There is different devices to measure pm2.5 and Pm10. These devices use Laser technique in the Measurement.

I agree. I have not seen a "estimation" technique but I have used outside field equipment such as Data Rams and Personal Data Rams and I have used hand held devices for indoor air quality measurements. The personal devices they sell for the average home owner are still in development. These devices have not been proven to be accurate.

The particulate sampler guide here may be helpful:

https://www.skcltd.com/knowledge-library/product-selection-guides/particulate-sampler-selection-guide.html

Respirable dust sampler or high volume samplers are used for measuring the concentration of coarse particulate matter in ambient air and further analysis of samples in the lab.

Wenming Shi - The ambient coarse particulate matter (PM2.5-10) can be obtained by subtracting PM2.5 from PM10 because PM10 should have included PM2.5.

PM2.5-10 = PM10 - PM2.5