PM2.5 Particles

This is one view of PM2.5 particles on a Teflon air filter. The bright particles are secondary salts that have grown in the filter during collection. They are primarily ammonium sulfate. The black particles are from coal dust, charred wood, natural minerals, and other combustion products. With linear polarized light some of the crystals are not visible and many are no brighter than the Teflon filter stress patterns in the background.