In cities, is the pollution level higher on higher floors?
Yes, the higher floors will have less pollution than the street level.
The wind rises as a function of altitude, so the street will have less new air coming in, on average:

Some of the pollutants are lighter than air, i.e. Ozone, and some are heavier, i.e. particulate matter, they tend to rise or fall very slowly, so they can be slightly different gradients at different floors of the building.
The pollutants are diffuse clouds that rely on ambient air currents for dispersion.