This is a common suggestion, and the answer is complicated. Evidence from research is mixed. However, even if trees did make a difference the effect would be very small in comparison to the size of the problem and would not deliver compliance in the shortest possible time. Trees of course offer multiple other benefits in urban environments.
Two recent (July 2018) reports from ONS (Office of National Statistics) and AQEG (Air Quality Expert Group) have looked at this issue.
The ONS report found that although vegetation removes significant quantities of air pollution from the atmosphere, and may reduce concentrations of particulate matter by 6 – 10%, the impact on NO2 was insignificant. The ONS report concludes: “Even though vegetation will not solve the whole issue of air quality in the UK, and in some cases vegetation can have adverse effects on air quality, the service of air pollution absorption by vegetation is nevertheless an important one.”
AQEG reviewed research and concluded that “vegetation and trees in particular are regarded as beneficial for air quality, but they are not a solution to the air quality problems at a city scale.”