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.

SOURCES:

Nowak 2006 — ufug_air_pollution_removal

Nowak 2002 — https://www.nrs.fs.fed.us/units/urban/local-resources/downloads/Tree_Air_Qual.pdf

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.