It is not dirt or dust but Ozone. Higher latitudes in winter you will be in it. Pax planes normally have ozone scrubbers or something that deals with it.I doubt the freighter does.
In previous life we used to have to fly lower altitudes transcon in US with 737 because it didn't have any system to remove Ozone.
From one of the manufacturers of the scrubbers:
http://www.catalysts.basf.com/Main/e...zone_catalysts