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

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.