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Sandeep Kumar Chaudhry

Your question makes little sense: 'Why we calculate particle size distribution (PSD) of PM particles of different sizes'

Different sizes are a distribution. You have a problem in logic.

Take a look here at these webinars (registration needed) for some guidance of the importance:

October 2nd, 2018 Particle Size Masterclass: Why Measure Particle Size?

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W181002PSMasterclass.html

and how:

October 23rd, 2018 How to measure particle size distribution

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W181023HowisPSmeasured.html