Trees absorb and capture particles in the air through their leaves, acting as a physical filter for molecules that can damage our bodies, but which trees can use efficiently to grow. Trees use carbon to grow wood and leaves, and nitrogen, sulfur and other molecules to make and store energy. When CO, CO2, NO2 and other pollutants are absorbed by trees and other plants, a biochemical process divides the oxygen from the other molecules and releases it back into the air. The oxygen released as a gas and as water vapor contributes to diluting the air as a consequence of filtration.

Active filtration is an important mitigation tactic in managing air pollution, but it’s not the only way that urban trees affect air quality. Tree crowns are physical barriers to heavier molecules that fall from the atmosphere onto the ground. The leaves collect dust and solid particles that build upon them, which then get rinsed away with rain.

Using all vertical layers of the understory, different sized trees capture falling particles along the way down, and dense shrubs and evergreens act as barriers that can block the horizontal spread of pollutants. They also slow down the wind, which interrupts the spread of dust and pollutants across an area. Trees keep an area slightly more humid, and soil more stable, further slowing the ability of fine particles to spread horizontally by air.

Another way that the canopy’s physical barrier affects air quality is by casting shade and lowering temperatures. In conjunction with raising the immediate area’s humidity, the cool cover that a tree’s crown provides reduces the dangerous formation of ground-level ozone concentrations. High temperatures create more ozone, which is formed when UV rays from sunlight create a gaseous chemical reaction with the pollutants released by cars and other

industrial emissions. As ozone builds, it absorbs more UV heat, raising the ambient temperature even more.

Plants that are effective in reducing smog typically have characteristics such as the ability to absorb pollutants, release oxygen, and provide shade. One mature tree can remove 0.2 tons (400 pounds) of carbon dioxide (CO2) per year. This figure doesn’t include the other main pollutants that are released by cars and industrial activities, such as carbon monoxide (CO), nitrogen dioxide (NO2) and sulfur dioxide (SO2). These gasses collect in the atmosphere and result in smog and ozone, which are toxin-rich air concentrations that can cause asthma, bronchitis and other lung diseases in humans. As it happens, trees make good use of these molecules as well, further reducing air pollution. Here is a list of plants that are known for their air-purifying properties and can contribute to reducing smog: