Why is the arctic brown?

I’m blogging from a 757, somewhere over the North pole, returning from a sustainability meeting in Iceland. The world below is a wilderness of sea ice and clouds. I’d expect brilliant white, but there’s actually a brown haze over the landscape. It’s stratified, much like the odd sight of half-white, half-brown clouds one occasionally sees when flying into a polluted city. Where does it come from? Chinese coal fumes? Russian fires? American SUV tailpipes? Icelandic airplane exhaust?