What units are represented by pr - micrometer?

In studying column abundances, I often encounter the units $$pr-\mu m$$. What does this stand for? Part per micrometer? how can this be an abundance unit?

Incredibility I cannot figure out what it means after quite a bit of online searching.

• Likely per micrometer. The pr is not standard. I imagine it would be commonly used in display technology and micofabrication for density. In molecular fields you have the related ppm for parts per million. – Paul Childs Jun 6 '19 at 1:03