[dognotdog]

This is probably a longer discussion, but PM0.3 (anything less than 0.3 microns) is quite difficult to measure, especially optically, as light scattering drops off very steeply once your particle sizes are around the wavelength of the light being used. Anything short of an SMPS class instrument that “grows” small particles through condensation so that they appear large enough to be counted, will not see much below 0.3 microns. Also, the total MASS of small particles might be low, but if you look at count, or lung-deposited surface area, we get a different picture. Especially around 0.3um, particulate matter is prone to get deep into the lungs, instead of being caught in the upper airways, and the small size means larger relative surface area, and thus higher reactivity. Even smaller nano particles might not go as deep, but are more likely to enter cells or the bloodstream due to their minuscule size.