What makes Mexico city the smoggiest?

Answer 1

It isn't the smoggiest -- at least not in the world. The city sits in a valley surrounded by mountains. Smog from manufacturing plants and automobiles is constantly trapped over the city with no place to go due to lack of winds, making up the layer of gloom that sits over the valley.

Also, temperature and humidity play a role in such "pressure pot" effect: during winter months pollution is more prevalent, while windy or rainy seasons tend to disperse these pollutants.