This is partly in response to Paris briefly being one of the most polluted cities in the world[1] by some measures last year. But generally, it seems Paris fares slight better than Los Angeles & Amsterdam and slightly worse than London and NYC[2].
Paris is built in a valley with Montmartre to the north and Montparnasse to the south. When the winds die down for more than a few days being in the city really sucks. All the diesel particulates just sit their. Like you can feel the smog and particulates accumulating on your skin.
You can't reasonably say that Paris is in a valley because of two small hills (altitude 130m, only 100m above the lowest point of Paris). When there is no wind pollution accumulate everywhere.
Paris is in a very good position for pollution: in a middle of a plain, not very far from sea with a regular wind from west.
[1] http://www.france24.com/en/20150320-paris-city-smog-pollutio...
[2] https://www.theguardian.com/cities/2015/dec/02/where-world-m...