While looking at a number of heavily used fluorescent ceiling fixtures, I have noticed that they typically have greyish black marks that apparently form on the surfaces above fluorescent tubes due to years of use. What causes them to form? Is it due to dust particles burning off the tubes and adhering to the surfaces above where the tubes are mounted?

Here is an example of the age darkening that I am talking about:

https://www.lighting-gallery.net/gallery/displayimage.php?album=6272&pos=3&pid=231474

In the picture, you can see some black lines that perfectly align with the locations where fluorescent tubes were once installed in the abandoned preheat F40 troffer fixtures.

Fluorescent lamps emit more heat than you would expect, combined with UV and visible spectrum, and will eventually degrade the paint or powder coat on a fixture. In some cases, the fixtures are more discoloured at the ends where the lamps emit more heat. All this combined with an ascending air current that carries and deposits dust and this is the result.

Sometimes it's just years of dust deposits on it and the paint is still in decent shape. My louvered fixture had some similar build up around the lampholders and in the ballast area but it all washed away once I gave it a proper wash.