The effect of vegetation in reducing air pollution in an urban environment: A review

The consequences of air pollution encompass a wide range of environmental, social, and health impacts. Environmental effects include smog formation, acid rain, and ecosystem degradation. Socially, air pollution leads to reduced visibility, malodorous conditions, and overall reduced quality of life. However, the most alarming consequences are associated with adverse health effects in humans and other living organisms. Air pollution has been linked to respiratory diseases, cardiovascular problems, allergies, and premature death. Therefore, a multifaceted approach is required to mitigate air pollution. This includes both preventive and active intervention. Preventive measures include reducing emissions from their sources through the implementation of stringent environmental regulations, adoption of cleaner technologies, and promotion of sustainable practices. Additionally, public awareness and education campaigns play a crucial role in encouraging individuals and communities to adopt eco-friendly behaviours and reduce their carbon footprint.


Introduction
One of the factors controlling air pollution is the availability of vegetation on roads and in the surrounding environment.The number of motor vehicles that continue to increase with the increase in emissions will also continue to reduce air quality if it is not matched by an increase in the amount of existing vegetation that absorbs pollutants [1], the vegetation in urban areas can improve air quality, moreover in some types of vegetation which have certain characteristics greatly affect the removal of urban pollutants such as particulate matter (PM) and nitrogen dioxide (NO2) from the air [2], Urban greening plays an important role in reducing ambient air pollution and the temperature in areas with greenery will be cooler [3], to identify tolerant plant species that can be useful for pollution removal using tool in air pollution monitoring and in decision making during urban development and urban greening [4], Air pollution that occurs in urban areas will get worse if it is not balanced with optimal air pollution control [5], the number of motor vehicles that continues to increase with the number of emissions that increase will also continue to reduce air quality if it is not matched by an increase in the amount of existing vegetation that absorbs pollutants [6], poor air quality is caused by pollutant loads that enter the atmosphere exceeding their natural dispersion and neutralization capabilities.Several incidents indicate that the air quality condition is at a moderate and unhealthy level, indicating an excessive load of pollutants entering the atmosphere [7].

Air pollutant sources
The main source of air pollution is transportation, especially motorized vehicles that use fuel containing contaminants; 60% of the pollutants produced consist of carbon monoxide, and approximately 15% consist of hydrocarbons [8].Other sources of pollutants include combustion, industrial processes, and waste disposal.In some urban areas, motorized vehicles account for 85% of all air pollutants.Motorized vehicles are mobile polluters that produce CO2 pollutants, hydrocarbons that do not burn completely, NOx, SOx, and particulate matter.Air pollutants that are commonly found in observable amounts in various places, especially in big cities, according to Molnar et al (1995) include: 1) Nitrogen Oxides (NOx) are gaseous compounds found in free air, mostly in the form of nitric oxide (NO) and nitrogen oxides (NO2) gases as well as various types of oxides in smaller amounts, NO gas is colorless and odorless, while NO2 gas is reddish-brown in color, has an unpleasant odor, and is quite pungent.Various types of NOx can be produced from the burning of fuel oil (BBM) and other fossil fuels (BB) at high temperatures, which are discharged into the environment through chimneys of factories in industrial areas, even NOx gas is harmful to health and livestock, and agricultural areas can damage crop yields [8].(2) Sulfur oxides (SOx), especially sulfur dioxide (SO2) and sulfur trioxide (SO3), are gaseous compounds with an unpleasant odor that are often found in industrial areas that use coal and corkscrew as fuel and the main energy source.Sulfur oxide is also a form of gas resulting from volcanic activity, volcanic eruptions, natural gas sources of sulfur (sulphatar), hot springs, and natural hot steam (fumaroles), these oxides are the main cause of rust because they are highly reactive with many types of metals (forming metal sulfide compounds).It also interferes with health, especially with the sense of sight and mucous membranes around the respiratory tract (nose, esophagus, and stomach).In agricultural areas, sulfur oxide gases can damage crop yields [1].(3) Particles, these can come from smoke (especially the results of burning wood, garbage, coal, coke, and fuel oil, which form soot) and can also be in the form of fine and coarse dust particles originating from various natural and human activities.The most important property of these particles is their size, which ranges from 0.0002 µm to 500 µm.In this size range, these particles can take the form of suspended particulates whose presence in the air ranges from a few seconds to several months depending on the dynamics of the atmosphere [9].

Air pollutant components
According to Zheng et al. (2021), there are two forms of emissions from two air pollutant elements or compounds: 1) Primary Air Pollutants, namely emissions of air pollutant elements directly into the atmosphere from stationary or moving sources.This primary air pollutant also has a high half-life in the atmosphere, such as CO, CO2, NO2, SO2, CFC, Cl2, and dust particles.2) Secondary Air Pollution, namely air pollutant emissions resulting from physical and chemical processes in the atmosphere in the form of photochemistry, which are generally reactive and undergo physicochemical transformations into elements or compounds [10].The form is also different/changes from when it is emitted until after it is in the atmosphere, such as ozone (O3), aldehydes, acid rain, etc.Based on spatial distribution, air pollutant sources can be grouped into point sources, regional sources, and line sources.According to the source of pollution, air pollutant emissions can be divided into stationary and moving sources.Silent sources are usually in the form of industrial and household activities (settlements), but experts consider settlements as nonpoint sources of air pollution.Mobile sources are mainly motorized vehicles, which are related to transportation [11].Air pollutant compounds based on their nature into three groups as proposed by [1], namely (1) Compounds, which are reactive.(2) Fine particles that are suspected in the atmosphere for a long period of time.(3) Coarse particles that immediately fall to the ground.The air polluting compounds include SO2, SO3, CO2, ammonia (NH3), hydrochloric acid, flour compounds and radioactive elements.Fine particles are mainly in the form of fog from incomplete combustion of fuels, while coarse particles are mainly in the form of organic compounds.SO2, smoke and dust compounds can serve as prototypes for other air pollutants.

Air pollution distribution factors and air pollution impacts
Wind speed, air temperature, and air humidity are meteorological parameters that can affect the concentration of pollutant gases in the air [12].

Topography
The shape of the land surface can be flat land, sloping land, and highlands or lowlands.Contoured landform conditions affect the different microclimates of a location [13].Industry and transportation constitute the majority of community activities in urban areas that have the potential to generate many pollutants.During the day when the air conditions are unstable, pollutants are dispersed either horizontally or vertically.At night, when air conditions are stable, pollutants tend to concentrate in one place, especially in rural areas.This is because rural areas tend to have higher air temperatures when compared to urban areas, so the air pressure in rural areas will be smaller and trigger winds to move towards rural areas.In addition, with the gravitational force pulling pollutant particles downward, rural areas tend to have higher pollutant concentrations than urban areas do.

Weather and wind direction
Weather and wind cause the spread of pollutants in the air.If wind blows from industrial areas to urban areas, the level of Air Pollution tends to be higher in urban areas (air pollution, undated).On sunny days, pollutants from vehicles can react in the presence of sunlight to form ozone. Pollutants that cause ozone formation are usually generated from vehicles in urban areas.In cloudy weather, rain can reduce the concentration of pollutants.Water particles in the air can absorb certain pollutants, such as dust, and then carry them down to Earth [9].

Air Pressure
Certain air pressures can accelerate or even inhibit the occurrence of a chemical reaction between pollutants and contaminants in the air or substances in the air (pollutants), thereby increasing or decreasing pollutants [14].

Temperature
Urban areas are vulnerable to temperature change [14].The air quality in urban areas is synonymous with hotter air temperatures.Air temperature can affect pollutant concentrations.High air temperatures cause the air to stretch more, decreasing the pollutant concentrations.Conversely, at cold temperatures, the air is denser and the concentration of pollutants in the air is higher.

Humidity
Air humidity can affect pollutant concentrations by expressing the amount of water vapor in air [15].At high humidity levels, water vapor can react with pollutants.Humid air conditions will help the process of pollutant deposition because under humid air conditions, some pollutants in the form of particles such as dust will bind with water in the air and form larger particles so that they easily settle to the earth's surface due to gravity.

Plant response to air pollution
Some plants have the ability to absorb and remove certain pollutants from air.Some examples of plants that are known to have phytoremediation abilities against air pollution are large trees such as oak, teak, and fir trees, which have the ability to absorb dust particles and pollutant gases such as nitrogen dioxide (NO2) and sulfur dioxide (SO2) [16].Broadleaf plants such as betel ivory (Scindapsus aureus) and mother-in-law's tongue (Sansevieria trifasciata) can help reduce the concentration of formaldehyde, which is a common pollutant in indoor air; chrysanthemum (Chrysanthemum morifolium), absorb ammonia and formaldehyde, bamboo plants have the ability to absorb nitrogen dioxide and dust particles from the air; aquatic plants such as water hyacinth (Eichhornia crassipes) and azolla (Azolla spp.) can help remove pollutants such as heavy metals and organic matter from water.Although plants can help reduce air pollution on a small scale, it is important to recognize that they cannot completely solve a serious air pollution problem [5].Therefore, other measures, such as the use of effective pollution control technologies and environmental policies, are still needed to deal with the problem of air pollution as a whole [17].According to Gogoi et al (2021), plants have several functions that can help to reduce air pollution.Some of the main functions of plants in reducing air pollution are as follows:

Carbon dioxide (CO2) absorption
Through photosynthesis, plants absorb carbon dioxide from the air and convert it into oxygen.In this process, plants help reduce the concentration of carbon dioxide, a greenhouse gas that contributes to climate change.

Figure 1. The Process of Photosynthesis
Based on this, CO2 can be utilized by plants, through the process of photosynthesis.For incomplete gasoline oxidation reactions, unoxidized CO gas will always be formed.For this reason, the Center for Road and Bridge Research and Development has conducted research to minimize the concentration of CO by examining the types of plants that have positive (good) potential.This is actually indirectly a solution to reducing CO2 concentration [16].

Removal of dust particles
Plant leaves and stems can capture dust particles and other pollutants from the air.When dust particles are trapped on plant surfaces, they are no longer present in the air.This helps reduce air pollution and improves the air quality around plants.

Figure 2. Interaction Diagram between Pollutant Gas Molecules and Leaves
In dicotyledonous plants, the stomata are generally located more in the lower epidermal layer than in the upper epidermis.In broad-leaved dicots, stomata were randomly distributed.The number of stomata affects the volume of incoming gas [18].

Absorption of air pollutants
Some plants can absorb air pollutants such as nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), and formaldehyde.Plants use the stomata on their leaves to absorb and remove these pollutants through chemical processes within plant tissues [19].

Moisturizing the air
Plants release water vapor through transpiration.This can help to increase the humidity of the surrounding air.Balanced humidity can help reduce irritation and respiratory problems caused by dry air as well as control pollutants such as dust and smaller particles [14].Transpiration is the process by which plants release water vapor through their leaves and stems and contributes to the humidity of the surrounding air, which is essential for plants as it helps in the uptake of nutrients from the soil and also cools down the plant through evaporation [7].According to Leuzinger et al. (2010), the release of water vapor during transpiration can indeed increase the humidity of the surrounding air, which can be beneficial for human comfort and health in several ways:1) dust and particle control and higher humidity can help control dust and smaller particles in the air; when the air is dry, these particles can remain suspended for longer periods, leading to increased respiratory irritation, and moist air helps to weigh down and settle these particles, reducing their presence in the air [13].2) Temperature regulation, transpiration, and the subsequent increase in humidity can have a cooling effect on the surrounding environment.As plants release water vapor, they absorb heat energy from the environment, which helps to lower the temperature, which can be particularly beneficial in hot and arid regions [20].

Heat reduction
Plants can also help reduce the air pollution generated by heat.Plant leaves absorb heat energy from sunlight and reduce the surrounding temperature through water evaporation through the stomata.This can help reduce the urban heat effect and heat-related pollution [7].Not all trees are in good shade and function as pollutant absorbers, there are several requirements that must be met so that trees planted along the road can function and not add unwanted problems.Tree selection for greening must be based on resistance to air pollutant particles.Many types of plants are good enough to be planted as protective trees in certain places, because they have the ability to absorb pollutants that are quite high [12].
The ability of plants to absorb pollutants is influenced by both environmental and plant factors.Plant factors that absorb and accumulate pollutants are influenced by the morphological characteristics of the plant, such as the size and shape of the leaves, presence of hair on the leaf surface, and the texture of the leaves.In addition, the anatomical arrangement of the leaves also acts as a barrier to the entry of material from the outside, which affects the amount of contaminants that accumulate in the leaves [4].

Conclusion
Air pollution is a complex and pervasive issue with severe consequences for the environment and human health.Addressing this problem requires a multi-pronged approach involving governmental policies, technological advancements, and collective efforts from individuals and communities.By taking decisive actions to reduce air pollution, we can safeguard the environment, protect public health, and ensure a sustainable future for generations to come.