Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 124

As the particle size increases relative to the wavelength, the molecular electrons throughout the particle no longer see a uniform field. The intensity and field direction vary at different points within the particle and the electrons will accelerate in different directions, generating a complex scattering pattern. In fact, the scattered light waves can add together constructively or can subtract destructively as they come from the separate locations within the particle. This can lead to light scattering to the back, to the side, and to the front of the particle as shown in Figure 4‐11.

The size range of pollutant particles emitted to the atmosphere often corresponds to the wavelengths of visible light (400–700 nm). Bag houses and electrostatic precipitators used to control the emission of particulate matter will effectively collect particles that are greater than 1 μm (1000 nm) in diameter. It is more difficult, however, to collect particles in the submicron range (<1 μm). These are the particles that will have a higher probability of escaping into the atmosphere. Visible light scattering from these particles is, therefore, typically due to Mie scattering.

Figure 4‐11 Mie scattering. Destructive and constructive interference of light waves scattered from molecular electrons when the particle radius and light wavelength are comparable.