According to the US Department of Agriculture, Forest Service publication THE EFFECTS OF URBAN TREES ON AIR QUALITY
Trees remove gaseous air pollution primarily by uptake via leaf stomata,
though some gases are removed by the plant surface. Once inside the leaf, gases diffuse into intercellular
spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces. Trees also
remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree,
though most particles that are intercepted are retained on the plant surface. The intercepted particle
often is resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig
fall. Consequently, vegetation is only a temporary retention site for many atmospheric particles.
Pollutants removed by trees include ozone, sulfur dioxide, particulate matter,
nitrogen dioxide, and carbon monoxide.
However, "Emissions of volatile organic compounds by
trees can contribute to the formation of ozone and carbon monoxide."
VOC emission rates also vary by species. Nine genera that have the highest standardized isoprene
emission rate and therefore the greatest relative effect among genera on increasing ozone, are:
beefwood (Casuarina spp.), Eucalyptus spp., sweetgum (Liquidambar spp.), black gum (Nyssa
spp.), sycamore (Platanus spp.), poplar (Populus spp.), oak (Quercus spp.), black locust (Robinia
spp.), and willow (Salix spp.). However, due to the high degree of uncertainty in atmospheric modeling,
results are currently inconclusive as to whether these genera will contribute to an overall net formation of
ozone in cities (i.e., ozone formation from VOC emissions are greater than ozone removal). Some
common genera in Brooklyn, NY, with the greatest relative effect on lowering ozone were mulberry
(Morus spp.), cherry (Prunus spp.), linden (Tilia spp.) and honey locust (Gleditsia sp.)
See also: Tree and forest effects on air quality and human health in the United States Environmental Pollution (2014) vol. 193, pages 119-129:
The average annual percent air quality improvement due to trees varied among pollutants and ranged from a low of 0.13% in urban areas for PM2.5 to a high of 0.51% in rural areas for O3
Modeled PM2.5 removal by trees in ten U.S. cities and associated health effects Environmental Pollution, 178 (2013), pp. 395–402
Average annual percent air quality improvement ranged between
0.05% in San Francisco and 0.24% in Atlanta.
Atlanta is 52.1% covered in trees, the most of the cities studied.
So even if a city is more than half covered with trees, the reduction in pollution is less than 1%.