the seed is about 2 mm long, surrounded by something like cotton. A lot of this is flying in the air

That is seed from a Cottonwood tree (Populus deltoides) or from a closely related European counterpart. Cottonwoods are not native to Europe, but there are some growing there.

When I drive between 2 forest preserves near me, it literally can look like it's snowing if the wind is in the right direction. It's a bit freaky when you see it for the first time in May or June.