[simias]

That seems very far fetched to me. Why would the pollution created by german coal power plants accumulate in and around Paris?

In my experience the big pollution episodes in Paris are when there's no wind to wash the smog away. It's definitely localized over the city (not that the air is not polluted elsewhere, it's just generally much worse in Paris).

[cataflam]

It travels all over Europe depending on the wind. There is just extra pollution around Paris (mostly because of traffic) so the two of them combined can make it exceed noticeable thresholds.

Another example of a recent particulate matter concentration map : http://www2.prevair.org/sites/prevair.org/files/styles/scald...