I don't know how true this is but I've heard the older diesels (2000 era) actually had lower NOx emissions than the newer models due to them running at lower te

I don't know how true this is but I've heard the older diesels (2000 era) actually had lower NOx emissions than the newer models due to them running at lower temperatures as they weren't trying to hit newer regulations on particulate emissions. This also then led to newer models emitting finer particulate which was more easily picked up by the wind and could travel into the lungs more easily instead of denser particulate which fell to the roadside.