Pan film wasn't introduced in 1928. Maybe it's a particular pan film Hurrell settled on?
Stieglitz needed pan film for his equivalents/songs of the sky series which started in 1923.
wikipedia claims pan plates were commercially available in 1906.
http://en.wikipedia.org/wiki/Panchromatic_film also claims kodak's ortho motion picture film was discontinued in 1930. Perhaps Hurrell wanted to use the same film characteristics as would be used in the motion pictures his subjects would be better known in? Perhaps he was forced to change because of this?