The relationship between trees and air pollution is complex. Contaminated air contains suspended particles that can easily attach to the leaves
The stomata of the leaves absorb gases such as nitrogen dioxide, slightly reducing the level of air pollution. However, plants can also block the surrounding airflow, making the clean airflow impossible to dilute.
Planting trees with very dense foliage on the roadside where many people walk will produce a roof-like effect, trapping polluted air on the ground.