431

). The total PM and sPM are higher in the samples of trees taken from the streets, which confirms, again, the high level of PM pollution in street areas compared with parks. For example, Platanus orientalis recorded an sPM of 17.14 μm (54.1%). Yerevan’s dry climate restricts PM retention on trees, as low summer rainfall prevents natural cleaning. It is important to note here that the high content of wPM proves that even in the case of heavy rains, PM is not washed off the leaves of trees, thus causing the long-term disruption of leaf surface gas exchange and metabolic changes, ultimately leading to the drying of the whole plant [70].

In general, large amounts of total PM and sPM were found in the tree species on Yerevan’s streets, possibly due to the comparatively low precipitation levels and along which there is continuous large-scale building and road construction. The other reason is due to the size of the leaf surface, which is also clearly seen in the leaf samples of Aesculus hypocastanus (total PM = 23.0864 μm, sPM = 14.2845 μm) taken in the city of Vanadzor. As for the wPM out of total PM, this was 38.3–50.4% in leaves, with the maximum dose found in the leaves of Quercus robur (wPM% of total PM = 50.4%) in Yerevan, which is directly related to the dryness of the climate.

The ratio of different sizes of PM in total PM, PM10–100, PM2.5–10, and PM0.2–2.5, was analyzed, and a very interesting picture was obtained (

Table 5. Total amount and division into categories (sPM and wPM) of large (10–100 μm), coarse (2.5–10 μm), and fine (0.2–2.5 μm) PM accumulated on the foliage of examined plant species.

City	Tree Species	Location	PM10-100	PM10–100% of Total PM	PM2.5–10	PM2.5–10% of Total PM	PM2.5–10 μm	PM0.2–2.5% of Total PM	Total PM
Yerevan	Quercus robur	Park	11.2832	68.6	4.0565	24.7	1.1138	6.8	16.4535
		Street	20.1934	71.7	5.8448	20.7	2.1474	7.6	28.1856
	Platanus orientalis	Park	12.9926	70.7	4.5033	24.5	0.8701	4.7	18.3659
		Street	22.1507	69.9	6.7193	21.2	2.7969	8.8	31.6669
Gymri	Fraxinus excelsior	Park	8.4343	67.7	2.6397	21.2	1.3826	11.1	12.4566
		Street	16.0633	70.9	4.5343	20.0	2.0533	9.1	22.6509
	Tilia caucasica	Park	7.9068	74.7	1.6852	15.9	0.9917	9.3	10.5837
		Street	10.9152	74.0	2.3334	15.8	1.4970	10.1	14.7457
Vanadzor	Aesculus hypocastanus	Park	12.6986	73.4	3.7473	21.7	0.8350	4.8	17.2809
		Street	17.1180	74.1	4.1168	17.8	1.8516	8.0	23.0864
	Acer pseudoplatanus	Park	10.2454	73.3	2.7293	19.5	0.9903	7.1	13.9649
		Street	13.7683	64.8	5.6930	26.8	1.7818	8.3	21.2431

). Predominant PM10-100-micron particles in total PM were 64.8–74.7%, with maximum values recorded from the park and streets of Gyumri city (Tilia cordata—74.7%, 74.0%) and Vanadzor Street (Aesculus hypocastanum—74.1%) in the studied samples. Other PM2.5–10-micron particles made up 15.8–26.8% according to the percentage, with the highest values recorded from the street plantations of Vanadzor (Acer pseudoplatanus—26.8%) and the park of Yerevan city (Quercus robur—24.7%; Platanus orientalis—24.5%) in samples.

Taking into account the size of the particles, based on which the degree of PM danger is assessed, its high content in Yerevan, especially in the park, can cause serious health risks from the point of view of the health of the population, especially since the Yerevan Park is functionally a children’s park, where the main visitors are young children. As for the percentage of PM2, which is considered the most dangerous of the PM particles, these particles made up 4.8–11.1% of the total PM. The highest values were recorded in the samples taken from the parks of Gyumri city (Fraxinus excelsior—11.1%) and the streets (Tilia cordata—10.1%). Here, taking into account the smallest size of PM particles and, therefore, the high degree of danger from the point of view of public health, it is necessary to take measures to reduce PM emissions.

Generalizing, we can say that the large dose of large PM particles was caused by the location because the trees were located on the street. The relatively heavy PM particles accumulated in the vicinity of PM10–100 μm, the lighter (PM2.5–10 μm and PM0.2–2.5 μm) particles partially settled to the leaves of nearby trees, and most of it was carried by the airflow to more distant areas, in this case to the parks. An important factor here is also the presence of winds (mountain valley winds), which are very frequent in the summer, especially in the cities of Yerevan and Gyumri; one can even say almost every day.

As for the absorption of different particles of PM by tree species, trees with relatively large and smooth leaf surfaces have a high potential for PM accumulation. So, leaf density may influence a tree’s ability to accumulate PM, and leaf hair helps to increase plants’ ability to accumulate PM.

4. Conclusions

• The summarized data reveal a diverse taxonomic composition in the studied cities, including both broadleaf and evergreen tree species, as well as shrubs, attributable to their distinct climate conditions. It is crucial to consider not only the climatic conditions but also the degree of pollution in these areas. Some tree species were found in the three studied cities, showing stability to both climate and high levels of environmental pollution, which is important to consider when planning landscaping works.
• Summarizing the green plantations of the studied cities, it can be noted that the species composition has not undergone significant changes over time, except for the newly planted trees, which have been planted especially in recent years in the city of Yerevan, which is the reason for such a variety of trees in Yerevan (74 genera, 152 species). All parks and street trees in the studied cities were assessed as satisfactory, although some streets had old and dry trees, sometimes infected with insects and fungi, caused by improper tree care. Otherwise, the trees in all cities were in relatively good condition, especially in the city of Yerevan, where the irrigation system is already fully operational, which is a very important factor for cities with a dry climate.
• In general, significant accumulations of PM were found in four species of the three studied cities (Quercus robur, Platanus orientalis, Aesculus hypocastanus, and Fraxinus excelsior). The highest amounts of PM particles were recorded in the city of Yerevan, which has a desert climate and a limited amount of precipitation, especially in summer. To reduce environmental pollution for urban greening, it is very important to choose tree species that are tolerant to environmental pollution and can show stability in accumulating such large amounts of PM particles. For this purpose, we separated the trees according to their PM particle absorption potential.
  Streets: Quercus robur (31.67 µg/cm²), Platanus orientalis (28.18 µg/cm²), Aesculus hypocastanus (23.08 µg/cm²), Fraxinus excelsior (22.65 µg/cm²), Acer pseudoplatanus (21.24 µg/cm²), and Tilia caucasica (14.75 µg/cm²)
  Parks: Quercus robur (18.36 µg/cm²), Aesculus hypocastanus (17.28 µg/cm²), Platanus orientalis (16.45 µg/cm²), Acer pseudoplatanus (13.96 µg/cm²), Fraxinus excelsior (12.46 µg/cm²), and Tilia caucasica (10.58 µg/cm²).
  Considering the aforementioned circumstance, as well as previous studies in which significant amounts of various toxic substances were also confirmed in these tree species, it is recommended to separate the sections of the street sidewalks with a barrier layer when greening, thereby ensuring the health and safety of pedestrians.
• In general, Yerevan’s tree species showed high levels of total PM and sPM, probably due to limited summer rainfall and the size of leaf surfaces. For instance, leaf samples from Aesculus hypocastanus in Vanadzor revealed total PM levels of 23.0864 μm and sPM of 14.2845 μm. Meanwhile, wPM constituted 38.3–50.4% of total PM in leaves, with the highest proportion found in Quercus robur in Yerevan, where wPM made up 50.4% of total PM. This accumulation is directly linked to the region’s dry climate.
• Overall, the high concentration of large PM particles is attributed to the trees’ location on streets, where heavier particles (PM10–100 μm) accumulate. Meanwhile, lighter particles (PM2.5–10 μm and PM0.2–2.5 μm) partially settle on nearby leaves, while most are carried by airflow to more distant areas, such as parks. Additionally, frequent, almost daily, mountain valley winds, especially in Yerevan and Gyumri during the summer, contribute significantly to this distribution.

5. Recommendations

By adopting the following recommendations, cities may enhance their ecological resilience, improve air quality, and create healthier, more sustainable urban environments.

To improve urban sustainability and air quality, urban planning should prioritize sustainable tree selection, focusing on pollution-tolerant and drought-resistant species like Quercus robur, Platanus orientalis, Fraxinus Excelsior, and Aesculus hypocastanus. Additionally, cities should invest in regular maintenance and monitoring of urban trees, particularly in high-traffic areas, to address pest issues and ensure tree health. The integration of plant barrier layers in sidewalk greening projects, especially in areas with high particulate matter (PM) accumulation, can help protect public health.

In cities with dry climates, such as Yerevan, it is crucial to expand irrigation systems, including drip irrigation, to ensure the longevity and environmental benefits of trees. Finally, continued research and data collection on the impact of tree species on air quality and PM absorption will help refine urban greening strategies, optimizing their effectiveness in different climatic conditions.

Key areas for further studies may include long-term monitoring of PM accumulation, impact on urban microclimates, biodiversity and ecosystem services, and maintenance practices.