With "dirty air effect" do you mean the drafting-effect?
That would be these lines:
BaseDropoff=0.175 // RFM Drafting override: Higher number -> more drafting effect (default=0.185)
LeadingExponent=2.10 // RFM Drafting override: Higher number -> lower effect on leader (default=2.0)
FollowingExponent=1.90 // RFM Drafting override: Higher number -> lower effect on followers (default=2.0)
VehicleWidth=1.96 // RFM Drafting override: Helps determine base width of wake (default=1.9)
SideEffect=0.35 // RFM Drafting override: Negative effects of side-by-side drafting (default=0.35, used to be ~3.0 which was way too strong!)
SideLeadingExponent=2.0 // RFM Drafting override: Added to regular LeadingExponent to affect the side wake
SideFollowingExponent=3.5 // RFM Drafting override: Added to regular FollowingExponent to affect the side wake
But attention, some mods have these lines in the *.rfm file which will override the ones in the *.hdv.