Pollution removal by urban trees in the US is estimated at 651,000 metric tons per year ($4.7 billion value for removal of ozone, nitrogen dioxide, sulfur dioxide and particulate matter less than 2.5 micrometers), or the equivalent carbon dioxide emissions from 138,510 cars.
Dense barriers of vegetation along roadways increase vertical mixing and concentrations of pollution on the road and generally reduces ground level pollution directly behind barriers relative to a clearing with no barrier.
Volatile Organic Compounds
While the emission of biogenic volatile organic compounds (BVOCs) from trees can increase ozone, other beneficial effects of trees tend to reduce ozone, making urban tree canopy cover a viable strategy to improve air quality and help meet clean air standards. BVOCs are highly species dependent.