Light extinction of particles
Not all particles are the same, many of them have different shape, size, and composition. Some of them reflect or scatter light, and others absorb it. Two instruments on the image, photometer and nephelometer, measure the amount of light absorbed and scattered by particles.
Photometer measures the amount of light absorbed by particles. If they are too dark, they are going to absorb light.
Some other particles don't absorb light, but they reflect and scatter it in different directions. In this case, nephelometer measures the amount of light scattered by particles.
With the scattering and the absorption, the researchers can estimate the particle's light extinction, the optical property of particles. It is important to know the light extinction of particles, because it is related to visibility, chemical reactions and heat transfer in the atmosphere.