Trees: Natural Air Filters Against Freeway Pollution

Trees and bushes near highways can significantly reduce air pollution caused by motor vehicles, which has been linked to several illnesses. Trees can improve air quality by reducing air temperature, reducing energy consumption in buildings, and directly removing pollutants from the air. They can also act as barriers between sources and populations, and their complex and porous structure can modify near-road concentrations via pollutant capture or through altering airflow. However, it's important to note that trees don't solve the problem of air pollution caused by vehicles and that factors like wind patterns, nearby buildings, and time of day can also impact pollution dispersion.

Trees can intercept airborne particles

Trees contribute to improved air quality by reducing air temperature and energy consumption in buildings, which, in turn, reduces air pollutant emissions from power sources. Urban forests can remove multiple tons of ozone, gaseous air pollution, and particulate matter annually, either through direct uptake of gases or by temporarily intercepting airborne particles.

Trees remove gaseous molecules in the air through tiny pores on their leaf surfaces called stomata. Once inside the leaf, the gases diffuse into intercellular spaces and react with the inner-leaf surfaces, permanently converting pollutants like SO2, NO2, CO, and ozone.

While trees can help reduce the severity of air pollution caused by motor vehicles, they do not solve the problem. Properly designed roadside vegetation and noise barriers can play a role in reducing downwind pollution concentrations near roadways by altering airflow and intercepting pollution. Vegetation barriers can also provide additional benefits to communities, including noise reduction and water runoff control.

They can reduce air temperature

Trees and bushes near highways can significantly reduce air pollution. They do this in several ways, one of which is by reducing air temperature.

Trees have a natural cooling effect, which makes them a simple and effective way to reduce heat islands. Strategically planting deciduous trees around buildings can provide shade and cool the air by several degrees. This, in turn, reduces the energy used for air conditioning inside buildings. NASA found that temperatures in downtown Atlanta were 5 to 8 degrees Celsius hotter than in the suburbs. Trees can help minimize this heat island effect that saddles many cities.

Trees store less heat and energy than many artificial surfaces found in metropolitan areas, such as concrete and asphalt. They also contribute to cooling through the process of evapotranspiration.

In addition to reducing air temperature, trees can reduce air pollution by directly removing pollutants from the air. They can intercept airborne particles and permanently remove gaseous pollutants through leaf stomata on the plant surface.

They can reduce energy consumption in buildings

Trees and other vegetation can significantly reduce air pollution near freeways and roads. They achieve this through several mechanisms, including the interception of airborne particles, the uptake of gaseous air pollution through leaf stomata, and the alteration of airflow, which reduces wind speed and turbulence.

Trees can also play a crucial role in reducing energy consumption in buildings. Here are some ways in which trees can help:

Cooling and Air Conditioning

Trees can provide shade and reduce air and surface temperatures, leading to lower energy costs for cooling. Large trees can significantly lower temperatures, with shaded surfaces being 11-25°C cooler than unshaded surfaces. Deciduous trees are particularly effective in the summer, intercepting up to 90% of sunlight to provide natural cooling. Planting trees near air conditioners and paved areas can also create a cooler microclimate, reducing the energy required for cooling.

Heating

In the colder months, deciduous trees allow sunlight to penetrate buildings, providing natural heating. Evergreen trees can also serve as windbreaks, blocking cold winds and reducing heat loss. Properly positioned trees can reduce heating costs and increase the comfort of those inside.

Energy Savings

The strategic placement of trees can lead to significant energy savings. Studies have shown that urban trees can reduce annual heating costs by 10-30% in the US. Additionally, computer models from the U.S. Department of Energy suggest that just three carefully placed trees can save an average household up to $250 in energy costs annually.

Evapotranspiration and Humidification

Trees release water vapor through evapotranspiration, which helps to reduce peak summer temperatures. Additionally, trees can humidify the air, contributing to a more comfortable indoor environment and potentially reducing the need for artificial humidification.

By incorporating smart landscaping and carefully considering the placement of trees, homeowners and urban planners can harness the benefits of trees to reduce energy consumption in buildings, leading to both financial savings and a positive environmental impact.

They can act as barriers between sources and populations

Trees and other vegetation have been shown to act as barriers between sources of pollution and populations. While trees do not solve the problem of air pollution caused by motor vehicles, they can help reduce its severity. A growing body of evidence has linked motor vehicle pollution to conditions such as asthma, chronic bronchitis, lung cancer, and heart attacks. Therefore, solutions to the problem of elevated pollution levels near roadways are urgently needed.

Trees can reduce regional air pollution levels through the interception of airborne particles or through the uptake of gaseous air pollution. The complex and porous structure of trees and bushes can modify near-road concentrations via pollutant capture or through altering airflow, which can result in either reduced dispersion through the reduction of wind speed and boundary layer heights or in enhanced dispersion due to increased air turbulence and mixing as the pollutant plume is lofted up and over the vegetation.

Trees absorb gaseous molecules in the air. Tiny pores on tree leaf surfaces called stomata take in air that includes toxic pollutants. Once inside the leaf, the gases diffuse into intercellular spaces and may react with inner-leaf surfaces. This means pollutants like SO2, NO2, CO, and ozone are permanently converted when inside the leaf. However, studies have found that ground-level ozone significantly reduces tree growth, injures the foliage, and predisposes trees to insect and disease attacks.

Trees can also remove particulate matter by "catching" them temporarily. Fine particulate matter is deposited on tree surfaces, clinging to leaves and stems instead of floating in the air. Most PM2.5 will only remain on surfaces. With rain or precipitation, the particulates can be dissolved in stormwater runoff or transferred to the soil. Particulates can also be re-suspended or re-enter the atmosphere, making the tree only a temporary retention site for many atmospheric particles.

They can reduce mixing and turbulence

Trees and bushes near highways can significantly reduce air pollution. Urban forests can remove multiple tons of ozone, gaseous air pollution, and particulate matter each year through direct uptake of gases or by temporarily intercepting airborne particles.

Trees can reduce air pollution through several means. They can reduce air temperature, thereby altering pollution concentrations. They can also reduce energy consumption in buildings, which reduces air pollutant emissions from power sources. Most notably, trees can directly remove pollutants from the air.

Trees have tiny pores on their leaf surfaces called stomata that take in air, including toxic pollutants. Once inside the leaf, the gases diffuse into the intercellular spaces and react with the inner-leaf surfaces. This means pollutants like SO2, NO2, CO, and ozone are permanently converted when inside the leaf. However, ground-level ozone can significantly reduce tree growth, injure foliage, and make trees more susceptible to insect and disease attacks.

Trees can also remove particulate matter by "catching" them temporarily. Fine particulate matter is deposited on tree surfaces, clinging to leaves and stems instead of floating in the air. Most of these particles will only remain on the surfaces until they are dissolved in stormwater runoff or transferred to the soil during rain or precipitation.

The complex and porous structure of trees and bushes can modify near-road concentrations by capturing pollutants or altering airflow. This can result in either reduced dispersion through slower wind speeds and lower boundary layer heights or enhanced dispersion due to increased air turbulence and mixing as the pollutant plume is lifted over the vegetation.

Janhall (2015) summarized that low vegetation like hedges can filter out particulate matter when located close to emission sources such as roads, while high vegetation like trees can reduce mixing and turbulence, leading to increased concentration levels. The height and thickness of vegetation influence the extent of mixing and pollutant deposition, and the built environment also plays a role. Airflow and the impact of trees differ between a street canyon and an open highway environment.

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