Parameters vary depending the country, the basic is "statistics". Over a year someone computes parameter based on average, peaks days and others params related to one cities (we are talking ONLY for cities, not area of the country).
Having "some" days of full stop helps a lot lowering theese values; parameters are not true average, rather they are some sort of peak detection and absolute positive/negative records of fine power (PM-10).
The truth is: only numbers are mitigated, real pollution just came back in hte evening when... everybody would comeback to the cities. Yes, to not to stay at home they just leave in early morning and come back in the evening.
Take in accoun that most cars in the cities are commuter cars... so sadly only residents are paying for someoneelse pollution... :(
Truly talking: they should setup a better public transport, and set industries outside the cities, also some exchange parking linked to metro/bus station... just a number: in italy every year are sold more cars than new babies are born in a year... don't know other counties..