Many people may think that PM2.5 is haze, and haze is caused by PM2.5. Whether it is or not, let's first popularize some basic knowledge.

What is haze?

Haze, as the name suggests, is fog and smoke/haze, but there is a big difference between the two.

Fog is suspended tiny water droplets formed by the condensation of water vapor as air temperature drops near the ground. The presence of fog reduces air transparency and worsens visibility; it is a natural weather phenomenon.

Haze is a phenomenon where a large number of extremely fine dry dust particles float uniformly in the air, making the horizontal visibility less than 10 kilometers and causing general air turbidity; it is also called gray haze. It mainly consists of three items: sulfur dioxide, nitrogen oxides, and inhalable particulate matter. When combined with fog, they instantly make the sky gloomy and gray.

In meteorology, the identification standards for fog and haze are: haze when relative humidity is less than 80%; fog when relative humidity is greater than 90%; a mixture of fog and haze when relative humidity is between 80% and 90%, where the degree of fog and haze is identified according to the concentration of atmospheric fine particles PM2.5 and PM1.0 (referring to fine particles with aerodynamic diameters less than 2.5 and 1.0 microns, respectively).

The English abbreviation for particulate matter is PM. This particle itself is both a pollutant and a carrier of toxic substances such as heavy metals and polycyclic aromatic hydrocarbons.

It can be seen that haze cannot be simply equated to PM2.5; PM2.5 is only one of the main components of haze. PM2.5 refers to particulate matter with a diameter less than or equal to 2.5 microns. It has a small particle size, strong activity, and easily carries toxic and harmful substances. After being inhaled, it can directly enter the bronchi, interfere with gas exchange in the lungs, and may cause diseases such as asthma, bronchitis, and cardiovascular diseases. At the same time, it is also the cause of reduced visibility on haze days, making the sky look gray.

The PM2.5 value mentioned earlier represents the mass concentration of particulate matter. The larger the value, the more pollutants and the more serious the pollution. PM2.5 promotes the formation of haze weather, and haze weather can further intensify the accumulation of PM2.5. So actually, PM2.5 is just one of the components of haze. Haze ≠ PM2.5; the two belong to a containment relationship, not an equivalent relationship.

The main function of PM2.5 particulate concentration sensors is to detect PM2.5 concentrations in the air and remind people to pay attention to the air environment and quality.