The judicious placement of greenery in urban spaces reduces noxious gases and particulate matter at street level.
A study concluded that judicious placement of grass, climbing ivy and other plants in urban canyons can reduce the concentration at street level of NO2 by as much as 40 percent and PM by 60 percent, much more than previously believed. The authors even suggest building plant-covered “green billboards” in these urban canyons to increase the amount of foliage. Trees were also shown to be effective, but only if care is taken to avoid trapping pollutants beneath their crowns.
American Chemical Society News Service Weekly: http://www.acs.org/content/acs/en/pressroom/presspacs/2012/acs-presspac-august-29-2012/green-plants-reduce-pollution-on-city-streets-up-to-eight-times-more-than-previously-believed.html