Lots of JT3Ds still in operation today! Many, but not all, DC-8s were re-engined . . . lots of military 707 variants still using the JT3D today. Now, I love the old noisy classic engines as much as any red blooded aviation lover - but given the concern over the last 20+ years about emissions, including particulate emissions, I truly wonder why after nearly 50 years, the core of the JT3D engine has not been modified so that it would burn more cleanly at low altitudes.
Just going by what I've seen, it seems that the JT3D smokes least when operating in hot, dry conditions, and most when humidity is high and the temperatures are cool to cold. The heaviest smoke trails seemed to always form on damp winter days back when I'd see the classics coming in and out of YYZ.