In this article, you will find information about everything related to particulate matter. I will focus on PM2.5 as I believe it is the most universal indicator of air quality. Almost all scientific studies concentrate their attention to PM2.5 because there is a substantial availability of monitors/sensors. Some extracts were taken from my book See The Air – PROOF THAT AIR POLLUTION KILLS.
1) What is PM2.5?
Scientific explanation: PM2.5 is a mass concentration on basis of a separation curve, where the 50% value is at 2.5μm aerodynamic diameter. More precisely, the weighting function is much steeper (100% weighting <0.5μm, to 0% weighting> 3.5μm and 50% weighting at approximately 2.5μm). PM2.5 pollution can only be seen with an electron microscope.
Plain English explanation: PM2.5 referees to fine particles with a size of or less than 2.5μm (aka micron). By the way, soot, aerosols, dust, BC are other common terms to call particle pollution.

2) What is PM2.5 made of?
It is hard to determine a specific chemical composition, however, here is a list of all the chemicals that might be found in a single particle. Polycyclic aromatic hydrocarbons (naphthalene, acenaphthylene acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a) pyrene, dibenzo(a,h)anthracene, benzo(g,h,i)perylene, and indene), metals (Al, Si, P, S, CI, K, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Pb, Ca), others.
