The common range for soot particle size is between 0.25 to 1000 microns( see MERV RATING CHART). As for soot in the exhaust and lub oil it can be collected by paper filters and then measured under a microscope. But it is almost impossible to determine the size of soot particles inside a cylinder since formation of them and combustion takes place at the same time.

See my paper " Prediction of soot emission in a direct injection diesel engine" which is available within my publications in Researchgate.

P.M 2.5   P.M 10

Sometimes for technical purposes is assessed than soot particle has 0.1 um.

Seinfeld and Pandis (2006) gave 30 nm agglomerates, but to be honest soot is not defined by size of particle, rather by physical properties.(radiation absorption).

I agree but how to really differentiate particles basing only on size (I assumed that @Fadzli Ibrahim would like to have only approximate amount of soot basing on diameter).

However you've stressed the age of selected publications, in my opinion having only  size of particles is not enough to differentiate them.

Thanks @Damian Zasina, so instead of particle size what else more important need to consider in order to differentiate them?

As Artur said try to look at ftp://opgc16.univ-bpclermont.fr/ATMOS/aerosols/PIREP/Engine/Maricq2007b.pdf , however I am not very much familiar with measurements. I'm involved rather in emission inventory.

Engine exhaust particles are associated with a complex mixture of organic compounds and common terms used to describe engine exhaust particle include black carbon, soot, non-volatile and volatile organic carbons.  Fraction of soot is higher than 50% of engine exhaust particle. Studies have been done on soot locations/fractions in flames, soot processes (fuel pyrolysis, nucleation, coalescence, surface growth, agglomeration, and oxidation), and size distribution. Difference between soot in combustion chamber (primary soot) and soot in tailpipe is not fully understood and soot deposition in lubricant is a different story from that in combustion chamber and in the tailpipe, I guess. Exhaust diesel soot size depends on engine operating conditions as well as fuel properties, but is typically bimodal: one peak at 5-20nm and other at 40-60nm. I expect to see if you guys can share what is difference in soot size, structure and/or other properties  between primary and exhaust soot.

Thanks @Phuong X. Pham,

Here is an article about Comparison between In-Flame and Exhaust Soot Particles in a Single-Cylinder, Light-Duty Diesel Engine http://people.eng.unimelb.edu.au/imarusic/proceedings/19/59.pdf

@ Fadzli Ibrahim, thanks for the link. it is very informative. and one can easily say that soot has a particle less than 50 nm.