By Thomas Cahill et al
Sacramento, CA (April 30, 2008)- Measurements of diesel and smoking car exhaust identify that almost all resulting mass is smaller in size than even very fine particles including some of the most toxic outputs such as Polycyclic Aromatic Hydrocarbons (PAH). This makes their uptake to human lungs and ability to diffuse to surfaces very easy. However, a recent study showed that vegetation near very fine particle sources can be effective in removing some of the most toxic particles in the air before they get mixed into the regional air mass.
The study especially focused on the ability of finely needled and leaved trees suitable for removing the most dangerous highway pollutants from diesel and smoking cars near roadways. Results showed that all forms of vegetation were able to remove 30% to 80% of very fine particles at wind velocities below about 1.0 m/sec (roughly 2 mi/hr) during the 2 to 4 seconds in which the particles were within the vegetation chamber. Redwood and deodar were about twice as effective as live oak. The researchers also measured removal for wood smoke, which was about 30% less efficient removal rate than with the flares and on air filters.
The studies confirm the theoretical predictions that vegetation is highly effective in removing some of the most toxic components in the ambient atmosphere, namely diesel and smoking car exhaust. The effectiveness is greatest at low wind velocities and in configurations such that the vegetation is very close to the source. Moreover, the particles, once impacted onto vegetation, were not easily removed at low wind velocity in “shake-off” tests.
We also note that many of the lighter PAHs (many of which have a significant volatility) and other material from diesels and smoking cars have even higher diffusion rates than the heavy PAHs and transition metals studied in this work and thus should be even more efficiently removed onto vegetation.
Removal Rates of Particulate Matter onto Vegetation as a Function of Particle Size