If a 90s diesel is spewing foul-smelling black soot then it's 1. poorly maintained 2. illegally modified (those guys are pricks).
Also DEF, urea, is a NOx control mechanism and does nothing for the particulate matter.
Ironically, NOx emissions legislation must increase particulate emissions and worsen mpg (Less NOx needs lower T, lower T means more incomplete combustion, poorer combustion means particulates).
So DEF is an attempt by engine manufacturers to meet NOx emissions without increasing particular matter and destroying mpg.
But you'd use gov't overreach and not solve anything directly (obviously, indirectly, you'd get rid of ill-maintained and illegally modified trucks... but that's poor legislation)
Also DEF, urea, is a NOx control mechanism and does nothing for the particulate matter.
Ironically, NOx emissions legislation must increase particulate emissions and worsen mpg (Less NOx needs lower T, lower T means more incomplete combustion, poorer combustion means particulates).
So DEF is an attempt by engine manufacturers to meet NOx emissions without increasing particular matter and destroying mpg.
But you'd use gov't overreach and not solve anything directly (obviously, indirectly, you'd get rid of ill-maintained and illegally modified trucks... but that's poor legislation)