What percent of air pollution is caused by cars?
It depends where you live.
In the USA, CO (carbon monoxide), NOx (nitrogen oxides) and non methane hydrocarbons (NMHC) emission standards (grams per mile) from motor vehicles are provided below (Masters and Ela, 2008)
Type of vehicle CO NOx NMHC
Passenger cars 3.4 0.4 0.25
Light duty trucks 4.4 0.7 0.32
Moderate duty trucks 5.0 1.1 0.39
Motorcycles 19.3 2.24 (NOx+HC)
As far as I know, 260 million motor vehicles exist in the USA (out of 320 million people). However, type of city, total miles a vehicle takes per day, driving habits (sudden acceleration, sudden stop, etc.), etc. all are factors that change these figures provided above. In some cases, there are problematic vehicles emitting higher emissions per mile.
The air pollution problem in many urban centres is caused by emissions of carbon monoxide, nitrogen oxides, and various volatile organic compounds that are emitted from stationary sources (power plants, for instance) and motor vehicles. Photochemical smog is most closely associated with motor vehicles. Again topography and local climate play role in percentage of air pollution caused by traffic vs. stationary sources (space heating, power plants, industrial sites, etc.). There is no magic number for this question.
Reference: Masters, G.M. and Ela, W.P. (2008). Introduction to Environmental Engineering and Science (Third Edition). Pearson International Edition, New Jersey, USA.