Polluting agents generated by road traffic in a city
You might also want consider the metals which appear in road dust, I recall having seen some work on neutron activation anaylsis of dusts collected near roads. The rubber tyres are likely to contain some metals. Cars will be likely to release metals in other ways. I would consider the dust from the operation of disk brakes, the wear on tyres, fuel additives and also corrosion of bodywork and engine parts. I know that zinc and cadmium plated parts can release both ionic and particulate forms of these metals.

One paper on the subject was by Siddique et. al. im Radiochimica Acta, 2013, volume 101, pages 273-277.

It is interesting that lead levels in the air have fallen since leaded petrol was banned in the UK, but some workers have reported finding platinum group metals in the air near to roads.

Using ICPMS Simitchiev et. al, in TALANTA, 2008, volume 77, pages 889-896 found Pt, Rh and Pd in road dust.