PM (Particulate matter) is the general term used for a combination of solid particles and liquid droplets found in the air. PM can also be referred to as PM10 and PM2.5. PM10 and PM2.5 refer to the aerodynamic equivalent diameter of 10 microns or less and 2.5 microns or less, respectively. The quantity and characteristics of the material and particle size distribution are dependent on the process generating the particulate matter.
Some of the solids are visible with the naked eye, while others are detected by microscope. Regardless, the particles, consisting of different chemicals can be found in many shapes, sizes, and consistency. The three primary sources of PM are power plants, industries, and automobiles. Most of the PM is formed due to reactions of chemicals such as sulphur oxides (SOx) and nitrogen oxides (NOx).