Trees Do Reduce Air Pollution
The programme is no longer easily accessible but the results are summarised at http://www.bbc.co.uk/programmes/p01dgd9c/features/pollutionexperiment. Silver birches were planted along a busy road and the pollution measured inside neighbouring houses both with and without the trees outside. Those with the trees recorded pollution levels 50-60% lower than the houses without the trees.
Apparently birch trees are particularly effective in trapping pollution but all greening will improve air quality.
Usefully trees can also hold water in the event of heavy rain, to mitigate extremes of heat and cold. And they improve mental health. One study found that a child in Projects in Chicago who could see just one tree from their window would enjoy better mental health than a child who could see only buildings.
Moral, we need to plant more trees and green more walls and roofs.