WE CAN USE AIR POLLUTION SENSORS, OR REMOTE SENSING DATA(CAN BE GET BY THEIR COMPANY/GOVT),ALSO SMALL LAB INSTRUMENTS ARE AVAILABLE. SATELLITE PLATFORMS INCREASE OUR SPATIO TEMPORAL RANGE IN DATA COLLECTION
In order to assess the level air pollution of a city, you will be looking at annual averages of pollutant concentrations. Short term measurements may be misleading. For long term measurements of NO2 concentrations you may use passive collctors, which are very cost effective. This sensor type will allow you to collect sampes at various locations. For a description of the principle, look here
The connected question will be where to place the sensors in order to be representative for a larger area. Local effects, like nearby roads or in street canyons may affect your measurement. As a general guideline, I would refer to the European directives on air quality, e.g.