Function: Retention of small particles

Vegetation acts as a filter by adsorbing and absorbing impurities from the air. Especially trees are effective due to their large canopies and great surface roughness. Also some soil microorganisms improve air quality.

BROADLEAVES

Broadleaves are efficient in absorbing air pollution especially because of their  large leaf area. The leaves are renewed annually so the accumulated toxicity does not rise very high. Nevertheless, it has been argued that as the leaves decompose the toxicity accumulates in the soil and thus may damage the root system.

CONIFERS

Conifers have large surface area, they grow fast and are evergreen, which altogether make them effective at absorbing airborne particles. However, the accumulation of toxins is higher in conifers than in broadleaves because the needles are perennial. This may result in more severe physiological damage.

PHYSIOLOGICAL DAMAGE ON TREES

Large concentration of air pollutants may cause variety of physiological damage in trees in addition to the increased toxicity mentioned above. Fine particles may block the stomata, which decreases the gas exchange of the plant and thus its vital functions. The particle cover on vegetation surfaces may disrupt also pollination, bud break and light reflectance, increase sensitivity to pathogens and alter genetic structure in the long run.
 

Beckett, K. P., Freer-Smith, P. H., Taylor, G. 1998. Urban woodlands: their role in reducing the effects of particulate pollution. Environmental pollution. Volume 99, Issue 3. pp. 347–360.