Maybe an ash. Foliage from tree would help.

Definitely ash seeds but I can't get more specific about species.

Poplar seeds are downy little wisps like dandelion fuzz.....with a small seed attached.

Yes, Ash keys. The native one would be Fraxinus exelsior but in a park there could be other species, hybrids or cultivars.

My friend confirmed that Fraxinus excelsior was planted in that park so I think you are all correct. Thanks for your help!