Air Quality and Trees
The relationship between air quality and trees is a complicated subject. Many of us learn at an early age that trees “inhale” carbon dioxide and “exhale” oxygen which allows us to breathe. This is true! But what about other air pollutants and particulate matter? Plenty of scientists are researching how trees can impact air quality.
The main air pollutants that are studied in relation to trees are ozone (O₃), nitrogen dioxide (NO₂), carbon monoxide (CO), sulfur dioxide (SO₂), and particulate matter measuring 10 micrometers or less (PM10). According to a 2006 study, urban trees in the lower 48 states of the US remove an estimated 711,000 metric tons of these pollutants each year. That’s a $3.8 billion value (Nowak 2006)! Gaseous pollutants like O₃ are absorbed through the small openings in a tree’s leaves, or stomata. Particulate matter is intercepted and stored on the tree’s leaf and bark surfaces (Nowak 2002).
Trees also impact air quality indirectly by mitigating air temperature. Smog effects are more likely to occur in cities on very hot days, but extensive tree canopy can lower air temperatures through shade and evapotranspiration (Akbari et al 2001).
However, while trees can improve air quality they can sometimes make it worse. Trees produce pollen, which is an airborne allergen that causes the yellow haze covering your car or front porch. In addition, some trees emit volatile organic compounds (VOCs), which interact with emissions from cars and contribute to the creation of ozone (Chameides et al 1992).
Clearly trees and air quality have a complicated relationship. Thankfully, scientists are continuing to do research to fill the gaps in our knowledge. In the meantime, we can be thankful for one indisputable fact: trees provide the oxygen that supports life on our planet!
Want to learn more? Check out this article from the BBC that covers a lot of the research and contradictions around trees and air quality.
Nowak 2006 — ufug_air_pollution_removal
Akbari et al 2001 — https://doi.org/10.1016/S0038-092X(00)00089‑X
Chameides et al 1992 — Chameides, W., F. Fehsenfeld, M. Rodgers, C. Cardelino, J. Martinez, D. Parish, W. Lonneman, D. Lawson, R. Rasmussen, and P. Zimmerman. 1992. Ozone precursor relationships in the ambient atmosphere. J. Geophys. Res. Atmos. 97: 6037–6055.