Here is a good article (open access at the website) on the plantower sensors used in PurpleAir monitors.
It demonstrates that the sensors are good measures of light scattering, but they do not count individual particles nor are they specific to certain size fractions. Light scattering usually correlates well with PM2.5, but the conversions from scattering are highly dependent on the particles shapes, indices of refraction, and size distributions. It is good to see that PurpleAir is now reporting scattering and visual range.
Ouimette, J.R., Malm, W.C., Schichtel, B.A., Sheridan, P.J., Andrews, E., Ogren, J.A., Arnott, W.P., (2022). Evaluating the PurpleAir monitor as an aerosol light scattering instrument. Atmospheric Measurement Techniques, 15, 655-676. 10.5194/amt-15-655-2022. AMT - Evaluating the PurpleAir monitor as an aerosol light scattering instrument