NO2 uptake by plants
It has been observed that plants can lead to a reduction of atmospheric NO2 concentrations [Breuninger et al, 2013]. Currently, it is assumed that NO2 is taken up through stomata of plant leaves when the atmospheric NO2 concentrations are above a certain threshold. However, when the atmospheric NO2 concentrations are below this threshold, plant leave stomata act as sources for NO2. Therefore, this process is not fixation in the sense as N2 is fixed by legumes.
See Breuninger et al.  and refernces therein for details
Plants do not only take up (excess) NO2 actively but they decrease the concentrations of several air pollutants by providing a lot of surface for dry deposition as Christoph stated in his comment. After deposition they may revolatilize or be washed down by rain. See e.g. Grundström and Pleijel  for details.
Impact on air quality
The presence of plants/trees in urban environments can reduce ambient NO2 concentrations. However, this effect is not due to NO2 fixation but due to the canopy reduction/enhanced dry deposition rate, which also yields a reduction of other air pollutants. See e.g. Grundström and Pleijel , Yang et al , or Yang et al.  for details.
The canopy reduction is also considered in recent regional modeling studies - see e.g. Arndt et al.  (Disclaimer: I was member of that working group until recently). It can be seen like increasing the surface roughness.