PM2.5 has become a widely concerned social issue. In recent years, the revised "Ambient Air Quality Standards" have included monitoring and control requirements for PM2.5 and established monitoring and reporting in major cities nationwide. Currently, particulate matter monitoring methods mainly include light scattering, gravimetric, micro-oscillating balance, and β-ray absorption methods.

1. Light Scattering Method

Light is scattered by particles and focused by a mirror, then received by a detector positioned at a specific angle to the laser beam. Based on the number and intensity of pulse signals, particle mass concentration is calculated. The light scattering method is an indirect measurement method, often used in portable instruments.

2. Radiation Method

The β-ray attenuation method is used to obtain particle concentration. The device contains a β-ray source and a sensitive detector. The β-ray source emits high-energy electrons (β-rays), and the detector counts the number of β particles. The sampling filter is between the source and detector; the number of β particles passing through decreases as particle concentration increases. The β-ray attenuation formula calculates the mass concentration of particles on the filter and in the sampled air. The β-ray method is recommended by the Ministry of Environmental Protection for automatic PM2.5 mass concentration monitoring.

3. Gravimetric Method

Not affected by particle shape, size, or color, this method is the most reliable when balances are accurate and operated correctly. However, it is cumbersome, time-consuming, not portable, noisy, and cannot provide real-time online monitoring results.

4. Micro-Oscillating Balance Method

TEOM is a classical online particulate concentration monitoring technique, widely adopted globally due to its good agreement with the gravimetric method. The FDMS (Filter Dynamics Measurement System) recovers volatilized particles during sampling for accurate measurement. TEOM+FDMS technology is validated as a more precise PM2.5 monitoring equivalent method and recognized by the US EPA. TEOM is recommended by the Ministry of Environmental Protection for automatic PM2.5 mass concentration monitoring.

Guangzhou Lefmai develops and produces the LD15 laser PM2.5 sensor module using Mie scattering of spherical particles. The laser sensor illuminates airborne particles, and a photoelectric detector collects scattered light at a certain angle. The scattered light intensity is linearly converted into voltage, which is processed according to Mie scattering theory to obtain particle equivalent diameter and particle count per unit volume.