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.
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.