[Bevdeforges]

This site explains the low emission zones in Rouen: Rouen

You buy a one-time Crit'Air sticker based on the emissions rating of your vehicle. (Includes a link to where foreigners can register their car and get their sticker.) The stickers cost a little less than 5€ and are a one-time deal. As soon as you register, you can get a receipt that includes a downloadable certificate that works like a sticker - but they will mail you an actual sticker for your windshield/windscreen.

 

[Bevdeforges]

I think there is also a Crit'Air sticker site for "international" folks - and it is (or was when I found it) in English. But as long as your car is registered in France, they just take the information directly off your vehicle registration and it's a pretty easy process. And as BiF said, it's valid in all the various zones around France.

 

[Bevdeforges]

Does the 'Crit Air system only apply to towns and cities, can anyone confirm? I will be driving from Biarritz to Italy in January, all the way on motorways.

This is the "official" Bison Futé map with the little icons indicating where the various ZFE are located: Trafic - Zones à faibles émissions mobilité (ZFE-m) - Bison Futé
The range (and applicable times) of the restrictions aren't directly indicated on the map, so you have to read the little descriptions of the restrictions for the various towns you will be passing through on your journey. If you read the detail and follow the available links, it does seem that the main autoroutes passing through the zones are often exempt. But considering that the sticker only costs a whopping 4€ or so, you may want to just get a sticker now so you don't have to worry about it any more. The process for getting the sticker is pretty quick and easy - and you can download a pdf version of your sticker to stick on your windscreen until the actual sticker arrives in the mail (which only takes a couple days).