On the ice just south of the airport, along with the large flock of Ring-billeds and many more Herring Gulls than usual; the closest point from which we could view it was the Indigo Landing restaurant area - still a long way away. Stood out very clearly despite the distance, though, as a very large, all-white gull. Much larger (say, half as large again) as adjacent Ring-billeds; appeared about the same size as a nearby imm Great Black-backed. Both body and head shape seemed elongated; certainly not rounded. We took some record photos (through bins!) which show both head shape and size comparison to nearby Ring-billeds. One caveat: one of the photos seems to show slight dark wingtips (we didn't notice these while we were watching the bird itself) - possibly some kind of Glaucous x Herring hybrid? It took off and flew off down the Potomac after we'd watched it for about five minutes - appeared very white in flight (again, we didn't notice any dark on the wingtips when it was flying), and again, very large.