I am guessing that it is coated in some type of clear coating and that is what is scratched, but getting more agressive with it will most likely go through the coating (like using steel wool) and ruin it. I would start with some wet dry sand paper, 1500 and if that doesn't work, drop down to 1000 or 800, then work your way back up with 1500 then 2000 to get the sandpaper lines out and polish, but be careful, the coating is probably fairly thin.
Just start with the cheapest easiest piece to replace in case you sand through it!
2008 335i Vert Montego/Saddle/Grey Poplar
2011 Audi S4 Deep See Blue/Black/Stainless
2014 X3 35i M-Sport Space Grey/Chestnut/Fineline Wave