How would the "polluter-pays principle" apply to the case of mobile-source air pollution?
I suppose forms of transport would be an example of mobile-source air pollution, in particular looking at air travel. Well, this is a very interesting case to apply to the "polluter-pays principle," as you put it. I think I am right in saying that some countries are already talking about imposing a "green tax" on the cost of tickets that will mean that those who are travelling by air will have to pay more to take into account the pollution that is being caused by their travel. However, to my mind, this does not go far enough, as the real people who should be having to pay something is the airline itself. Perhaps there should be a law forcing them to invest a certain amount of their profits into environmental relief or some kind of green energy research.
I suppose the same could be said for shipping companies who transport vast quantities of goods both within countries and between them by road. Maybe we should have to pay more for products that have to be shipped this way, but also maybe the companies themselves that do the shipping should be forced to pay for the pollution that they are causing. It is a difficult moral issue in a sense, because such companies wouldn't exist if we didn't want such services, so maybe we should all have to pay a price that reflects the environmental impact of such activities.