Review: Implications of Air Pollution on Trees Located in Urban Areas

Abstract

Air pollution in cities is intensifying, inevitably affecting all living organisms, gincluding trees. Urban trees are vital for cities because they improve air quality and regulate the climate; however, like all living organisms, they are affected by the environment to which they are exposed. In cities, the primary atmospheric pollutants of inorganic origin include NO, SO_X, CO_X, O₃, and suspended particulate matter (PM_2.5 and PM₁₀). Each of these pollutants impacts population health, with urban trees undergoing a series of consequent alterations. In this study, we review the inorganic pollutants identified by the World Health Organization (WHO) as impacting air quality in cities in different regions of the world; discuss the regulations that govern NO₂, SO₂, CO, O₃, and PM_2.5 and PM₁₀ emissions and their impact they have on urban trees; analyze the processes involved in pollutant–tree interactions and the related tolerance and/or resistance mechanisms; and determine the tree species with the best tolerance, classified using an air pollution tolerance index (APTI).

1. Introduction

Trees are essential for life on Earth as we know it. They play a crucial role in capturing and retaining water, acting as climate regulators, and providing shelter for many species of animals, birds, and insects; they also serve as habitats for various species of microorganisms. Trees purify the air through the direct removal of air pollutants and the uptake of reactive oxygen species (ROS) and reactive nitrogen species (RNS); in addition, they detoxify the air through leaf stomata, while tree bark can also absorb air pollutants. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with the inner leaf surface. Additionally, some particles can be absorbed by the tree, although most intercepted particles are retained on the leaf surface. Trees are thus essential to our planet and are often planted in urban settings as an effective strategy to assess air quality in polluted urban areas [1,2,3], while they contribute to reducing CO₂ levels by sequestering and storing it in their biomass [4].

Air pollution is a global environmental problem that is worsening due to the increase in pollutant gas emissions into the atmosphere; this is mainly caused by anthropogenic activities, the growth of cities around the world, and the use of fossil fuels [5] and poses a severe challenge for the health of humans, animals, and plant species.

Poor air quality in urban areas is mainly caused by the following pollutants: ozone (O₃), carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), total suspended particles (TSP), and particulate material (PM_2.5 and PM₁₀). These pollutants are included in the World Health Organization Global Air Quality Guidelines [6] and the Metropolitan Air Quality Index (IMECA) in Mexico, which is based on the federal air quality standard of the United States National Ambient Air Quality Standards (NAAQS). In Europe, the European Environmental Agency regulates the permissible levels of these contaminants and has recently made modifications to its regulations, promoting the Green Pact that seeks to reduce the emissions of these pollutants by up to 55% by 2030 (compared to 1990) [7]. Other relevant pollutants are heavy metals such as mercury and lead, and the burning of fossil fuels is the primary source of heavy metal pollution [8]. Globally, 64% of countries have legislation regarding air quality, meaning 34% still lack any regulation, although 31% have established air quality standards. However, these standards are often not applied because an adequate administrative process for their implementation has not been developed [9]. The air quality is generally cleaner in areas with abundant trees, such as urban forests, parks, greenbelts, and schools, but this depends on the types of trees, their distribution and quantity, and the available forest space [10]. While some tree species are stimulated by pollutants, growth is affected in others [11]. Trees can develop sicknesses and suffer metabolic alterations due to the exposure to and buildup of polluting particles, which mainly accumulate in the foliage and allow diseases caused by bacteria, fungi, and viruses to take hold, causing a decrease in the total chlorophyll content, the inhibition of photosynthesis, acidification in cells, and the failure of physiological activities, mainly impacting the respiration system and gas exchange [12]. There are many tree species reported to have characteristics that allow them to develop tolerance to various environmental contaminants in urban environments, such as higher concentrations of chlorophyll and ascorbic acid [13], which has opened up the possibility that these organisms could be used to mitigate the effects of pollutants. A study that used computer simulations with local environmental data revealed that trees and forests in the conterminous United States removed 17.4 million tons (t) of air pollution in 2010, with the human health effects valued at USD 6.8 billion [1]. Another study, conducted in a zone with a total of 5193 trees of 66 different species within the campus of the Institute of Technology Shenyang in northern China, determined an annual carbon sequestration of 105.3 T, equivalent to the elimination of 509.6 kg of atmospheric pollutants [4]. Various research groups worldwide have focused their studies on the potential use of trees in urban development, for example, as bioindicators of pollution and/or as green belts that can purify the air [12,14].

The health benefits of trees extend not only to the prevention of physical illnesses but also to mental illnesses. A study in London evaluated the relationship between the number of trees in different areas of the city and the prescription rate of antidepressants, and it was found that in areas with a higher density of trees, there is less use of antidepressants. It is thought that contact with trees reduces people’s stress levels, causing a protective effect on mental health, particularly in individuals with a lower socioeconomic status [15]. Below are the most common air pollutants in urban environments in various regions of the world, the implications these pollutants have for urban trees, the responses of trees to the different pollutants with which they interact, and the legislation and international agreements proposed for the conservation of urban ecosystems.

2. Atmospheric Pollution

Air is naturally mainly composed of three gases—nitrogen, oxygen, and carbon dioxide—along with traces of other gases; the non-alteration of this composition can be considered a sign of good quality air. However, in the last 10 years, air quality has been declining due to the addition of other compounds, such as the inorganics NO_X, SO_X, and CO_X, greenhouse gases (CO₂ and CH₄), and metals including mercury, lead, and volatile organic compounds that hurt the planet’s regulation cycles in such a way as to cause climate change. A contaminant is any substance at a concentration level that negatively alters the environment in which it is found. Atmospheric pollution encompasses any alteration that the layers of the earth’s atmosphere have undergone, from the biosphere to the stratosphere; each of these alterations represents problems that have come to trigger climate change and thereby affect the Earth’s life cycles, where the impact on all species of living organisms is felt at the biosphere level. The most notorious impact is the effect on humans and on some higher organisms like animals and plants [5,16,17]. Aerosols are pollutants suspended as particles in a liquid or solid state and have different shapes and sizes ranging from 3 nm to 100 µm [18]. Among these, particulate matter (PM) includes microparticles of sizes of 10 and 2.5 microns; PM is another type of pollutant generated from trace elements, including metals, microplastics, and bioaerosols. The bacteria contained in particulate matter play a role in the planet’s biogeochemical cycles, with a study in China finding that of the 404 genera of bacteria identified, more than 90% had the potential to participate in at least one metabolic pathway of the carbon, nitrogen, and sulfur cycles [19].

According to the World Health Organization (WHO), air pollution is considered the leading environmental health risk in the Americas. It is estimated that 76% of the population of the Americas lives in urban environments, meaning there is a high demand for energy, provisions, and transportation (which in most of the territory is private), as well as unregulated garbage dumps and the burning of crop fields (in addition to accidental and intentional fires), which all contribute primarily to poor air quality [20]. In Asia, increasing levels of pollution are mainly due to population growth brought about by rapid urbanization and industrialization, coupled with coal-fired power plants and the use of solid fuel in homes; this has resulted in 59% of total global deaths due to poor air quality occurring in this region of the world [21]. In the European Union, seven pollutants have been identified to have a more significant presence in urban environments: ammonia (NH₃), nitrogen oxides (NO_X), carbon monoxide (CO), particles with aerodynamic diameters of less than 2.5 µm and 10 µm (PM_2.5 and PM₁₀), sulfur oxides (SO_X), tropospheric ozone (O₃), and non-methane volatile organic compounds (NMVOCs) [6]. In sub-Saharan Africa, 40% of the population lives in urban areas, with this percentage predicted to increase in the coming years. Currently, the primary sources of air pollution are biomass burning in homes, road transport, and the particulate matter present in dust from the Sahara Desert [22]. Although the WHO has established permissible limits for air pollutants, countries independently legislate in their own territories. The table below compares allowable air pollutant limits in some regions of the world (

Table 1. Permissible limits of air pollutants in urban environments in different regions of the world.

Air Quality Standard	PM10	PM2.5	Ozone (O3)	Nitrogen Dioxide (NO2)	Sulfur Dioxide (SO2)	Carbon Monoxide (CO)	References
WHO	Annual mean (20 µg m−3) Number of exceedances of 24 h mean (50 µg m−3)	Annual mean (10 µg m−3) Number of exceedances of 24 h mean (25 µg m−3)	Number of exceedances of maximum daily 8 h mean (100 µg m−3)	Annual mean (40 µg m−3) Number of exceedances of 1 h mean (200 µg m−3)	Number of exceedances of 24 h mean (20 µg m−3)	Maximum daily 8 h (10,000 µg m−3)	[6]
Europe	Annual mean (40 µg m−3)	Annual mean (25 µg m−3)	Number of exceedances of maximum daily 8 h mean (120 µg m−3)	Annual mean (40 µg m−3) Number of exceedances of 1 h mean (200 µg m−3)	Number of exceedances of the 24 h mean (125 µg m−3)	Maximum daily 8 h (10,000 µg m−3)	[6]
	Number of exceedances of 24 h mean (50 µg m−3)
EU	Number of exceedances of the 24 h mean (150 µg m−3)	Annual mean (9 µg m−3) Number of exceedances of 24 h mean (35 µg m−3)	Number of exceedances of maximum daily 8 h mean (0.070 ppm)	Annual mean (53 ppm) Number of exceedances of 1 h mean (100 ppb)	Number of exceedances of 1 h mean (75 ppb)	Maximum daily 8 h (9 ppm)	[23]
China	Annual mean (40 µg m−3) Number of exceedances of 24 h mean (50 µg m−3)	Annual mean (15 µg m−3) Number of exceedances of 24 h mean (35 µg m−3)	Number of exceedances of maximum daily 8 h mean (100 µg m−3)	Annual mean (40 µg m−3) Number of exceedances of the 24 h mean (80 µg m−3) Number of exceedances of 1 h mean (200 µg m−3)	Annual mean (20 µg m−3) Number of exceedances of 24 h mean (50 µg m−3) Number of exceedances of 1 h mean (150 µg m−3)	Maximum daily 8 h (4000 µg m−3)	[24]
Mexico	Annual mean (36 µg m−3) Number of exceedances of the 24 h mean (10 µg m−3)	Annual mean (10 µg m−3)| Number of exceedances of the 24 h mean (41 µg m−3)	Number of exceedances of maximum daily 8 h mean (0.065 ppm)	Annual mean (0.021 ppm) Number of exceedances of 1 h mean (0.0.106 ppm) Number of exceedances of 24 h mean (0.040 ppm)	Number of exceedances of 1 h mean (0.075 ppm) Number of exceedances of 24 h mean (0.040 ppm)	Maximum daily 8 h (9 ppm)	[25]
India	Number of exceedances of the 24 h mean (150 µg m−3)	Annual mean (9 µg m−3) Number of exceedances of 24 h mean (35 µg m−3)	Number of exceedances of maximum daily 8 h mean (0.070 ppm)	Annual mean (53 ppm) Number of exceedances of 1 h mean (100 ppb)	Number of exceedances of 1 h mean (75 ppb)	Maximum daily 8 h (9 ppm)	[26]
Japan	Number of exceedances of 24 h mean (100 µg m−3)	Annual mean (15 µg m−3) Number of exceedances of 24 h mean (35 µg m−3)	Number of exceedances of 1 h mean (60 ppb)	Number of exceedances of 24 h mean (60 ppb)	-	-	[27]

).

Pollution Sources and Types of Pollutants

Pollutant emissions, e.g., volatile organic compounds (VOCs), inorganic compounds, and particulate matter, are distributed according to the predominant activities in the area. The increase in emissions due to anthropogenic activities has caused negative alterations in ecosystems, including those involving trees. Simultaneously, in cities, other types of pollutants are found, such as the gases NO_X, SO_X, CO_X, and O₃; metals; soot; and PM₁₀ and PM_2.5 microparticles, with the burning of fossil fuels being the primary emission source [28,29]. In areas where mining is a significant economic activity, there is typically a higher concentration of metals in the air. In Colombia’s San Martín de la Loba mining district, concentrations of up to 278 ng/m³ of Hg have been recorded, exceeding the permissible international limit of 200 ng/m³ [30]. In Zambia, mining activity is the country’s primary source of foreign currency; consequently, there is a significant amount of SO₂ and PM emissions, including Pb, Zn, Cu, Cd, As, Co, and Fe, and in 2000, releases of 406.8 kt/year of PM₁₀ and 359.6 kt/year of SO₂ were reported [31]. Elsewhere, the steel industry generates emissions of trace elements, including As, Cd, Cr, Co, Cu, Hg, Ni, Pb, V, Zn, and Fe; the petrochemical industry generates emissions of Cd, Cu, Ni, Zn, and V; and the cement industry generates Pb, V, and Al [32]. The presence of heavy metals in the air can affect the vegetation in the area, reducing or limiting the growth and development of plants. Additionally, the deposition of these pollutants in the soil negatively impacts the soli’s physicochemical parameters, such as pH, and can inhibit vegetation growth [31].

Table 2. Types and sources of air pollutants [33].

Pollutant	Type	Source
Particulate matter (PM10, PM2.5)	Suspended particles	Burning fossil fuels, road dust, fires, and land burning.
Sulfur dioxide (SO2)	Gas	Coal and oil power plants, oil refineries, metallurgical industry, and volcanic eruptions.
Nitrogen dioxide (NO2)	Gas	Motor vehicles, industries, and power generation.
Carbon monoxide (CO)	Gas	Internal combustion engines, forest fires, and industrial processes.
Tropospheric ozone (O3)	Secondary gas	Chemical reactions between NOx and VOCs in the presence of sunlight.
Methane (CH4)	Greenhouse gas	Agriculture, waste decomposition, and oil and gas extraction.
Volatile organic compounds (VOCs)	Gases	Emissions from solvents, paints, gasoline, and diesel combustion.
As Cd, Cr, Co, Cu, Hg, Ni, Pb, V, Zn, and Fe	Heavy metal	Metallurgical industries, the combustion of leaded gasoline (in some countries), electronic waste, and the mining industry.

summarizes the types of contaminants and their emission sources.

The number of pollutants emitted is a function of industrial activities, population size, and the use of fossil fuels (primarily for transportation and power generation), which is particularly prevalent in various global metropolises, where the highest levels of emissions are detected. Moreover, the location and climate of cities directly influence the health impacts of pollution, and a common factor for poor air quality is the dry season [34,35]; however, it is not the only factor contributing to emitted pollutants. With its 9,209,944 inhabitants, Mexico City is one of the most polluted cities on the planet, and there are a further 16,992,944 people in the State of Mexico [36]. As the country’s capital, Mexico City boasts significant economic and industrial activity; however, it is located in the Neovolcanic axis within a valley defined by a mountain range that delimits it from the northwest, west, and southwest and thus favors the accumulation of pollutants. The city’s air quality is generally assessed as fair to poor, with the highest peaks of contaminant accumulation observed in the transition from the dry to the rainy season, where the limits established by the WHO are often exceeded (O₃: 100 μg m⁻³; SO₂: 20 μg m⁻³; NO₂: 200 μg m⁻³; PM₁₀: 50 μg m⁻³), except for NO₂, which remains below the limits throughout the year [37]. On the other hand, in Middle Eastern cities, such as Riyadh, the capital of Saudi Arabia, the concentration of atmospheric pollutants increases in the summer when the weather is hot and there is minimal rainfall and an ambient humidity of around 12%. However, the sandstorms typical of this geographical area are an important factor regarding the pollution in this region; an increase in PM of up to 84% has been observed in summer compared to winter [38].

In addition to the physicochemical characteristics of the particulate material, there are microorganisms that are generally known as bioaerosols. It is estimated that up to 25% of the PM is of biological origin, comprising 80–86% bacteria, 13–18% eukaryotes, 0.1% viruses, and 0.8% archaea, as determined by massive gene sequencing [34]. Bacteroidetes, Actinobacteria, Firmicutes, Chloroflexi, and Proteobacteria are among the groups of bacteria. A metagenomic study conducted in Mexico City during the dry season (March 2016) identified 14 bacterial phyla, with Actinobacteria (38%), Proteobacteria (38%), and Firmicutes (15%) being the most abundant. A total of 147 bacterial genera were found, including Microbispora (9%), Paracoccus (6%), Exiguobacterium (6%), Kocuria (3%), Friedmanniella (3%), Rubellimicrobium (2%), Sphingomonas (2%), and Methylobacterium (2%). Regarding fungi, a total of 211 genera were found, with Ascomycota (81%) and Basidiomycota (8%) being the most predominant [39].

In 2015, microplastics were first detected in the air [40]. Microplastics with sizes below 10 µm are classified as particulate matter, and due to their composition and size, microplastics can absorb persistent organic and inorganic pollutants. In terrestrial environments, microplastics alter the physicochemical properties of the soil and have the potential to affect plant growth. In a pilot study, microplastic beads measuring 5 to 50 μm in size were labeled with a fluorescent dye and introduced into the soil of Betula pendula Roth (silver birch) seedlings during the growing season. After five months, the roots were examined via fluorescence and laser scanning confocal microscopy, which showed the incorporation of microplastics in the root tissues of woody plants [41]; however, relatively little is known about how microplastics interact with higher-order terrestrial plants and what effects this interaction could have.

3. Effects of Air Pollution on Trees

Trees are organisms that are directly affected by pollution, adsorbing or accumulating pollutants mainly in their leaves as the most exposed part. Visible changes in leaves may therefore be the first indication of injury, depending on tree sensitivity [42]. The contaminants present in air enter trees through leaf stomata; when encountering the water found in leaves, a series of chemical reactions are activated that contribute to decreasing the pH, generating an acidic environment, and triggering a series of metabolic conditions ranging from changing stomatal activity to reactions with enzymes inhibiting processes such as the conversion of hexose into ascorbic acid [13]. The emissions produced by motor vehicles affect tree health. The loss of foliage in Tilia cordata has been evaluated in sites with low and high vehicular loads; in sites where there was less traffic, the loss of foliage was lower, while in sites with high traffic levels, the loss of leaves in the crowns was more significant. A scale from 1 to 5 was established, with 5 representing the best condition, characterized by a foliage loss of 10%, and 1 representing a foliage loss of 90% or more. The authors obtained scores of 4.5 and 3.6 for trees in areas with less traffic and more traffic, respectively [43].

Among the organisms that have been studied to show the effects of environmental pollution are urban trees, which, like all living beings, have nutrient requirements for their survival. Trees feed on CO₂, light, macronutrients (N, P, K), and micronutrients (Mg, Ca, Fe, etc.); they obtain CO₂ from the air, while the other nutrients are mainly obtained from the soil through the roots but can also be absorbed from the air. Some polluting gases contain elements necessary for and can thus stimulate tree growth, as is the case for phosphorus (P) and SO₂, where it has been shown that at high concentrations of P [11] or moderate concentrations of SO₂ [44], tree growth was favored. In contrast, tree growth is hindered by high concentrations of Al, Ba, and Zn in the environment: Al is toxic to plants, as it interferes with root growth and inhibits the absorption of essential nutrients, such as phosphorus; Zn is a micronutrient that, when it exceeds the limits necessary for plants, begins to have a toxic effect; and Ba is an indicator of a polluted environment [11]. A high concentration of SO₂ contributes to the partial denaturation of chloroplasts, as it replaces Mg²⁺ with two H⁺ atoms, degrading the chlorophyll molecules into pheophytin. This reduction in photosynthetic pigments negatively affects the photosynthesis process of trees [13,45]. Heavy metals are part of the particulate matter and tend to accumulate on the surface of tree leaves; the presence of Cd, Pb, and Cu has been reported in E. globulus and F. nítida [46]. In the Gulf of Fos, France, the accumulation of heavy metals in the bark of pine (Pinus halepenis) and poplar (Populus nigra) was evaluated. A significant difference was observed between the trees located in an industrial area and those in a rural area 20 km away, with industrial area/rural area ratios of 1.1 (Cu) and 3.9 (Al) for poplars and 0.8 (Cu, Zn) and 2.0 (Al, Hg) for pines [32].

The accumulation of contaminants in tree rings can reveal information about the presence of contaminants during their development [47]. Heavy metals have been found in pine (Pinus halepenis) and poplar (Populus nigra) rings corresponding to the years between 1975 and 2015 [32]. This phenomenon demonstrates the bioaccumulation of cations in tree rings, which can occur through the contact of contaminants with the bark and, subsequently, via the stomatal pathway, resulting in their transport to the rings [32].

Microparticles were analyzed from a physicochemical perspective, where a series of elements, including C, O, Na, Mg, Al, Si, K, Ca, Ti, Fe, Cl, Nb, S, F, and P [35], were measured. However, microparticles contain not only small-sized materials but also many bioaerosols [34]. In another study on the physical and chemical effects of particulate matter on the foliage of trees, the authors identified that the reduction in tree growth rates due to particulate matter is mainly due to the accumulation of this material on the surface of the leaves, which reduces the availability of light and thereby decreases photosynthetic activity; in addition, the stomata are obstructed, modifying gas exchange processes [11]. These effects are determined by the number of particles that the trees can retain, which is directly related to the type of leaf, with trees with broad and rigid leaves exhibiting a greater retention capacity than coniferous trees that are characterized by perennial needle-shaped or scale-like leaves [48]. Ozone fulfills an essential function for life in the stratosphere, as it filters UV rays from the sun; however, it is an unstable radical at the biosphere level and a harmful oxidizing agent [49]. A study of stomata damage in three Chinese tree species (Ailanthus altissima, Fraxinus chinensis, and Platanus orientalis) showed that although no changes were observed in stomatal density, there was a decrease in photosynthetic activity [49]. It has been observed that chronic exposure to ozone generates a considerable amount of reactive oxygen species (ROS), which cause stomata to close, altering stomatal conductance and thereby resulting in slow and inefficient stomatal activity [50]. Consequently, carbon assimilation is diminished [51]. The enzymatic activities of superoxide dismutase, ascorbate peroxidase, and catalase in the leaves of Platanus orientalis have been observed to increase in the presence of Cd, Pb, Ni, and Cr [52]. Other reports indicate that tree leaves undergo structural changes when pollution levels increase, such as in the case of R. pulchrum, which showed a reduction in its stomatal density in sites with high pollution levels (NO 27.6 ± 1.2 ppb, NO₂ 25.8 ± 0.7 ppb, PM_2.5 13.7 ± 0.4 µg m⁻³, O_X 28.9 ± 1.2 ppb) and a strong negative correlation between stomatal density and NOx. In sites with low pollution (NO 9.8 ppb and NO₂ 15.7 ppb), a stomatal density of 480 was obtained, while in sites with high pollution (NO 28.2 ppb and NO₂ 25.5 ppb), the stomatal density was 318 [53].

NO_X are nitrogenous compounds, mainly NO and NO₂, that are found in the atmosphere as byproducts from the use of fossil fuels; meanwhile, nitrogen is a macronutrient for plants and is used to synthesize proteins, enzymes, amino acids, nucleic acids, and chlorophyll, among others. It has been shown that plants are capable of fixing nitrogen from NO_X in the environment by absorbing these gases through the stomata of the leaves, incorporating them into the nitrogen cycle; NO₂ is better assimilated than NO because the latter has low solubility in water [54]. In the coniferous forests of California, United States, where Pinus ponderosa predominates, foliar lesions have been observed as well as the premature loss of needles and a decrease in the state of the crown due to the pollution generated in the California valley, although a positive impact has been observed in the combination of NO₂ and O₃ deposition, which has the effect of increasing the radial growth of pine trees [47].

As mentioned above, one of the first modifications in trees when they encounter contaminants is the generation of an acidic environment within the leaves [55]; this action, in addition to triggering biochemical reactions, contributes to the significant growth of the microbial populations present in the individual. In microecosystems of Azadirachta indica, it was observed that the presence of bacterial species such as Salmonella and Proteus increased by a factor of 10 to 100 times in sites with high concentrations of contaminants compared to those with less contamination. While the genus Salmonella is an opportunistic bacteria that lives in stressed plants, the genus Proteus is a symbiotic bioindicator related to the presence of heavy metals such as Cu, Cr (VI), Co, Cd, and Zn [56]. However, when colonizing a plant, these microorganisms overcome defense mechanisms and promote the invasion of other pathogenic microorganisms and the appearance of diseases.

The bacteria located in the phyllosphere of Populus cathayana have low nutrient availability since they are in an oligotrophic environment, which limits their growth; among the dominant genera are Betaproteobacteria (44.7%), Gammaproteobacteria (29.9%), Firmicutes (10.9%), Alphaproteobacteria (8.9%), and, to a lesser extent, Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Deltaproteobacteria, and Gemmatimonadetes. When the ecosystem is subjected to changes, in this case, an increase in O₃ (40 ppb), a significant decrease in the populations of Deltaproteobacteria, Gammaproteobacteria, and Sphingobacteriia is observed, while with an increase in supplementary N, the genera Actinobacteria, Alphaproteobacteria, and Betaproteobacteria increase in abundance [57].

Acid rain is mainly caused by emissions of sulfur and nitrogen and their interaction with water vapor in the atmosphere, which produces sulfuric acid and nitric acid and causes the pH of rain to decrease to values below 5.6. A simulation was performed to observe the effects of acid rain on one-year-old Liquidambar styraciflua and Fraxinus uhdei, two of the most abundant tree species in the urban landscapes of Mexico City. The trees were sprayed with sulfuric acid at pH values of 3.8, the value reported for acid rain in CDMX, and 2.5, to mimic a higher acidity scenario. There was visible leaf damage in both species with the pH 2.5 treatment; a reduction of at least 1.5 was also observed compared to the control for both acidic treatments, indicating that physiological damage occurs before morphological damage [58].

Air pollution in cities, combined with the low diversity of tree species, has led to the loss of thousands of trees within urban ecosystems. Among the diseases that can affect trees and cause their death is Dutch Elm Disease (DED), which, throughout history, has caused devastating losses in Europe and the USA, mainly during the 20th century [59]. In addition to DED in elms, wood rot disease and insect pests have been shown to increase tree mortality in both forests and urban environments; poor air quality has also been observed to be a factor contributing to the onset of these diseases [59,60]. Another case is that of Platanus acerifolia, a species resistant to urban conditions, which showed higher susceptibility to diseases such as mildew and masaria when grown in environments with high vehicular traffic compared to environments where there was no passage of motor vehicles [43]. The susceptibility of different plants to air pollutants varies based on their responses to specific types of contaminants. Pollutants exhibit diverse deleterious effects on plant growth and metabolism by hindering photosynthesis, altering the biochemical composition, affecting flowering patterns, and modifying the lengths of petioles and stomata [61].

4. Stress-Tolerant Tree Species

While some species of trees are greatly affected by environmental pollutants, others show tolerance and adaptation to them. In 2020, Roy et al. [14] studied three species of trees with a high capacity for tolerance to air pollution, metal accumulation, and particle capture, namely Magnifera indica, Azadirachta indica, and Ficus religiosa, and the authors observed no differences in this adaptation due to seasonal changes. Other trees like Betula pendula, Tilia cordata, Salix alba, Robinia pseudoacacia, Populus alba, Populus simonii, and Populus nigra have been reported to be tolerant to and accumulators of SO₂ [62]. In these trees, there is a relationship between the amount of SO₂ accumulated in the leaves and a higher production of glutathione via the transformation and incorporation of the sulfur from SO₂ in the form of sulfates into the tree’s metabolic cycle, i.e., the presence of sulfur gases in the atmosphere stimulates glutathione synthesis. Glutathione levels are therefore indicative of the SO₂ levels in the environment [62].

Among the predominant tree species in central Europe are Alnus glutinosa, Acer platanoides, Acer pseudoplatanus, Quercus robur, Quercus petraea, and Betula pendula. These species have shown relevant NO_X deposition rates, with Carpinus betulus exhibiting the fastest rate at 0.15 and 0.56 mm s⁻¹ for NO and 0.45 and 0.95 mm s⁻¹ for NO₂ [55].

The microstructures of tree leaves, such as the grooves, trichomes, glands, and epicuticular wax layer, are associated with the tree’s particle retention capacity. Some trees reduce their accumulation of particulate matter by self-cleaning their leaves through washing with precipitation, as is the case for G. biloba [63].

Air Pollution Tolerance Index and Anticipated Performance Index (APTI and API)

All plants can tolerate environmental factors, including the concentrations of environmental pollutants. This property is known as the air pollution tolerance index (APTI), and it is one of the most widely used indices for analyzing how plants behave in the context of urban air pollution [64]. One of the uses of this index is the selection of plants or trees based on their ability to mitigate the effects of pollution in urban areas [42], this being of particular interest in the study of tree species located along the sides of roads [55]. Evaluating this index enables us to determine the relative water content of the fresh leaves, pH and content of ascorbic acid, and total chlorophyll of the leaf extract; it is used to indicate a plant’s sensitivity or tolerance to air pollution. APTI values range from 1 to 100, where less than 1 = very sensitive, 1–16 = sensitive, 17–29 = intermediate, and 30–100 = tolerant [13,14,64,65]. Some tree species and their respective APTI values are shown in

Table 3. Values of APTI reported in some species of trees; the classification is shown based on the reported values in [3,13,14,38,42,45,50,55,63,64,65,66,67,68,69,70].

Species	APTI Value	Classification	References
Morus alba	14.08	Sensitive	[3]
Ailanthus altissima	11.15	Sensitive
Salix babylonica	11.08	Sensitive
Swietenia mahagoni	17–29	Intermediate	[13]
Cassia fistula	17–29	Intermediate
Ficus benghalensis	1–16	Sensitive
Polyalthia longifolia	1–16	Sensitive
Mesua fera sp.	1–16	Sensitive
Mimusops elengi	1–16	Sensitive
Lagerstroemia speciosa	1–16	Sensitive
Saraca asoca	1–16	Sensitive
Duranta repens	1–16	Sensitive
Manilkara hexandra	1–16	Sensitive
Ailanthus excelsa	24.05	Intermediate	[14] Values from industrial site post-monsoon
Alstonia scholaris	18.27	Intermediate
Azadirachta indica	28.62	Intermediate
Ficus benghalensis	17.29	Intermediate
Ficus religiosa	17.27	Intermediate
Mangifera indica	26.43	Intermediate
Psidium guajava	18.19	Intermediate
Saraca asoca	16.46	Sensitive
Tectona grandis	11.79	Sensitive
Albizia lebbeck	36.9	Tolerant	[38]
Eucalyptus camaldulensis	21.8	Intermediate
Ficus altissima	23.1	Intermediate
Prosipis juliflora	14.8	Sensitive
Ziziphus spina-christi	58.5	Tolerant
Alstonia scholaris	60.03	Tolerant	[42]
Nerium oleander	82.14	Tolerant
Tabernaemontana coronaria	73.13	Tolerant
Thevetia peruviana	68.22	Tolerant
Celtis occidentalis	12.9	Sensitive	[45]
Tilia × europaea	8.7	Sensitive
Taxus cuspidata	6.1–10	Sensitive	[50]
Pinus densiflora	6.1–10	Sensitive
Chionanthus retusus	6.1–10	Sensitive
Prunus yedoensis	6.1–10	Sensitive
Zelkova serrata	6.1–10	Sensitive
Ginkgo biloba	6.1–10	Sensitive
Cassia fistula	11.83	Sensitive	[55]
Madhuca longifolia	24.76	Intermediate
Pongamia pinnata	10.41	Sensitive
Peltophorum pterocarpum	16.83	Sensitive
Terminalia catappa	10.59	Sensitive
Pinus densiflora	8.9	Sensitive	[63]
Prunus yedoensis	8.7	Sensitive
Zelkova serrata	8.4	Sensitive
Platanus occidentalis	9.3	Sensitive
Ginkgo biloba	8.4	Sensitive
Ficus religiosa	15.23–82.12	Sensitive–tolerant	[64]
Anthocephalus cadamba	59.34–121.11	Tolerant
Lagerstroemia speciosa	14.77–181.42	Sensitive–tolerant
Cassia siamea	12.25–18.02	Sensitive–tolerant
Eucalyptus globus	95.20	Tolerant	[65]
Ficus religiosa	85.45	Tolerant
Mangifera indica	80.52	Tolerant
Polyalthia longifolia	79.01	Tolerant
Phyllanthus emblica	57.88	Tolerant
Citrus limon	43.57	Tolerant
Lantana camara	18.14	Intermediate
Ficus benghalensis	26.01	Intermediate	[66]
Cassia fistula L.	24.52	Intermediate
Ficus religiosa	23.35	Intermediate
Polyalthia longifolia	22.88	Intermediate
Drypetes roxburghii	22.11	Intermediate
Zizyphus jujuba Lamk.	21.37	Intermediate
Delonix regia	20.58	Intermediate
Terminalia arjuna	20.54	Intermediate
Psidium guajava L.	20.38	Intermediate
Albizia lebbeck Linn.	20.00	Intermediate
Kigelia pinnata	19.87	Intermediate
Ficus glomerata (Roxb.)	19.22	Intermediate
Millingtonia hortensis	17.64	Intermediate
Anthocephalus indicus	17.36	Intermediate
Mangifera indica	16.89	Sensitive
Dalbergia sissoo	15.95	Sensitive
Nerium indicum	15.09	Sensitive
Azadirachta indica Juss	15.08	Sensitive
Artocarpus heterophyllus	14.55	Sensitive
Bauhinia purpurea	14.02	Sensitive
Alstonia scholaris	13.90	Sensitive
Tectona grandis	13.29	Sensitive
Bauhinia variegata	12.11	Sensitive
Holoptelea integrifolia	11.72	Sensitive
Cassia siamea	11.63	Sensitive
Terminalia bellirica	11.60	Sensitive
Murraya peniculata	11.17	Sensitive
Syzygium cumini	11.09	Sensitive
Madhuca indica	11.01	Sensitive
Mangifera indica	26.66	Intermediate	[67]
Ficus religiosa	18.4	Intermediate
Plumeria rubra	13.36	Sensitive
Lagestroemia speciosa	13.66	Sensitive
Alstonia scholaris	11.18	Sensitive
Butea monosperma	18.84	Intermediate
Polyalthia longifolia	16.28	Sensitive
Ricinus communis	24.75	Intermediate	[68]
Bougainvillea glabra	20.92	Intermediate
Myoporum pictum	14.18	Sensitive
Juniperus procera	13.76	Sensitive
Phoenix caespitosa	13.63	Sensitive
Shinusmolle	13.57	Sensitive
Hibiscus rosa-sinensis	12.38	Sensitive
Catharanthus roseus	11.2	Sensitive
Tagetes tenuifolia	9.93	Sensitive
Vitis vinifera	9.69	Sensitive
Mangífera indica	18.00	Intermediate	[69]
Tabebuia chrysantha-rosea	12.00	Sensitive
Erythrina fusca	11.00	Sensitive
Jacaranda mimosifolia	9.00	Sensitive
Fraxinus uhdei	12.00	Sensitive
Spathodea campanulata	10.00	Sensitive
Azadirachta indica	15.84	Sensitive	[70]
Bougainvillea spectabilis	16.0	Sensitive
Ficus benghalensis	26.40	Intermediate
Ficus religiosa	29.34	Intermediate
Nerium indicum	17.8	Intermediate
Plumeria rubra	25.4	Intermediate
Saraca asoca	21.99	Intermediate
Tabernaemontana divaricata	20.88	Intermediate
Terminalia arjuna	15.29	Sensitive

.

The Anticipated Performance Index (API) is determined from the APTI value and takes into account biological and socioeconomic aspects; that is, factors such as the habitat of the tree, size (small, medium, or large), canopy structure (irregular globular, open semi-dense, or spreading dense), type of plant (evergreen or deciduous), leaf lamina (size, texture, hardiness), use, and socioeconomic value are considered, facilitating the selection of species for afforestation, reforestation, and implementation of green belts while considering parameters such as esthetics and recreation [42]. In addition to the API, a new index called the Air Quality Impact Index (AQII) has been proposed considering other factors such as aerodynamic properties, leaf structure, pollutant absorption potential, pollution tolerance, ozone and aerosol precursor emissions, and the impact of pollen allergy. The index provides a more comprehensive assessment of the impact of urban trees on air quality as compared to the API [71]. Compared to the API, the AQII is a relatively new index that proposes a more comprehensive approach to decision making regarding tree planting in urban areas.

5. Regulation and World Problems

Countries around the world have specific regulations regarding the maximum allowable concentrations of atmospheric pollutants; however, many of these limits are exceeded, and it is known that South Asia and Africa are the regions most polluted by PM_2.5 particles and ozone, with India (83.2 µg m⁻³), Nepal (83.1 µg m⁻³), Niger (80.1 µg m⁻³), Qatar (76.0 µg m⁻³), Nigeria (70.4 µg m⁻³), Egypt (67.9 µg m⁻³), Mauritania (66.8 µg m⁻³), Cameroon (64.5 µg m⁻³), Bangladesh (63.4 µg m⁻³), and Pakistan (62.6 µg m⁻³) being the 10 countries most polluted with PM_2.5, and Qatar (67.2 ppb), Nepal (67.0 ppb), India (66.2 ppb), Bangladesh (64.6 ppb), Bahrain (64.0 ppb), Pakistan (63.3 ppb), Kuwait (62.1 ppb), Iraq (59.5 ppb), Republic of Korea (57.9 ppb), and Saudi Arabia (58.2 ppb) the 10 countries with the highest concentrations. However, it is considered that the existing monitoring methods do not provide a sufficiently accurate picture of air pollution in the world, much less in cities [72]. Various international organizations are addressing the problem of environmental pollution; the measures proposed to mitigate the emission of pollutants into the atmosphere are the transition to the use of clean energies, reducing the burning of garbage, avoiding the spread of fires, reducing burning practices in agriculture, and the reduction in emissions from the use of fossil fuels in transport [73]. The Paris Agreement, signed by 196 countries in 2015 and which entered into force on 4 November 2016, seeks to stop climate change. The countries that signed this treaty have committed to reducing their greenhouse gas emissions to comply with this agreement; within this, a socioeconomic transformation based on scientific development is sought [74]. The member countries of the ONU adopted the 2030 Agenda for Sustainable Development, establishing 17 Sustainable Development Goals aimed at achieving global development in economic, social, and sustainable environmental matters, for example, the protection of the climate and life in terrestrial ecosystems [75]. Despite the existence of air pollution policies and legislation in many countries, recent studies estimate that 80% of the world population is still exposed to pollutant levels that exceed the air quality guidelines recommended by the WHO. The introduction of various air quality management strategies has not prevented increases in the levels of pollutants such as NO₂, O₃, PST, and CO in the urban areas of developing countries. In different parts of the world, some policies have been implemented, such as (i) strengthening the capacity of institutions, through financial support, to measure, reduce, and evaluate the effects of air pollution, including those that affect human health, animals, and plants; (ii) promoting inter-institutional cooperation at the international, national, and regional levels to reduce air pollution; (iii) implementing or strengthening state and regional programs (including through public sector and private sector associations) to expand the atmospheric monitoring network in countries; (iv) strengthening and supporting efforts to reduce emissions through the use of cleaner fuels; (v) designing programs that provide affordable energy to rural communities to reduce dependence on traditional fuel sources used for cooking and heating; (vi) the effective reduction, prevention, and control of forest fires and open burning of waste; (vii) legislating on air quality, as well as the maximum permissible limits of pollutants by emission source; and (viii) carrying out vehicular circulation restriction programs for a determined time [76]. Many governments around the world have made significant efforts to reduce the generation of air pollutants, but testing has shown that since 2005, these pollutants have deteriorated human health [77]. A guide developed by the Stockholm Environment Institute (SEI), IKEA, and Climate and Clean Air Coalition (CCAC) Group instructs companies to measure air pollutant emissions in value chains, which helps them to measure pollutant emissions and develop actions to reduce them; the guide focuses on quantifying the emissions of direct emitted particulate matter (P.M2.5, P.M10), black carbon, organic carbon, sulfur dioxide, nitrogen oxides, ammonia, non-methane volatile organic compounds, and carbon monoxide and introduces approaches for mitigation and implementation, as well as how utilizing an emissions inventory can inform decision making by different companies. This demonstrates commitment on the part of companies to reduce these pollutants [78].

Despite these efforts, a report issued by the Intergovernmental Panel on Climate Change (IPCC) in August 2021 showed that the intentions and actions so far are not sufficient to stop the crisis. It is estimated that reducing CO₂ and greenhouse gas emissions drastically enough would require changes taking between 20 and 30 years [79], which would put at risk many species of trees throughout the world and reduce the quality of life of plants and animals, including humans.

6. Conclusions

Air pollution impacts the health of urban trees, though not always negatively. Some pollutants, such as NO_X, SO_X, and CO_X, are assimilated and incorporated into nitrogen, sulfur, and carbon cycles, respectively, with the concentration of pollutants being the main factor that determines whether these compounds are beneficial or harmful to trees; however, it is important to consider that the assimilation of some compounds, especially NO_X or SO_X, should not interfere with the carbon fixation capacity, since this would be counterproductive for trees. On the other hand, PM_2.5 and PM₁₀ can be considered the contaminants that most harm these organisms because they accumulate in the leaves and prevent them from adequately performing their physiological functions, thus promoting diseases or the invasion of pests. Trees perform a series of essential processes contributing to ecosystem balance around the world, in addition to other processes that mitigate the impact of atmospheric pollution. For these reasons, strategies are proposed where trees can be used to reverse or reduce the effects of pollution. Trees are of considerable interest in urban planning as they can be used as biofilters in cities. However, this should not be the primary focus, with the preservation of established international agreements instead taking center stage. While it is essential to plan cities by considering vegetation which benefits all inhabitants, all actions that involve trees should be prioritized.

7. Future Directions

The effects of climate change are currently being experienced in various parts of the world, from natural disasters to the displacement, reduction, and extinction of species. The preservation of ecosystems has therefore become a priority; however, this presents a series of challenges, some of which are directly related to the management of cities and are the responsibility of the relevant authorities. It is thus important to continue conducting research regarding the behavior of urban ecosystems, which are mostly controlled and modified areas where strategies can be implemented to guarantee the coexistence of species. Research on the beneficial dynamic interactions of various species of trees and plants with different characteristics in the face of air pollution in cities is an area that continues to be explored. Obtaining information in this area could be key to establishing relationships and systems that allow sensitive and tolerant organisms to coexist. Observing and describing these multispecies processes can generate proposals for the sustainable planning and design of cities. The above factors are essential in influencing the management of public policies to prioritize and guarantee the conservation of biodiversity in cities.