PM refers to the particulate matter in the atmosphere, which is a relatively complex multi-dispersed mixture.
Definition of PM2.5：
Refers to the airborne particles whose aerodynamic equivalent diameter is less than 2.5 microns, also known as fine particles.
Form： Smoke，dust，fog
Size distribution：total suspended particulates TSP（dp＜100 μm），floating dust（dp＜30μm），PM10, PM2.5……
Source: mainly industrial production, motor vehicle exhaust, road and construction dust，ect.