Air and water meet over most of the earth’s surface, but exactly where one ends and the other begins turns out to be a surprisingly subtle question.

A new study in Nature narrows the boundary to just one quarter of water molecules in the uppermost layer – those that happen to have one hydrogen atom in water and the other vibrating freely above.

Such molecules straddle gas and liquid phases, according to senior author and USC Dornsife College of Letters, Arts and Sciences associate professor Alexander Benderskii, who said the free hydrogen behaves like an atom in gas phase, while its twin below acts much like the other atoms that make up “bulk” water.

The finding matters for theoretical reasons and for practical studies of reactions at the water’s surface, including the processes that maintain a vital supply of nitrogen, oxygen and carbon dioxide in the atmosphere.