"The vehicle was a light-duty diesel-engined truck with a displacement of 4800 cc. A diesel oxidation catalyst was provided as an after-treatment device." https://doi.org/10.1016/j.proci.2010.07.001

From a review: "Diesel engines produce in-cylinder-formed soot particles consisting of 10- to 20-nm primary carbon spherules agglomerated to...a single-mode size distribution that is close to lognormal (e.g., Harris and Maricq35) with a GMD of 30 –100 nm" https://doi.org/10.3155/1047-3289.60.10.1245

However! There can also be nucleation (as the first tweet in this thread indicated) but at a size close to 10 nm. https://doi.org/10.3155/1047-3289.60.10.1245