Are there any high-resolution urban CO2 concentration data sets?
Are there papers or datasets of average CO2 concentrations down at
  ground level (say 0-20 metres upwards from the ground), in the spaces
  between buildings - streets, parks and so on?


Yes, THE URBAN CO2 DOME OF PHOENIX, ARIZONA has measurements at a height of 2 meters above ground level, 1.6 km horizontal resolution. CO2 concentrations are 555 ppm near the center of the city.

An intensive two-week study of an urban CO2 dome in Phoenix, Arizona, USA has higher resolution data along selected transects.