Challenges in Non-Viable Air Sampling in Compressed Air-Nitrogen Lines

I test compressed/ nitrogen air for quality. Sometimes on smaller systems with 10 drops others on 50 plus access ports.  When performing Non-Viable sampling I will get a burst of particulate read by a particle counter connected to the system.  I use a high pressure diffuser to keep positive pressure on the line, my incoming PSI to the high pressure diffuser can be as high as 50 psi.  The particle counter I'm taking a 28.3L sample generally.  I've struggled with explaining the burst of particulate.  I correlate it to opening of solenoid valves or labs using adjacent air for processes.  EG...particle counter will run for 30 secs with no issues and very close to zero particulate when a valve is opened I see the counts blow up to 30K-500K or more. I usually only pay attention to .5 micron and 5 Micron sizes.  Some Lines have inline filters of .2 micron and i will still see a burst.  Usually the burst will spike the P/C and then be gone immediately...Dirty Lines??? Pressure drop/exchange?

It sounds like pockets of material are being disturbed when turbulence is created within the line by other users.  This activity is probably representative of what is happening during production normally -- so I don't see it as a 'problem'.